Astronomy Picture of the Day Search Results for "moon OR satellite OR asteroid OR comet"

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.

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.

APOD: 2024 March 11 – A Full Plankton Moon
Explanation: What glows in the night? This night featured a combination of usual and unusual glows. Perhaps the most usual glow was from the Moon, a potentially familiar object. The full Moon's nearly vertical descent results from the observer being near Earth's equator. As the Moon sets, air and aerosols in Earth's atmosphere preferentially scatter out blue light, making the Sun-reflecting satellite appear reddish when near the horizon. Perhaps the most unusual glow was from the bioluminescent plankton, likely less familiar objects. These microscopic creatures glow blue, it is thought, primarily to surprise and deter predators. In this case, the glow was caused primarily by plankton-containing waves crashing onto the beach. The image was taken on Soneva Fushi Island, Maldives just over one year ago.

APOD: 2024 February 13 – A January Wolf Moon
Explanation: Did you see the full moon last month? During every month, on average, a full moon occurs in the skies over planet Earth. This is because the Moon takes a month to complete another orbit around our home planet, goes through all of its phases, and once again has its entire Earth-facing half lit by reflected sunlight. Many indigenous cultures give each full moon a name, and this past full moon's names include the Ice Moon, the Stay at Home Moon, and the Quiet Moon. Occurring in January on the modern western calendar, several cultures have also named the most recent full moon the Wolf Moon, in honor of the famous howling animal. Featured here above the Italian Alps mountains, this past Wolf Moon was captured in combined long and short exposure images. The image is striking because, to some, the surrounding clouds appear as a wolf's mouth ready to swallow the Wolf Moon, while others see the Moon as a wolf's eye.

APOD: 2024 January 24 – Earth and Moon from Beyond
Explanation: What do the Earth and Moon look like from beyond the Moon? Although frequently photographed together, the familiar duo was captured with this unusual perspective in late 2022 by the robotic Orion spacecraft of NASA's Artemis I mission as it looped around Earth's most massive satellite and looked back toward its home world. Since our Earth is about four times the diameter of the Moon, the satellite’s seemingly large size was caused by the capsule being closer to the smaller body. Artemis II, the next launch in NASA’s Artemis series, is currently scheduled to take people around the Moon in 2025, while Artemis III is planned to return humans to lunar surface in late 2026. Last week, JAXA's robotic SLIM spacecraft, launched from Japan, landed on the Moon and released two hopping rovers.

APOD: 2023 November 4 - Dinkinesh Moonrise
Explanation: Last Wednesday the voyaging Lucy spacecraft encountered its first asteroid, 152830 Dinkinesh, and discovered the inner-main belt asteroid has a moon. From a distance of just over 400 kilometers, Lucy's Long-Range Reconnaissance Imager captured this close-up of the binary system during a flyby at 4.5 kilometer per second or around 10,000 miles per hour. A marvelous world, Dinkinesh itself is small, less than 800 meters (about 0.5 miles) across at its widest. Its satellite is seen from the spacecraft's perspective to emerge from behind the primary asteroid. The asteroid moon is estimated to be only about 220 meters wide.

APOD: 2023 August 27 – Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007.

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.

APOD: 2023 June 3 - Charon: Moon of Pluto
Explanation: A darkened and mysterious north polar region known to some as Mordor Macula caps this premier high-resolution view. The portrait of Charon, Pluto's largest moon, was captured by New Horizons near the spacecraft's closest approach on July 14, 2015. The combined blue, red, and infrared data was processed to enhance colors and follow variations in Charon's surface properties with a resolution of about 2.9 kilometers (1.8 miles). A stunning image of Charon's Pluto-facing hemisphere, it also features a clear view of an apparently moon-girdling belt of fractures and canyons that seems to separate smooth southern plains from varied northern terrain. Charon is 1,214 kilometers (754 miles) across. That's about 1/10th the size of planet Earth but a whopping 1/2 the diameter of Pluto itself, and makes it the largest satellite relative to its parent body in the Solar System. Still, the moon appears as a small bump at about the 1 o'clock position on Pluto's disk in the grainy, negative,telescopic picture inset at upper left. That view was used by James Christy and Robert Harrington at the U.S. Naval Observatory in Flagstaff to discover Charon in June of 1978.

APOD: 2023 May 28 – Ida and Dactyl: Asteroid and Moon
Explanation: This asteroid has a moon. The robot spacecraft Galileo on route to Jupiter in 1993 encountered and photographed two asteroids during its long interplanetary voyage. The second minor planet it photographed, 243 Ida, was unexpectedly discovered to have a moon. The tiny moon, Dactyl, is only about 1.6 kilometers across and seen as a small dot on the right of the sharpened featured image. In contrast, the potato-shaped Ida is much larger, measuring about 60 kilometers long and 25 km wide. Dactyl is the first moon of an asteroid ever discovered -- now many asteroids are known to have moons. The names Ida and Dactyl are from Greek mythology.

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

APOD: 2022 September 29 - DART Asteroid Impact from Space
Explanation: Fifteen days before impact, the DART spacecraft deployed a small companion satellite to document its historic planetary defense technology demonstration. Provided by the Italian Space Agency, the Light Italian CubeSat for Imaging Asteroids, aka LICIACube, recorded this image of the event's aftermath. A cloud of ejecta is seen near the right edge of the frame captured only minutes following DART's impact with target asteroid Dimorphos while LICIACube was about 80 kilometers away. Presently about 11 million kilometers from Earth, 160 meter diameter Dimorphos is a moonlet orbiting 780 meter diameter asteroid Didymos. Didymos is seen off center in the LICIACube image. Over the coming weeks, ground-based telescopic observations will look for a small change in Dimorphos' orbit around Didymos to evaluate how effectively the DART impact deflected its target.

APOD: 2022 August 18 - Full Moon Perseids
Explanation: The annual Perseid meteor shower was near its peak on August 13. As planet Earth crossed through streams of debris left by periodic Comet Swift-Tuttle meteors rained in northern summer night skies. But even that night's nearly Full Moon shining near the top of this composited view couldn't hide all of the popular shower's meteor streaks. The image captures some of the brightest perseid meteors in many short exposures recorded over more than two hours before the dawn. It places the shower's radiant in the heroic constellation of Perseus just behind a well-lit medieval tower in the village of Sant Llorenc de la Muga, Girona, Spain. Observed in medieval times, the Perseid meteor shower is also known in Catholic tradition as the Tears of St. Lawrence, and festivities are celebrated close to the annual peak of the meteor shower. Joining the Full Moon opposite the Sun, bright planet Saturn also shines in the frame at the upper right.

APOD: 2022 July 3 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of our Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning color image from the robotic Mars Reconnaissance Orbiter, with objects as small as 10 meters visible. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. In perhaps 50 million years, Phobos is expected to disintegrate into a ring of debris.

APOD: 2022 January 28 - Western Moon, Eastern Sea
Explanation: The Mare Orientale, Latin for Eastern Sea, is one of the most striking large scale lunar features. The youngest of the large lunar impact basins it's very difficult to see from an earthbound perspective. Still, taken during a period of favorable tilt, or libration of the lunar nearside, the Eastern Sea can be found near top center in this sharp telescopic view, extremely foreshortened along the Moon's western edge. Formed by the impact of an asteroid over 3 billion years ago and nearly 1000 kilometers across, the impact basin's concentric circular features are ripples in the lunar crust. But they are a little easier to spot in more direct images of the region taken from lunar orbit. So why is the Eastern Sea at the Moon's western edge? The Mare Orientale lunar feature was named before 1961. That's when the convention labeling east and west on lunar maps was reversed.

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.

