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Astronomy Picture of the Day
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Thumbnail image of picture found for this day. APOD: 2024 April 8 – The Changing Ion Tail of Comet Pons Brooks
Explanation: How does a comet tail change? It depends on the comet. The ion tail of Comet 12P/Pons–Brooks has been changing markedly, as detailed in the featured image sequenced over nine days from March 6 to 14 (top to bottom). On some days, the comet's ion tail was relatively long and complex, but not every day. Reasons for tail changes include the rate of ejection of material from the comet's nucleus, the strength and complexity of the passing solar wind, and the rotation rate of the comet. Over the course of a week, apparent changes even include a change of perspective from the Earth. In general, a comet's ion tail will point away from the Sun, as gas expelled is pushed out by the Sun's wind. Today, Pons-Brooks may become a rare comet suddenly visible in the middle of the day for those able to see the Sun totally eclipsed by the Moon.

Thumbnail image of picture found for this day. APOD: 2024 March 26 – Comet Pons Brooks Ion Tail
Explanation: Comet Pons-Brooks has quite a tail to tell. First discovered in 1385, this erupting dirty snowball loops back into our inner Solar System every 71 years and, this time, is starting to put on a show for deep camera exposures. In the featured picture, the light blue stream is the ion tail which consists of charged molecules pushed away from the comet's nucleus by the solar wind. The ion tail, shaped by the Sun's wind and the comet's core's rotation, always points away from the Sun. Comet 12P/Pons–Brooks is now visible with binoculars in the early evening sky toward the northwest, moving perceptibly from night to night. The frequently flaring comet is expected to continue to brighten, on the average, and may even become visible with the unaided eye -- during the day -- to those in the path of totality of the coming solar eclipse on April 8.

Thumbnail image of picture found for this day. APOD: 2024 March 18 – Comet Pons Brooks Swirling Coma
Explanation: A bright comet will be visible during next month's total solar eclipse. This very unusual coincidence occurs because Comet 12P/Pons-Brooks's return to the inner Solar System places it by chance only 25 degrees away from the Sun during Earth's April 8 total solar eclipse. Currently the comet is just on the edge of visibility to the unaided eye, best visible with binoculars in the early evening sky toward the constellation of the Fish (Pisces). Comet Pons-Brooks, though, is putting on quite a show for deep camera images even now. The featured image is a composite of three very specific colors, showing the comet's ever-changing ion tail in light blue, its outer coma in green, and highlights some red-glowing gas around the coma in a spiral. The spiral is thought to be caused by gas being expelled by the slowly rotating nucleus of the giant iceberg comet. Although it is always difficult to predict the future brightness of comets, Comet Pons-Brook has been particularly prone to outbursts, making it even more difficult to predict how bright it will actually be as the Moon moves in front of the Sun on April 8.

Thumbnail image of picture found for this day. APOD: 2024 March 9 - Comet 12P/Pons-Brooks in Northern Spring
Explanation: As spring approaches for northern skygazers, Comet 12P/Pons-Brooks is growing brighter. Currently visible with small telescopes and binoculars, the Halley-type comet could reach naked eye visibility in the coming weeks. Seen despite a foggy atmosphere, the comet's green coma and long tail hover near the horizon in this well-composed deep night skyscape from Revuca, Slovakia recorded on March 5. M31, also known as the Andromeda galaxy, and bright yellowish star Mirach, beta star of the constellation Andromeda, hang in the sky above the comet. The Andromeda galaxy is some 2.5 million light-years beyond the Milky Way. Comet Pons-Brooks is a periodic visitor to the inner Solar System and less than 14 light-minutes away. Reaching its perihelion on April 21, this comet should be visible in the sky during the April 8 total solar eclipse.

Thumbnail image of picture found for this day. APOD: 2023 December 8 - Vega and Comet 12P/Pons-Brooks
Explanation: On December 4, periodic Comet 12P/Pons-Brooks shared this telescopic field of view with Vega, alpha star of the northern constellation Lyra. Fifth brightest star in planet Earth's night, Vega is some 25 light-years distant while the much fainter comet was about 21 light-minutes away. In recent months, outbursts have caused dramatic increases in brightness for Pons-Brooks though. Nicknamed the Devil Comet for its hornlike appearance, fans of interstellar spaceflight have also suggested the distorted shape of this large comet's central coma looks like the Millenium Falcon. A Halley-type comet, 12P/Pons-Brooks last visited the inner Solar System in 1954. Its next perihelion passage or closest approach to the Sun will be April 21, 2024. That's just two weeks after the April 8 total solar eclipse path crosses North America. But, highly inclined to the Solar System's ecliptic plane, the orbit of periodic Comet 12P/Pons-Brooks will never cross the orbit of planet Earth.

Thumbnail image of picture found for this day. APOD: 2023 November 26 – A Dust Jet from the Surface of Comet 67P
Explanation: Where do comet tails come from? There are no obvious places on the nuclei of comets from which the jets that create comet tails emanate. In 2016, though, ESA's Rosetta spacecraft not only imaged a jet emerging from Comet 67P/Churyumov-Gerasimenko, but flew right through it. Featured is a telling picture showing a bright plume emerging from a small circular dip bounded on one side by a 10-meter high wall. Analyses of Rosetta data show that the jet was composed of both dust and water-ice. The rugged but otherwise unremarkable terrain indicates that something likely happened far under the porous surface to create the plume. This image was taken about two months before Rosetta's mission ended with a controlled impact onto Comet 67P's surface.

Thumbnail image of picture found for this day. APOD: 2023 September 9 – Comet Nishimura Grows
Explanation: Comet Nishimura is growing. More precisely, the tails C/2023 P1 (Nishimura) are growing as it nears the Sun. Discovered only last month, the comet is already near naked eye brightness as it now moves inside the Earth's orbit. The comet will be nearest the Earth next week, but nearest the Sun the week after -- on September 17. Speculation holds that expelled ice and dust from Comet Nishimura's last visit to the inner Solar System may have created the Sigma Hydrids meteor shower which peaks yearly in December. If so, then this meteor shower may become more active, refreshed with new comet debris. Pictured, Comet Nishimura was captured from Edgewood, New Mexico, USA four nights ago, showing a long ion tail structured by interactions with the Sun's wind. Look for this comet near your eastern horizon just before sunrise for the next few mornings, but very near your western horizon just after sunset next week -- as its coma continues to brighten and its tails continue to grow.

Thumbnail image of picture found for this day. APOD: 2023 August 21 – Introducing Comet Nishimura
Explanation: Will Comet Nishimura become visible to the unaided eye? Given the unpredictability of comets, no one can say for sure, but it currently seems like a good bet. The comet was discovered only ten days ago by Hideo Nishimura during 30-second exposures with a standard digital camera. Since then, C/2023 P1 Nishimura has increased in brightness and its path across the inner Solar System determined. As the comet dives toward the Sun, it will surely continue to intensify and possibly become a naked-eye object in early September. A problem is that the comet will also be angularly near the Sun, so it will only be possible to see it near sunset or sunrise. The comet will get so close to the Sun -- inside the orbit of planet Mercury -- that its nucleus may break up. Pictured, Comet Nishimura was imaged three days ago from June Lake, California, USA while sporting a green coma and a thin tail.

Thumbnail image of picture found for this day. APOD: 2023 July 14 - Comet C/2023 E1 ATLAS near Perihelion
Explanation: Comet C/2023 E1 (ATLAS) was just spotted in March, another comet found by the NASA funded Asteroid Terrestrial-impact Last Alert System. On July 1 this Comet ATLAS reached perihelion, its closest approach to the Sun. Days later the telescopic comet was captured in this frame sporting a pretty greenish coma and faint, narrow ion tail against a background of stars in the far northern constellation Ursa Minor. This comet's closest approach to Earth is still to come though. On August 18 this visitor to the inner Solar System will be a mere 3 light-minutes or so from our fair planet. Based on its inclination to the ecliptic plane and orbital period of about 85 years C/2023 E1 (ATLAS) is considered a Halley-type comet.

Thumbnail image of picture found for this day. APOD: 2023 March 24 - Outbound Comet ZTF
Explanation: Former darling of the northern sky Comet C/2022E3 (ZTF) has faded. During its closest approach to our fair planet in early February Comet ZTF was a mere 2.3 light-minutes distant. Then known as the green comet, this visitor from the remote Oort Cloud is now nearly 13.3 light-minutes away. In this deep image, composed of exposures captured on March 21, the comet still sports a broad, whitish dust tail and greenish tinted coma though. Not far on the sky from Orion's bright star Rigel, Comet ZTF shares the field of view with faint, dusty nebulae and distant background galaxies. The telephoto frame is crowded with Milky Way stars toward the constellation Eridanus. The influence of Jupiter's gravity on the comet's orbit as ZTF headed for the inner solar system, may have set the comet on an outbound journey, never to return.

Thumbnail image of picture found for this day. APOD: 2023 February 7 – A Comet and Two Dippers
Explanation: Can you still see the comet? Yes. Even as C/2022 E3 (ZTF) fades, there is still time to see it if you know where and when to look. Geometrically, Comet ZTF has passed its closest to both the Sun and the Earth and is now headed back to the outer Solar System. Its orbit around the Sun has it gliding across the northern sky all month, after passing near Polaris and both the Big and Little Dippers last month. Pictured, Comet ZTF was photographed between the two dippers in late January while sporting an ion tail that extended over 10 degrees. Now below naked-eye visibility, Comet ZTF can be found with binoculars or a small telescope and a good sky map. A good time to see the comet over the next week is after the Sun sets -- but before the Moon rises. The comet will move nearly in front of Mars in a few days

Thumbnail image of picture found for this day. APOD: 2023 January 31 – A Triple View of Comet ZTF
Explanation: Comet ZTF has a distinctive shape. The now bright comet visiting the inner Solar System has been showing not only a common dust tail, ion tail, and green gas coma, but also an uncommonly distinctive antitail. The antitail does not actually lead the comet -- it is just that the head of the comet is seen superposed on part of the fanned-out and trailing dust tail. The giant dirty snowball that is Comet C/2022 E3 (ZTF) has now passed its closest to the Sun and tomorrow will pass its closest to the Earth. The main panel of the featured triple image shows how Comet ZTF looked last week to the unaided eye under a dark and clear sky over Cáceres, Spain. The top inset image shows how the comet looked through binoculars, while the lower inset shows how the comet looked through a small telescope. The comet is now visible all night long from northern latitudes but will surely fade from easy observation during the next few weeks.

Thumbnail image of picture found for this day. APOD: 2023 January 28 - Comet ZTF over Mount Etna
Explanation: Comet-like plumes are blowing over the volcanic peaks of Mount Etna in this wintry mountain-and-skyscape from planet Earth. The stacked and blended combination of individual exposures recorded during the cold night of January 23, also capture naked-eye Comet ZTF just above Etna's snowy slopes. Of course the effect of increasing sunlight on the comet's nucleus and the solar wind are responsible for the comet's greenish coma and broad dusty tail. This weekend Comet ZTF is dashing across northern skies between north star Polaris and the Big Dipper. From a dark site you can only just spot it as a fuzzy patch though. That's still an impressive achievement if you consider you are gazing at a visitor from the distant Oort cloud with your own eyes. But binoculars or a small telescope will make for an even more enjoyable view of this Comet ZTF in the coming days.

Thumbnail image of picture found for this day. APOD: 2023 January 27 - Comet ZTF: Orbital Plane Crossing
Explanation: The current darling of the northern night, Comet C/2022 E3 ZTF is captured in this telescopic image from a dark sky location at June Lake, California, USA. Of course Comet ZTF has been growing brighter in recent days, headed for its closest approach to Earth on February 1. But this view was recorded on January 23, very close to the time planet Earth crossed the orbital plane of long-period Comet ZTF. The comet's broad, whitish dust tail is still curved and fanned out away from the Sun as Comet ZTF sweeps along its orbit. Due to perspective near the orbital plane crossing, components of the fanned out dust tail appear on both sides of the comet's green tinted coma though, to lend Comet ZTF a visually striking (left) anti-tail. Buffeted by solar activity the comet's narrower ion tail also streams away from the coma diagonally to the right, across the nearly three degree wide field of view.

Thumbnail image of picture found for this day. APOD: 2023 January 9 – Tails of Comet ZTF
Explanation: Comet ZTF may become visible to the unaided eye. Discovered early last year, this massive snowball has been brightening as it approaches the Sun and the Earth. C/2022 E3 (ZTF) will be closest to the Sun later this week, at which time it may become visible even without binoculars to northern observers with a clear and dark sky. As they near the Sun, comet brightnesses are notoriously hard to predict, though. In the featured image taken last week in front of a picturesque star field, three blue ion tails extend to the upper right, likely the result of a variable solar wind on ions ejected by the icy comet nucleus. The comet's white dust tail is visible to the upper left and much shorter. The green glow is the comet's coma, caused by glowing carbon gas. Comet ZTF is expected to pass nearest the Earth in early February, after which it should dim dramatically.

Thumbnail image of picture found for this day. APOD: 2022 July 21 - Messier 10 and Comet
Explanation: Imaged on July 15 2022, comet C/2017 K2 (PanSTARRS) had a Messier moment, sharing this wide telescopic field of view with globular star cluster Messier 10. Of course M10 was cataloged by 18th century comet hunter Charles Messier as the 10th object on his list of things that were definitely not comets. While M10 is about 14 thousand light-years distant, this comet PanSTARRS was about 15 light-minutes from our fair planet following its July 14 closest approach. Its greenish coma and dust tail entertaining 21st century comet watchers, C/2017 K2 is expected to remain a fine telescopic comet in northern summer skies. On a maiden voyage from our Solar System's remote Oort Cloud this comet PanSTARRS was discovered in May 2017 when it was beyond the orbit of Saturn. At the time that made it the most distant active inbound comet known. Its closest approach to the Sun will be within 1.8 astronomical units on December 19, beyond the orbital distance of Mars.

Thumbnail image of picture found for this day. APOD: 2022 March 5 - Interstellar Comet 2I Borisov
Explanation: From somewhere else in the Milky Way galaxy, Comet 2I/Borisov was just visiting the Solar System. Discovered by amateur astronomer Gennady Borisov on August 30, 2019, the first known interstellar comet is seen in these two Hubble Space Telescope images from November and December 2019. On the left, a distant background galaxy near the line-of-sight to Borisov is blurred as Hubble tracked the speeding comet and dust tail about 327 million kilometers from Earth. At right, 2I/Borisov appears shortly after perihelion, its closest approach to Sun. European Southern Observatory observations indicate that this comet may never have passed close to any star before its 2019 perihelion passage. Borisov's closest approach to our fair planet, a distance of about 290 million kilometers, came on December 28, 2019. Even though Hubble's sharp images don't resolve the comet's nucleus, they did lead to estimates of less than 1 kilometer for its diameter.

Thumbnail image of picture found for this day. APOD: 2022 January 12 - Comet Leonard Closeup from Australia
Explanation: What does Comet Leonard look like up close? Although we can't go there, imaging the comet's coma and inner tails through a small telescope gives us a good idea. As the name implies, the ion tail is made of ionized gas -- gas energized by ultraviolet light from the Sun and pushed outward by the solar wind. The solar wind is quite structured and sculpted by the Sun's complex and ever changing magnetic field. The effect of the variable solar wind combined with different gas jets venting from the comet's nucleus accounts for the tail's complex structure. Following the wind, structure in Comet Leonard's tail can be seen to move outward from the Sun even alter its wavy appearance over time. The blue color of the ion tail is dominated by recombining carbon monoxide molecules, while the green color of the coma surrounding the head of the comet is created mostly by a slight amount of recombining diatomic carbon molecules. Diatomic carbon is destroyed by sunlight in about 50 hours -- which is why its green glow does not make it far into the ion tail. The featured image was taken on January 2 from Siding Spring Observatory in Australia. Comet Leonard, presently best viewed from Earth's Southern Hemisphere, has rounded the Sun and is now headed out of the Solar System.

Thumbnail image of picture found for this day. APOD: 2021 December 30 - The Further Tail of Comet Leonard
Explanation: Comet Leonard, brightest comet of 2021, is at the lower left of these two panels captured on December 29 in dark Atacama desert skies. Heading for its perihelion on January 3 Comet Leonard's visible tail has grown. Stacked exposures with a wide angle lens (also displayed in a reversed B/W scheme for contrast), trace the complicated ion tail for an amazing 60 degrees, with bright Jupiter shining near the horizon at lower right. Material vaporizing from Comet Leonard's nucleus, a mass of dust, rock, and ices about 1 kilometer across, has produced the long tail of ionized gas fluorescing in the sunlight. Likely flares on the comet's nucleus and buffeting by magnetic fields and the solar wind in recent weeks have resulted in the tail's irregular pinched and twisted appearance. Still days from its closest approach to the Sun, Comet Leonard's activity should continue. The comet is south of the Solar System's ecliptic plane as it sweeps through the southern constellation Microscopium.

Thumbnail image of picture found for this day. APOD: 2021 December 27 - Comet Leonard behind JWST Launch Plume
Explanation: Which one of these two streaks is a comet? Although they both have comet-like features, the lower streak is the only real comet. This lower streak shows the coma and tail of Comet Leonard, a city-sized block of rocky ice that is passing through the inner Solar System as it continues its looping orbit around the Sun. Comet Leonard has recently passed its closest to both the Earth and Venus and will round the Sun next week. The comet, still visible to the unaided eye, has developed a long and changing tail in recent weeks. In contrast, the upper streak is the launch plume of the Ariane V rocket that lifted the James Webb Space Telescope (JWST) off the Earth two days ago. The featured single-exposure image was taken from Thailand, and the foreground spire is atop a pagoda in Doi Inthanon National Park. JWST, NASA's largest and most powerful space telescope so far, will orbit the Sun near the Earth-Sun L2 point and is scheduled to start science observations in the summer of 2022.

Thumbnail image of picture found for this day. APOD: 2021 December 25 - The Tail of a Christmas Comet
Explanation: The tail of a comet streams across this three degree wide telescopic field of view captured under dark Namibian skies on December 21. In outburst only a few days ago and just reaching naked eye visibility Comet Leonard (C/2021 A1) is this year's brightest comet. Binoculars will make the diffuse comet easier to spot though, close to the western horizon after sunset. Details revealed in the sharp image show the comet's coma with a greenish tinge, and follow the interaction of the comet's ion tail with magnetic fields in the solar wind. After passing closest to Earth on December 12 and Venus on December 18, Comet Leonard is heading toward perihelion, its closest approach to the Sun on January 3rd. Appearing in late December's beautiful evening skies Comet Leonard has also become known as 2021's Christmas Comet.

Thumbnail image of picture found for this day. APOD: 2021 December 20 - The Comet and the Fireball
Explanation: This picture was supposed to feature a comet. Specifically, a series of images of the brightest comet of 2021 were being captured: Comet Leonard. But the universe had other plans. Within a fraction of a second, a meteor so bright it could be called a fireball streaked through just below the comet. And the meteor's flash was even more green than the comet's coma. The cause of the meteor's green was likely magnesium evaporating from the meteor's pebble-sized core, while the cause of the comet's green was likely diatomic carbon recently ejected from the comet's city-sized nucleus. The images were taken 10 days ago over the Sacramento River and Mt. Lassen in California, USA. The fireball was on the leading edge of this year's Geminid Meteor Shower -- which peaked a few days later. Comet Leonard is now fading after reaching naked-eye visibility last week -- but now is moving into southern skies.

Thumbnail image of picture found for this day. APOD: 2021 December 12 - Comet Leonard Before Star Cluster M3
Explanation: Comet Leonard is now visible to the unaided eye -- but just barely. Passing nearest to the Earth today, the comet is best seen this week soon after sunset, toward the west, low on the horizon. Currently best visible in the north, by late December the comet will best be seen from south of Earth's equator. The featured image of Comet C/2021 A1 (Leonard) was taken a week ago from California, USA. The deep exposure shows in great detail the comet's green gas coma and developing dust tail. The comet -- across our inner Solar System and only light-minutes away -- was captured passing nearly in front of globular star cluster M3. In contrast, M3 is about 35,000 light-years away. In a week, Comet Leonard will pass unusually close to Venus, but will continue on and be at its closest to the Sun in early January.

Thumbnail image of picture found for this day. APOD: 2021 December 8 - Comet Hale Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp, the Great Comet of 1997, became much brighter than any surrounding stars. It was seen even over bright city lights. Away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail, consisting of ions from the comet's nucleus, is pushed out by the solar wind. The white dust tail is composed of larger particles of dust from the nucleus driven by the pressure of sunlight, that orbit behind the comet. Comet Hale-Bopp (C/1995 O1) remained visible to the unaided eye for 18 months -- longer than any other comet in recorded history. The large comet is next expected to return around the year 4385. This month, Comet Leonard is brightening and may soon become visible to the unaided eye.

Thumbnail image of picture found for this day. APOD: 2021 December 3 - Comet Leonard and the Whale Galaxy
Explanation: Sweeping through northern predawn skies, on November 24 Comet Leonard (C/2021 A1) was caught between two galaxies in this composite telescopic image. Sporting a greenish coma the comet's dusty tail seems to harpoon the heart of NGC 4631 (top) also known as the Whale Galaxy. Of course NGC 4631 and NGC 4656 (bottom, aka the Hockey Stick) are background galaxies some 25 million light-years away. On that date the comet was about 6 light-minutes from our fair planet. Its closest approach to Earth (and even closer approach to Venus) still to come, Comet Leonard will grow brighter in December. Already a good object for binoculars and small telescopes, this comet will likely not return to the inner Solar System. Its perihelion, or closest approach to the Sun, will be on January 3, 2022.