APOD: 2021 October 10 - Full Moon Silhouettes
Explanation: Have you ever watched the Moon rise? The slow rise of a nearly full moon over a clear horizon can be an impressive sight. One impressive moonrise was imaged in early 2013 over Mount Victoria Lookout in Wellington, New Zealand. With detailed planning, an industrious astrophotographer placed a camera about two kilometers away and pointed it across the lookout to where the Moon would surely soon be making its nightly debut. The featured single shot sequence is unedited and shown in real time -- it is not a time lapse. People on Mount Victoria Lookout can be seen in silhouette themselves admiring the dawn of Earth's largest satellite. Seeing a moonrise yourself is not difficult: it happens every day, although only half the time at night. Each day the Moon rises about fifty minutes later than the previous day, with a full moon always rising at sunset. This Saturday, October 16, is International Observe the Moon Night, where you observe a first-quarter Moon along with other lunar enthusiasts.

APOD: 2021 September 5 - Earth and Moon
Explanation: The Earth and Moon are rarely photographed together. One of most spectacular times this occurred was about 30 years ago when the Jupiter-bound Galileo spacecraft zoomed past our home planetary system. Then, robotic Galileo watched from about 15-times the Earth-Moon separation as our only natural satellite glided past our home world. The featured video combines 52 historic color-enhanced images. Although our Moon may appear small next to the Earth, no other planet in our Solar System has a satellite so comparable in size . The Sun, far off to the right, illuminated about half of each sphere, and shows the spinning Earth's white clouds, blue oceans, and tan continents.

APOD: 2021 February 3 - Found on the Moon: Candidate for Oldest Known Earth Rock
Explanation: Was the oldest known rock on Earth found on the Moon? Quite possibly. The story opens with the Apollo 14 lunar mission. Lunar sample 14321, a large rock found in Cone crater by astronaut Alan Shepard, when analyzed back on Earth, was found to have a fragment that was a much better match to Earth rocks than other Moon rocks. Even more surprising, that rock section has recently been dated back 4 billion years, making it older, to within measurement uncertainty, than any rock ever found on Earth. A leading hypothesis now holds that an ancient comet or asteroid impact launched Earth rocks into the Solar System, some of which fell back to the Moon, became mixed with heated lunar soil and other rocks, cooled, and re-fragmented. The video features an internal X-ray scan of 14321 showing multiple sections with markedly different chemistries. Moon rocks will continue to be studied to learn a more complete history of the Moon, the Earth, and the early Solar System. Friday marks the 50th Anniversary of the Apollo 14 landing on the Moon.

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.

APOD: 2020 November 11 - Colors of the Moon
Explanation: What color is the Moon? It depends on the night. Outside of the Earth's atmosphere, the dark Moon, which shines by reflected sunlight, appears a magnificently brown-tinged gray. Viewed from inside the Earth's atmosphere, though, the moon can appear quite different. The featured image highlights a collection of apparent colors of the full moon documented by one astrophotographer over 10 years from different locations across Italy. A red or yellow colored moon usually indicates a moon seen near the horizon. There, some of the blue light has been scattered away by a long path through the Earth's atmosphere, sometimes laden with fine dust. A blue-colored moon is more rare and can indicate a moon seen through an atmosphere carrying larger dust particles. What created the purple moon is unclear -- it may be a combination of several effects. The last image captures the total lunar eclipse of 2018 July -- where the moon, in Earth's shadow, appeared a faint red -- due to light refracted through air around the Earth. The next full moon will occur at the end of this month (moon-th) and is known in some cultures as the Beaver Moon.

APOD: 2020 November 8 - Martian Moon Phobos from Mars Express
Explanation: Why is Phobos so dark? Phobos, the largest and innermost of two Martian moons, is the darkest moon in the entire Solar System. Its unusual orbit and color indicate that it may be a captured asteroid composed of a mixture of ice and dark rock. The featured picture of Phobos near the limb of Mars was captured in 2010 by the robot spacecraft Mars Express currently orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From images like this, Phobos has been determined to be covered by perhaps a meter of loose dust. Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all. Phobos' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years.

APOD: 2020 September 25 - Moon over Andromeda
Explanation: The Great Spiral Galaxy in Andromeda (also known as M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (below and right).

APOD: 2020 September 5 - A Falcon 9 Moon
Explanation: Illuminating planet Earth's night, full moons can have many names. This year the last full moon of northern hemisphere summer was on September 2, known to some as the Full Corn Moon. A few days earlier on August 30 this almost full moon rose just before sunset though, shining through cloudy skies over Cape Canaveral Air Force Station on Florida's Space Coast. A well-timed snapshot caught the glare of rocket engines firing below the lunar disk, a Falcon 9 rocket's first stage successfully returning to Cape Canaveral's landing zone 1. About 9 minutes earlier, the same SpaceX Falcon 9 rocket had launched the SAOCOM 1B satellite toward polar orbit. The fourth launch for this reusable Falcon 9 first stage, it was the first launch to a polar orbit from Cape Canaveral since 1969.

APOD: 2020 July 27 – Comet and Lightning Beyond Bighorn Mountains
Explanation: Normally, Steamboat Point looks cool -- but not this cool. Every day, the iconic peak of the Bighorn Mountains is an interesting sight, in particular from US Highway 14 in Wyoming. On some rare days, the rocky vertical ridges look even more incredible when seen in front of a distant lightning storm. Earlier this month, though, something even more unusual happened -- the naked-eye Comet NEOWISE rose above it in the middle of the night. Just as a distant lightning storm was occurring in the background. Recognizing a rare opportunity, a determined astrophotographer spent a sleepless night capturing over 1400 images of this unusual triple conjunction. The featured image is among the best of them, with the foreground lit by the Moon off to the right. Comet C/2020 F3 (NEOWISE) is now headed back to the outer Solar System, destined to return only in about 6700 years.

APOD: 2020 July 19 - Rotating Moon from LRO
Explanation: No one, presently, sees the Moon rotate like this. That's because the Earth's moon is tidally locked to the Earth, showing us only one side. Given modern digital technology, however, combined with many detailed images returned by the Lunar Reconnaissance Orbiter (LRO), a high resolution virtual Moon rotation movie has been composed. The featured time-lapse video starts with the standard Earth view of the Moon. Quickly, though, Mare Orientale, a large crater with a dark center that is difficult to see from the Earth, rotates into view just below the equator. From an entire lunar month condensed into 24 seconds, the video clearly shows that the Earth side of the Moon contains an abundance of dark lunar maria, while the lunar far side is dominated by bright lunar highlands. Currently, over 19 new missions to the Moon are under active development from eight different countries, most of which have expected launch dates in the next three years.

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.

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.

APOD: 2019 December 13 - Full Moon Geminids
Explanation: The dependable annual Geminid meteor shower will be near its peak tonight (December 13/14) and before tomorrow's dawn. As Earth crosses through the dusty trail of active asteroid 3200 Phaethon the meteors will flash through the sky from the shower's radiant in Gemini. Gemini will be pretty easy for skygazers to find too as it won't be far from a nearly full waning gibbous Moon. You don't have look at the shower's radiant to see meteors though. The almost full moonlight won't hide the brightest of the Geminids from view either, but it will substantially reduce the rate of visible meteors for those who are counting. In fact, the 2019 Geminids should look a lot like the 2016 meteor shower. This composite image from the 2016 Geminids aligns individual short exposures to capture many of the brighter Geminid meteors, inspite of a Full Moon shining near the constellation of the Twins. Along the horizon are the Teide Observatory's Solar Laboratory (right) and the Teide volcano on the Canary Island of Tenerife.

APOD: 2019 October 5 - Jupiter and the Moons
Explanation: After sunset on October 3, some of the Solar System's largest moons stood low along the western horizon with the largest planet. Just after nightfall, a pairing of the Moon approaching first quarter phase and Jupiter was captured in this telephoto field of view. A blend of short and long exposures, it reveals the familiar face of our fair planet's own large natural satellite in stark sunlight and faint earthshine. At lower right are the ruling gas giant and its four Galilean moons. Left to right, the tiny pinpricks of light are Ganymede, [Jupiter], Io, Europa, and Callisto. Our own natural satellite appears to loom large because it's close, but Ganymede, Io, and Callisto are actually larger than Earth's Moon. Water world Europa is only slightly smaller. Of the Solar System's six largest planetary satellites, only Saturn's moon Titan, is missing from this scene. But be sure to check for large moons in your sky tonight.

APOD: 2019 September 13 - A Harvest Moon
Explanation: Famed in festival, story, and song the best known full moon is the Harvest Moon. For northern hemisphere dwellers that's a traditional name of the full moon nearest the September equinox. In most North America time zones this year's Harvest Moon will officially rise on Friday, September 13. In fact the same Harvest Moon will rise on September 14 for much of the planet though. Of course the Moon will look almost full in the surrounding days. Regardless of your time zone the Harvest Moon, like any other full moon, will rise just opposite the setting Sun. Near the horizon, the Moon Illusion might make it appear bigger and brighter to you but this Harvest Moon will be near lunar apogee. That's the farthest point in its orbit, making it the most distant, and so the smallest, full moon of the year. On August 15 a wheat field harvested in south central France made this a harvest moon scene too, the full moon shining on with beautiful iridescent clouds at sunset.

APOD: 2018 December 7 - December's Comet Wirtanen
Explanation: Coming close in mid-December, Comet 46P Wirtanen hangs in this starry sky over the bell tower of a Romanesque church. In the constructed vertical panorama, a series of digital exposures capture its greenish coma on December 3 from Sant Llorenc de la Muga, Girona, Catalonia, Spain, planet Earth. With an orbital period that is now about 5.4 years, the periodic comet's perihelion, its closest approach, to the Sun will be on December 12. On December 16 it will be closest to Earth, passing at a distance of about 11.6 million kilometers or 39 light-seconds. That's close for a comet, a mere 30 times the Earth-Moon distance. A good binocular target for comet watchers, Wirtanen could be visible to the unaided eye from a dark sky site. To spot it after dusk on December 16, look close on the sky to the Pleiades star cluster in Taurus.

APOD: 2018 November 25 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. The origin of the Martian moons is unknown, though, with a leading hypothesis holding that they are captured asteroids. The larger moon, at 25-kilometers across, is Phobos, and is indeed seen to be a cratered, asteroid-like object in this false-colored image mosaic taken by the robotic Viking 1 mission in 1978. A recent analysis of the unusual long grooves seen on Phobos indicates that they may result from boulders rolling away from the giant impact that created the crater on the upper left: Stickney Crater. Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. The ultimate result will be for Phobos to break up in orbit and then crash down onto the Martian surface in about 50 million years. Well before that -- tomorrow, in fact, if everything goes according to plan -- NASA's robotic InSight lander will touch down on Mars and begin investigating its internal structure.

APOD: 2018 November 16 - The Hill, The Moon, and Saturn
Explanation: Last Sunday when the Moon was young its sunlit crescent hung low near the western horizon at sunset. With strong earthshine it was joined by Saturn shining in the early evening sky for a beautiful conjunction visible to skygazers around our fair planet. On that clear evening on a hill near Veszprem, Hungary mother, daughter, bright planet, and young Moon are framed in this quiet night skyscape taken with a telephoto lens. Of course the Moon ages too quickly for some, and by tonight the sunlit part has reached its first quarter phase. This weekend skygazers spending quality time under Moon and stars might expect to see the annual rain of comet dust otherwise known as the Leonid meteor shower.

APOD: 2018 November 15 - Comet 46P Wirtanen
Explanation: Periodic Comet 46P/Wirtanen is now the brightest comet in the night sky, but too faint to be seen by eye. From dark sky sites it could just become naked-eye visible though, as its 5.4 year long looping orbit takes it closest to Earth and the Sun in mid December. Fluorescing in sunlight, its spherical coma is about half the angular size of a full moon in this southern hemisphere telescopic view from November 7. Then the comet was about 2 light-minutes away or 35 million kilometers from Earth-bound telescopes, so the pretty greenish coma seen here is around 150,000 kilometers across. That makes it about the size of Jupiter. The stack of digital images also reveals a very faint tail extending toward 4 o'clock with a distant background galaxy notable at the upper left. As a regular visitor to the inner Solar System, comet 46P/Wirtanen was once the favored rendezvous target for ESA's comet exploring Rosetta mission.

APOD: 2018 November 13 - Rotating Asteroid Bennu from OSIRIS REx
Explanation: Could this close-by asteroid ever hit the Earth? Eventually yes -- but probably not for a very long time, even though the asteroid is expected to pass inside the orbit of the Moon next century. However, to better understand the nature and orbit of all near-Earth asteroids, NASA sent the robotic Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) to investigate this one: the 500-meter across asteroid 101955 Bennu. Launched in 2016, OSIRIS-REx is now approaching Bennu, and is first scheduled to map the minor planet's rough surface. The featured time-lapse video taken earlier this month compacts Bennu's 4.25-hour rotation period into about 7 seconds. Bennu's diamond-like appearance is similar to asteroid Ryugu currently being visited by the Japanese spacecraft Hayabusa2. The exact future orbit of Bennu is a bit uncertain due to close passes near the Earth and the Yarkovsky effect: a slight force created by an object's rotationally-induced, asymmetric infrared glow. If all goes according to plan, ORISIS-Rx will actually touch the asteroid in 2020, collect soil samples, and return them to Earth in 2023 for detailed analyses.

APOD: 2018 November 10 - The Old Moon in the Young Moon's Arms
Explanation: Tonight the Moon is young again, but this stunning image of a young Moon near the western horizon was taken just after sunset on October 10. On the lunar disk Earthshine, earthlight reflected from the Moon's night side, is embraced by the slim, sunlit crescent just over 2 days old. Along the horizon fading colors of twilight silhouette the radio telescope dish antennas of the Very Large Array, New Mexico, planet Earth. The view from the Moon would be stunning, too. When the Moon appears in Earth's sky as a slender crescent, a dazzlingly bright, nearly full Earth would be seen from the lunar surface. A description of earthshine, in terms of sunlight reflected by Earth's oceans in turn illuminating the Moon's dark surface, was written 500 years ago by Leonardo da Vinci.

APOD: 2018 October 8 - Comet 21P Between Rosette and Cone Nebulas
Explanation: Small bits of this greenish-gray comet are expected to streak across Earth's atmosphere tonight. Specifically, debris from the eroding nucleus of Comet 21P / Giacobini-Zinner, pictured, causes the annual Draconids meteor shower, which peaks this evening. Draconid meteors are easy to enjoy this year because meteor rates will likely peak soon after sunset with the Moon's glare nearly absent. Patience may be needed, though, as last month's passing of 21P near the Earth's orbit is not expected to increase the Draconids' normal meteor rate this year of (only) a few meteors per hour. Then again, meteor rates are notoriously hard to predict, and the Draconids were quite impressive in 1933, 1946, and 2011. Featured, Comet 21P gracefully posed between the Rosette (upper left) and Cone (lower right) nebulas two weeks ago before heading back out to near the orbit of Jupiter, to return again in about six and a half years.

APOD: 2018 July 6 - Charon: Moon of Pluto
Explanation: A darkened and mysterious north polar region known to some as Mordor Macula caps this premier high-resolution view. The portrait of Charon, Pluto's largest moon, was captured by New Horizons near the spacecraft's closest approach on July 14, 2015. The combined blue, red, and infrared data was processed to enhance colors and follow variations in Charon's surface properties with a resolution of about 2.9 kilometers (1.8 miles). A stunning image of Charon's Pluto-facing hemisphere, it also features a clear view of an apparently moon-girdling belt of fractures and canyons that seems to separate smooth southern plains from varied northern terrain. Charon is 1,214 kilometers (754 miles) across. That's about 1/10th the size of planet Earth but a whopping 1/2 the diameter of Pluto itself, and makes it the largest satellite relative to its parent body in the Solar System. Still, the moon appears as a small bump at about the 1 o'clock position on Pluto's disk in the grainy, negative,telescopic picture inset at upper left. That view was used by James Christy and Robert Harrington at the U.S. Naval Observatory in Flagstaff to discover Charon 40 years ago in June of 1978.