Thumbnail image of picture found for this day. APOD: 2021 November 28 - A High Cliff on Comet Churyumov Gerasimenko
Explanation: This high cliff occurs not on a planet, not on a moon, but on a comet. It was discovered to be part of the dark nucleus of Comet Churyumov-Gerasimenko (CG) by Rosetta, a robotic spacecraft launched by ESA that rendezvoused with the Sun-orbiting comet in 2014. The ragged cliff, as featured here, was imaged by Rosetta in 2014. Although towering about one kilometer high, the low surface gravity of Comet CG would likely make it an accessible climb -- and even a jump from the cliff survivable. At the foot of the cliff is relatively smooth terrain dotted with boulders as large as 20 meters across. Data from Rosetta indicates that the ice in Comet CG has a significantly different deuterium fraction -- and hence likely a different origin -- than the water in Earth's oceans. Rosetta ended its mission with a controlled impact onto Comet CG in 2016. Comet CG has just completed another close approach to Earth and remains visible through a small telescope.

Thumbnail image of picture found for this day. APOD: 2021 November 21 - Introducing Comet Leonard
Explanation: Here comes Comet Leonard. Comet C/2021 A1 (Leonard) was discovered as a faint smudge in January 2021 when it was out past Mars -- but its orbit will take the giant shedding ice-ball into the inner Solar System, passing near both Earth and Venus in December before it swoops around the Sun in early January 2022. Although comets are notoriously hard to predict, some estimations have Comet Leonard brightening to become visible to the unaided eye in December. Comet Leonard was captured just over a week ago already sporting a green-tinged coma and an extended dust tail. The featured picture was composed from 62 images taken through a moderate-sized telescope -- one set of exposures tracking the comet, while another set tracking the background stars. The exposures were taken from the dark skies above the Eastern Sierras (Mountains), near June Lake in California, USA. Soon after passing near the Earth in mid-December, the comet will shift from northern to southern skies.

Thumbnail image of picture found for this day. APOD: 2021 October 22 - A Comet and a Crab
Explanation: This pretty field of view spans over 2 degrees or 4 full moons on the sky, filled with stars toward the constellation Taurus, the Bull. Above and right of center in the frame you can spot the faint fuzzy reddish appearance of Messier 1 (M1), also known as the Crab Nebula. M1 is the first object in 18th century comet hunter Charles Messier's famous catalog of things which are definitely not comets. Made from image data captured this October 11, there is a comet in the picture though. Below center and left lies the faint greenish coma and dusty tail of periodic comet 67P Churyumov-Gerasimenko, also known as Rosetta's comet. In the 21st century, it became the final resting place of robots from planet Earth. Rosetta's comet is now returning to the inner solar system, sweeping toward its next perihelion or closest approach to the Sun, on November 2. Too faint to be seen by eye alone, the comet's next perigee or closest approach to Earth will be November 12.

Thumbnail image of picture found for this day. APOD: 2021 September 10 - Rosetta's Comet in View
Explanation: Faint comet Churyumov-Gerasimenko (67P) sweeps past background stars in the constellation Taurus and even fainter distant galaxies in this telescopic frame from September 7. About 5 years ago, this comet's 4 kilometer spanning, double-lobed nucleus became the final resting place of robots from planet Earth, following the completion of the historic Rosetta mission to the comet. After wandering out beyond the orbit of Jupiter, Churyumov-Gerasimenko is now returning along its 6.4 year periodic orbit toward its next perihelion or closest approach to the Sun, on November 2. On November 12, the comet's perigee, its closest approach to Earth, will bring it within about 0.42 astronomical units. Telescopes should still be required to view it even at its brightest, predicted to be in late November and December. On September 7 Rosetta's comet was about 0.65 astronomical units away or about 5.4 light-minutes from our fair planet.

Thumbnail image of picture found for this day. APOD: 2021 March 8 - Three Tails of Comet NEOWISE
Explanation: What created the unusual red tail in Comet NEOWISE? Sodium. A spectacular sight back in the summer of 2020, Comet NEOWISE, at times, displayed something more than just a surprisingly striated white dust tail and a pleasingly patchy blue ion tail. Some color sensitive images showed an unusual red tail, and analysis showed much of this third tail's color was emitted by sodium. Gas rich in sodium atoms might have been liberated from Comet NEOWISE's warming nucleus in early July by bright sunlight, electrically charged by ultraviolet sunlight, and then pushed out by the solar wind. The featured image was captured in mid-July from Brittany, France and shows the real colors. Sodium comet tails have been seen before but are rare -- this one disappeared by late July. Comet C/2020 F3 (NEOWISE) has since faded, lost all of its bright tails, and now approaches the orbit of Jupiter as it heads back to the outer Solar System, to return only in about 7,000 years.

Thumbnail image of picture found for this day. APOD: 2020 November 12 - Comet ATLAS and Orion's Belt
Explanation: With its closest approach to planet Earth scheduled for November 14, this Comet ATLAS (C/2020 M3) was discovered just this summer, another comet found by the NASA funded Asteroid Terrestrial-impact Last Alert System. It won't get as bright as Comet NEOWISE but it can still be spotted using binoculars, as it currently sweeps through the familiar constellation of Orion. This telephoto field from November 8, blends exposures registered on the comet with exposures registered on Orion's stars. It creates an effectively deep skyview that shows colors and details you can't quite see though, even in binoculars. The comet's telltale greenish coma is toward the upper left, above Orion's three belt stars lined-up across the frame below center. You'll also probably spot the Orion Nebula, and famous Horsehead Nebula in the stunning field of view. Of course one of Orion's belt stars is nearly 2,000 light-years away. On November 14, this comet ATLAS will fly a mere 2.9 light-minutes from Earth.

Thumbnail image of picture found for this day. APOD: 2020 August 3 - Comet NEOWISE over Vikos Gorge
Explanation: Did the Earth part to show us this comet? Of course not, even if this image makes it seem that way. Pictured far in the background is Comet NEOWISE as it appeared about two weeks ago over northern Greece. Above the comet are many stars including the bright stars of the Big Dipper (also the Sorcerer, in Aztec mythology), an asterism that many people around the world used to find the naked-eye comet as it hovered in the northern sky over the past month. In the foreground is Vikos Gorge, the deepest gorge on Earth, relative to its width. The gorge was slowly created by erosion from the Voidomatis River over the past few million years. Capturing this image took a lot of planning, waiting, luck, braving high winds, and avoiding local wolves. Comet C/2020 F3 (NEOWISE) continues to fade and is now best visible through binoculars as it coasts back to the outer Solar System.

Thumbnail image of picture found for this day. APOD: 2020 July 22 – The Structured Tails of Comet NEOWISE
Explanation: What is creating the structure in Comet NEOWISE's tails? Of the two tails evident, the blue ion tail on the left points directly away from the Sun and is pushed out by the flowing and charged solar wind. Structure in the ion tail comes from different rates of expelled blue-glowing ions from the comet's nucleus, as well as the always complex and continually changing structure of our Sun's wind. Most unusual for Comet C/2020 F3 (NEOWISE), though, is the wavy structure of its dust tail. This dust tail is pushed out by sunlight, but curves as heavier dust particles are better able to resist this light pressure and continue along a solar orbit. Comet NEOWISE's impressive dust-tail striations are not fully understood, as yet, but likely related to rotating streams of sun-reflecting grit liberated by ice melting on its 5-kilometer wide nucleus. The featured 40-image conglomerate, digitally enhanced, was captured three days ago through the dark skies of the Gobi Desert in Inner Mongolia, China. Comet NEOWISE will make it closest pass to the Earth tomorrow as it moves out from the Sun. The comet, already fading but still visible to the unaided eye, should fade more rapidly as it recedes from the Earth.

Thumbnail image of picture found for this day. APOD: 2020 July 16 - The Long Tails of Comet NEOWISE
Explanation: This Comet NEOWISE (C/2020 F3) now sweeps through our fair planet's northern skies. Its long tails stretch across this deep skyview from Suchy Vrch, Czech Republic. Recorded on the night of July 13/14, the composite of untracked foreground and tracked and filtered sky exposures teases out details in the comet's tail not visible to the unaided eye. Faint structures extend to the top of the frame, over 20 degrees from the comet's bright coma. Pushed out by the pressure of sunlight itself, the broad curve of the comet's yellowish dust tail is easy to see by eye. But the fainter, more bluish tail is separate from the reflective comet dust. The fainter tail is an ion tail, formed as ions from the cometary coma are dragged outward by magnetic fields in the solar wind and fluoresce in the sunlight. Outbound NEOWISE is climbing higher in northern evening skies, coming closest to Earth on July 23rd.

Thumbnail image of picture found for this day. APOD: 2020 July 14 - Comet NEOWISE over Stonehenge
Explanation: Have you ever seen a comet? Tonight -- and likely the next few nights -- should be a good chance. Go outside just at sunset and look to your northwest. The lower your horizon, the better. Binoculars may help, but if your sky is cloudless and dark, all you should need is your unaided eyes and patience. As the Sun sets, the sky will darken, and there will be an unusual faint streak pointing diagonally near the horizon. That is Comet NEOWISE. It is a 5-kilometer-wide evaporating dirty iceberg visiting from -- and returning to -- the outer Solar System. As the Earth turns, the comet will soon set, so you might want to take a picture. In the featured image, Comet C/2020 F3 (NEOWISE) was captured two mornings ago rising over Stonehenge in the UK. Discovered with the NASA satellite NEOWISE toward the end of March, Comet NEOWISE has surprised many by surviving its closest approach to the Sun, brightening dramatically, and developing impressive (blue) ion and (white) dust tails.

Thumbnail image of picture found for this day. APOD: 2020 July 13 - Comet NEOWISE Rising over the Adriatic Sea
Explanation: This sight was worth getting out of bed early. Comet C/2020 F3 (NEOWISE) has been rising before dawn during the past week to the delight of northern sky enthusiasts awake that early. Up before sunrise, the featured photographer was able to capture in dramatic fashion one of the few comets visible to the unaided eye this century, an inner-Solar System intruder that might become known as the Great Comet of 2020. The resulting video details Comet NEOWISE from Italy rising over the Adriatic Sea. The time-lapse video combines over 240 images taken over 30 minutes. The comet is seen rising through a foreground of bright and undulating noctilucent clouds, and before a background of distant stars. Comet NEOWISE has remained unexpectedly bright, so far, with its ion and dust tails found to emanate from a nucleus spanning about five kilometers across. Fortunately, starting tonight, northern observers with a clear and dark northwestern horizon should be able to see the sun-reflecting interplanetary snowball just after sunset.

Thumbnail image of picture found for this day. APOD: 2020 July 12 - Comet CG Creates Its Dust Tail
Explanation: Where do comet tails come from? There are no obvious places on the nuclei of comets from which the jets that create comet tails emanate. One of the best images of emerging jets is shown in the featured picture, taken in 2015 by ESA's robotic Rosetta spacecraft that orbited Comet 67P/Churyumov-Gerasimenko (Comet CG) from 2014 to 2016. The picture shows plumes of gas and dust escaping numerous places from Comet CG's nucleus as it neared the Sun and heated up. The comet has two prominent lobes, the larger one spanning about 4 kilometers, and a smaller 2.5-kilometer lobe connected by a narrow neck. Analyses indicate that evaporation must be taking place well inside the comet's surface to create the jets of dust and ice that we see emitted through the surface. Comet CG (also known as Comet 67P) loses in jets about a meter of radius during each of its 6.44-year orbits around the Sun, a rate at which will completely destroy the comet in only thousands of years. In 2016, Rosetta's mission ended with a controlled impact onto Comet CG's surface.

Thumbnail image of picture found for this day. APOD: 2020 July 11 - The Tails of Comet NEOWISE
Explanation: Comet NEOWISE (C/2020 F3) is now sweeping through northern skies. Its developing tails stretch some six degrees across this telescopic field of view, recorded from Brno, Czech Republic before daybreak on July 10. Pushed out by the pressure of sunlight itself, the comet's broad, yellowish dust tail is easiest to see. But the image also captures a fainter, more bluish tail too, separate from the reflective comet dust. The fainter tail is an ion tail, formed as ions from the cometary coma are dragged outward by magnetic fields in the solar wind and fluoresce in the sunlight. In this sharp portrait of our new visitor from the outer Solar System, the tails of comet NEOWISE are reminiscent of the even brighter tails of Hale Bopp, the Great Comet of 1997.

Thumbnail image of picture found for this day. APOD: 2020 July 7 - Comet NEOWISE over Lebanon
Explanation: A comet has suddenly become visible to the unaided eye. Comet C/2020 F3 (NEOWISE) was discovered in late March and brightened as it reached its closest approach to the Sun, inside the orbit of Mercury, late last week. The interplanetary iceberg survived solar heating, so far, and is now becoming closer to the Earth as it starts its long trek back to the outer Solar System. As Comet NEOWISE became one of the few naked-eye comets of the 21st Century, word spread quickly, and the comet has already been photographed behind many famous sites and cities around the globe. Featured, Comet NEOWISE was captured over Lebanon two days ago just before sunrise. The future brightness of Comet NEOWISE remains somewhat uncertain but the comet will likely continue to be findable not only in the early morning sky, but also next week in the early evening sky.

Thumbnail image of picture found for this day. APOD: 2020 June 6 - Comet PanSTARRs and the Galaxies
Explanation: Comet PanSTARRs, C/2017 T2, shared this stunning telescopic field of view with galaxies M81 and M82 on May 22/23. Of course, the galaxies were some 12 million light-years distant and the comet about 14 light-minutes away, seen in planet Earth's sky toward the Big Dipper. A new visitor from the Oort Cloud, this Comet PanSTARRs was discovered in 2017 by the PanSTARRs survey telescope when the comet was over 1 light-hour from the Sun, almost as distant as the orbit of Saturn. With a beautiful coma and dust tail, this comet has been a solid northern hemisphere performer for telescope wielding comet watchers this May, following its closest approach to the Sun on May 4. In this deep image from dark California skies the outbound comet even seems to develop a short anti-tail as it leaves the inner Solar System.

Thumbnail image of picture found for this day. APOD: 2020 May 14 - Comet Halley vs Comet SWAN
Explanation: The pre-dawn hours of May 3rd were moonless as grains of cosmic dust streaked through southern skies above Reunion Island. Swept up as planet Earth plowed through dusty debris streams left behind periodic Comet 1/P Halley, the annual meteor shower is known as the Eta Aquarids. This inspired exposure captures a bright aquarid meteor flashing left to right over a sea of clouds. The meteor streak points back to the shower's radiant in the constellation Aquarius, well above the eastern horizon and off the top of the frame. Known for speed Eta Aquarid meteors move fast, entering the atmosphere at about 66 kilometers per second, visible at altitudes of 100 kilometers or so. Then about 6 light-minutes from Earth, the pale greenish coma and long tail of Comet C/2020 F8 SWAN were not to be left out of the celestial scene, posing above the volcanic peaks left of center. Now in the northern sky's morning twilight near the eastern horizon Comet SWAN has not become as bright as anticipated though. This first time comet made its closest approach to planet Earth only two days ago and reaches perihelion on May 27.

Thumbnail image of picture found for this day. APOD: 2020 April 16 - Comet ATLAS Breaks Up
Explanation: Cruising through the inner solar system, Comet ATLAS (C/2019 Y4) has apparently fragmented. Multiple separate condensations within its diffuse coma are visible in this telescopic close-up from April 12, composed of frames tracking the comet's motion against trailing background stars. Discovered at the end of December 2019, this comet ATLAS showed a remarkably rapid increase in brightness in late March. Northern hemisphere comet watchers held out hope that it would become a bright naked-eye comet as it came closer to Earth in late April and May. But fragmenting ATLAS is slowly fading in northern skies. The breakup of comets is not uncommon though. This comet ATLAS is in an orbit similar to the Great Comet of 1844 (C/1844 Y1) and both may be fragments of a single larger comet.

Thumbnail image of picture found for this day. APOD: 2020 March 21 - Comet ATLAS and the Mighty Galaxies
Explanation: Comet ATLAS C/2019 Y4 was discovered by the NASA funded Asteroid Terrestrial-impact Last Alert System, the last comet discovery reported in 2019. Now growing brighter in northern night skies, the comet's pretty greenish coma is at the upper left of this telescopic skyview captured from a remotely operated observatory in New Mexico on March 18. At lower right are M81 and M82, well-known as large, gravitationally interacting galaxies. Seen through faint dust clouds above the Milky Way, the galaxy pair lies about 12 million light-years distant, toward the constellation Ursa Major. In bound Comet ATLAS is about 9 light-minutes from Earth, still beyond the orbit of Mars. The comet's elongated orbit is similar to orbit of the Great Comet of 1844 though, a trajectory that will return this comet to the inner Solar System in about 6,000 years. Comet ATLAS will reach a perihelion or closest approach to the Sun on May 31 inside the orbit of Mercury and may become a naked-eye comet in the coming days.

Thumbnail image of picture found for this day. APOD: 2020 January 27 - Comet CG Evaporates
Explanation: Where do comet tails come from? There are no obvious places on the nuclei of comets from which the jets that create comet tails emanate. One of the best images of emerging jets is shown in the featured picture, taken in 2015 by ESA's robotic Rosetta spacecraft that orbited Comet 67P/Churyumov-Gerasimenko (Comet CG) from 2014 to 2016. The picture shows plumes of gas and dust escaping numerous places from Comet CG's nucleus as it neared the Sun and heated up. The comet has two prominent lobes, the larger one spanning about 4 kilometers, and a smaller 2.5-kilometer lobe connected by a narrow neck. Analyses indicate that evaporation must be taking place well inside the comet's surface to create the jets of dust and ice that we see emitted through the surface. Comet CG (also known as Comet 67P) loses in jets about a meter of radius during each of its 6.44-year orbits around the Sun, a rate at which will completely destroy the comet in only thousands of years. In 2016, Rosetta's mission ended with a controlled impact onto Comet CG's surface.

Thumbnail image of picture found for this day. APOD: 2019 February 19 - Comet Iwamoto Before Spiral Galaxy NGC 2903
Explanation: It isn't every night that a comet passes a galaxy. Last Thursday, though, binocular comet C/2018 Y1 (Iwamoto) moved nearly in front of a spiral galaxy of approximately the same brightness: NGC 2903. Comet Iwamoto was discovered late last year and orbits the Sun in a long ellipse. It last visited the inner Solar System during the Middle Ages, around the year 648. The comet reached its closest point to the Sun -- between Earth and Mars -- on February 6, and its closest point to Earth a few days ago, on February 13. The featured time-lapse video condenses almost three hours into about ten seconds, and was captured last week from Switzerland. At that time Comet Iwamoto, sporting a green coma, was about 10 light minutes distant, while spiral galaxy NGC 2903 remained about 30 million light years away. Two satellites zip diagonally through the field about a third of the way through the video. Typically, a few comets each year become as bright as Comet Iwamoto.

Thumbnail image of picture found for this day. APOD: 2019 February 9 - Comet Iwamoto and the Sombrero Galaxy
Explanation: Comet Iwamoto (C/2018 Y1), shows off a pretty, greenish coma at the upper left in this telescopic field of view. Taken on February 4 from the Mount John Observatory, University of Canterbury, the 30 minute long total exposure time shows the comet sweeping quickly across a background of stars and distant galaxies in the constellation Virgo. The long exposure and Iwamoto's rapid motion relative to the stars and galaxies results in the noticeable blurred streak tracing the the comet's bright inner coma. In fact, the streaked coma gives the comet a remarkably similar appearance to Messier 104 at lower right, popularly known as the Sombrero Galaxy. The comet, a visitor to the inner Solar System, is a mere 4 light-minutes away though, while majestic Messier 104, a spiral galaxy posing edge-on, is 30 million light-years distant. The first binocular comet of 2019, Iwamoto will pass closest to Earth on February 12. This comet's highly elliptical orbit around the Sun stretches beyond the Kuiper belt with an estimated 1,371 year orbital period. That should bring it back to the inner Solar System in 3390 AD.

Thumbnail image of picture found for this day. APOD: 2017 November 13 - Comet Machholz Approaches the Sun
Explanation: Why is Comet Maccholz so depleted of carbon-containing chemicals? Comet 96P/Machholz's original fame derives from its getting closer to the Sun than any other short period comet -- half as close as Mercury -- and doing so every five years. To better understand this unusual comet, NASA's Sun-monitoring SOHO spacecraft tracked the comet during its latest approach to the Sun in October. The featured image composite shows the tail-enhanced comet swooping past the Sun. The Sun's bright surface is hidden from view behind a dark occulter, although parts of the Sun's extended corona are visible. Neighboring stars dot the background. One hypothesis holds that these close solar approaches somehow cause Comet Machholz to shed its carbon, while another hypothesis posits that the comet formed with this composition far away -- possibly even in another star system.

Thumbnail image of picture found for this day. APOD: 2017 April 25 - A Split Ion Tail for Comet Lovejoy E4
Explanation: What's happened to Comet Lovejoy? In the pictured image, a processed composite, the comet was captured early this month after brightening unexpectedly and sporting a long and intricate ion tail. Remarkably, the typically complex effect of the Sun's wind and magnetic field here caused the middle of Comet Lovejoy's ion tail to resemble the head of a needle. Comet C/2017 E4 (Lovejoy) was discovered only last month by noted comet discoverer Terry Lovejoy. The comet reached visual magnitude 7 earlier this month, making it a good target for binoculars and long duration exposure cameras. What's happened to Comet Lovejoy (E4) since this image was taken might be considered even more remarkable -- the comet's nucleus appeared to be disintegrating and fading as it neared its closest approach to the Sun two days ago.