APOD: 2018 January 20 - Old Moon in the New Moon's Arms
Explanation: Also known as the Moon's "ashen glow" or the "Old Moon in the New Moon's arms", earthshine is earthlight reflected from the Moon's night side. This stunning image of earthshine from a young crescent moon was taken from Las Campanas Observatory, Atacama Desert, Chile, planet Earth near moonset on January 18. Dramatic atmospheric inversion layers appear above the Pacific Ocean, colored by the sunset at the planet's western horizon. But the view from the Moon would have been stunning, too. When the Moon appears in Earth's sky as a slender crescent, a dazzlingly bright, nearly full Earth would be seen from the lunar surface. A description of earthshine, in terms of sunlight reflected by Earth's oceans in turn illuminating the Moon's dark surface, was written 500 years ago by Leonardo da Vinci.

APOD: 2018 January 14 - Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007.

APOD: 2017 December 4 - Earth and Moon
Explanation: On rare occasions, the Earth and Moon are photographed together. One of most spectacular times this occurred was 25 years ago this month when the Jupiter-bound Galileo spacecraft zoomed past our home planetary system. Then, robotic Galileo watched from about 15-times the Earth-Moon separation as our only natural satellite glided past our home world. The featured video combines 52 historic color-enhanced images. Although our Moon may appear small next to the Earth, no other planet in our Solar System has a satellite so comparable in size . The Sun, far off to the right, illuminated about half of each sphere, and shows the spinning Earth's white clouds, blue oceans, and tan continents. Tonight, a nearly full Oak supermoon will be visible from all of Earth from sunset to sunrise.

APOD: 2017 December 3 - Full Moon Silhouettes
Explanation: Have you ever watched the Moon rise? The slow rise of a nearly full moon over a clear horizon can be an impressive sight. One impressive moonrise was imaged in early 2013 over Mount Victoria Lookout in Wellington, New Zealand. With detailed planning, an industrious astrophotographer placed a camera about two kilometers away and pointed it across the lookout to where the Moon would surely soon be making its nightly debut. The featured single shot sequence is unedited and shown in real time -- it is not a time lapse. People on Mount Victoria Lookout can be seen in silhouette themselves admiring the dawn of Earth's largest satellite. Seeing a moonrise yourself is not difficult: it happens every day, although only half the time at night. Each day the Moon rises about fifty minutes later than the previous day, with a full moon always rising at sunset. A good time to see a full moonrise will occur tonight at sunset as the Moon's relative closeness to Earth during a full phase -- called a supermoon -- will cause it to appear slightly larger and brighter than usual.

APOD: 2017 July 17 - Moon Shadow versus Sun Reflection
Explanation: What are those lights and shadows crossing the Earth? As the featured five-second time-lapse video progresses, a full day on planet Earth is depicted as seen from Japan's Himawari-8 satellite in geostationary orbit high above the Pacific Ocean. The Sun rises to the right and sets to the left, illuminating the half of Earth that is most directly below. A reflected image of the Sun -- a Sun glint -- is visible as a bright spot that moves from right to left. More unusual, though, is the dark spot that moves from the lower left to upper right That is the shadow of the Moon, and it can only appear when the Moon goes directly between the Earth and the Sun. Last year, on the day these images were taken, the most deeply shadowed region experienced a total eclipse of the Sun. Next month a similarly dark shadow will sweep right across the USA.

APOD: 2017 April 20 - Asteroid 2014 JO25
Explanation: A day before its closest approach, asteroid 2014 JO25 was imaged by radar with the 70-meter antenna of NASA's Goldstone Deep Space Communications Complex in California. This grid of 30 radar images, top left to lower right, reveals the two-lobed shape of the asteroid that rotates about once every five hours. Its largest lobe is about 610 meters across. On the list of Potentially Hazardous Asteroids, this space rock made its close approach to our fair planet on April 19, flying safely past at a distance of 1.8 million kilometers. That's over four times the distance from the Earth to the Moon. The asteroid was a faint and fast moving speck visible in backyard telescopes. Asteroid 2014 JO25 was discovered in May 2014 by A. D. Grauer of the Catalina Sky Survey, a project of NASA's Near-Earth Objects Observations Program in collaboration with the University of Arizona.

APOD: 2017 January 26 - GOES-16: Moon over Planet Earth
Explanation: Launched last November 19 from Cape Canaveral Air Force Station, the satellite now known as GOES-16 can now observe planet Earth from a geostationary orbit 22,300 miles above the equator. Its Advanced Baseline Imager captured this contrasting view of Earth and a gibbous Moon on January 15. The stark and airless Moon is not really the focus of GOES-16, though. Capable of providing a high resolution full disk image of Earth every 15 minutes in 16 spectral channels, the new generation satellite's instrumentation is geared to provide sharper, more detailed views of Earth's dynamic weather systems and enable more accurate weather forecasting. Like previous GOES weather satellites, GOES-16 will use the moon over our fair planet as a calibration target.

APOD: 2016 November 13 - Super Moon vs Micro Moon
Explanation: What is so super about tomorrow's supermoon? Tomorrow, a full moon will occur that appears slightly larger and brighter than usual. The reason is that the Moon's fully illuminated phase occurs within a short time from perigee - when the Moon is its closest to the Earth in its elliptical orbit. Although the precise conditions that define a supermoon vary, tomorrow's supermoon will undoubtedly qualify because it will be the closest, largest, and brightest full moon in over 65 years. One reason supermoons are popular is because they are so easy to see -- just go outside at sunset and watch an impressive full moon rise! Since perigee actually occurs tomorrow morning, tonight's full moon, visible starting at sunset, should also be impressive. Pictured here, a supermoon from 2012 is compared to a micromoon -- when a full Moon occurs near the furthest part of the Moon's orbit -- so that it appears smaller and dimmer than usual. Given many definitions, at least one supermoon occurs each year, with another one coming next month (moon-th). However, a full moon will not come this close to Earth again until 2034.

APOD: 2016 October 12 - Penumbral Lunar Eclipse
Explanation: Does this Moon look a little different to you? Although shown in spectacular detail, the full face of Earth's most familiar satellite appears slightly darker than usual, in particular on the upper left, because it is undergoing a penumbral lunar eclipse. The image was captured in Hong Kong, China, on September 16 when the Moon crossed through part of Earth's shadow -- but not the darkest where the Earth shades the entire Sun. A lunar eclipse can only occur during a full moon, and many know this particular full moon as the Harvest moon for its proximity to northern harvests. The next full moon will occur this coming Sunday. Some cultures refer to it as a Leaf Falling Moon, named for its proximity to northern autumn. The second full moon of the same month ("moonth") is sometimes called a Blue moon; meanwhile, this month features a rare second new moon, an event known to some as a Black moon.

APOD: 2016 August 7 - Io: Moon over Jupiter
Explanation: How big is Jupiter's moon Io? The most volcanic body in the Solar System, Io (usually pronounced "EYE-oh") is 3,600 kilometers in diameter, about the size of planet Earth's single large natural satellite. Gliding past Jupiter at the turn of the millennium, the Cassini spacecraft captured this awe inspiring view of active Io with the largest gas giant as a backdrop, offering a stunning demonstration of the ruling planet's relative size. Although in the featured picture Io appears to be located just in front of the swirling Jovian clouds, Io hurtles around its orbit once every 42 hours at a distance of 420,000 kilometers or so from the center of Jupiter. That puts Io nearly 350,000 kilometers above Jupiter's cloud tops, roughly equivalent to the distance between Earth and Moon. In July, NASA's Juno satellite began orbiting Jupiter and will sometimes swoop to within 5,000 kilometers of Jupiter's cloud tops.

APOD: 2016 June 17 - Comet PanSTARRS in the Southern Fish
Explanation: Now approaching our fair planet this Comet PanSTARRS (C/2013 X1) will come closest on June 21-22, a mere 5.3 light-minutes away. By then its appearance low in northern hemisphere predawn skies (high in the south), will be affected by the light of a nearly Full Moon, though. Still the comet's pretty green coma is about the apparent size of the Full Moon in this telescopic portrait, captured on June 12 from the southern hemisphere's Siding Spring Observatory. The deep image also follows a broad, whitish dust tail up and toward the left in the frame, sweeping away from the Sun and trailing behind the comet's orbit. Buffeted by the solar wind, a fainter, narrow ion tail extends horizontally toward the right. On the left edge, the brightest star is bluish Iota Piscis Austrini. Shining at about fourth magnitude, that star is visible to the unaided eye in the constellation of the Southern Fish.

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.

APOD: 2015 December 12 - Comet Meets Moon and Morning Star
Explanation: A crescent Moon and brilliant Venus met in predawn skies on December 7, a beautiful conjunction of planet Earth's two brightest celestial beacons after the Sun. Harder to see but also on the scene was Comet Catalina (C/2013 US10). The fainter comet clearly sporting two tails, lunar night side, bright sunlit lunar crescent, and brilliant morning star, are all recorded here by combining short and long exposures of the same field of view. Pointing down and right, Catalina's dust tail tends to trail behind the comet's orbit. Its ion tail, angled toward the top left of the frame, is blowing away from the Sun. Discovered in 2013, the new visitor from the Oort cloud was closest to the Sun on November 15 and is now outbound, headed for its closest approach to Earth in mid-January.

APOD: 2015 December 7 - Comet Catalina Emerges
Explanation: Comet Catalina is ready for its close-up. The giant snowball from the outer Solar System, known formally as C/2013 US10 (Catalina), rounded the Sun last month and is now headed for its closest approach to Earth in January. With the glow of the Moon now also out of the way, morning observers in Earth's northern hemisphere are getting their best ever view of the new comet. And Comet Catalina is not disappointing. Although not as bright as early predictions, the comet is sporting both dust (lower left) and ion (upper right) tails, making it an impressive object for binoculars and long-exposure cameras. The featured image was taken last week from the Canary Islands, off the northwest coast of Africa. Sky enthusiasts around the world will surely be tracking the comet over the next few months to see how it evolves.

APOD: 2015 November 22 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning color image from the robotic Mars Reconnaissance Orbiter, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. A recent analysis of the long grooves indicates that they may result from global stretching caused by tides -- the differing force of Mars' gravity on different sides of Phobos. These grooves may then be an early phase in the disintegration of Phobos into a ring of debris around Mars.

APOD: 2015 November 12 - Kenya Morning Moon, Planets and Taurid
Explanation: On November 8, a waning crescent Moon joined the continuing parade of planets in Earth's early morning skies. Captured here from Amboseli National Park, Kenya, even the overexposed moonlight can't washout brilliant Venus though, lined up near the ecliptic plane with faint Mars and bright Jupiter above. As if Moon and planets aren't enough, a comparably bright Taurid meteor also streaks through the scene. In fact November's Taurid meteor showers have had a high proportion of bright fireballs. Apparently streaming from radiants in Taurus, the meteors are caused by our fair planet's annual passage through debris from Comet 2P/Encke. The comet's dust grains are catching up with Earth's atmosphere at a relatively low speed of about 27 kilometers per second.

APOD: 2015 October 2 - Charon: Moon of Pluto
Explanation: A darkened and mysterious north polar region informally known as Mordor Macula caps this premier high-resolution portrait of Charon, Pluto's largest moon. Captured by New Horizons near its closest approach on July 14, the image data was transmitted to Earth on September 21. The combined blue, red, and infrared data is processed to enhance colors, following variations in surface properties with a resolution of about 2.9 kilometers (1.8 miles). In fact, Charon is 1,214 kilometers (754 miles) across, about 1/10th the size of planet Earth but a whopping 1/2 the diameter of Pluto itself. That makes it the largest satellite relative to its planet in the solar system. This remarkable image of Charon's Pluto-facing hemisphere shows a clearer view of an apparently moon-girdling belt of fractures and canyons that seems to separate smooth southern plains from varied northern terrain.

APOD: 2015 July 23 - Comet PanSTARRS, Moon, and Venus
Explanation: It is the object to the left of the big tree that's generating much recent excitement. If you look closely, there you can see Comet PanSTARRS, complete with two tails. During July, this comet has increased markedly in brightness and has just passed its closest approach to Earth. The statuesque tree in the center is a Norfolk Island Pine, and to either side of this tree are New Zealand Pohutukawa trees. Over the trees, far in the distance, are bright Venus and an even brighter crescent Moon. If you look even more closely, you can find Jupiter hidden in the branches of the pine. The featured image was taken a few days ago in Fergusson Park, New Zealand, looking over Tauranga Harbour Inlet. In the coming days and weeks, Comet C/2014 Q1 (PANSTARRS) will slowly move away from the Sun and the Earth, drift deep into southern skies, and fade.

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.

APOD: 2015 February 18 - Dark Craters and Bright Spots Revealed on Asteroid Ceres
Explanation: What are those bright spots on asteroid Ceres? As the robotic spacecraft Dawn approaches the largest asteroid in the asteroid belt, the puzzle only deepens. Sharper new images taken last week and released yesterday indicate, as expected, that most of the surface of dwarf planet Ceres is dark and heavily cratered like our Moon and the planet Mercury. The new images do not clearly indicate, however, the nature of comparatively bright spots -- although more of them are seen to exist. The enigmatic spots were first noticed on Texas-sized Ceres a few weeks ago during Dawn's approach. The intriguing mystery might well be solved quickly as Dawn continues to advance toward Ceres, being on schedule to enter orbit on March 6.

APOD: 2015 January 17 - Comet Lovejoy's Tail
Explanation: Sweeping north in planet Earth's sky, Comet Lovejoy's greenish coma and blue tinted ion tail stretched across this field of stars in the constellation Taurus on January 13. The inset at the upper left shows the 1/2 degree angular size of the full moon for scale. So Lovejoy's coma appears only a little smaller (but much fainter) than a full moon on the sky, and its tail is visible for over 4 degrees across the frame. That corresponds to over 5 million kilometers at the comet's estimated distance of 75 million kilometers from Earth. Blown by the solar wind, the comet's tenuous, structured ion tail streams away from the Sun, growing as this Comet Lovejoy heads toward perihelion, its closest approach to the Sun, on January 30. While diatomic carbon (C2) gas fluorescing in sunlight produces the coma's green color, the fainter bluish tail is tinted by emission from ionized carbon monoxide (CO+).

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.

APOD: 2014 September 8 - Super Moon vs Micro Moon
Explanation: What is so super about tomorrow's supermoon? Tomorrow, a full moon will occur that appears slightly larger and brighter than usual. The reason is that the Moon's fully illuminated phase occurs within a short time from perigee - when the Moon is its closest to the Earth in its elliptical orbit. Although the precise conditions that define a supermoon vary, given one definition, tomorrow's will be the third supermoon of the year -- and the third consecutive month that a supermoon occurs. One reason supermoons are popular is because they are so easy to see -- just go outside and sunset and watch an impressive full moon rise! Since perigee actually occurs today, tonight's sunset moonrise should also be impressive. Pictured above, a supermoon from 2012 is compared to a micromoon -- when a full Moon occurs near the furthest part of the Moon's orbit -- so that it appears smaller and dimmer than usual. Given many definitions, at least one supermoon occurs each year, with the next being 2015 August 30.

APOD: 2014 September 7 - Full Moon Silhouettes
Explanation: Have you ever watched the Moon rise? The slow rise of a nearly full moon over a clear horizon can be an impressive sight. One impressive moonrise was imaged in early 2013 over Mount Victoria Lookout in Wellington, New Zealand. With detailed planning, an industrious astrophotographer placed a camera about two kilometers away and pointed it across the lookout to where the Moon would surely soon be making its nightly debut. The above single shot sequence is unedited and shown in real time -- it is not a time lapse. People on Mount Victoria Lookout can be seen in silhouette themselves admiring the dawn of Earth's largest satellite. Seeing a moonrise yourself is not difficult: it happens every day, although only half the time at night. Each day the Moon rises about fifty minutes later than the previous day, with a full moon always rising at sunset. A good time to see a moonrise will occur at sunset on Tuesday as the Moon's relative closeness to Earth during a full phase -- called a supermoon -- will cause it to appear slightly larger and brighter than usual.