Thumbnail image of picture found for this day. APOD: 2017 April 9 - Comet Hale Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp, the Great Comet of 1997, became much brighter than any surrounding stars. It was seen even over bright city lights. Away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail, consisting of ions from the comet's nucleus, is pushed out by the solar wind. The white dust tail is composed of larger particles of dust from the nucleus driven by the pressure of sunlight, that orbit behind the comet. Comet Hale-Bopp (C/1995 O1) remained visible to the unaided eye for 18 months -- longer than any other comet in recorded history. This year marks the 20th anniversary of Comet Hale-Bopp's last trip to the inner Solar System. The large comet is next expected to return around the year 4385.

Thumbnail image of picture found for this day. APOD: 2017 March 24 - The Comet, the Owl, and the Galaxy
Explanation: Comet 41P/Tuttle-Giacobini-Kresak poses for a Messier moment in this telescopic snapshot from March 21. In fact it shares the 1 degree wide field-of-view with two well-known entries in the 18th century comet-hunting astronomer's famous catalog. Sweeping through northern springtime skies just below the Big Dipper, the faint greenish comet was about 75 light-seconds from our fair planet. Dusty, edge-on spiral galaxy Messier 108 (bottom center) is more like 45 million light-years away. At upper right, the planetary nebula with an aging but intensely hot central star, the owlish Messier 97 is only about 12 thousand light-years distant though, still well within our own Milky Way galaxy. Named for its discoverer and re-discoverers, this faint periodic comet was first sighted in 1858 and not again until 1907 and 1951. Matching orbit calculations indicated that the same comet had been observed at widely separated times. Nearing its best apparition and closest approach to Earth in over 100 years on April 1, comet 41P orbits the Sun with a period of about 5.4 years.

Thumbnail image of picture found for this day. APOD: 2017 February 20 - Almost Three Tails for Comet Encke
Explanation: How can a comet have three tails? Normally, a comet has two tails: an ion tail of charged particles emitted by the comet and pushed out by the wind from the Sun, and a dust tail of small debris that orbits behind the comet but is also pushed out, to some degree, by the solar wind. Frequently a comet will appear to have only one tail because the other tail is not easily visible from the Earth. In the featured unusual image, Comet 2P/Encke appears to have three tails because the ion tail split just near to the time when the image was taken. The complex solar wind is occasionally turbulent and sometimes creates unusual structure in an ion tail. On rare occasions even ion-tail disconnection events have been recorded. An image of the Comet Encke taken two days later gives a perhaps less perplexing perspective.

Thumbnail image of picture found for this day. APOD: 2017 January 2 - Comet 45P Returns
Explanation: An old comet has returned to the inner Solar System. Not only is Comet 45P/HondaMrkosPajdukov physically ancient, it was first discovered 13 orbits ago in 1948. Comet 45P spends most of its time out near the orbit of Jupiter and last neared the Sun in 2011. Over the past few months, however, Comet 45P's new sunward plummet has brightened it considerably. Two days ago, the comet passed the closest part of its orbit to the Sun. The comet is currently visible with binoculars over the western horizon just after sunset, not far from the much brighter planet Venus. Pictured, Comet 45P was captured last week sporting a long ion tail with impressive structure. Comet 45P will pass relatively close to the Earth early next month.

Thumbnail image of picture found for this day. APOD: 2016 October 3 - Explore Rosetta's Comet
Explanation: What would it be like to fly around a comet nucleus? To find out, just wait for your WebGL-compatible browser to load a detailed digital model of Comet 67P and then -- go exploring! With a standard mouse, the left button allows you to rotate the comet, the right button allows you to move the comet around, and the scroll wheel allows you to zoom in. ESA's robotic Rosetta spacecraft orbited Comet C67/P Churyumov-Gerasimenko from mid-2014 until last Friday, when, after a remarkable and successful mission, it was intentionally set down on the surface and powered down. Among many notable scientific achievements, Rosetta allowed humanity to better understand where comet jets form on comets as they near the Sun.

Thumbnail image of picture found for this day. APOD: 2016 March 25 - Close Comet and the Milky Way
Explanation: Comet 252P/Linear's lovely greenish coma is easy to spot in this expansive southern skyscape. Visible to the naked eye from the dark site near Flinders, Victoria, Australia, the comet appears tailless. Still, its surprisingly bright coma spans about 1 degree, posed here below the nebulae, stars, and dark rifts of the Milky Way. The five panels used in the wide-field mosaic were captured after moonset and before morning twilight on March 21. That was less than 24 hours from the comet's closest approach, a mere 5.3 million kilometers from our fair planet. Sweeping quickly across the sky because it is so close to Earth, the comet should be spotted in the coming days by northern hemisphere comet watchers. In predawn but moonlit skies it will move through Sagittarius and Scorpius seen toward the southern horizon. That's near the triangle formed by bright, yellowish, Mars, Saturn, and Antares at the upper left of this frame.

Thumbnail image of picture found for this day. APOD: 2016 March 17 - Close Comet and Large Magellanic Cloud
Explanation: Sporting a surprisingly bright, lovely green coma Comet 252P/Linear poses next to the Large Magellanic Cloud in this southern skyscape. The stack of telephoto exposures was captured on March 16 from Penwortham, South Australia. Recognized as a Jupiter family periodic comet, 252P/Linear will come close to our fair planet on March 21, passing a mere 5.3 million kilometers away. That's about 14 times the Earth-Moon distance. In fact, it is one of two comets that will make remarkably close approaches in the next few days as a much fainter Comet Pan-STARRS (P/2016 BA14) comes within 3.5 million kilometers (9 times the Earth-Moon distance) on March 22. The two have extremely similar orbits, suggesting they may have originally been part of the same comet. Sweeping quickly across the sky because of their proximity to Earth, both comets will soon move into northern skies.

Thumbnail image of picture found for this day. APOD: 2016 February 2 - Comet 67P from Spacecraft Rosetta
Explanation: Spacecraft Rosetta continues to circle and map Comet Churyumov-Gerasimenko. Crossing the inner Solar System for ten years to reach the vicinity of the comet in 2014, the robotic spacecraft continues to image the unusual double-lobed comet nucleus. The featured image, taken one year ago, shows dust and gas escaping from the comet's nucleus. Although appearing bright here, the comet's surface reflects only about four percent of impinging visible light, making it as dark as coal. Comet 67P/ChuryumovGerasimenko spans about four kilometers in length and has a surface gravity so low that an astronaut could jump off of it. With Rosetta in tow, Comet 67P passed its closest to the Sun last year and is now headed back to the furthest point -- just past the orbit of Jupiter.

Thumbnail image of picture found for this day. APOD: 2015 November 18 - A Sudden Jet on Comet 67P
Explanation: There she blows! A dramatic demonstration of how short-lived some comet jets can be was documented in late July by the robotic Rosetta spacecraft orbiting the nucleus of Comet 67P/Churyumov-Gerasimenko. The featured animation depicts changes in the rotating comet with three illuminating stills. Although the first frame shows nothing unusual, the second frame shows a sudden strong jet shooting off the 67P's surface only 20 minutes later, while the third frame -- taken 20 minutes after that -- shows but a slight remnant of the once-active jet. As comets near the Sun, they can produce long and beautiful tails that stream across the inner Solar System. How comet jets produce these tails is a topic of research -- helped by images like this. Another recent Rosetta measurement indicates that the water on Earth could not have come from comets like 67P because of significant differences in impurities. Comet 67P spans about four kilometers, orbits the Sun between Earth and Jupiter, and has been the home for ESA's Rosetta spaceship since 2014 August. Rosetta is currently scheduled to make a slow crash onto Comet 67P's surface in late 2016.

Thumbnail image of picture found for this day. APOD: 2015 August 18 - Announcing Comet Catalina
Explanation: Will Comet Catalina become visible to the unaided eye? Given the unpredictability of comets, no one can say for sure, but it seems like a good bet. The comet was discovered in 2013 by observations of the Catalina Sky Survey. Since then, Comet C/2013 US10 (Catalina) has steadily brightened and is currently brighter than 8th magnitude, making it visible with binoculars and long-duration camera images. As the comet further approaches the inner Solar System it will surely continue to intensify, possibly becoming a naked eye object sometime in October and peaking sometime in late November. The comet will reside primarily in the skies of the southern hemisphere until mid-December, at which time its highly inclined orbit will bring it quickly into northern skies. Featured above, Comet Catalina was imaged last week sporting a green coma and two growing tails.

Thumbnail image of picture found for this day. APOD: 2015 July 21 - Comet Tails and Star Trails
Explanation: After grazing the western horizon on northern summer evenings Comet PanSTARRS (also known as C/2014 Q1) climbed higher in southern winter skies. A visitor to the inner Solar System discovered in August 2014 by the prolific panSTARRS survey, the comet was captured here on July 17. Comet and colorful tails were imaged from Home Observatory in Mackay, Queensland, Australia. The field of view spans just over 1 degree. Sweeping quickly across a the sky this comet PanSTARRS was closest to planet Earth about 2 days later. Still, the faint stars of the constellation Cancer left short trails in the telescopic image aligned to track the comet's rapid motion. PanSTARRS' bluish ion tails stream away from the Sun, buffetted by the solar wind. Driven by the pressure of sunlight, its more diffuse yellowish dust tail is pushed outward and lags behind the comet's orbit. A good target for binoculars from southern latitudes, in the next few days the comet will sweep through skies near Venus, Jupiter, and bright star Regulus.

Thumbnail image of picture found for this day. APOD: 2015 July 20 - Comet PanSTARRS and a Crescent Moon
Explanation: A comet has brightened quickly and unexpectedly. Discovered last year, Comet C/2014 Q1 (PanSTARRS) is expected to be visible now for a few days to the unaided eye, just after sunset, from some locations. The comet rounded the Sun on July 6 and apparently has shed quite a bit of gas and dust. Today it is now as close as it will ever get to the Earth, which is another factor in its recent great apparent brightness and the large angular extent of its tails. In the featured image taken two days ago, Comet PanSTARRS is seen sporting a short white dust tail fading to the right, and a long blue ion tail pointing away from the recently set Sun. A crescent moon dominates the image center. Tomorrow, Comet PannSTARRS will pass only 7 degrees away from a bright Jupiter, with even brighter Venus nearby. Due to its proximity to the Sun, the comet and its tails may best be seen in the sunset din with binoculars or cameras using long-duration exposures.

Thumbnail image of picture found for this day. APOD: 2015 April 29 - Comet Churyumov Gerasimenko in Crescent
Explanation: What's happening to Comet 67P/ChuryumovGerasimenko? As the 3-km wide comet moves closer to the Sun, heat causes the nucleus to expel gas and dust. The Rosetta spacecraft arrived at the comet's craggily double nucleus last July and now is co-orbiting the Sun with the giant dark iceberg. Recent analysis of data beamed back to Earth from the robotic Rosetta spacecraft has shown that water being expelled by 67P has a significant difference with water on Earth, indicating that Earth's water could not have originated from ancient collisions with comets like 67P. Additionally, neither Rosetta nor its Philae lander detected a magnetic field around the comet nucleus, indicating that magnetism might have been unimportant in the evolution of the early Solar System. Comet 67P, shown in a crescent phase in false color, should increase its evaporation rate as it nears its closest approach to the Sun in 2015 August, when it reaches a Sun distance just a bit further out than the Earth.

Thumbnail image of picture found for this day. APOD: 2015 February 3 - Jets from Comet Churyumov Gerasimenko
Explanation: Where do comet tails come from? Although it is common knowledge that comet tails and comas originate from comet nuclei, exactly how that happens is an active topic of research. One of the best images yet of emerging jets is shown in the featured image, taken last November by the robotic Rosetta spacecraft in orbit around the Comet 67P/Churyumov-Gerasimenko (Comet CG), and released last month. The overexposed picture shows plumes of gas and dust escaping numerous places from the Comet CG's nucleus as it nears the Sun and heats up. Although Comet CG is currently further out from the Sun than Mars, its orbit will take it almost as close as the Earth this coming August, at which time its jet activity is expected to increase by a factor of about 100. You've likely seen some debris from comet nuclei before but in another form -- when sand-sized bits end their journey through the Solar System by impacting the atmosphere of Earth as meteors.

Thumbnail image of picture found for this day. APOD: 2015 January 21 - The Complex Ion Tail of Comet Lovejoy
Explanation: What causes the structure in Comet Lovejoy's tail? Comet C/2014 Q2 (Lovejoy), which is currently at naked-eye brightness and near its brightest, has been showing an exquisitely detailed ion tail. As the name implies, the ion tail is made of ionized gas -- gas energized by ultraviolet light from the Sun and pushed outward by the solar wind. The solar wind is quite structured and sculpted by the Sun's complex and ever changing magnetic field. The effect of the variable solar wind combined with different gas jets venting from the comet's nucleus accounts for the tail's complex structure. Following the wind, structure in Comet Lovejoy's tail can be seen to move outward from the Sun even alter its wavy appearance over time. The blue color of the ion tail is dominated by recombining carbon monoxide molecules, while the green color of the coma surrounding the head of the comet is created mostly by a slight amount of recombining diatomic carbon molecules. The featured three-panel mosaic image was taken nine days ago from the IRIDA Observatory in Bulgaria. Comet Lovejoy made it closest pass to the Earth two weeks ago and will be at its closest to the Sun in about ten days. After that, the comet will fade as it heads back into the outer Solar System, to return only in about 8,000 years.

Thumbnail image of picture found for this day. APOD: 2014 December 31 - Comet Lovejoy before a Globular Star Cluster
Explanation: Comet Lovejoy has become visible to the unaided eye. To see the comet, just go outside an hour or so after sunset and look for a fuzzy patch to the right of Orion's belt. Binoculars and a star chart may help. Pictured here, Comet C/2014 Q2 (Lovejoy) was captured three days ago passing nearly in front of M79, the globular star cluster visible as the bright spot slightly above and to the left of the comet's green-hued coma. The nucleus of Comet Lovejoy is a giant dirty iceberg that is shedding gas into a long and intricate ion tail, extending across the image, as it nears the Sun. The comet is expected to become even easier to spot for northern observers during January, as it is rises earlier and, hopefully, continues to brighten.

Thumbnail image of picture found for this day. APOD: 2014 December 25 - This Comet Lovejoy
Explanation: Comet Lovejoy, C/2014 Q2, is framed like a cosmic Christmas tree with starry decorations in this colorful telescopic portrait, snapped on December 16th. Its lovely coma is tinted green by diatomic C2 gas fluorescing in sunlight. Discovered in August of this year, this Comet Lovejoy is currently sweeping north through the constellation Columba, heading for Lepus south of Orion and bright enough to offer good binocular views. Not its first time through the inner Solar System, this Comet Lovejoy will pass closest to planet Earth on January 7, while its perihelion (closest point to the Sun) will be on January 30. Of course, planet Earth's own 2015 perihelion passage is scheduled for January 4. A long period comet, this Comet Lovejoy should return again ... in about 8,000 years.

Thumbnail image of picture found for this day. APOD: 2014 December 23 - The Cliffs of Comet Churyumov Gerasimenko
Explanation: These high cliffs occur on the surface of a comet. They were discovered to be part of the dark nucleus of Comet ChuryumovGerasimenko (CG) by Rosetta, a robotic spacecraft launched by ESA which began orbiting the comet in early August. The ragged cliffs, as featured here, were imaged by Rosetta about two weeks ago. Although towering about one kilometer high, the low surface gravity of Comet CG would likely make a jump from the cliffs, by a human, survivable. At the foot of the cliffs is relatively smooth terrain dotted with boulders as large as 20 meters across. Data from Rosetta indicates that the ice in Comet CG has a significantly different deuterium fraction -- and hence likely a different origin -- than the water in Earth's oceans. The Rosetta spacecraft is scheduled to continue to accompany the comet as it makes its closest approach to the Sun in 2015 August.

Thumbnail image of picture found for this day. APOD: 2014 October 20 - Comet Siding Spring Passes Mars
Explanation: Yesterday, a comet passed very close to Mars. In fact, Comet C/2013 A1 (Siding Spring) passed closer to the red planet than any comet has ever passed to Earth in recorded history. To take advantage of this unique opportunity to study the close interaction of a comet and a planet, humanity currently has five active spacecraft orbiting Mars: NASA's MAVEN, MRO, Mars Odyssey, as well as ESA's Mars Express, and India's Mars Orbiter. Most of these spacecraft have now sent back information that they have not been damaged by small pieces of the passing comet. These spacecraft, as well as the two active rovers on the Martian surface -- NASA's Opportunity and Curiosity -- have taken data and images that will be downloaded to Earth for weeks to come and likely studied for years to come. The featured image taken yesterday, however, was not taken from Mars but from Earth and shows Comet Siding Spring on the lower left as it passed Mars, on the upper right.

Thumbnail image of picture found for this day. APOD: 2014 October 19 - Comet McNaught Over New Zealand
Explanation: Comet McNaught was perhaps the most photogenic comet of modern times -- from Earth. After making quite a show in the northern hemisphere in early January of 2007, the comet moved south and developed a long and unusual dust tail that dazzled southern hemisphere observers. In late January 2007, Comet McNaught was captured between Mount Remarkable and Cecil Peak in this spectacular image taken from Queenstown, South Island, New Zealand. The bright comet dominates the right part of the above image, while the central band of our Milky Way Galaxy dominates the left. Careful inspection of the image will reveal a meteor streak just to the left of the comet. Today, Comet Siding Spring may become the most photogenic comet of modern times -- from Mars.

Thumbnail image of picture found for this day. APOD: 2014 October 17 - Messier 6 and Comet Siding Spring
Explanation: This looks like a near miss but the greenish coma and tail of Comet Siding Spring (C/2013 A1) are really 2,000 light-years or so away from the stars of open cluster Messier 6. They do appear close together though, along the same line-of-sight in this gorgeous October 9th skyscape toward the constellation Scorpius. Still, on Sunday, October 19th this comet really will be involved in a near miss, passing within only 139,500 kilometers of planet Mars. That's about 10 times closer than any known comet flyby of planet Earth, and nearly one third the Earth-Moon distance. While an impact with the nucleus is not a threat the comet's dust, moving with a speed of about 56 kilometers per second relative to the Red Planet, and outskirts of its gaseous coma could interact with the thin Martian atmosphere. Of course, the comet's close encounter will be followed intently by spacecraft in Martian orbit and rovers on the surface.

Thumbnail image of picture found for this day. APOD: 2014 September 15 - 62 Kilometers above Comet Churyumov Gerasimenko
Explanation: Spacecraft Rosetta continues to approach, circle, and map Comet Churyumov-Gerasimenko. Crossing the inner Solar System for ten years to reach the vicinity of the comet last month, the robotic spacecraft continues to image the unusual double-lobed comet nucleus. The reconstructed-color image featured, taken about 10 days ago, indicates how dark this comet nucleus is. On the average, the comet's surface reflects only about four percent of impinging visible light, making it as dark as coal. Comet 67P/ChuryumovGerasimenko spans about four kilometers in length and has a surface gravity so low that an astronaut could jump off of it. In about two months, Rosetta is scheduled to release the first probe ever to attempt a controlled landing on a comet's nucleus.

Thumbnail image of picture found for this day. APOD: 2014 August 11 - Rosetta Approaches Comet Churyumov Gerasimenko
Explanation: What does it look like to approach a comet? Early this month humanity received a new rendition as the robotic Rosetta spacecraft went right up to -- and began orbiting -- the nucleus of Comet 67P/Churyumov-Gerasimenko. This approach turned out to be particularly fascinating because the comet nucleus first revealed itself to have an unexpected double structure, and later showed off an unusual and craggily surface. The above 101-frame time-lapse video details the approach of the spacecraft from August 1 through August 6. The icy comet's core is the size of a mountain and rotates every 12.7 hours. Rosetta's images and data may shed light on the origin of comets and the early history of our Solar System. Later this year, Rosetta is scheduled to release the Philae lander, which will attempt to land on Comet ChuryumovGerasimenko's periphery and harpoon itself to the surface.

Thumbnail image of picture found for this day. APOD: 2014 July 21 - Spacecraft Rosetta Shows Comet has Two Components
Explanation: Why does this comet's nucleus have two components? The surprising discovery that Comet 67P/ChuryumovGerasimenko has a double nucleus came late last week as ESA's robotic interplanetary spacecraft Rosetta continued its approach toward the ancient comet's core. Speculative ideas on how the double core was created include, currently, that Comet ChuryumovGerasimenko is actually the result of the merger of two comets, that the comet is a loose pile of rubble pulled apart by tidal forces, that ice evaporation on the comet has been asymmetric, or that the comet has undergone some sort of explosive event. Pictured above, the comet's unusual 5-km sized comet nucleus is seen rotating over the course of a few hours, with each frame taken 20-minutes apart. Better images -- and hopefully more refined theories -- are expected as Rosetta is on track to enter orbit around Comet ChuryumovGerasimenko's nucleus early next month, and by the end of the year, if possible, land a probe on it.

Thumbnail image of picture found for this day. APOD: 2013 December 9 - Comet Lovejoy over a Windmill
Explanation: Lovejoy continues to be an impressive camera comet. Pictured above, Comet C/2013 R1 (Lovejoy) was imaged above the windmill in Saint-Michel-l'Observatoire in southern France with a six-second exposure. In the foreground is a field of lavender. Comet Lovejoy should remain available for photo opportunities for northern observers during much of December and during much of the night, although it will be fading as the month progresses and highest in the sky before sunrise. In person, the comet will be best viewed with binoculars. A giant dirty snowball, Comet Lovejoy last visited the inner Solar System about 7,000 years ago, around the time that humans developed the wheel.

Thumbnail image of picture found for this day. APOD: 2013 November 27 - Comet ISON Rising
Explanation: Will Comet ISON survive tomorrow's close encounter with the Sun? Approaching to within a solar diameter of the Sun's surface, the fate of one of the most unusual comets of modern times will finally be determined. The comet could shed a great amount of ice and dust into a developing tail -- or break apart completely. Unfortunately, the closer Comet ISON gets to the Sun, the harder it has been for conventional telescopes to see the brightening comet in the glare of the morning Sun. Pictured in the above short time lapse video, Comet ISON was captured rising over the Canary Islands just above the morning Sun a few days ago. If the comet's nucleus survives, the coma and the tails it sheds might well be visible rising ahead of the Sun in the next few days or weeks. Alternatively, satellites watching the Sun might document one of the larger comet disintegrations yet recorded. Stay tuned!