APOD: 2014 September 4 - Cloud, Clusters and Comet Siding Spring
Explanation: On October 19th, a good place to watch Comet Siding Spring will be from Mars. Then, this inbound visitor (C/2013 A1) to the inner solar system, discovered in January 2013 by Robert McNaught at Australia's Siding Spring Observatory, will pass within 132,000 kilometers of the Red Planet. That's a near miss, equivalent to just over 1/3 the Earth-Moon distance. Great views of the comet for denizens of planet Earth's southern hemisphere are possible now, though. This telescopic snapshot from August 29 captured the comet's whitish coma and arcing dust tail sweeping through southern skies. The fabulous field of view includes, the Small Magellanic Cloud and globular star clusters 47 Tucanae (right) and NGC 362 (upper left). Worried about all those spacecraft in Martian orbit? Streaking dust particles from the comet could pose a danger and controllers plan to position Mars orbiters on the opposite side of the planet during the comet's close flyby.

APOD: 2014 August 22 - Comet Jacques, Heart and Soul
Explanation: On July 13th, a good place to watch Comet Jacques was from Venus. Then, the recently discovered visitor (C/2014 E2) to the inner solar system passed within about 14.5 million kilometers of our sister planet. But the outbound comet will pass only 84 million kilometers from our fair planet on August 28 and is already a fine target for telescopes and binoculars. Two days ago Jacques' greenish coma and straight and narrow ion tail were captured in this telescopic snapshot, a single 2 minute long exposure with a modified digital camera. The comet is flanked by IC 1805 and IC 1848, also known as Cassiopeia's Heart and Soul Nebulae. If you're stuck on planet Earth this weekend you can hunt for Comet Jacques in evening skies, or spot a Venus, Jupiter, crescent Moon triangle before the dawn.

APOD: 2014 July 16 - The Moon Eclipses Saturn
Explanation: What happened to half of Saturn? Nothing other than Earth's Moon getting in the way. As pictured above on the far right, Saturn is partly eclipsed by a dark edge of a Moon itself only partly illuminated by the Sun. This year the orbits of the Moon and Saturn have led to an unusually high number of alignments of the ringed giant behind Earth's largest satellite. Technically termed an occultation, the above image captured one such photogenic juxtaposition from Buenos Aires, Argentina that occurred early last week. Visible to the unaided eye but best viewed with binoculars, there are still four more eclipses of Saturn by our Moon left in 2014. The next one will be on August 4 and visible from Australia, while the one after will occur on August 31 and be visible from western Africa at night but simultaneously from much of eastern North America during the day.

APOD: 2014 June 6 - Comet PanSTARRS with Galaxy
Explanation: Sweeping slowly through northern skies, the comet PanSTARRS C/2012 K1 posed for this telescopic portrait on June 2nd in the constellation Ursa Major. Now within the inner solar system, the icy body from the Oort cloud sports two tails, a lighter broad dust tail and crooked ion tail extending below and right. The comet's condensed greenish coma makes a nice contrast with the spiky yellowish background star above. NGC 3319 appears at the upper left of the frame that spans almost twice the apparent diameter of the full Moon. The spiral galaxy is about 47 million light-years away, far beyond the stars in our own Milky Way. In comparison, the comet was a mere 14 light-minutes from our fair planet. This comet PanSTARRS will slowly grow brighter in the coming months remaining a good target for telescopic comet watchers and reaching perihelion, its closest approach to the Sun, while just beyond Earth's orbit in late August.

APOD: 2014 January 21 - Micro Moon over Super Moon
Explanation: Did you see the big, bright, beautiful Full Moon last Wednesday night? That was actually a Micro Moon! On that night, the smallest Full Moon of 2014 reached its full phase only a few hours from lunar apogee, the time of its the most distant point from Earth in the Moon's elliptical orbit. Of course, last year on the night of June 22, a Full Super Moon was near perigee, the closest point in its orbit. The relative apparent size of January 15's Micro Moon is compared to the June 22 Super Moon in the above composite image digitally superimposing telescopic images from Perugia, Italy. The difference in apparent size represents a difference in distance of just under 50,000 kilometers between apogee and perigee, given the Moon's average distance of about 385,000 kilometers. How long do you have to wait to see another Full Micro Moon? Until March 5, 2015, when the lunar full phase will again occur within a few hours of lunar apogee.

APOD: 2013 December 4 - Comet Lovejoy through Morby Castle Ruins
Explanation: This new comet is quite photogenic. Comet Lovejoy, discovered only three months ago, was imaged through ruins of ancient Mörby Castle in Sweden last week sporting a green-glowing coma and tails trailing several degrees. The past few weeks have been an unusually active time for comet watchers as four comets were visible simultaneously with binoculars: ISON, Lovejoy, Encke, and LINEAR. C/2013 R1 (Lovejoy) comet is currently visible to the unaided eye from a dark location. As Monday's new Moon will provide little glare, the next few days provide a good time to see Comet Lovejoy as it reaches its peak brightness. In two and a half weeks, Comet Lovejoy will reach its closest approach to the Sun at a distance just inside the orbital distance of the Earth.

APOD: 2013 October 20 - Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. The scene also captures the broad tail and bright coma of Comet McNaught, the Great Comet of 2007. Currently, many sky enthusiasts are following the development of Comet ISON, a comet which might become the Great Comet of 2013.

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.

APOD: 2013 August 1 - Moon Over Andromeda
Explanation: The Great Spiral Galaxy in Andromeda (aka M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (bottom).

APOD: 2013 July 20 - Comet Lemmon and the Deep Sky
Explanation: Now sweeping high above the ecliptic plane, Comet Lemmon has faded dramatically in planet Earth's night sky as it heads for the outer solar system. Some 16 light-minutes (2 AU) from the Sun, it still sports a greenish coma though, posing on the right in this 4 degree wide telescopic view from last Saturday with deep sky star clusters and nebulae in Cassiopeia. In fact, the rich background skyscape is typical within the boundaries of the boastful northern constellation that lie along the crowded starfields of the Milky Way. Included near center is open star cluster M52 about 5,000 light-years away. Around 11,000 light-years distant, the red glowing nebula NGC 7635 below and left of M52 is well-known for its appearance in close-up images as the Bubble Nebula. But the fading Comet Lemmon is not the only foreground object on the scene. A faint streak on the right is an orbiting satellite caught crossing through the field during the long exposure, glinting in the sunlight and winking out as it passes into Earth's shadow.

APOD: 2013 July 16 - The Moon from Zond 8
Explanation: Which moon is this? Earth's. Our Moon's unfamiliar appearance is due partly to an unfamiliar viewing angle as captured by a little-known spacecraft -- the Soviet Union's Zond 8 that circled the Moon in October of 1970. Pictured above, the dark-centered circular feature that stands out near the top of the image is Mare Orientale, a massive impact basin formed by an ancient collision with an asteroid. Mare Orientale is surrounded by light colored and highly textured highlands. Across the image bottom lies the dark and expansive Oceanus Procellarum, the largest of the dark (but dry) maria that dominate the side of the Moon that always faces toward the Earth. Originally designed to carry humans, robotic Zond 8 came within 1000 km of the lunar surface, took about 100 detailed photographs on film, and returned them safely to Earth within a week.

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.

APOD: 2013 March 14 - Clouds, Comet, and Crescent Moon
Explanation: In silhouette against the colorful evening twilight, clouds part for this much anticipated magic moment. The scene captures naked-eye Comet PanSTARRS peeking into northern hemisphere skies on March 12. The comet stands over the western horizon after sunset, joined by the thin, flattened crescent of a day old Moon. Posing for its own beauty shot, the subtly lit dome of the 4.2 meter William Herschel Telescope is perched above cloud banks on the Canary Island of La Palma. While PanSTARRS has not quite developed into the spectacular comet once hoped for, it is still growing easier to see in the north. In coming days it will steadily climb north, farther from the Sun into darker western evening skies.