Thumbnail image of picture found for this day. APOD: 2013 November 24 - Comet Hale Bopp Over Indian Cove
Explanation: Comet Hale-Bopp, the Great Comet of 1997, was quite a sight. In the above photograph taken on 1997 April 6, Comet Hale-Bopp was imaged from the Indian Cove Campground in the Joshua Tree National Park in California, USA. A flashlight was used to momentarily illuminate foreground rocks in this six minute exposure. An impressive blue ion tail was visible above a sunlight-reflecting white dust tail. Comet Hale-Bopp remained visible to the unaided eye for over a year before returning to the outer Solar System and fading. As Comet ISON approaches the Sun this week, sky enthusiasts around the Earth are waiting to see if its tails will become even more spectacular than those displayed by Comet Hale-Bopp.

Thumbnail image of picture found for this day. APOD: 2013 November 17 - The Magnificent Tail of Comet McNaught
Explanation: Comet McNaught, the Great Comet of 2007, grew a spectacularly long and filamentary tail. The magnificent tail spread across the sky and was visible for several days to Southern Hemisphere observers just after sunset. The amazing tail showed its greatest extent on long-duration, wide-angle camera exposures. During some times, just the tail itself estimated to attain a peak brightness of magnitude -5 (minus five), was caught by the comet's discoverer in the above image just after sunset in January 2007 from Siding Spring Observatory in Australia. Comet McNaught, the brightest comet in decades, then faded as it moved further into southern skies and away from the Sun and Earth. Within the next two weeks of 2013, rapidly brightening Comet ISON might sprout a tail that rivals even Comet McNaught.

Thumbnail image of picture found for this day. APOD: 2013 November 9 - Comet Lovejoy with M44
Explanation: While anxiously waiting for Comet ISON to brighten further as it falls toward the Sun, northern skygazers can also find three other bright comets in the east before dawn. In fact, Comet Lovejoy C/2013 R1 is currently the morning sky's brightest. Only discovered in September and not a sungrazing comet, this Comet Lovejoy is nearing the edge of naked-eye visibility and might be spotted from very dark sky sites. Sporting a greenish coma and tail in this telescopic view taken on November 7, Comet Lovejoy is about 0.5 AU from our fair planet and 1.2 AU from the Sun. The comet is having a photogenic Messier moment, sweeping past well known star cluster M44, the Beehive in Cancer. Yellowish bright star Delta Cancri is near the bottom of the frame.

Thumbnail image of picture found for this day. APOD: 2013 October 28 - The Great Comet of 1680 Over Rotterdam
Explanation: Was there ever another comet like ISON? Although no two comets are exactly alike, one that appears to have had notable similarities was Comet Kirch, the Great Comet of 1680. Like approaching Comet ISON, Comet Kirch was a bright sungrazer, making a very close approach to the surface of the Sun. Neither comet, coincidently, is a member of the most common group of sungrazers -- the Kreutz group -- populated by remnants of a comet that disintegrated near the Sun hundreds of years ago. The long tail of Comet Kirch is depicted in the above painting by Lieve Versheier. As pictured, some members of the foreground crowd of Rotterdam in the Netherlands are holding cross-staffs, an angle measuring device that predated the sextant. No one knows how Comet ISON will develop, but like Comet Kirch, it is expected to be brightest when very near the Sun, in ISON's case during last few days of November.

Thumbnail image of picture found for this day. APOD: 2013 October 13 - Hale Bopp: The Great Comet of 1997
Explanation: Sixteen years ago, Comet Hale-Bopp rounded the Sun and offered a dazzling spectacle in planet Earth's night. This stunning view, recorded shortly after the comet's 1997 perihelion passage, features the memorable tails of Hale-Bopp -- a whitish dust tail and blue ion tail. Here, the ion tail extends well over ten degrees across the northern sky, fading near the double star clusters in Perseus, while the head of the comet lies near Almach, a bright star in the constellation Andromeda. Do you remember Hale-Bopp? The photographer's sons do, pictured in the foreground at ages 12 and 15. In all, Hale-Bopp was reported as visible to the naked eye from roughly late May 1996 through September 1997. Currently, sky enthusiasts await Comet ISON's continued brightening in the coming weeks, unsure how interesting its first journey to the inner Solar System will be.

Thumbnail image of picture found for this day. APOD: 2013 October 7 - Comet ISON Approaches
Explanation: How impressive will Comet ISON become? No one is sure, but unfortunately, as the comet approaches the inner Solar System, it is brightening more slowly than many early predictions. Pictured above, Comet ISON is seen about two weeks ago as it continued to develop a tail. Last week the comet passed relatively close to Mars, and was directly imaged by the Mars Reconnaissance Orbiter. When Comet ISON dives to within a few solar radii of the Sun's surface in late November, it may become brighter than the Moon and sport a long and flowing tail -- or it may appear somewhat less spectacular. Either way, sky enthusiasts hope that whatever comet parts survive will put on quite an impressive show, as viewed from Earth, through at least the rest of the year.

Thumbnail image of picture found for this day. APOD: 2013 May 18 - Comet PanSTARRS with Anti Tail
Explanation: Once the famous sunset comet, PanSTARRS (C/2011 L4) is now visible all night from much of the northern hemisphere, bound for the outer solar system as it climbs high above the ecliptic plane. Dimmer and fading, the comet's broad dust tail is still growing, though. This widefield telescopic image was taken against the starry background of the constellation Cepheus on May 15. It shows the comet has developed an extensive anti-tail, dust trailing along the comet's orbit (to the left of the coma), stretching more than 3 degrees across the frame. Since the comet is just over 1.6 astronomical units from planet Earth, that corresponds to a distance of over 12 million kilometers. In late May Comet PanSTARRS will pass within a few degrees of the north celestial pole.

Thumbnail image of picture found for this day. APOD: 2013 May 6 - Tails of Comet Lemmon
Explanation: What caused the interestingly intricate tails that Comet Lemmon displayed earlier this year? First of all, just about every comet that nears the Sun displays two tails: a dust tail and an ion tail. Comet Lemmon's dust tail, visible above and around the comet nucleus in off-white, is produced by sun-light reflecting dust shed by the comet's heated nucleus. Flowing and more sculptured, however, is C/2012 F6 (Lemmon)'s blue ion tail, created by the solar wind pushing ions expelled by the nucleus away from the Sun. Also of note is the coma seen surrounding Comet Lemmon's nucleus, tinted green by diatomic C2 gas fluorescing in sunlight. The above image was taken from the dark skies of Namibia in mid-April. Comet Lemmon is fading as it now heads back to the outer Solar System.

Thumbnail image of picture found for this day. APOD: 2013 April 5 - Comet of the North
Explanation: It looks like a double comet, but Comet PanSTARRS (C/2011 L4) is just offering skygazers a Messier moment. Outward bound and fading in this starry scene, the well-photographed comet is remarkably similar in brightness to M31, the Andromeda Galaxy. Tracking through northern skies just below the galaxy, the comet was captured as local midnight approached on April 3. Both comet and galaxy were visible to the eye and are immersed in the faint glow of northern lights. Our own Milky Way galaxy arcs over the snowy field near Tänndalen, Sweden. Double star cluster h and chi Persei can be spotted along the Milky Way's arc high above the comet/galaxy pair. Follow the arc to bright Deneb, alpha star of the constellation Cygnus, at the right edge of the frame.

Thumbnail image of picture found for this day. APOD: 2013 March 22 - Comet Castle
Explanation: The broad dust tail of Comet PanSTARRS (C/2011 L4) has become a familiar sight for many northern hemisphere comet watchers, as the comet fades but rises higher above the western horizon after sunset. This view of the popular comet may seem a little fantastic, though. Sweeping away from the Sun and trailing behind the comet's orbit, the curving dust tail also seems to stream away from a shining mountaintop castle. Comet Castle might be an appropriate name in this scene, but its traditional name is Castle Hohenzollern. Taken on March 15 with an extreme telephoto lens, the Comet Castle image was captured in exceptionally clear skies about 80 kilometers away from Stuttgart, Germany.

Thumbnail image of picture found for this day. APOD: 2013 March 18 - Comet PANSTARRS Just After Sunset
Explanation: Have you seen the comet? As Comet PANSTARRS fades, careful observers -- even with unaided eyes -- should still be able to find the shedding ice ball on the western horizon just after sunset. Pictured above, Comet PANSTARRS (C/2011 L4) was pointed out from a hilltop last week on First Encounter Beach in Massachusetts, USA. The comet was discovered by -- and is named for -- the Pan-STARRS astronomical sky survey that discovered it. As the comet now recedes from both the Earth and the Sun, it will remain visible further into the night, although binoculars or a small telescope will soon to be needed to find it.

Thumbnail image of picture found for this day. APOD: 2013 March 15 - CME, Comet, and Planet Earth
Explanation: After appearing in a popular photo opportunity with a young crescent Moon near sunset, naked-eye Comet PanSTARRS continues to rise in northern hemisphere skies. But this remarkable interplanetary perspective from March 13, finds the comet posing with our fair planet itself - as seen from the STEREO Behind spacecraft. Following in Earth's orbit, the spacecraft is nearly opposite the Sun and looks back toward the comet and Earth, with the Sun just off the left side of the frame. At the left an enormous coronal mass ejection (CME) is erupting from a solar active region. Of course, CME, comet, and planet Earth are all at different distances from the spacecraft. (The comet is closest.) The processed digital image is the difference between two consecutive frames from the spacecraft's SECCHI Heliospheric Imager, causing the strong shadowing effect for objects that move between frames. Objects that are too bright create the sharp vertical lines. The processing reveals complicated feather-like structures in Comet PanSTARRS's extensive dust tail.

Thumbnail image of picture found for this day. APOD: 2013 February 7 - Comet Lemmon near the South Celestial Pole
Explanation: Currently sweeping through southern skies, Comet Lemmon (C/2012 F6) was named for its discovery last year as part of the Mount Lemmon (Arizona) Survey. Brighter than expected but still just below naked-eye visibility, Comet Lemmon sports a stunning lime green coma and faint divided tail in this telescopic image from February 4. The greenish tint comes from the coma's diatomic C2 gas fluorescing in sunlight. Captured from an observatory near Sydney, Australia, the color composite is constructed from a series of individual exposures registered on the comet. Across the 1 degree wide field of view, the star trails are a consequence of the comet's relatively rapid motion against the background of stars near the South Celestial Pole. Moving north, the comet should grow brighter, reaching a peak (3rd magnitude or so) when it is closest to the Sun in late March. By early April it should be visible from the northern hemisphere. Of course, this year Comet Lemmon may be just another pretty comet as skygazers on planet Earth also eagerly anticipate views of Comet PANSTARRS and Comet ISON.

Thumbnail image of picture found for this day. APOD: 2013 January 27 - Comet McNaught Over Chile
Explanation: Comet McNaught of 2007 has been, so far, the most photogenic comet of our time. After making quite a show in the northern hemisphere in early 2007 January, the comet moved south and developed a long and unusual dust tail that dazzled southern hemisphere observers. In this image, Comet McNaught was captured above Santiago, Chile. The bright comet dominates on the left while part of its magnificent tail spreads across the entire frame. From this vantage point in the Andes Mountains, one looks up toward Comet McNaught and a magnificent sky, across at a crescent moon, and down on clouds, atmospheric haze, and the city lights. The current year -- 2013 -- holds promise to be even better for comets than 2007. In early March, Comet PANSTARRS is on track to become visible to the unaided eye, while at the end of the year Comet ISON shows possibilities that include casting a tail that spreads across the sky, breaking up, and even becoming one of the brightest comets in recorded history.

Thumbnail image of picture found for this day. APOD: 2012 December 23 - Comet Hale Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp, the Great Comet of 1997, became much brighter than any surrounding stars. It was seen even over bright city lights. Away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail, consisting of ions from the comet's nucleus, is pushed out by the solar wind. The white dust tail is composed of larger particles of dust from the nucleus driven by the pressure of sunlight, that orbit behind the comet. Observations showed that Comet Hale-Bopp's nucleus spins about once every 12 hours. A comet that may well exceed Hale-Bopp's peak brightness is expected to fall into the inner Solar System next year.

Thumbnail image of picture found for this day. APOD: 2012 March 3 - Another Tail for Comet Garradd
Explanation: Remarkable comet Garradd (C2009/P1) has come to be known for two distinctive tails. From the perspective of earthbound comet watchers the tails are visible on opposite sides of its greenish coma. Seen here in a telescopic view, the recognizable dust tail fans out to the right, trailing the comet nucleus in its orbit. Streaming away from the sunward direction, a familiar bluish ion tail sweeps to the left. But the comet also seems to have, at least temporarily, sprouted a second ion tail recorded in this image from February 24. Other comet imagers have recently captured changing structures in Garradd's ion tail created as the plasma is buffeted by the magnetic fields in the solar wind. Now moving more quickly through northern skies, on March 5th comet Garradd will reach its closest approach to planet Earth, about 10.5 light-minutes distant.

Thumbnail image of picture found for this day. APOD: 2012 February 28 - The Opposing Tails of Comet Garradd
Explanation: Why does Comet Garradd have two tails? Visible on the left, Comet Garradd's dust tail is composed of ice and dust bits that trail the comet in its orbit around the Sun. Visible on the right, Comet Garradd's ion tail, is composed of ionized gas blown directly out from the Sun by the solar wind. Most comets show two tails, although it is unusual for them to appear to point in nearly opposite directions. Comet Garradd is currently showing opposing tails because of the Earth's opportunistic intermediate viewing angle. Subtle hues in the above image captured last week show the dust tail as slightly yellow as its large grains reflecting sunlight achromatically, while the ion tail shines slightly blue as the carbon monoxide ions reflect blue sunlight more efficiently. In the center, surrounding the comet's nucleus, is the green-tinted coma, so colored as it is a mix of dust and gasses that include green-emitting cyanogen. Although now drifting out from the Sun, Comet Garradd will make its closest approach to the Earth next week.

Thumbnail image of picture found for this day. APOD: 2011 December 31 - Comet Lovejoy and the ISS
Explanation: On December 24, Comet Lovejoy rose in dawn's twilight, arcing above the eastern horizon, its tails swept back by the solar wind and sunlight. Seen on the left is the comet's early morning appearance alongside the southern Milky Way from the town of Intendente Alvear, La Pampa province, Argentina. The short star trails include bright southern sky stars Alpha and Beta Centauri near the center of the frame, but the long bright streak that crosses the comet tails is a little closer to home. Waiting for the proper moment to start his exposure, the photographer has also caught the International Space Station still glinting in the sunlight as it orbits (top to bottom) above the local horizon. The right panel is the near horizon view of Comet Lovejoy from the space station itself, captured only two days earlier. In fact, Dan Burbank, Expedition 30 commander, recorded Comet Lovejoy rising just before the Sun in a spectacular video (linked here). Even considering the other vistas available from low Earth orbit, Burbank describes the comet as "the most amazing thing I have ever seen in space."

Thumbnail image of picture found for this day. APOD: 2011 October 7 - The Comet Hartley 2 Cruise
Explanation: Early last November, small but active Comet Hartley 2 (103/P Hartley) became the fifth comet imaged close-up by a spacecraft from planet Earth. Still cruising through the solar system with a 6 year orbital period, Hartley 2 is making astronomical headlines again. New Herschel Space Observatory measurements indicate that the water found in this comet's thin atmosphere or coma has the same ratio of the hydrogen isotope deuterium (in heavy water) as the oceans of our fair planet. Hartley 2 originated in the distant Kuiper Belt, a region beyond the orbit of Neptune that is a reservoir of icy cometary bodies and dwarf planets. Since the ratio of deuterium is related to the solar system environment where the comet formed, the Herschel results indicate that Kuiper Belt comets could have contributed substantial amounts of water to Earth's oceans. Comet Hartley 2 appears in this starry skyscape from last November sporting a tantalizing greenish coma appropriately sailing through the nautical constellation Puppis. Below the comet are open star clusters M47 (right) and M46 (left).

Thumbnail image of picture found for this day. APOD: 2011 August 6 - Comet Garradd and Messier 15
Explanation: Recorded on August 2, this telescopic composite image catches Comet Garradd (C/2009 P1) in the same field of view as globular star cluster M15. The celestial scene would have been a rewarding one for influential 18th century comet hunter Charles Messier. While Messier scanned French skies for comets, he carefully cataloged positions of things which might be fuzzy and comet-like in appearance but did not move against the background stars and so were definitely not comets. M15 (lower right), the 15th entry in his famous not-a-comet catalog, is now understood to be a cluster of over 100,000 stars some 35,000 light-years distant. The comet, discovered in August 2009 by astronomer G. J. Garradd (Siding Spring Observatory, Australia) is currently sweeping across the constellation Pegasus, some 13 light-minutes from Earth. Shining faintly around 9th magnitude, comet Garradd will brighten in the coming months, predicted to be just below naked eye visibility near its peak in February 2012.

Thumbnail image of picture found for this day. APOD: 2011 February 16 - Comet Tempel 1 from Stardust NeXT Spacecraft
Explanation: No comet has ever been visited twice before. Therefore, the unprecedented pass of the Stardust-NeXT spacecraft near Comet Tempel 1 earlier this week gave humanity a unique opportunity to see how the nucleus of a comet changes over time. Changes in the nucleus of Comet Tempel 1 were of particular interest because the comet was hit with an impactor from the passing Deep Impact spacecraft in 2005. Pictured above is one digitally sharpened image of Comet Tempel 1 near the closest approach of Stardust-NeXT. Visible are many features imaged in 2005, including craters, ridges, and seemingly smoother areas. Few firm conclusions are yet available, but over the next few years astronomers who specialize in comets and the understanding the early Solar System will be poring over these images looking for new clues as to how Comet Tempel 1 is composed, how the 2005 impact site now appears, and how general features of the comet have evolved.

Thumbnail image of picture found for this day. APOD: 2010 November 23 - Gas and Snow Jets from Comet Hartley 2
Explanation: Unusual jets have been discovered emanating from Comet Hartley 2. The EPOXI spacecraft imaged the jets in unprecedented detail during its flyby of the comet earlier this month. Pictured above, sun-illuminated jets shoot away from the two-kilometer long decaying iceberg that orbits the Sun between Earth and Jupiter. Comet Hartley 2 became active recently as it neared the Sun and sunlight warmed the comet. Preliminary analyses of images like that shown above indicate that the smooth regions around the middle are porous and leak frozen water vapor directly out into space. Unexpectedly, however, the rough regions at either end appear to shoot carbon dioxide jets that expel fluffy snowballs, some as large as basketballs, from the nucleus. Many of the dots in the above image are thought to be snowballs. The unusual jets will continue to be studied, and may yield further clues as to how comets and asteroids formed and evolved during the early years of our Solar System. Comet Hartley 2 is slowly evaporating and may completely break up over the next 1,000 years.

Thumbnail image of picture found for this day. APOD: 2010 November 8 - 700 Kilometers Below Comet Hartley 2
Explanation: What kind of comet is this? Last week, NASA's robotic EPOXI spacecraft whizzed past Comet 103P/Hartley, also known as Comet Hartley 2, and recorded images and data that are both strange and fascinating. EPOXI was near its closest approach -- about 700 kilometers away -- when it snapped the above picture. As expected, the comet has indeed shown itself to be a tumbling iceberg orbiting the Sun between Earth and Jupiter. However, unexpected features on the images have raised many questions. For example, where are all the craters? Why is there a large smooth area around the middle? How much of Comet Hartley 2 is a loose pile of dust and ice shards? Future analyses and comparisons to other comet nuclei may answer some of these questions and, hopefully, lead to a better general understanding of comets, meteors, and the early Solar System.

Thumbnail image of picture found for this day. APOD: 2010 June 17 - Comet McNaught Passes NGC 1245
Explanation: Of the many comets named for discoverer Robert McNaught, the one cataloged as C/2009 R1 is gracing dawn skies for northern hemisphere observers this month. Seen here on June 13th from southern New Mexico, this Comet McNaught's long ion tail sweeps across the telescopic field of view (a negative image is inset). Remarkably, the ion tail easily stretches past background star cluster NGC 1245 (upper left) in the constellation Perseus, about 1.5 degrees from the comet's lovely greenish head or coma. The coma also sports a short, stubby, dust tail. Of course, the comet and background stars move at different rates through planet Earth's skies. But a digital processing of many short exposures allowed frames of comet and stars to be separated, registered, and recombined in the final image. To see the comet separate from the background stars, just slide your cursor over the image. The recombined frames show off both the rich star field and faint details of the comet. Easy to spot in binoculars for now, McNaught will sink into the twilight along the eastern horizon in the coming days as it heads toward perihelion (closest approach to the Sun) on July 2.

Thumbnail image of picture found for this day. APOD: 2009 December 16 - Comet Hyakutake Passes the Earth
Explanation: In 1996, an unexpectedly bright comet passed by planet Earth. Discovered less than two months before, Comet C/1996 B2 Hyakutake came within only 1/10th of the Earth-Sun distance from the Earth in late March. At that time, Comet Hyakutake, dubbed the Great Comet of 1996, became the brightest comet to grace the skies of Earth in 20 years. During its previous visit, Comet Hyakutake may well have been seen by the stone age Magdalenian culture, who 17,000 years ago were possibly among the first humans to live in tents as well as caves. Pictured above near closest approach as it appeared on 1996 March 26, the long ion and dust tails of Comet Hyakutake are visible flowing off to the left in front of a distant star field that includes both the Big and Little Dippers. On the far left, the blue ion tail appears to have recently undergone a magnetic disconnection event. On the far right, the comet's green-tinted coma obscures a dense nucleus of melting dirty ice estimated to be about 5 kilometers across. A few months later, Comet Hyakutake began its long trek back to the outer Solar System. Because of being gravitationally deflected by massive planets, Comet Hyakutake is not expected back for about 100,000 years.