APOD: 2013 February 17 - Asteroid 2012 DA14 Passes the Earth
Explanation: There it goes. That small spot moving in front of background stars in the above video is a potentially dangerous asteroid passing above the Earth's atmosphere. This past Friday, the 50-meter wide asteroid 2012 DA14 just missed the Earth, passing not only inside the orbit of the Moon, which is unusually close for an asteroid of this size, but also inside the orbit of geosynchronous satellites. Unfortunately, asteroids this big or bigger strike the Earth every 1000 years or so. Were 2012 DA14 to have hit the Earth, it could have devastated a city-sized landscape, or stuck an ocean and raised dangerous tsunamis. Although finding and tracking potentially dangerous asteroids is a primary concern of modern astronomy, these small bodies or ice and rock are typically so dim that only a few percent of them have been found, so far. Even smaller chunks of ice and rock, like the (unrelated) spectacular meteors that streaked over Russia and California over the past few days, are even harder to find -- but pose less danger.

APOD: 2013 January 30 - Full Moon Silhouettes
Explanation: Have you ever watched the Moon rise? The slow rise of a nearly full moon over a clear horizon can be an impressive sight. One impressive moonrise was imaged two nights ago over Mount Victoria Lookout in Wellington, New Zealand. With detailed planning, an industrious astrophotographer placed a camera about two kilometers away and pointed it across the lookout to where the Moon would surely soon be making its nightly debut. The above single shot sequence is unedited and shown in real time -- it is not a time lapse. People on Mount Victoria Lookout can be seen in silhouette themselves admiring the dawn of Earth's largest satellite. Seeing a moonrise yourself is not difficult: it happens every day, although only half the time at night. Each day the Moon rises about fifty minutes later than the previous day, with a full moon always rising at sunset.

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.

APOD: 2012 November 29 - Super Moon vs Micro Moon
Explanation: Did you see the big, bright, beautiful Full Moon Wednesday night? That was actually a Micro Moon! On that night, the smallest Full Moon of 2012 reached its full phase only about 4 hours before apogee, the most distant point from Earth in the Moon's elliptical orbit. Of course, earlier this year on May 6, a Full Super Moon was near perigee, the closest point in its orbit. The relative apparent size of November 28's Micro Moon (right) is compared to the famous May 6 Super Moon in these two panels, matching telescopic images from Bucharest, Romania. The difference in apparent size represents a difference in distance of just under 50,000 kilometers between apogee and perigee, given the Moon's average distance of about 385,000 kilometers. How long do you have to wait to see another Full Micro Moon? Until January 16, 2014, when the lunar full phase will occur within about 3 hours of apogee.

APOD: 2012 November 6 - Methone: Smooth Egg Moon of Saturn
Explanation: Why is this moon shaped like a smooth egg? The robotic Cassini spacecraft completed the first flyby ever of Saturn's small moon Methone in May and discovered that the moon has no obvious craters. Craters, usually caused by impacts, have been seen on every moon, asteroid, and comet nucleus ever imaged in detail -- until now. Even the Earth and Titan have craters. The smoothness and egg-like shape of the 3-kilometer diameter moon might be caused by Methone's surface being able to shift -- something that might occur were the moon coated by a deep pile of sub-visual rubble. If so, the most similar objects in our Solar System would include Saturn's moons Telesto, Pandora, Calypso, as well as asteroid Itokawa, all of which show sections that are unusually smooth. Methone is not entirely featureless, though, as some surface sections appear darker than others. Although flybys of Methone are difficult, interest in the nature and history of this unusual moon is sure to continue.

APOD: 2012 October 28 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning color image from the Mars Reconnaissance Orbiter, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. In 100 million years or so Phobos will likely be shattered by stress caused by the relentless tidal forces, the debris forming a decaying ring around Mars.

APOD: 2012 October 1 - Introducing Comet ISON
Explanation: Could this dim spot brighten into one of the brightest comets ever? It's possible. Alternatively, the comet could break up when it gets closer to the Sun, or brighten much more modestly. Sky enthusiasts the world over are all abuzz, though, from the more optimistic speculations -- that the newly discovered C/2012 S1 (ISON) could develop a spectacular tail or briefly approach the brightness of the full Moon toward the end of 2013. Comet ISON currently is very faint but is just visible at magnitude 18 in the above image. The comet, discovered just over a week ago from Russia by Vitali Nevski (Belarus) and Artyom Novichonok (Russia), is currently falling toward the Sun from between the orbits of Jupiter and Saturn. In early 2013 October it will pass very near Mars and possibly be visible to rovers and orbiting spacecraft. Comet ISON appears on course to achieve sungrazer status as it passes within a solar diameter of Sun's surface in late 2013 November. Whatever survives will then pass nearest the Earth in late 2013 December. Astronomers around the world will be tracking this large dirty snowball closely to better understand its nature and how it might evolve during the next 15 months.

APOD: 2012 April 24 - Rosetta Approaches Asteroid Lutetia
Explanation: What would it look like to approach an asteroid in a spaceship? In 2010, ESA's robotic Rosetta spacecraft zipped past the asteroid 21 Lutetia taking data and snapping images in an effort to better determine the history of the asteroid and the origin of its unusual colors. Recently, many images from a camera always facing the asteroid were compiled into the above video. Although of unknown composition, Lutetia is not massive enough for gravity to pull it into a sphere. The 100-kilometer across Lutetian was at that time the largest asteroid or comet nucleus that had been visited by a human-launched spacecraft. Orbiting in the main asteroid belt, Lutetia shows itself to be a heavily cratered remnant of the early Solar System. Now well past Lutetia, the Rosetta spacecraft is continuing onto comet Churyumov-Gerasimenko where a landing is planned for 2014.

APOD: 2012 April 8 - Io: Moon Over Jupiter
Explanation: How big is Jupiter's moon Io? The most volcanic body in the Solar System, Io (usually pronounced "EYE-oh") is 3,600 kilometers in diameter, about the size of planet Earth's single large natural satellite. Gliding past Jupiter at the turn of the millennium, the Cassini spacecraft captured this awe inspiring view of active Io with the largest gas giant as a backdrop, offering a stunning demonstration of the ruling planet's relative size. Although in the above picture Io appears to be located just in front of the swirling Jovian clouds, Io hurtles around its orbit once every 42 hours at a distance of 420,000 kilometers or so from the center of Jupiter. That puts Io nearly 350,000 kilometers above Jupiter's cloud tops, roughly equivalent to the distance between Earth and Moon. The Cassini spacecraft itself was about 10 million kilometers from Jupiter when recording the image data.

APOD: 2012 March 24 - The New Moon in the Old Moon's Arms
Explanation: Also known as the Moon's ashen glow, Earthshine is Earthlight illuminating the Moon's night side. Taken on Nowruz, the March 20 equinox, from Esfahan, Iran, planet Earth, this telescopic image captures strong Earthshine from an old Moon. The darker earthlit disk is in the arms of a bright sunlit crescent. But the view from the Moon would have been enchanting too. When the Moon appears in Earth's sky as a slender crescent, a dazzlingly bright, nearly full Earth would be seen from the lunar surface. The Earth's brightness due to reflected sunlight is known to be strongly influenced by cloud cover. Still, a description of Earthshine, in terms of sunlight reflected by Earth's oceans in turn illuminating the Moon's dark surface, was written 500 years ago by Leonardo da Vinci.