Thumbnail image of picture found for this day. APOD: 2009 December 6 - The Magnificent Tail of Comet McNaught
Explanation: Comet McNaught, the Great Comet of 2007, was the brightest comet of the last 40 years. Its spectacular tail spread across the sky and was breathtaking to behold from dark locations for many Southern Hemisphere observers. The head of the comet remained quite bright and was easily visible to even city observers without any optical aide. Part of the spectacular tail was visible just above the horizon after sunset for many northern observers as well. Comet C/2006 P1 (McNaught), which reached an estimated peak brightness of magnitude -6 (minus six), was caught by the comet's discoverer in the above image soon after sunset in 2007 January from Siding Spring Observatory in Australia. The robotic Ulysses spacecraft fortuitously flew through Comet McNaught's tail and found, unexpectedly, that the speed of the solar wind dropped significantly.

Thumbnail image of picture found for this day. APOD: 2009 October 2 - Comet and Orion
Explanation: These colorful panels both feature a familiar astronomical sight: the stellar nursery known as the Great Orion Nebula. They also offer an intriguing and unfamiliar detail of the nebula rich skyscape -- a passing comet. Recorded this weekend with a remotely operated telescope in New Mexico, the right hand image was taken on September 26 and the left on September 27. Comet 217P Linear sports an extended greenish tail and lies above the bluish Running Man reflection nebula near the top of both frames. Nearby and moving rapidly through the night sky, the comet's position clearly shifts against the cosmic nebulae and background stars from one night to the next. In fact, the comet was a mere 5 light-minutes away on September 27, compared to 1,500 light-years for the Orion Nebula. Much too faint to be seen with the unaided eye, Comet 217P Linear is a small periodic comet with an orbital period of about 8 years. At its most distant point from the Sun, the comet's orbit is calculated to reach beyond the orbit of Jupiter At its closest point to the Sun, the comet still lies just beyond the orbit of planet Earth.

Thumbnail image of picture found for this day. APOD: 2009 February 25 - Two Tails of Comet Lulin
Explanation: Go outside tonight and see Comet Lulin. From a dark location, you should need only a good star map and admirable perseverance -- although wide-field binoculars might help. Yesterday, Comet Lulin passed its closest to Earth, so that the comet will remain near its brightest over the next few days. The comet is currently almost 180 degrees around from the Sun and so visible nearly all night long, but will appear to move on the sky about 10 full moons a night. In this image, Comet Lulin was captured in spectacular form two nights ago from New Mexico, USA. The central coma of the comet is appearing quite green, a color likely indicating glowing molecular carbon gasses. Bright stars and a distant spiral galaxy are clearly visible in the image background. The yellow dust tail, reflecting sunlight, is visible sprawling to the coma's left trailing behind the comet, while the textured bluish-glowing ion tail is visible to the coma's right, pointing away from the Sun. Over the past few weeks, from the current vantage point of Earth, these two tails appeared to point in opposite directions. Comet Lulin is expected to slowly fade over the next few weeks.

Thumbnail image of picture found for this day. APOD: 2009 February 21 - The Swift View of Comet Lulin
Explanation: Now growing brighter, Comet Lulin is headed for its closest approach to planet Earth early next week. But the comet's greenish glow, familiar to earthbound skygazers, is replaced by false colors in this premier view from the orbiting Swift satellite. Image data from the Swift detectors, normally intended to follow cosmic gamma-ray bursts, were recorded on January 28. The data are combined here, along with a sky survey image of background stars, to show optical and ultraviolet light in green-blue hues and x-rays from the comet in red. The result maps remarkable x-ray emission on the comet's sunward side as incoming solar wind ions interact with gases in the swollen coma. It also shows substantial ultraviolet emission opposite the Sun, in the direction of motion and the comet's tail. The ultraviolet emission is from the OH molecule derived from the breakup of water, an indicator of the copius amounts of water produced by this extremely active comet. In fact, astronomers estimate Lulin was releasing about 800 gallons of water each second, enough to fill an Olympic-size swimming pool in less than 15 minutes.

Thumbnail image of picture found for this day. APOD: 2009 February 7 - Comet Lulin Tails
Explanation: Sweeping through the inner solar system, Comet Lulin is easily visible in both northern and southern hemispheres with binoculars or a small telescope. Recent changes in Lulin's lovely greenish coma and tails are featured in this two panel comparison of images taken on January 31st (top) and February 4th. Taken from dark New Mexico Skies, the images span over 2 degrees. In both views the comet sports an apparent antitail at the left -- the comet's dust tail appearing almost edge on from an earth-based perspective as it trails behind in Lulin's orbit. Extending to the right of the coma, away from the Sun, is the beautiful ion tail. Remarkably, as captured in the bottom panel, Comet Lulin's ion tail became disconnected on February 4, likely buffeted and torn away by magnetic fields in the solar wind. In 2007 NASA satellites recorded a similar disconnection event for Comet Encke. Don't worry, though. Comet tails can grow back.

Thumbnail image of picture found for this day. APOD: 2009 February 2 - Comet Lulin Approaches
Explanation: How bright will Comet Lulin become? No one knows for sure. Although it is notoriously difficult to accurately predict the brightness of newly discovered comets, Comet Lulin could well become visible to the unaided eye later this month. As Comet Lulin moves into the northern sky in mid February to rise around midnight, it should at least be spotted by comet watchers with binoculars and a good sky chart. Tracking observations indicate that the comet officially designated C/2007 N3 (Lulin) has now swung by the Sun and is approaching Earth on a trajectory that will bring it within half the Earth-Sun distance in late February. Comet Lulin's orbit indicates that this is likely the comet's first trip into the inner Solar System. The comet was discovered by Quanzhi Ye of Sun Yat-sen University on images obtained by Chi-Sheng Lin at the Lu-Lin Observatory of National Central University. In this picture, taken from Italy last Friday, are Comet Lulin's coma and tails, one tail pointing away from the Sun, and an anti-tail -- dust that trails the comet in its orbit and may appear to point toward the Sun.

Thumbnail image of picture found for this day. APOD: 2008 March 2 - Comet Hale Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp, the Great Comet of 1997, became much brighter than any surrounding stars. It was seen even over bright city lights. Away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail, consisting of ions from the comet's nucleus, is pushed out by the solar wind. The white dust tail is composed of larger particles of dust from the nucleus driven by the pressure of sunlight, that orbit behind the comet. Observations showed that Comet Hale-Bopp's nucleus spins about once every 12 hours.

Thumbnail image of picture found for this day. APOD: 2008 February 5 - Three Month Composite of Comet Holmes
Explanation: How has Comet Holmes changed? Since brightening unexpectedly by nearly one million fold in late October, the last three months have found the coma of Comet 17P/Holmes both expanding and fading. This spectacular composite image shows how the coma and tail of Comet Holmes have changed. Due to Earth's changing vantage point, Comet Holmes, out beyond the orbit of Mars, was seen in November nearly head-on, but in recent months is seen more from the side. Additionally, the comet's motion, when combined with Earth's changing perspective, has caused the comet to have shifted relative to the background stars. The curved path of Comet Holmes shows it to be undergoing apparent retrograde motion as the Earth orbits quickly in front of it. The extent of the coma currently makes Comet Holmes over five times the physical size of our Sun. Anecdotal evidence holds that the comet is hard to see without long photographic exposures, but on such exposures the comet may still be an impressive sight.

Thumbnail image of picture found for this day. APOD: 2008 January 20 - Comet McNaught Over Chile
Explanation: Comet McNaught was perhaps the most photogenic comet of our time. After making quite a show in the northern hemisphere in early January, the comet moved south and developed a long and unusual dust tail that dazzled southern hemisphere observers. In this image, Comet McNaught was captured one year ago above Chile. The bright comet dominates on the left while part of its magnificent tail spreads across the entire picture. From this vantage point in the Andes Mountains, one looks up toward Comet McNaught and a magnificent sky, across at a crescent moon, and down on clouds, atmospheric haze, and the city lights of Santiago. Comet McNaught has glided into the outer Solar System and is now only visible as a speck in a large telescope. The other spectacular comet of 2007, Comet Holmes, has also faded from easy view.

Thumbnail image of picture found for this day. APOD: 2008 January 2 - A Galaxy is not a Comet
Explanation: This gorgeous galaxy and comet portrait was recorded on December 30th, in the skies over Hoogeveen, The Netherlands. The combined series of 60 x 60 second exposures finds the lovely green coma of Comet 8P/Tuttle near its predicted conjunction with the Triangulum Galaxy. Aligning each exposure with the stars shows the comet as a streak, slowly moving against the background stars and galaxy. An alternative composition with exposures centered on the comet, shows the background stars and galaxy as streaks. The alluring celestial scene would also have been a rewarding one for the influential 18th century comet hunter Charles Messier. While Messier scanned French skies for comets, he carefully cataloged positions of things which were fuzzy and comet-like in appearance but did not move against the background stars and so were definitely not comets. The Triangulum Galaxy, also known as M33, is the 33rd object in his famous not-a-comet catalog. The modern understanding holds that the Triangulum Galaxy is a large spiral galaxy some 3 million light-years distant. Comet 8P/Tuttle, just bright enough to be visible to the unaided eye in dark, northern skies, is about 40 million kilometers (2 light-minutes) away.

Thumbnail image of picture found for this day. APOD: 2007 November 13 - The Inner Coma of Comet Holmes
Explanation: What's happening to Comet Holmes? The rare comet remains visible to the unaided eyes of northern observers as an unusual small puff ball in the constellation of Perseus. A high resolution set of images of the comet's inner coma, taken last week and shown above, reveals significant detail. Close inspection shows numerous faint streamers that are possibly the result of jets emanating from the comet's nucleus. Comet Holmes has remained surprisingly bright over the past week, with luminosity estimates ranging from between visual magnitudes 2 to 3, making it brighter than most stars visible on a dark sky. The above image of Comet Holmes was made with a small automated 0.38-meter telescope hirable over the web for a small fee.

Thumbnail image of picture found for this day. APOD: 2007 November 10 - A Tale of Comet Holmes
Explanation: A beautiful blue ion tail has become visible in deep telescopic images of Comet Holmes. Pointing generally away from the Sun and also planet Earth, the comet's ion tail is seriously foreshortened by our extreme viewing angle. Still, enthusiastic comet watchers have remarked that on the whole, the compact but tentacled appearance suggests a jellyfish or even a cosmic calamari. This stunning view of the comet's greenish coma and blue tail was recorded on November 4 in clear skies near Budapest, Hungary. The colors are caused by molecules in the tenuous gas, like C2 (green) and CO+ (blue), fluorescing in sunlight. In a more recent development, the dramatic inset is a deep image from L'Aquila in central Italy on November 8, showing the ion tail disconnecting from the comet.

Thumbnail image of picture found for this day. APOD: 2007 November 5 - Comet Holmes Grows a Tail
Explanation: Comet Holmes continues to be an impressive sight to the unaided eye. The comet has diminished in brightness only slightly, and now clearly appears to have a larger angular extent than stars and planets. Astrophotographers have also noted a distinctly green appearance to the comet's coma over the past week. Pictured above over Spain in three digitally combined exposures, Comet 17P/Holmes now clearly sports a tail. The blue ion tail is created by the solar wind impacting ions in the coma of Comet Holmes and pushing them away from the Sun. Comet Holmes underwent an unexpected and dramatic increase in brightness starting only two weeks ago. The detail visible in Comet Holmes' tail indicates that the explosion of dust and gas that created this dramatic brightness increase is in an ongoing and complex event. Comet Holmes will move only slightly on the sky over during the next month.

Thumbnail image of picture found for this day. APOD: 2007 October 30 - Comet Holmes' Coma Expands
Explanation: Go outside tonight and see Comet Holmes. No binoculars or telescopes are needed -- just curiosity and a sky map. Last week, Comet 17P/Holmes underwent an unusual outburst that vaulted it unexpectedly from obscurity into one of the brightest comets in recent years. Sky enthusiasts from the northern hemisphere have been following the comet's progress closely. In this animation recorded from Quebec, Canada, the coma of Comet Holmes is seen noticeably expanding over the past few days. Jupiter has been placed artificially nearby to allow for a comparison of angular sizes and scaled to the size it would appear at the current location of Comet Holmes. How Comet Holmes will further evolve is unknown, with one possibility being that the expanding gas cloud that started from its recent outburst will slowly disperse and fade.

Thumbnail image of picture found for this day. APOD: 2007 October 29 - A Telescopic View of Erupting Comet Holmes
Explanation: What's happened to Comet Holmes? A normally docile comet discovered over 100 years ago, Comet 17P/Holmes suddenly became nearly one million times brighter last week, possibly over just a few hours. In astronomical terms, the comet brightened from magnitude 17, only visible through a good telescope, to magnitude 3, becoming visible with the unaided eye. Comet Holmes had already passed its closest to the Sun in 2007 May outside the orbit of Mars and was heading back out near Jupiter's orbit when the outburst occurred. The comet's sudden brightening is likely due to some sort of sunlight-reflecting outgassing event, possibly related to ice melting over a gas-filled cavern, or possibly even a partial breakup of the comet's nucleus. Pictured above through a small telescope last Thursday, Comet Holmes appeared as a fuzzy yellow spot, significantly larger in angular size than Earth-atmosphere blurred distant stars. Although Comet Holmes' orbit will place it in northern hemisphere skies for the next two years, whether it will best be viewed through a telescope or sunglasses remains unknown.

Thumbnail image of picture found for this day. APOD: 2007 October 26 - Comet Holmes in Outburst
Explanation: Comet 17P/Holmes stunned comet watchers across planet Earth earlier this week. On October 24, it increased in brightness over half a million times in a matter of hours. The outburst transformed it from an obscure and faint comet quietly orbiting the Sun with a period of about 7 years to a naked-eye comet rivaling the brighter stars in the constellation Perseus. Recorded on that date, this view from Tehran, Iran highlights the comet's (enhanced and circled) dramatic new visibility in urban skies. The inset (left) is a telescopic image from a backyard in Buffalo, New York showing the comet's greatly expanded coma, but apparent lack of a tail. Holmes' outburst could be due to a sudden exposure of fresh cometary ice or even the breakup of the comet nucleus. The comet may well remain bright in the coming days.

Thumbnail image of picture found for this day. APOD: 2007 February 12 - Comet McNaught Over New Zealand
Explanation: Comet McNaught is perhaps the most photogenic comet of our time. After making quite a show in the northern hemisphere in mid January, the comet moved south and developed a long and unusual dust tail that dazzled southern hemisphere observers starting in late January. Comet McNaught was imaged two weeks ago between Mount Remarkable and Cecil Peak in this spectacular image taken from Queenstown, South Island, New Zealand. The bright comet dominates the right part of the above image, while the central band of our Milky Way Galaxy dominates the left. Careful inspection of the image will reveal a meteor streak just to the left of the comet. Comet McNaught continues to move out from the Sun and dim, but should remain visible in southern skies with binoculars through the end of this month.

Thumbnail image of picture found for this day. APOD: 2007 January 24 - A Comet Tail Horizon
Explanation: What's happening over the horizon? Many a sky enthusiast who thought they had seen it all had never seen anything like this. To the surprise of many Northern Hemisphere observers, the tail of Comet McNaught remained visible even after the comet's head set ahead of the Sun. What's more, visible were bright but extremely rare filamentary striae from the comet's expansive dust tail. The cause of dust tail striae are not known for sure, but are possibly related to fragmentation of comet's nucleus. The last comet to show prominent striae was Comet Hale-Bopp in 1997. Pictured above, the tail of Comet McNaught was caught just after sunset last Friday above the Carnic Alps of northern Italy.

Thumbnail image of picture found for this day. APOD: 2007 January 22 - The Magnificent Tail of Comet McNaught
Explanation: Comet McNaught, the Great Comet of 2007, has grown a long and filamentary tail. The spectacular tail spreads across the sky and is visible to Southern Hemisphere observers just after sunset. The head of the comet remains quite bright and easily visible to even city observers without any optical aide. The amazing tail is visible on long exposures and even to the unaided eye from a dark location. Reports even have the tail visible just above the horizon after sunset for many northern observers as well. Comet McNaught, estimated at magnitude -2 (minus two), was caught by the comet's discoverer in the above image just after sunset last Friday from Siding Spring Observatory in Australia. Comet McNaught, the brightest comet in decades, is now fading as it moves further into southern skies and away from the Sun and Earth.

Thumbnail image of picture found for this day. APOD: 2007 January 17 - Comet McNaught from New STEREO Satellite
Explanation: The brightest comet of recent decades was a surprising first sight for a new camera in space. The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument onboard the Solar TErrestrial RElations Observatory (STEREO) satellite had just opened up on January 11 when it snapped the above image of Comet McNaught. Visible was a spectacular view of the ion tail of Comet McNaught being swept away from the Sun by the solar wind in filamentary rays. The comet tail is seen to extend at least seven degrees across the above image, while the central coma is so bright it saturates. Comet McNaught is now reportedly so bright that it is visible even in broad daylight by blocking out the Sun with your hand. Comet McNaught has rounded the Sun and will slowly fade away for observers in Earth's Southern Hemisphere as it recedes from the Sun.

Thumbnail image of picture found for this day. APOD: 2007 January 15 - Comet McNaught Over Catalonia
Explanation: This past weekend Comet McNaught peaked at a brightness that surpassed even Venus. Fascinated sky enthusiasts in the Earth's northern hemisphere were treated to an instantly visible comet head and a faint elongated tail near sunrise and sunset. Recent brightness estimates had Comet McNaught brighter than magnitude -5 (minus five) over this past weekend, making it the brightest comet since Comet Ikeya-Seki in 1965, which was recorded at -7 (minus seven). The Great Comet of 2007 reached its brightest as it rounded the Sun well inside the orbit of Mercury. Over the next week Comet McNaught will begin to fade as it moves south and away from the Sun. The unexpectedly bright comet should remain visible to observers in the southern hemisphere with unaided eyes for the rest of January. The above image, vertically compressed, was taken at sunset last Friday from mountains above Catalonia, Spain.

Thumbnail image of picture found for this day. APOD: 2007 January 5 - Comet McNaught Heads for the Sun
Explanation: Early morning risers with a clear and unobstructed eastern horizon can enjoy the sight of Comet McNaught (C/2006 P1) in dawn skies over the next few days. Discovered in August by R. H. McNaught (Siding Spring Survey) the comet has grown bright enough to see with the unaided eye but will soon be lost in the glare of the Sun. Still, by January 11 sun-staring spacecraft SOHO should be able to offer web-based views as the comet heads toward a perihelion passage inside the orbit of Mercury. This image captures the new naked-eye comet at about 2nd magnitude in twilight skies near sunset on January 3rd. After rounding the Sun and emerging from the solar glare later this month, Comet McNaught could be even brighter.

Thumbnail image of picture found for this day. APOD: 2006 October 4 - Comet SWAN Brightens
Explanation: A newly discovered comet has brightened enough to be visible this week with binoculars. The picturesque comet is already becoming a favored target for northern sky imagers. Pictured above just last week, Comet SWAN showed a bright blue-green coma and an impressive tail. Comet C/2006 M4 (SWAN) was discovered in June in public images from the Solar Wind Anisotropies (SWAN) instrument of NASA and ESA's Sun-orbiting SOHO spacecraft. Comet SWAN, near magnitude six, will be visible with binoculars in the northeastern sky not far from the Big Dipper over the next few days before dawn. The comet is expected to reach its peak brightness this week. Passing its closest to the Sun two days ago, Comet SWAN and will be at its closest to the Earth toward the end of this month. Comet SWAN's unusual orbit appears to be hyperbolic, meaning that it will likely go off into interstellar space, never to return.

Thumbnail image of picture found for this day. APOD: 2006 May 23 - Comet Schwassmann Wachmann 3 Passes the Earth
Explanation: Rarely does a comet pass this close to Earth. Last week, dedicated astrofilmographers were able to take advantage of the close approach of crumbling 73P / Comet Schwassmann-Wachmann 3 to make time-lapse movies of the fast-moving comet. Large comet fragments passed about 25 times the Moon's distance from the Earth. The above time lapse movie of Fragment B of Comet Schwassmann-Wachmann 3 over Colorado, USA was taken during a single night, May 16, with 83 consecutive 49-second exposures. Some observers report being able to perceive the slight motion of the comet with respect to the background stars using only their binoculars and without resorting to the creation of fancy digital time-lapse movies. Fragment B of Comet Schwassmann-Wachmann 3 became just barely visible to the unaided eye two weeks ago but now is appearing to fade as the comet has moved past the Earth and nears the Sun. Many sky enthusiasts will be on the watch for a particularly active meteor shower tonight as the Earth made its closest approach to orbit of Comet Schwassmann-Wachmann 3 late yesterday.

Thumbnail image of picture found for this day. APOD: 2006 May 13 - Crumbling Comet
Explanation: This false-color mosaic of crumbling comet Schwassmann-Wachmann 3 spans about 6 degrees (12 full moons) along the comet's orbit. Recorded on May 4-6 by an infrared camera on board the Spitzer Space Telescope, the picture captures about 45 of the 60 or more alphabetically cataloged large comet fragments. The brightest fragment at the upper right of the track is Fragment C. Bright Fragment B is below and left of center. Looking for clues to how the comet broke up, Spitzer's infrared view also captures the trail of dust left over as the comet deteriorated during previous passes. Emission from the dust particles warmed by sunlight appears to fill the space along the cometary orbit. The fragments are near their closest approach in the coming days, about 10 million kilometers away, and none pose any danger to our fair planet.

Thumbnail image of picture found for this day. APOD: 2006 May 11 - Comet Meets Ring Nebula: Part I
Explanation: As dawn approached on May 8, astronomer Stefan Seip carefully watched Fragment C of broken comet 73P/Schwassmann-Wachmann 3 approach M57 - the Ring Nebula, and faint spiral galaxy IC 1296. Of course, even though the trio seemed to come close together in a truly cosmic photo opportunity, the comet is in the inner part of our solar system, a mere 0.5 light-minutes or so from Seip's telescope located near Stuttgart, Germany, planet Earth. The Ring Nebula (upper right) is more like 2,000 light-years distant, well within our own Milky Way Galaxy. At a distance of 200 million light-years, IC 1296 (between comet and ring) is beyond even the Milky Way's boundaries. Because the comet is so close, it appears to move relatively rapidly against the distant stars. This dramatic telescopic view was composited from two sets of images; one compensating for the comet's apparent motion and one recording the background stars and nebulae.

Thumbnail image of picture found for this day. APOD: 2006 April 26 - Crumbling Comet Schwassmann Wachmann 3 Approaches
Explanation: A crumbling comet will soon pass near the Earth. Comet 73P/Schwassmann-Wachmann 3 is brightening and may even be visible to the unaided eye when the fragmented comet zooms past Earth during the middle of next month. Still, the small comet poses no Earth hazard, since it will pass the Earth at about 25 times the distance of the Moon. Exactly how bright Comet Schwassman-Wachmann 3 will get is unknown. It is even possible, althought unlikely, that debris from the comet will have spread out enough to cause a notable meteor shower. Pictured above, Fragment B of Comet Schwassman-Wachmann 3 was photographed two nights ago by a 8.2-meter Very Large Telescope in Chile. Visible to the lower right of the large B fragment are many mini-comets that have broken off and now orbit the Sun separately. Each mini-comet itself sheds gas and dust and so appears to have its own hazy coma. The comet will pass closest to the Sun on June 7.

Thumbnail image of picture found for this day. APOD: 2006 March 6 - Unexpected Comet Pojmanski Now Visible
Explanation: Have you ever seen a comet? Comets bright enough to be visible to the unaided eye appear only every few years. Right now, however, a new comet has brightened unexpectedly and is visible as a faint streak to the unaided northern observer in the eastern morning sky just before sunrise. Binoculars may help. Comet Pojmanski, officially designated C/2006 A1 and discovered only in January, now sports a turquoise tail several times longer than the full moon. Comet Pojmanski's ion tail is due to gas particles expelled by the comet being pushed away from the Sun by the solar wind, the same wind that ionizes gas in the tail causing its blue tint. Pictured above as it appeared only last week, Comet Pojmanski has now begun to fade as its orbit around the Sun takes it further from the Earth.

Thumbnail image of picture found for this day. APOD: 2006 March 3 - Venus and Comet Pojmanski
Explanation: Shining brightly in the east at dawn, Venus dominates the sky in this view over a suburban landscape from Bursa, Turkey. An otherwise familiar scene for astronomer Tunc Tezel, his composite picture of the morning sky recorded on March 2nd also includes a surprise visitor to the inner solar system, Comet Pojmanski. Cataloged as C/2006 A1, the comet was discovered on January 2nd by Grzegorz Pojmanski of Warsaw University Astronomical Observatory in Poland. At the time very faint and tracking through southern skies, the comet has now moved north and grown just bright enough to be a good target for early-rising skygazers with binoculars. Enhanced and framed in this picture, the comet's tail has also grown to a length of several degrees. The comet will be at its closest approach to planet Earth, just over 100 million kilometers away, on March 5. For northern hemisphere observers in the next few days, the beginning of morning twilight really will be the best time to spot Comet Pojmanski.

Thumbnail image of picture found for this day. APOD: 2005 September 4 - Comet Hale Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp became much brighter than any surrounding stars. It was seen even over bright city lights. Out away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail was created when fast moving particles from the solar wind struck expelled ions from the comet's nucleus. The white dust tail is composed of larger particles of dust and ice expelled by the nucleus that orbit behind the comet. Observations showed that Comet Hale-Bopp's nucleus spins about once every 12 hours.

Thumbnail image of picture found for this day. APOD: 2005 July 6 - The Landscape on Comet Tempel 1
Explanation: This diverse landscape is the surface of comet Temple 1's nucleus as seen by the Deep Impact probe's Impactor Targeting Sensor. Within minutes of recording the rugged view, the landscape had changed dramatically though, as the impactor smashed into the surface near the two large, half kilometer-sized craters at picture center. Indications are that the probe penetrated well below the surface before vaporizing, sending a relatively narrow plume of debris blasting back into space. Researchers are still speculating on the final size of the crater produced by the July 4th comet crash, but material continues to spew from the impact site and has caused the faint comet to brighten significantly. Determining the crater dimensions and analyzing the debris ejected from the comet's interior will provide premier insights into the formation of comet Tempel 1, a primordial chunk of our own solar system.

Thumbnail image of picture found for this day. APOD: 2005 May 22 - The Dust and Ion Tails of Comet Hale-Bopp
Explanation: In 1997, Comet Hale-Bopp's intrinsic brightness exceeded any comet since 1811. Since it peaked on the other side of the Earth's orbit, however, the comet appeared only brighter than any comet in two decades. Visible above are the two tails shed by Comet Hale-Bopp. The blue ion tail is composed of ionized gas molecules, of which carbon monoxide particularly glows blue when reacquiring electrons. This tail is created by the particles from the fast solar wind interacting with gas from the comet's head. The blue ion tail points directly away from the Sun. The light colored dust tail is created by bits of grit that have come off the comet's nucleus and are being pushed away by the pressure of light from the Sun. This tail points nearly away from the Sun. The above photograph was taken in March 1997.

Thumbnail image of picture found for this day. APOD: 2005 January 5 - Comet Machholz in View
Explanation: Good views of Comet Machholz are in store for northern hemisphere comet watchers in January. Now making its closest approach to planet Earth, the comet will pass near the lovely Pleiades star cluster on January 7th and the double star cluster in Perseus on January 27th as Machholz moves relatively quickly through the evening sky. Currently just visible to the unaided eye from dark locations, the comet should be an easy target in binoculars or a small telescope. In fact, this telephoto time exposure from January 1 shows Comet Machholz sporting two lovely tails in skies over Colorado, USA. Extending to the left, strands of the comet's ion or gas tail are readily affected by the solar breeze and point away from the Sun. Dust, which tends to trail along the comet's orbit, forms the tail jutting down and to the right.

Thumbnail image of picture found for this day. APOD: 2004 December 13 - Announcing Comet Machholz
Explanation: A comet discovered only this summer is brightening quickly and already visible to the unaided eye. Comet C/2004 Q2 (Machholz) is currently best visible in Earth's Southern Hemisphere where some observers report it brighter than magnitude 5. The comet is moving rapidly to northern skies and should continue to brighten until early January. By coincidence, Comet Machholz will be easy to view as it will be nearly opposite the Sun when appearing its brightest. How bright Comet Machholz will become then remains uncertain, but it will surely stay in northern skies for much of 2005, even approaching Polaris in early March. Pictured above, Comet Machholz was captured in early December already sporting a bright surrounding coma, a white oblong dust tail fading off toward the bottom, and a long wispy ion tail toward the right with a kink near the end.

Thumbnail image of picture found for this day. APOD: 2004 August 30- Announcing Comet C 2003 K4 LINEAR
Explanation: A comet discovered last year has brightened unexpectedly and now may become visible to the unaided eye within the next month. Designated Comet C/2003 K4 (LINEAR), the comet was discovered in 2003 May by project LINEAR. Many reports already place the comet as brighter than magnitude 7, meaning that it can now be seen with binoculars. Reports also indicate the comet already has a visible tail nearly the length of a full Moon. Since predicting the future brightness of comets is a very tricky business, there remains the possibility that K4 might never become very bright. Current predictions, however, estimate the comet may approach fifth magnitude in October. K4 passes closest to the Sun on October 12 and then closest to the Earth on December 23. Comet K4 was photographed above from Van Buren, Arkansas, USA on August 17.

Thumbnail image of picture found for this day. APOD: 2004 May 12 - The Tails of Comet NEAT Q4
Explanation: Comet NEAT (Q4) is showing its tails. As the large snowball officially dubbed Comet C/2001 Q4 (NEAT) falls toward the inner Solar System, it has already passed the Earth and will reach its closest approach to the Sun this coming Saturday. Reports place the comet at third magnitude, making it easily visible to the unaided eye to northern sky gazers observing from a dark location just after sunset. The above image was captured last Saturday from Happy Jack, Arizona, USA. Visible is a long blue ion tail, a blue coma surrounding the comet's nucleus, and a shorter but brighter sunlight reflecting dust tail. Q4 will likely drop from easy visibility during the next month as it recedes from both the Earth and the Sun. Another separate naked-eye comet, Comet Linear (T7), is also as bright as third magnitude and should remain bright into June.

Thumbnail image of picture found for this day. APOD: 2004 May 7 - Look West for a NEAT Comet
Explanation: On May 5th, while scanning western skies after sunset, astronomer Jimmy Westlake was glad to spot a visitor from the outer solar system, Comet NEAT, with his own eyes. Taken with a normal lens, the picture records his memorable view of comet, clouds, and Colorado Rocky Mountains against a backdrop of many faint stars (most not visible to the unaided eye) and one very bright one. In the three minute time exposure, the comet is seen as a fuzzy greenish smudge left of center, with brilliant Sirius, alpha star of the constellation Canis Major, just above the low cloud bank on the right. Comet NEAT (C/2001 Q4) is now near its closest aproach to planet Earth and tonight will lie well above bright Sirius. Look for the comet - the third naked-eye comet in as many weeks - after sunset in clear, dark, western skies.

Thumbnail image of picture found for this day. APOD: 2004 April 22 - Comet C/2002 T7 (LINEAR)
Explanation: Discovered by the the Lincoln Near Earth Asteroid Research (LINEAR) project in October of 2002, comet C/2002 T7 is now visiting the inner solar system, making its closest approach (see animation by L. Koehn) to the Sun tomorrow, April 23rd. Emerging from the solar glare, the comet is now just visible to the unaided eye in the constellation Pisces, near the eastern horizon in morning twilight. In this gorgeous telescopic view recorded before dawn yesterday, the clearly active comet has developed an extensive, complex tail extending over 2 degrees in the anti-sunward direction, and a pronounced anti-tail or anomalous tail. Later next month this comet should appear brighter, making its closest approach to planet Earth on May 19th. In fact, it could share southern skies with another naked-eye comet, also anticipated to brighten in May, designated C/2001 Q4 (NEAT).

Thumbnail image of picture found for this day. APOD: 2004 April 20 - Comet Hale Bopp Over Indian Cove
Explanation: Comet Hale-Bopp, the Great Comet of 1997, was quite a sight. No comets of comparable brightness have graced the skies of Earth since then. During this next month, however, even besides the fleeting Comet Bradfield, two comets have a slight chance of rivaling Hale-Bopp and a good chance of putting on a memorable sky show. Unfortunately, most of the show will be confined to sky gazers in Earth's southern hemisphere. Both comets are already visible to the unaided eye from there. The first, Comet C/2002 T7 (LINEAR), should be at its best before dawn during the first weeks of May from the south. The second, Comet C/2001 Q4 (NEAT), should be visible in early May from all over the Earth. Both comets appear to be approaching the inner Solar System for the first time and so it is very hard to predict how bright each will become. In the above photograph taken 1997 April 6, Comet Hale-Bopp was imaged from the Indian Cove Campground in the Joshua Tree National Forest in California, USA. A flashlight was used to momentarily illuminate foreground rocks during this six minute exposure.

Thumbnail image of picture found for this day. APOD: 2004 April 19 - Comet Bradfield Passes the Sun
Explanation: Today, Comet Bradfield is passing the Sun. The above image, taken yesterday in the direction of the Sun by the SOHO LASCO instrument, shows the comet and its dust tail as the elongated white streak. The Sun would normally be seen in the very center but has been blocked from view. Comet C/2004 F4 (Bradfield) was discovered just one month ago and has brightened dramatically as it neared the Sun. Careful sky gazers can see Comet Bradfield with the unaided eye near the Sun, although NASA's sun-orbiting SOHO satellite has the best view. During the day, Comet Bradfield will continually shift inside the LASCO frame as it rounds the Sun. There is even the possibility that the comet will break up. If not, the bright comet's trajectory will carry it outside the field of LASCO sometime tomorrow. Along with T7 and Q4, Comet Bradfield is now the third comet that is currently visible on the sky with the unaided eye, the most ever of which we are aware and quite possibly the most in recorded history.

Thumbnail image of picture found for this day.APOD: 2004 March 14 - Comet Hale Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp became much brighter than any surrounding stars. It was seen even over bright city lights. Out away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail was created when fast moving particles from the solar wind struck expelled ions from the comet's nucleus. The white dust tail is composed of larger particles of dust and ice expelled by the nucleus that orbit behind the comet. Observations showed that Comet Hale-Bopp's nucleus spins about once every 12 hours.

Thumbnail image of picture found for this day. APOD: 2004 February 9 - Announcing Comet C 2002 T7 LINEAR
Explanation: A newly discovered comet may outshine most stars in the sky by May. Designated Comet C/2002 T7 (LINEAR), the comet was discovered in 2002 October by project LINEAR. Many reports already place the comet as brighter than magnitude 7, meaning that it can now be seen with binoculars. Reports also indicate the comet already has a visible tail nearly the length of a full Moon. Since predicting the future brightness of comets is a very tricky business, there remains the possibility that T7 might never become visible to the unaided eye. Alternatively, another comet, C/2001 Q4 (NEAT), may also reach naked eye visibility at nearly the same time, making 2004 April and May two of the busiest bright-comet months in centuries. Comet T7 can be seen on the above right on January 20, while an airplane trail is visible on the left.

Thumbnail image of picture found for this day. APOD: 2004 January 31 - A Galaxy is not a Comet
Explanation: This gorgeous galaxy and comet portrait was recorded on April 5th, 2002, in the skies over the Oriental Pyrenees near Figueres, Spain. From a site above 1,100 meters, astrophotographer Juan Carlos Casado used a guided time exposure, fast film, and a telephoto lens to capture the predicted conjunction of the bright Comet Ikeya-Zhang (right) and the Andromeda Galaxy (left). This stunning celestial scene would also have been a rewarding one for the influential 18th century comet hunter Charles Messier. While Messier scanned French skies for comets, he carefully cataloged positions of things which were fuzzy and comet-like in appearance but did not move against the background stars and so were definitely not comets. The Andromeda Galaxy, also known as M31, is the 31st object in his famous not-a-comet catalog. Not-a-comet object number 110, a late addition to Messier's catalog, is one of Andromeda's small satellite galaxies, and can be seen here just below M31. Our modern understanding holds that the Andromeda galaxy is a large spiral galaxy some 2 million light-years distant. The photogenic Comet Ikeya-Zhang, then a lovely sight in early morning skies was about 80 million kilometers (4 light-minutes) from planet Earth.

Thumbnail image of picture found for this day. APOD: 2004 January 3 - Comet Wild 2's Nucleus from Stardust
Explanation: What does a comet nucleus look like? Yesterday the robot spacecraft Stardust answered this question by returning the most detailed images yet of the center of a comet. The icy centers of comets are usually hidden from Earth-bound telescopes by opaque dust and gas that boils off during approach to the Sun. Twice before, however, in the cases of Comet Halley and Comet Borrelly, spacecraft dove through the debris cloud of a comet's coma to image the nucleus. Pictured above is the nucleus of Comet Wild 2 taken by Stardust when passing within 500 kilometers. Clearly visible are numerous craters and hilly terrain. The Stardust mission is yet more ambitious -- it has captured particles from the coma and will jettison them to Earth in 2006. Analyses of the images and returned particles will likely give fresh information about our Solar System back near its beginning, when Comet Wild 2 formed.

Thumbnail image of picture found for this day. APOD: 2003 December 23 - Comet Encke Returns
Explanation: It's back. Every 3.3 years, Comet Encke swoops back into our inner Solar System. First officially discovered in 1786, Comet Encke is on its 59 th documented return, making it one of the best-studied comets on the sky. Mysteriously, Comet Encke should have been discovered millennia earlier, since it likely became bright enough to see unaided many times over the past few thousand years. Comet Encke's elliptical trajectory reaches from outside the orbit of Mars to inside the orbit of Mercury. It passed relatively close to the Earth on Nov. 17 and will reach its closest to the Sun on Dec 29. Recent observations place Comet Encke as bright as visual magnitude six during early December, making it just on the verge of unaided human vision. Pictured above, the diffuse smudge of periodic Comet Encke was imaged through a small telescope on November 29 from Arkansas, USA.

Thumbnail image of picture found for this day. APOD: 2003 October 3 - Cold Comet Halley
Explanation: While this may not be the most esthetic image of Comet Halley that you have ever seen, it is likely the most unique. The tiny cluster of pixels circled is the famous comet along its orbit over 4 billion (4,000,000,000) kilometers or 28 AU from the Sun -- a record distance for a comet observation. Its last passage through our neck of the woods in 1986, Comet Halley presently cruises through the dim reaches of the outer solar system, almost as far away as outermost gas giant Neptune, and shows no sign of activity. Captured in March, this negative image is a composite of digital exposures made with three of ESO's Very Large Telescopes. The exposures are registered on the moving comet, so the picture shows background stars and galaxies as elongated smudges. An earth-orbiting satellite appears as a dark streak at the top. Comet Halley is clearly extremely faint here, but large earthbound telescopes will be able to follow it as it grows fainter still, reaching the most distant point in its orbit, more than 5 billion kilometers (35 AU) from the Sun, in 2023.

Thumbnail image of picture found for this day. APOD: 2003 March 6 - Comet NEAT in Southern Skies
Explanation: After last month's dramatic swoop past the Sun, Comet NEAT (C/2002 V1) appeared as a naked-eye comet, emerging from the evening twilight in planet Earth's southern skies. On March 1st, New Zealand photographer Noel Munford captured this telephoto view of the outbound comet close to the southwestern horizon against the faint stars of the constellation Sculptor. He reports that the picture is a good representation of the comet's visual appearance on that date and estimates the impressive tail to be five or six degrees long. Discovered last November as part of the Near-Earth Asteroid Tracking program, there was some speculation that this comet would not survive its close encounter with the Sun. However, Comet NEAT is now returning to the outer solar system, diving southward and fading fast.

Thumbnail image of picture found for this day. APOD: 2003 February 24 - Comet NEAT Passes an Erupting Sun
Explanation: As Comet NEAT flared last week, the Sun roared. Just as the comet swooped inside the orbit of Mercury and developed a long and flowing tail of gas and dust, the Sun emitted a huge Coronal Mass Ejection (CME). Neither the fortuitous hot ball of solar gas nor the intense glare of sunlight appeared to disrupt the comet's nucleus. The action was too close to the Sun to be easily visible by humans, but the orbiting Sun-pointing SOHO satellite had a clear view of the celestial daredevil show. The above image was taken on February 18 when the comet was so bright it created an artificial horizontal streak on the camera image. During the encounter, Comet NEAT, official designation (C/2002 V1), brightened to second magnitude. An opaque disk blocked the Sun's image. The now-outbound comet remains bright but will surely fade as it moves away from the Sun. Nevertheless, Comet NEAT will likely be visible with binoculars to southern hemisphere observers for the next month.

Thumbnail image of picture found for this day. APOD: 2003 February 10 - Comet NEAT Approaches the Sun
Explanation: A comet may likely become visible to the unaided eye over the next few days above the horizon where the Sun has just set. Comet NEAT (C/ 2002 V1), discovered last November, has brightened dramatically as it approached the Sun. Over the next few days, the quickly setting comet could appear as bright as second magnitude. On February 18 it will round the Sun well within the orbit of Mercury. During surrounding days, the Sun's glare will effectively hide the comet to human observers. It is quite probable, though, that Comet NEAT will standout prominently in images taken by the Sun-looking SOHO satellite. Pictured above, Comet NEAT's complex and developing tail was photographed on January 29 (top) and February 2. Sky enthusiasts should remember to never look directly at the Sun.

Thumbnail image of picture found for this day. APOD: 2003 January 30 - Comet Kudo-Fujikawa: Days in the Sun
Explanation: Cruising through the inner Solar System, new Comet Kudo-Fujikawa reached perihelion, its closest approach to the Sun, yesterday, January 29. Passing within 28.4 million kilometers of the Sun, this comet came much closer than innermost planet Mercury basking only 57.9 million kilometers from our parent star. So close to the Sun, comet Kudo-Fujikawa was extremely bright but impossible for earthbound observers to see against the solar glare. Still, the space-based SOHO observatory captured these views of the comet as it neared perihelion by using a coronograph's occulting disk to block the overwhelming sunlight. In the series of images, the size and location of the blocked-out Sun is indicated by white circles, while arrows point to the traveling comet's bright coma and developing tail. Though fading on its outbound journey, Kudo-Fujikawa should soon be visible to southern hemisphere comet-watchers in February's evening skies.

Thumbnail image of picture found for this day. APOD: 2002 August 2 - Comet 57P Falls to Pieces
Explanation: Comet 57P has fallen to pieces, at least 19 of them. Orbiting the Sun every 6 years or so this faint comet - also christened Comet 57P/du Toit-Neujmin-Delporte for its three 1941 co-discoverers - is simply 57th on the list of comets known to be periodic, beginning with Comet 1P/Halley. In mid July, responding to reports of a new object possibly associated with Comet 57P, astronomers were able to construct this mosaic of deep sky images identifying a surprising 19 fragments (circled) strung out behind the cometary coma and nucleus itself (far left). The full mosaic spans about a million kilometers at the distance of the comet, while the individual pieces detected are probably a few tens to a few hundred meters across. Stress produced as sunlight warmed the icy, rocky nucleus likely contributed to the fragmentation. In fact, when last seen passing through the inner solar system in 1996, Comet 57P brightened unexpectedly, indicating a sudden increase in surface activity.

Thumbnail image of picture found for this day. APOD: 2002 April 22 - Comet and Aurora Over Alaska
Explanation: Can you spot the comet? Flowing across the frozen Alaskan landscape is an easily visible, colorful aurora. Just to the lower left, however, well in the background, is something harder to spot: Comet Ikeya-Zhang, the brightest comet of recent years. Although the aurora faded in minutes, the comet is just now beginning to fade. It remains just barely visible without aid, however, before sunrise in the East. The comet is actually a giant dirt-covered snowball that spends most of its time in the outer Solar System -- to where it is now returns. The above photograph was taken on March 20 when Comet Ikeya-Zhang was near its brightest. Careful inspection of the photo will uncover several other sky delights, including the giant galaxy M31.

Thumbnail image of picture found for this day. APOD: 2002 April 12 - A Galaxy is not a Comet
Explanation: This gorgeous galaxy and comet portrait was recorded on April 5th in the skies over the Oriental Pyrenees near Figueres, Spain. From a site above 1,100 meters, astrophotographer Juan Carlos Casado used a guided time exposure, fast film, and a telephoto lens to capture the predicted conjunction of the bright Comet Ikeya-Zhang (right) and the Andromeda Galaxy (left). This stunning celestial scene would also have been a rewarding one for the influential 18th century comet hunter Charles Messier. While Messier scanned French skies for comets, he carefully cataloged positions of things which were fuzzy and comet-like in appearance but did not move against the background stars and so were definitely not comets. The Andromeda Galaxy, also known as M31, is the 31st object in his famous not-a-comet catalog. Not-a-comet object number 110, a late addition to Messier's catalog, is one of Andromeda's small satellite galaxies, and can be seen here just below M31. Our modern understanding holds that the Andromeda galaxy is a large spiral galaxy some 2 million light-years distant. The photogenic Comet Ikeya-Zhang, now a lovely sight in early morning skies, is about 80 million kilometers (4 light-minutes) from planet Earth.

Thumbnail image of picture found for this day. APOD: 2002 April 4 - Ikeya-Zhang: Comet Over Colorado
Explanation: Comet Ikeya-Zhang ("ee-KAY-uh JONG") has become a most photogenic comet. This lovely early evening view of the comet in Rocky Mountain skies looks northwest over ridges and low clouds. The time exposure was recorded on March 31st from an 8,000 foot elevation near Yampa, Colorado, USA. Sporting a sweeping yellowish dust tail and blue ion tail eight to ten degrees long, Ikeya-Zhang is nestled near the horizon in the northern constellation of Andromeda. To the comet's left is the bright star Mirach or Beta Andromedae while the stretched celestial fuzzball to the comet's right is M31 or the Andromeda galaxy, the nearest bright spiral galaxy to our own Milky Way. As the days pass, Comet Ikeya-Zhang's apparent motion through the sky is towards the right in this image. Tonight, comet-watchers blessed with clear skies should find Ikeya-Zhang posing perfectly for binoculars and cameras just above M31, less than two degrees from the center of the bright galaxy.

Thumbnail image of picture found for this day. APOD: 2002 March 26 - Comet Ikeya-Zhang over Tenerife
Explanation: Comet Ikeya-Zhang has become bright enough to stand out in the night sky. Discovered February 1, the comet has now just rounded the Sun and has likely attained its peak brightness. The comet appears near the Sun and over the next week moves from the evening sky (just after sunset) to the morning sky (just before sunrise). Many observers report a current brightness approaching third magnitude. The comet is actually a giant snowball created during the early days of our Solar System and pushed out by the gravitational tugs by massive planets. Comet Ikeya-Zhang has been back to the inner Solar System at least once before in 1661. Above, the comet was photographed above Tenerife, one the Canary Islands, Spain.

Thumbnail image of picture found for this day. APOD: 2002 March 7 - Comet Ikeya-Zhang Brightens
Explanation: In the last week, Comet Ikeya-Zhang has become bright enough to be just visible to the unaided eye. Based on its present activity, observers are optimistic that Ikeya-Zhang will become substantially brighter. This composite color image from March 3rd, captured with a wide-field telescope, shows this active comet's bright, condensed coma and a delightful array of subtle structures in its developing tail. The bluish tail stretches for 5 degrees or so against a background of stars in the constellation Pisces. In the coming days look for the comet hanging low in the western evening sky (below a bright yellowish Mars), eventually becoming difficult to see in the March twilight. But after April begins, Ikeya-Zhang will become a predawn object climbing higher into the morning sky as the month progresses. Cataloged as comet C/2002 C1, improved orbit determinations now make it seem very likely that Comet Ikeya-Zhang has been around here before. Refined calculations indicate this comet's last trip through the inner Solar System was 341 years ago, in 1661, when it was recorded as a bright comet.

Thumbnail image of picture found for this day. APOD: 2002 February 21 - Comet Ikeya-Zhang
Explanation: Comet Ikeya-Zhang is presently heading north in planet Earth's sky, framed by stars of the constellation Cetus. The comet was discovered as a faint, telescopic object near the western horizon on the evening of February 1st independently by Kaoru Ikeya of Shizuoka prefecture, Japan, Daqing Zhang in Henan province, China, and later by observer Paulo Raymundo of Salvador, Brazil. But Ikeya-Zhang is expected to brighten significantly and in March and April could become visible to the unaided eye. This picture, taken near Tucson, Arizona, USA on the evening of February 9th, covers a field a bit less than the width of the full moon showing the comet's condensed coma and narrow, developing tail. Ikeya-Zhang should pass closest to the Sun (perihelion) on March 18 at a point roughly midway between the orbits of Mercury and Venus. Based on preliminary calculations of this comet's orbit, Ikeya-Zhang is suspected of being a periodic comet, returning to the inner Solar System every 500 years or so. In fact, it is speculated that Ikeya-Zhang may be directly connected with a historic bright comet seen in 1532.

Thumbnail image of picture found for this day. APOD: 2001 December 6 - Comet Linear (WM1) Brightens
Explanation: A comet bright enough to be seen with binoculars is swooping into southern skies. Comet C/2000 WM1 (LINEAR) continues to brighten and develop tails as it nears its closest approach of the Sun in late January 2002. Comet LINEAR WM1 was discovered over a year ago when it was out past Jupiter and still very faint. In the above picture from the Curtis Schmidt 0.6-meter Telescope in Chile, a 30-second exposure in red on December 4 captured detail in Comet LINEAR WM1's emerging dust tail. Optimistic sky watchers hope that Comet LINEAR WM1 will undergo an even greater (and unexpected) brightening to the point where its coma and tails are easily discernable to the unaided eye. Comet LINEAR WM1 is being watched with particular interest by astronomers because its ion tail might yield clues to understanding the solar wind expelled from near the Sun's poles.

Thumbnail image of picture found for this day. APOD: 2001 November 9 - SOHO Comet 367: Sungrazer
Explanation: The most prolific comet discovering instrument in history rides aboard the sun-staring SOHO spacecraft, 1.5 million kilometers sunward of planet Earth. Of course, most of these SOHO comets have been sungrazers - like the one illustrated in the dramatic montage above. Three frames taken hours apart on October 23rd, show bright SOHO comet number 367 plunging toward the fiery solar surface, its tail streaming away from the Sun located just beyond the left hand border. Each panel spans about one million kilometers at the distance of the Sun. From bottom to top, the comet's tail grows as the intensifying solar radiation heats the frozen comet material and increases the outflow of gas and dust. Because of their orbits, sungrazers are believed to belong to a family of comets produced by the breakup of a single much larger comet. Coincidentally, this sungrazer was discovered shortly after solar active regions blasted out clouds of energetic particles, like those that triggered the recent spectacular auroral storms. And like all SOHO sungrazers so far, comet number 367 was not seen to survive its close solar encounter.

Thumbnail image of picture found for this day. APOD: 2001 September 26 - Comet Borrelly's Nucleus
Explanation: What does a comet nucleus look like? To answer this question, NASA controllers drove an aging probe through the hostile environs of a distant comet, expecting that even if comet fragments disabled the spacecraft, it would be worth the risk. The probe, Deep Space 1, survived. Pictured above is the most detailed image ever taken of a comet nucleus, obtained Saturday by Deep Space 1 and released yesterday by NASA. Comet Borrelly's nucleus is seen to be about 8 kilometers long with mountains, faults, grooves, smooth rolling plains, and materials of vastly different reflectance. Light colored regions are present near the center and seem to give rise to dust jets seen in Borrelly's coma, visible in distant images of the comet. Previously, the best image of a comet nucleus came from the Giotto mission to Comet Halley in 1986. Deep Space 1 images of Borrelly add welcomed bedrock to understanding Solar System history and to the accurate prediction of future brightness changes of notoriously fickle comets.

Thumbnail image of picture found for this day. APOD: 2001 June 25 - A Brighter Comet LINEAR
Explanation: Brighter than ever expected, comet LINEAR -- you know, the one designated C/2001 A2 -- is a sight to see in southern skies. This comet LINEAR first brightened impressively in late March as its active nucleus began to fragment, prompting some speculation that the comet might soon break up completely. But still hanging together after its closest approach to the Sun, C/2001 A2 suddenly brightened again and was reported last week to have reached nearly 3rd magnitude, easily visible to the unaided eye. This delightful telescopic picture of the brighter coma of comet LINEAR was recorded from Australia on June 20. Stars seen through the tenuous coma and filamentary tail appear as a series of short trails in this three-color composite image registered on the comet. North is up and the scene covers about half the width of the full Moon. Now moving through the constellation Cetus, comet LINEAR will be north of the celestial equator by July 4 as it comes into view for eager northern sky-gazers.

Thumbnail image of picture found for this day. APOD: 2001 May 27 - Comet Hale-Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp became much brighter than any surrounding stars. It was seen even over bright city lights. Out away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail was created when fast moving particles from the solar wind struck expelled ions from the comet's nucleus. The white dust tail is composed of larger particles of dust and ice expelled by the nucleus that orbit behind the comet. Observations showed that Comet Hale-Bopp's nucleus spins about once every 12 hours.

Thumbnail image of picture found for this day. APOD: 2001 May 21 - Another Comet LINEAR Breaks Up
Explanation: Last year, a different comet LINEAR (C/1999 S4) broke up. This year, a comet first imaged by the Lincoln Near Asteroid Research (LINEAR) telescope in New Mexico on 2001 January 3, is also breaking up. This new Comet LINEAR (C/2001 A2) unexpectedly brightened to the edge of naked-eye visiblilty a few weeks ago when its nucleus broke in two. Observations taken just last week now indicate that one of the two remaining nuclear fragments has again fragmented. The first piece to break off is visible on the upper left of the above false-color image by a Very Large Telescope, while additional fragmentation is inferred from the brightness and elongation of the spot on the lower right. When a comet nucleus splits, new surfaces are exposed and previously trapped ice and gas are released that evaporate and brighten in the energetic sunlight. Comet LINEAR may remain visible with little or no optical aid into early June. In contrast, at least two other much dimmer Comet LINEARs discovered recently appear stable.

Thumbnail image of picture found for this day. APOD: 2001 March 26 - Comet Hale Bopp in the Outer Solar System
Explanation: Whatever became of Comet Hale-Bopp? The brightest comet in recent years has continued into the outer Solar System and is now farther from the Sun than Saturn. To the surprise of many, Comet Hale-Bopp is still active, continuing to spew gas, ice and dust particles out into space. Pictured above earlier this month, Comet Hale-Bopp can be seen in the Southern Hemisphere with a moderate sized-telescope. The continued activity of Comet Hale-Bopp may be due to the large size of its nucleus - estimated to be about 50 kilometers across. The unusual dotted appearance of most stars in the above image is due to the 14 discrete exposures that were centered on the comet and not the stars.

Thumbnail image of picture found for this day. APOD: 2001 March 14 - Comet McNaught-Hartley
Explanation: Outbound and climbing above the plane of our solar system, comet McNaught-Hartley (C/1999 T1) is presently soaring through northern skies. This telescopic picture, a composite of many 30 second exposures made through three color filters, recorded the delicate colors in its diminutive coma and faint tail on February 26th. Combining the exposures to produce the final image registered on the comet causes stars to appear as "dotted trails", evidence of the comet's motion relative to the distant stellar background. Discovered by southern hemisphere observers, this comet's closest approach to the Sun occurred in December last year as it passed just outside planet Earth's orbit. For now the brightest comet in the sky, this primordial chunk of solar system is crossing from the constellation Hercules to Draco and will continue to fade. Never visible to the unaided eye, McNaught-Hartley is still at about 10th magnitude and can be viewed by comet seekers using small telescopes.

Thumbnail image of picture found for this day. APOD: 2000 December 27 - The Dust and Ion Tails of Comet Hale Bopp
Explanation: In 1997, Comet Hale-Bopp's intrinsic brightness exceeded any comet since 1811. Since it peaked on the other side of the Earth's orbit, however, the comet appeared only brighter than any comet in two decades. Visible above are the two tails shed by Comet Hale-Bopp. The blue ion tail is composed of ionized gas molecules, of which carbon monoxide particularly glows blue when reacquiring electrons. This tail is created by the particles from the fast solar wind interacting with gas from the comet's head. The blue ion tail points directly away from the Sun. The white dust tail is created by bits of grit that have come off the comet's nucleus and are being pushed away by the pressure of light from the Sun. This tail points nearly away from the Sun. The above photograph was taken in March 1997.

Thumbnail image of picture found for this day. APOD: 2000 August 11 - Fragments of Comet LINEAR
Explanation: What do you call a bunch of comet fragments anyway ... a flock, a covey, a swarm? The question is definitely relevant to comet LINEAR (C/1999 S4 LINEAR) whose nucleus apparently fragmented late last month during its first trip through the inner solar system. This computer enhanced composite image shows faint stars as trails and the remnants of LINEAR's nucleus as a flock of "mini-comets" embedded in a cloud of gas and dust. It was recorded by astronomers using the European Southern Observatory's Antu telescope about a day after the Hubble Space Telescope (HST) was also able to image the covey of condensations. A comparison of the HST and the subsequent Antu images reveals that the swarm of cometary debris has changed markedly in 24 hours demonstrating the very dynamic behavior of comet LINEAR's remains. Astronomers intend to keep watching as comet LINEAR's fragments continue to lose dust and gas and fade from view. As a result, LINEAR's legacy may well be insight into the make-up of a primordial piece of our solar system. If pictures of comet LINEAR have piqued your curiosity about fragments of a comet, why not watch the Perseid meteor shower this weekend?

Thumbnail image of picture found for this day. APOD: 2000 July 31 - Comet LINEAR Breaks Up
Explanation: Unexpectedly, Comet LINEAR is breaking up. In retrospect, clues of its demise have been surfacing all month as the new comet has been approaching the Sun and brightening with dramatic flares. Above, the Hubble Space Telescope captured Comet C/1999 S4 LINEAR early this month blowing off a large piece of its crust. Recent speculation holds that the nucleus completely disrupted on or about July 24. If true, the elongated train of material should continue to ablate and orbit the Sun, but may now fade much more quickly. The break up of a bright comet is unusual but not unprecedented, as Comet Shoemaker-Levy 9 broke up before it struck Jupiter in 1994, and Comet Bennett broke apart as it neared the Sun in 1974. Future observations will tell if Comet LINEAR's first trip into the inner Solar System is its last.

Thumbnail image of picture found for this day. APOD: 2000 July 27 - Tails Of Comet LINEAR
Explanation: Comet C/1999 S4 LINEAR is only one of many comets discovered with the Lincoln Near Earth Asteroid Research (LINEAR) telescope operating near Soccoro, New Mexico, USA. Traveling steadily southward through Earth's night sky, C/1999 S4 passed perihelion (closest approach to the Sun) yesterday on what is likely its first trip through the inner solar system. Now fading, comet LINEAR became no brighter than about 6th magnitude, but is still easily visible with binoculars in northern hemisphere skies. While the memorable comets Hale-Bopp and Hyakutake were much brighter, comet LINEAR is displaying delightful tails evident in this false-color composite image from the Crni Vrh Observatory in Slovenia. The combined series of exposures made on July 22nd are registered on the comet. In the resulting picture, stars appear as rows of dots, but the faint structures in the comet's tail are beautifully recorded. Presently seen moving from Ursa Major to Leo this comet LINEAR will begin to shine in southern hemisphere skies in August.

Thumbnail image of picture found for this day. APOD: 2000 July 10 - Comet LINEAR Extends
Explanation: Comet LINEAR's tail appears to be extending. Many sky watchers are closely following Comet C/1999 S4 LINEAR and wondering if it will develop an impressive tail or become visible to the naked eye later this month. So far, the unpredictable comet is moving oddly indicating that exploding caverns of heated gas are causing the comet to shift slightly in its orbit around the Sun. This volatility contributes to Comet LINEAR newly visible two-degree tail, discernable in the above photographic negative taken Friday from California. Current brightness estimates indicate that Comet LINEAR will just barely become visible without binoculars in northern skies in the days surrounding July 23 during the early evening hours.

Thumbnail image of picture found for this day. APOD: 2000 July 4 - Comet LINEAR Approaches
Explanation: Just possibly, a new comet may become bright enough to see without binoculars later this month. Comet C/1999 S4 LINEAR is rapidly approaching both the Earth and the Sun from the outer Solar System, and should be at its brightest around 2000 July 25 in the early evening sky of northern observers. The comet was discovered by chance by project LINEAR last September. The above time-lapse sequence of Comet LINEAR was taken on 2000 July 2 from Arizona and shows the comet's movement over only 19 minutes. Although Comet LINEAR's positions will be known quite accurately, the comet's future brightness and tail length can only be guessed, and it is quite possible that neither will become very impressive.

Thumbnail image of picture found for this day. APOD: 2000 April 13 - Exploring Comet Tails
Explanation: Comets are known for their tails. In the spring of 1997 and 1996 Comet Hale-Bopp (above) and Comet Hyakutake gave us stunning examples as they passed near the Sun. These extremely active comets were bright, naked-eye spectacles offering researchers an opportunity to telescopically explore the composition of primordial chunks of our solar system by studying their long and beautiful tails. But it has only recently been discovered that surprising readings from experiments on-board the interplanetary Ulysses probe which lasted for several hours on May 1, 1996, indicate the probe passed through comet Hyakutake's tail! Ulysses experiments were intended to study the Sun and solar wind and the spacecraft-comet encounter was totally unanticipated. Relative positions of Ulysses and Hyakutake on that date indicate that this comet's ion tail stretched an impressive 360 million miles or about four times the Earth-Sun distance. This makes Hyakutake's tail the longest ever recorded and suggests that comet tails are much longer than previously believed.

Thumbnail image of picture found for this day. APOD: August 8, 1999 - Comet Hale-Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp became much brighter than any surrounding stars. It was seen even over bright city lights. Out away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail was created when fast moving particles from the solar wind struck expelled ions from the comet's nucleus. The white dust tail is composed of larger particles of dust and ice expelled by the nucleus that orbit behind the comet. Observations showed that Comet Hale-Bopp's nucleus spins about once every 12 hours.

Thumbnail image of picture found for this day. APOD: April 27, 1999 - Introducing Comet Lee
Explanation: Another large snowball is falling toward the Sun. Comet Lee was discovered two weeks ago by Steve Lee (AAO) in Australia, and is expected to brighten as it approaches the inner Solar System. Comet Lee is not expected to become a naked-eye object, though, with recent projections guessing it will peak at magnitude 7 in July. Starting in May, the large piece of dirty ice officially known as Comet C/1999 H1 Lee will be visible to northern observers. Comet Lee is the fuzzy patch in the above picture taken on April 16 from Loomberah, Australia.

Thumbnail image of picture found for this day. APOD: October 6, 1998 - Comet Williams in 1998
Explanation: The brightest comet in the sky right now is Comet Williams. Moving slowly though the constellation of Centaurus, Comet Williams, at magnitude 8, is visible to Southern Hemisphere observers with binoculars. In ten days, Comet Williams will reach its closest point to the Sun, although it will still be farther from the Sun than the Earth. Comet Williams should become visible to many Northern Hemisphere observers in late November. At magnitude 10, however, it might require a small telescope to see. Comet Williams was discovered in early August by Peter Williams. The above image was taken August 25th from Australia.

Thumbnail image of picture found for this day. APOD: June 28, 1998 - Comet Hale Bopp Over Val Parola Pass
Explanation: In 1997, Comet Hale-Bopp became much brighter than any surrounding stars. It could be seen even over bright city lights. Out away from city lights, however, it put on quite a spectacular show. Here Comet Hale-Bopp was photographed last March above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tail was created when fast moving particles from the solar wind strike ions expelled from the comet's nucleus. The white dust tail was composed of larger particles of dust and ice expelled by the nucleus that orbit behind the comet. Observations have shown that Comet Hale-Bopp's nucleus spins about once every 12 hours. Comet Hale-Bopp is still visible to those in the right place with a good telescope.

Thumbnail image of picture found for this day. APOD: May 21, 1998 - Bright Comet SOHO
Explanation: Discovered this month with an orbiting solar observatory, bright Comet SOHO has now emerged from the Sun's glare. This telephoto picture of the new naked-eye comet was taken by astrophotographer Michael Horn after sunset in the western twilight above Lake Samsonvale, Brisbane, Australia on May 18. The comet is seen in the constellation Orion. Its long lovely tail stretches nearly 5 degrees to the bright star Bellatrix, near the top of the image. For Southern Hemisphere comet watchers, views of Comet SOHO (1998J1) will improve as this month draws to a close and the comet climbs to the south and east on its journey outward bound. In February 1999, NASA plans to launch the Stardust mission to fly close to a comet and return samples of dust from a comet's tail.

Thumbnail image of picture found for this day. APOD: May 14, 1998 - Comet Stonehouse
Explanation: Comets move against a field of background stars. Their apparent motion is slow but carefull tracking reveals their orbits, allowing these visitors to the inner solar system to be identified as old or new acquaintances. Recently a new comet, designated 1998 H1, was discovered by observer Patrick L. Stonehouse of Wolverine, Michigan, USA, and announced on April 26. At 10th magnitude, comet Stonehouse is too faint to be seen by the unaided eye, but it is presently a popular object for telescope-equipped comet watchers. This false color picture of comet Stonehouse was taken on May 1st at Limber Observatory and is a composite of 8 exposures each 60 seconds long. The sequence of exposures was made with the telescope following the background stars. The individual pictures were then aligned on the comet and added together. Because of the comet's relative motion, the combined multiple-exposure shows trails of progressively offset star images but nicely captures the comet's coma and faint, tenuous tail.

Thumbnail image of picture found for this day. APOD: April 3, 1997 - Earth, Clouds, Sky, Comet
Explanation: Does a comet's dust tail always orbit behind it? Since comets rotate, they shed gas and dust in all directions equally. Small ice and dust particles expelled by the comet, however, are literally pushed around by sunlight. The smaller the particle, the greater the effect. When the comet is headed inward, sunlight slows down small particles so they orbit behind the comet. When the comet is headed back out though, sunlight speeds them up, so small particles orbit in front of the comet. Comet Hale-Bopp itself is too big to have its orbit affected by the momentum of sunlight. Therefore, since Comet Hale-Bopp started back out to the outer Solar System two days ago, we can expect the dramatic dust tail shown above to shift in front in the coming days.

Thumbnail image of picture found for this day. APOD: March 25, 1997 - Hale-Bopp Brightest Comet This Century
Explanation: A comet as bright as Comet Hale-Bopp is very rare indeed. No comet has emitted or reflected this much light since possibly the Great Comet of 1811. However, since Comet Hale-Bopp is across the inner Solar System from us, it does not appear as bright as Comet West did in 1975. The Great Comet of 1996, Comet Hyakutake, was relatively dim but also appeared bright since it passed close to the Earth. Above, Comet Hale-Bopp was photographed high over the town of Las Palmas of the Spanish Canary Islands, on March 11th.

Thumbnail image of picture found for this day. APOD: March 17, 1997 - Comet Hale-Bopp Over Val Parola Pass
Explanation: Comet Hale-Bopp is now much brighter than any surrounding stars. It can be seen even over bright city lights. Out away from city lights, however, it is putting on quite a spectacular show. Here Comet Hale-Bopp was photographed last week above Val Parola Pass in the Dolomite mountains surrounding Cortina d'Ampezzo, Italy. Comet Hale-Bopp's blue ion tale is created when fast moving particles from the solar wind strike recently expelled ions from the comet's nucleus. The white dust tail is composed of larger particles of dust and ice expelled by the nucleus that orbit behind the comet. Recent observations show that Comet Hale-Bopp's nucleus spins about once every 12 hours. Comet Hale-Bopp is now visible in both the early morning and early evening sky, and will continue to brighten this week.

Thumbnail image of picture found for this day. APOD: February 20, 1997 - Comet Hale-Bopp and the Dumbbell Nebula
Explanation: Comet Hale-Bopp is now slowly moving across the morning sky. During its trip to our inner Solar System, the comet passes in front of several notable objects. Here Comet Hale-Bopp was photographed on February 11th superposed nearly in front of the picturesque Dumbbell Nebula, visible on the upper right. Comet Hale-Bopp is now first magnitude - one of the brightest objects in the morning sky. APOD, always in search of interesting and accurate astronomy pictures, issues the following informal challenge: that Comet Hale-Bopp be photographed in color with both easily recognizable foreground and background objects. For instance, in late March, it might be possible to photograph the comet with the Eiffel Tower in the foreground and the Andromeda galaxy (M31) in the background. Such superpositions would not only contrast human and cosmic elements, but give angular perspective on the size of the comet's tail.

Thumbnail image of picture found for this day. APOD: December 10, 1996 - Comet Halley's Nucleus
Explanation: Here is what a comet nucleus really looks like. For all active comets except Halley, it was only possible to see the surrounding opaque gas cloud called the coma. During Comet Halley's most recent pass through the inner Solar System in 1986, however, spacecraft Giotto was able to go right up to the comet and photograph its nucleus. The above image is a composite of hundreds of these photographs. Although the most famous comet, Halley achieved in 1986 only 1/10th the brightness that Comet Hyakutake did last year, and a similar comparison is likely with next year's pass of Comet Hale-Bopp. Every 76 years Comet Halley comes around again, and each time the nucleus sheds about 6 meters of ice and rock into space. This debris composes Halley's tails and leaves an orbiting trail that, when falling to Earth, are called the Orionids Meteor Shower.

Thumbnail image of picture found for this day. APOD: September 17, 1996 - Comet Hale-Bopp Fades
Explanation: Comet Hale-Bopp has faded in the past few weeks. For Hale-Bopp, promised as the Great Comet of 1997, this was a bit of a disappointment -- but not entirely unexpected. Comet Hale-Bopp continues to approach the Sun - making the comet itself brighten, but now the Earth is moving away from it - making the comet appear to dim. Experts disagree on just how bright Hale-Bopp will become. Optimists hope it will eventually outshine Comet Hyakutake, but some pessimists now expect no better than 3rd magnitude - hardly visible from well-lit cities. Comet Hale-Bopp still appears to be, however, a very large comet, and is sure to show much activity as it nears the Sun. The comet should reach peak brightness in March 1997. This image was taken on August 18th and shows gas shed from the nucleus of the comet.

Thumbnail image of picture found for this day. APOD: May 1, 1996 - Comet Hyakutake and a Cactus
Explanation: Comet Hyakutake is shown photographed the night of March 27 in Arizona, USA, with a cactus in the foreground. Polaris, the north star, is the bright star seen just to the upper right of the comet's head. Today Comet Hyakutake reaches its closest approach to the Sun. Comet Hyakutake is now at its intrinsic brightest, but because of its distance from the Earth, it will appear less bright to us than it did during its closest approach to the Earth in late March. In fact, due to the comet's angular proximity to the Sun, it will difficult to see at all from the Earth! Comet Hyakutake will reach less than one quarter of the Earth-Sun distance - inside the orbit of Mercury. Comet Hyakutake will not venture near the Sun again for another about 15,000 years.

Thumbnail image of picture found for this day. APOD: April 28, 1996 - The Sun Sets on Comet Hyakutake
Explanation: Comet Hyakutake is seen here just as the Sun sets on April 22. As April draws to a close, Comet Hyakutake will be visible only just after sunset and will be hard to discern against the brightly lit sky. Unfortunately, Comet Hyakutake did not brighten as much as hoped during its journey to the inner Solar System, and is now not supposed to get as bright as it did when it passed the Earth in late March. Nevertheless, Comet Hyakutake is still a bright comet and spectacular sight.

Thumbnail image of picture found for this day. APOD: April 14, 1996 - The Rotating Jets of Comet Hyakutake
Explanation: Comet Hyakutake will reach its closest point to the Sun on May 1, passing well inside the orbit of Mercury. At this time, the comet's dust and ion tail will be at their greatest physical length. As the comet nears the Sun, gas and dust are driven off the surface, sometimes being shot off in jets. Although much of this material ends up in the tail, some interesting features can be seen close to the comet's three kilometer nucleus. Because the comet's nucleus rotates, the jets can be seen to form arcs around the comet's center resembling a pinwheel. The above photograph, taken April 8, shows two expanding arcs of cometary material and two source jets. The outermost arc is at a projected distance of 12,000 kilometers from the nucleus. The inner is about 8,000 kilometers from the nucleus. They are expanding from the nucleus at 870 km per hour. The inner arc ends at the brightest of the Comet Hyakutake's many jets.

Thumbnail image of picture found for this day. APOD: March 31, 1996 - Comet Hyakutake Finder Chart for Early April
Explanation: During April Comet Hyakutake heads in toward the Sun after passing the Earth. At this time the comet's orbit places it north of the Earth. Remaining visible in the northern sky as it nears the Sun, it will set progressively earlier in the evening. Early in April, the Moon's glow will diminish viewing of the comet's tail - except during the lunar eclipse on April 3rd! As the comet recedes from the Earth it will appear dimmer even though it is getting intrinsically brighter as it nears the Sun. In late April the intrinsic brightening effect will "win" and the comet will again appear to brighten - possibly getting even brighter than it was last week. At this point the comet will appear near sunset low on the northwestern horizon (see above). So far Comet Hyakutake has exceeded most expectations in brightness and length of tail. If you haven't been impressed by Hyakutake, you probably haven't seen it from a dark location!

Thumbnail image of picture found for this day. APOD: March 30, 1996 - An Extreme UltraViolet View of the Comet
Explanation: As the Sun floods Comet Hyakutake with ultraviolet light gases in the coma scatter the radiation and fluoresce making the comet a bright source in the ultraviolet sky. The above image made using data from NASA's Extreme UltraViolet Explorer (EUVE) satellite, represents the intensity of the comet in this invisible high energy band in false color. The image is about 3/4 of a degree high and 2 degress wide and offers insights to the composition of this visitor from the distant solar system that can be obtained from the highest energy bands of the ultraviolet spectrum. The International Ultraviolet Explorer (IUE) satellite has also examined ultraviolet light from the comet and now reports the detection of many bands of molecular emission particularly those due to molecular carbon (C2), carbon monoxide (CO) and caron dioxide (C02) ions as well as indications of a rapid increase in the production of water (H20).

Thumbnail image of picture found for this day. APOD: March 27, 1996 - How Much is That Comet in the Window?
Explanation: The above true-color photo taken March 25th shows Comet Hyakutake passing below the stars of the Big Dipper. Many astronomy enthusiasts delight in helping people in their local community see the comet. Both Jerry Bonnell and I (RJN) from APOD have been so inclined - both now and when Comet Halley came by in 1986. During these sessions, many good questions are asked and occasionally a humorous situation will arise. One was with a little girl. She waited so patiently for her turn to look through the telescope, hardly able to contain her excitement. Finally her turn came. "Do you see the comet?" I asked. "Wow, wow, WOW!" She beamed. "You see it?" "No." One little boy seemed particularly bent on destruction. "This telescope looks like a big gun," he volunteered. "In some ways, it's even more powerful than a gun," I replied, hoping to challenge his imagination. "Really?" he countered. "Can we shoot down the comet?" "How expensive is the telescope?" is a fairly common question. But one time a real business-person showed up and, possibly feeling particularly affluent, asked "How much is the comet?"

Thumbnail image of picture found for this day. APOD: March 26, 1996 - What are Comet Tails Made Of?
Explanation: The tail of comet Hyakutake, visible in this recent color image, is composed of dust and gas driven off the icy comet nucleus by the Sun's heat and blown away by the solar wind. Bathed in solar ultraviolet light, the gas molecules break down and are excited, producing a characteristic glow. This glow is responsible for visible light from the tail and astronomers using spectroscopes can identify the compounds involved. The close passage of Hyakutake presents an excellent chance to use this technique to explore the composition of its tail. Typical comet gas tail constituents are simple combinations of hydrogen, carbon, nitrogen, and oxygen - for example, H20 (water), CO (carbon monoxide), and CN (cyanogen) are common. In fact, the poisonous CO and CN compounds were seen in the spectrum of Halley's comet during its 1910 apparition. This caused some public concern at the time as the Earth was expected to pass through Halley's tail! However, stretching for millions of miles, comet tails are extremely thin and tenuous and don't pose a danger to the Earth's atmosphere.

Thumbnail image of picture found for this day. APOD: March 25, 1996 - Comet Hyakutake Passes the Earth
Explanation: This picture of Comet Hyakutake taken the night of March 21/22 in Illinois, USA shows the enormous tail that has already developed. The silhouette on the right is a foreground tree, and the superposed green circle on the left shows the size of the full moon. Today Comet Hyakutake makes its closest approach to the Earth. As the comet moves into the inner Solar System, it will pass the Earth at about 40 times the distance of our Moon. This is not the closest a comet has ever come, though. As recently as 1983 Comet IRAS-Araki-Alcock came three times closer than Hyakutake, and in 1770 Comet Lexell got yet twice closer than that! Asteroids - usually less massive than comets - frequently whiz by inside the Moon's orbit, with four doing so far in this decade. In the distant past, asteroids have even struck the Earth. Comet Hyakutake is much brighter now than Comet IRAS-Araki-Alcock ever got, and in fact is the brightest since Comet West in 1976. Comet Hyakutake will be easily visible all week.

Thumbnail image of picture found for this day. APOD: March 24, 1996 - Comet Hyakutake's Closest Approach
Explanation: The above true color image of Comet Hyakutake was taken the night of March 21/22. Tonight, Comet Hyakutake will make its nearest approach to Earth, closing to a mere 10 million miles as it passes over the planet's Northern Hemisphere. From dark sky areas, it's tail may be seen to cover about 20 degrees on the sky (40 times the apparent diameter of the full moon) corresponding to well over 3 million miles. at the distance of the comet. The word comet, referring to the tail, derives from the Greek "aster kometes", meaning long-haired star - and the hair of comet Hyakutake continues to grow as it nears the Sun! The tail grows as the sun heats and sublimates (changes directly from solid to gas) the material on the icy surface of the comet nucleus, sending jets of gas and dust into space. The material is swept back by the solar wind, so comet tails usually point away from the sun rather than simply trailing along behind in the comets' orbit. Some predict the tail will grow over the next few days to nearly 50 degrees. For the rest of March and most of April Comet Hyakutake will be visible to Northern viewers (weather permitting). The tail will be most visible from dark sky areas. Moonlit skies will tend to washout the comet as the April 3rd full moon approaches - however, on April 3rd, there will be a lunar eclipse!

Thumbnail image of picture found for this day. APOD: March 23, 1996 - Comet Hyakutake's Past and Future
Explanation: The above false-color picture of Comet Hyakutake taken just two days ago shows its rapidly developing tail. The comet now has a substantial coma with a bright center, lending it a dramatic eye-like appearance. This is not Comet Hyakutake's first visit to the inner Solar System. Recent orbital determinations clearly show Comet Hyakutake's was here before, although the previous approach is estimated to be about 8600 years ago - during the epoch of the first recorded human cities. Were this the comet's first trip to the inner Solar System, it probably would not appear as bright as it does now - first time comets typically do not shed as much luminous gas as veterans. Before making any approach to the inner Solar System, Comet Hyakutake was dormant in the Oort cloud of the outer Solar System for a few billion years, along with hundreds of thousands of similar comets. Comet Hyakutake is predicted to become the brightest comet since Comet West in 1976, which rivaled the brightest stars in the sky. Tonight, Comet Hyakutake can be seen best from about 10 pm near the Big Dipper's handle.

Thumbnail image of picture found for this day. APOD: March 22, 1996 - Where to See Comet Hyakutake
Explanation: People the world over are preparing to witness the closest approach of the brightest comet of the past twenty years. Comet Hyakutake, discovered just two months ago, will pass nearest the Earth Monday morning. All during the coming week, Comet Hyakutake will be visible in the northern sky as an unusual extended fuzzy patch. To see the comet is not difficult - just go outside and look up - no telescope is required! The comet's location in the sky during late March is charted above. The horizon is drawn for about 8 pm in your local time. By about 11 pm, Comet Hyakutake will be high in the sky and well placed for viewing. Although Comet Hyakutake is whizzing past the Earth at a blistering speed of almost 100,000 miles per hour, and it is practically streaking across the sky by astronomical standards, it will appear to move only a few degrees on any given night. Each night this weekend and in the coming week, the comet will be visible. Please don't miss this rare and exciting astronomical event!

Thumbnail image of picture found for this day. APOD: March 19, 1996 - The Ion Tail of Comet Hyakutake
Explanation: This picture of Comet Hyakutake was taken on March 14, 1996. Structure in the ion tale of Comet Hyakutake is now clearly visible. An ion tale forms as a comet nears the Sun. Sunlight causes gas and dust to boil off the comet's solid nucleus. Charged gas - called ions - are then accelerated away from the Sun by the solar wind - fast moving particles streaming out from the Sun's corona. The ion tale will appear blue and glows by fluorescence. As Comet Hyakutake gets closer to the Sun during the next month, a dust tail is expected to be visible as well. Dust tails shine by light reflected from the Sun. Comet tails point away from the Sun, even as a comet moves away from the Sun. For observers in the Northern Hemisphere, Comet Hyakutake should appear tonight in the eastern part of the constellation of Virgo and should be about magnitude 2.5. The comet will look the most impressive in the darkest skies - in a city you are likely to see only a fuzzy blob!

Thumbnail image of picture found for this day. APOD: March 14, 1996 - Comet Hyakutake's Orbit
Explanation: Where did Comet Hyakutake come from? The orbits of the Earth and this brightening comet are shown in the above diagram. The blue disk is bounded by the circular orbit of the Earth about the central Sun. The comet's path outlines the green shape. The shape of the comet's orbit is close to a parabola. The comet has come in from the outer Solar System, will pass near the Earth in late March, and pass near the Sun in late April. Comet Hyakutake will appear bright in late March because it is so close to the Earth, and will again appear bright in late April because it is so close to the Sun. In late March, the comet will be "north" of the Earth and so only visible in the Northern hemisphere. Information about how to see Comet Hyakutake is available from many University astronomy departments and planetaria.

Thumbnail image of picture found for this day. APOD: March 13, 1996 - Here Comes Comet Hyakutake
Explanation: The reaction of ancient peoples to the appearance of bright comets has toppled empires, de-throned kings, and been taken as a sign of great things to come. Probably some of these comets did not get as bright as Comet Hyakutake ("hyah-koo-tah-kay") will in the next two weeks. It is likely that every major news organization will soon cover Comet Hyakutake extensively. Comet Hyakutake is shown above already showing a tail of dust. This image, taken directly from a photographic negative, shows stars as black spots and the bright comet coma and tail as dark clouds against the white background of space. During its closest approach to the Earth on March 25th, Comet Hyakutake will appear in the Northern hemisphere as a diffuse ball of light brighter than most stars. Comet Hyakutake will be visible then most of the night even without binoculars, passing above the stars in the handle of the Big Dipper. Comet Hyakutake will be best seen in dark skies far from city lights, where its tail - possibly extending 20 degrees or more - will be most easily visible. As Comet Hyakutake recedes from the Earth it will fade, but brighten again as it nears the sun later in April. At that time it will be best seen in the southern hemisphere. There is no chance Comet Hyakutake will hit the Earth.

Thumbnail image of picture found for this day. APOD: February 19, 1996 - Periodic Comet Swift-Tuttle
Explanation: Comet Swift-Tuttle, shown above in false color, is the largest object known to make repeated passes near the Earth. It is also one of the oldest known periodic comets with sightings spanning two millennia. Last seen in 1862, its reappearance in 1992 was not spectacular, but the comet did become bright enough to see from many locations with binoculars. To create this composite telescopic image, four separate exposures have been combined, compensating for the motion of the comet. As a result, the stars appear slightly trailed. The inset shows details of the central coma. The unseen nucleus itself is essentially a chunk of dirty ice about ten kilometers in diameter. Comets usually originate in the Oort cloud in the distant Solar System - well past Pluto, most never venturing into the inner Solar System. When perturbed - perhaps by the gravity of a nearby star - a comet may fall toward the Sun. As a comet approaches the Sun, rocks, ice-chunks, gas, and dust boil away, sometimes creating impressive looking tails. In fact, debris from Comet Swift-Tuttle is responsible for the Perseids meteor shower visible every July and August. Comet Swift-Tuttle is expected to make an impressive pass near the earth in the year 2126, possibly similar to Comet Hyakutake this year or Comet Hale-Bopp next year.

Thumbnail image of picture found for this day. APOD: February 8, 1996 - Hyakutake: The Great Comet of 1996?
Explanation: Get ready for one of the most impressive but least anticipated light shows in modern astronomical history. Next month, newly discovered Comet Hyakutake will pass closer to the Earth than any recent comet. Unknown before its discovery by Yuji Hyakutake on 30 January 1996, the fuzzy spot in the above photograph is a comet now predicted to become bright enough to see without a telescope. Although comets act in such diverse ways that predictions are frequently inaccurate, even conservative estimates indicate that this comet is likely to impress. For example, even if Comet Hyakutake remains physically unchanged, its close pass near the Earth in late March 1996 should cause it to appear to brighten to about 3rd magnitude - still bright enough to see with the unaided eye. In the next two months, though, the comet will continue to approach the Sun and hence should become brighter still. Optimistic predictions include that Comet Hyakutake will change physically, develop a larger coma and tail, brighten dramatically, move noticeably in the sky during a single night, and may ultimately become known as the "The Great Comet of 1996." Move over Hale-Bopp!

Thumbnail image of picture found for this day. APOD: August 26, 1995 - Two Tails of Comet West
Explanation: Here Comet West is seen showing two enormous tails that wrap around the sky. The ion tale of a comet usually appears more blue and always points away from the Sun. The dust tail trailing the comet's nucleus is the most prominent. Comet West was a visually spectacular comet, reaching its most picturesque in March of 1976. A comet this bright occurs only about once a decade. Comets are really just large dirty snowballs that shed material when they reach the inner solar-system. Many astronomers are hopeful that Comet Hale-Bopp will look as spectacular as this in the spring of 1997.


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