APOD: 2011 November 9 - Asteroid 2005 YU55 Passes the Earth
Explanation: Asteroid 2005 YU55 passed by the Earth yesterday, posing no danger. The space rock, estimated to be about 400 meters across, coasted by just inside the orbit of Earth's Moon. Although the passing of smaller rocks near the Earth is not very unusual -- in fact small rocks from space strike Earth daily -- a rock this large hasn't passed this close since 1976. Were YU55 to have struck land, it might have caused a magnitude seven earthquake and left a city-sized crater. A perhaps larger danger would have occurred were YU55 to have struck the ocean and raised a large tsunami. The above radar image was taken two days ago by the Deep Space Network radio telescope in Goldstone, California, USA. YU55 was discovered only in 2005, indicating that other potentially hazardous asteroids might lurk in our Solar System currently undetected. Objects like YU55 are hard to detect because they are so faint and move so fast. However, humanity's ability to scan the sky to detect, catalog, and analyze such objects has increased notably in recent years.

APOD: 2011 October 5 - Comet and CME on the Sun
Explanation: Did a sun-diving comet just cause a solar explosion? Probably not. This past weekend a comet dove toward the Sun and was followed very quickly by a Coronal Mass Ejection (CMEs) from the other side of the Sun. The first two sequences in the above video shows the spectacular unfolding of events as seen by the Sun-orbiting SOHO satellite, while the same events were also captured by both Sun-orbiting STEREO satellites. Now sungrazer comets that break up as they pass near the Sun are not all that rare -- hundreds have been cataloged over the past few years. CMEs are even more common, with perhaps three lesser events occurring even during the eight hours of the above time-lapse movie. Therefore, the best bet of solar scientists is that the two events were unrelated. Another basis for this judgment is that CMEs are caused by rapid changes in the Sun's magnetic field, changes that a small comet seem unlikely to make. Such coincidences are even more likely during periods of high activity on the Sun's surface -- like now.

APOD: 2011 September 19 - The South Pole of Asteroid Vesta
Explanation: What created the circular structure around the south pole of asteroid Vesta? Pictured above, the bottom of the second largest object in the asteroid belt was recently imaged for the first time by the robotic Dawn satellite that arrived last month. A close inspection of the 260-meter resolution image shows not only hills and craters and cliffs and more craters, but ragged circular features that cover most of the lower right of the 500-kilometer sized object. Early speculation posits that the structure might have been created by a collision and coalescence with a smaller asteroid. Alternatively, the features might have originated in an internal process soon after the asteroid formed. New clues might come in the next few months as Dawn spirals down toward the rocky world and obtains images of increasingly high resolution.

APOD: 2010 December 1 - Martian Moon Phobos from Mars Express
Explanation: Why is Phobos so dark? Phobos, the largest and innermost of two Martian moons, is the darkest moon in the entire Solar System. Its unusual orbit and color indicate that it may be a captured asteroid composed of a mixture of ice and dark rock. The above picture of Phobos near the limb of Mars was captured last month by the robot spacecraft Mars Express currently orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From images like this, Phobos has been determined to be covered by perhaps a meter of loose dust. Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all. Phobos' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years.

APOD: 2010 July 26 - Lutetia: The Largest Asteroid Yet Visited
Explanation: As humans explore the universe, the record for largest asteroid visited by a spacecraft has increased yet again. Earlier this month, ESA's robotic Rosetta spacecraft zipped past the asteroid 21 Lutetia taking data and snapping images in an effort to better determine the history of the asteroid and the origin of its unusual colors. Although of unknown composition, Lutetia is not massive enough for gravity to pull it into a sphere. Pictured above on the upper right, the 100-kilometer across Lutetia is shown in comparison with the other nine asteroids and four comets that have been visited, so far, by human-launched spacecraft. Orbiting in the main asteroid belt, Lutetia shows itself to be a heavily cratered remnant of the early Solar System. The Rosetta spacecraft is now continuing onto comet Churyumov-Gerasimenko where a landing is planned for 2014.

APOD: 2010 February 3 - P2010 A2: Unusual Asteroid Tail Implies Powerful Collision
Explanation: What is this strange object? First discovered on ground based LINEAR images on January 6, the object appeared unusual enough to investigate further with the Hubble Space Telescope last week. Pictured above, what Hubble saw indicates that P/2010 A2 is unlike any object ever seen before. At first glance, the object appears to have the tail of a comet. Close inspection, however, shows a 140-meter nucleus offset from the tail center, very unusual structure near the nucleus, and no discernable gas in the tail. Knowing that the object orbits in the asteroid belt between Mars and Jupiter, a preliminary hypothesis that appears to explain all of the known clues is that P/2010 A2 is the debris left over from a recent collision between two small asteroids. If true, the collision likely occurred at over 15,000 kilometers per hour -- five times the speed of a rifle bullet -- and liberated energy in excess of a nuclear bomb. Pressure from sunlight would then spread out the debris into a trailing tail. Future study of P/2010 A2 may better indicate the nature of the progenitor collision and may help humanity better understand the early years of our Solar System, when many similar collisions occurred.

APOD: 2009 November 18 - Water Discovered in Moon Shadow
Explanation: Why is there water on the Moon? Last month, the LCROSS mission crashed a large impactor into a permanently shadowed crater near the Moon's South Pole. A plume of dust rose that was visible to the satellite, although hard to discern from Earth. The plume is shown above in visible light. Last week, the results of a preliminary chemical analysis gave a clear indication that the dust plume contained water. Such water is of importance not only for understanding the history of the Moon, but as a possible reservoir for future astronauts trying to live on the Moon for long periods. The source of the lunar water is now a topic of debate. Possible origins include many small meteorites, a comet, or primordial moon soil.

APOD: 2009 September 28 - Water Discovered on the Moon
Explanation: Water has been discovered on the surface of the Moon. No lakes have been found, but rather NASA's Moon Mineralogy Mapper aboard India's new Chandrayaan-1 lunar orbiter radios back that parts of the Moon's surface absorb a very specific color of light identified previously only with water. Currently, scientists are trying to fit this with other facts about the Moon to figure out how much water is there, and even what form this water takes. Unfortunately, even the dampest scenarios leave our moon dryer than the driest of Earth's deserts. A fascinating clue being debated is whether the water signal rises and falls during a single lunar day. If true, the signal might be explainable by hydrogen flowing out from the Sun and interacting with oxygen in the lunar soil. This could leave an extremely thin monolayer of water, perhaps only a few molecules thick. Some of the resulting water might subsequently evaporate away in bright sunlight. Pictured above, the area near a crater on the far side of the Moon shows a relatively high abundance of water-carrying minerals in false-color blue. Next week, the new LCROSS satellite will release an impactor that will strike a permanently shadowed crater near the lunar south pole to see if any hidden water or ice sprays free there.

APOD: 2009 June 22 - Atlas 5 Rocket Launches to the Moon
Explanation: This rocket is headed for the Moon. Pictured above, a huge Altas V rocket roared off the launch pad last week to start NASA's first missions to Earth's Moon in 10 years. The rocket is carrying two robotic spacecraft. The Lunar Reconnaissance Orbiter (LRO) is scheduled to orbit and better map the Moon, search for buried and hidden ice, and return many high resolution images. Some images will be below one-meter in resolution and include images of historic Apollo landing sites. Exploratory data and images should allow a more informed choice of possible future astronaut landing sites. The Lunar CRater Observation and Sensing Satellite (LCROSS) is scheduled to monitor the controlled impact of the rocket's upper stage into a permanently shadowed crater near the Moon's south pole. This impact, which should occur in about three months, might be visible on Earth through small telescopes.

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.

APOD: 2008 September 8 - Rosetta Spacecraft Passes Asteroid Steins
Explanation: What's that diamond in the sky? Cruising though space, sometimes you'll come across an unusual object. Such was the case on Friday for ESA's Rosetta spacecraft on its way to comet Churyumov-Gerasimenko in 2014. Robotic Rosetta buzzed right by the main belt asteroid 2867 Šteins, taking many pictures, some of which have been compiled into a short video. At first glance, Steins looked like a 5-kilometer wide diamond, but as Rosetta shot by, craters and a more extended shape become evident. In the above sequence of six images, a notable chain of craters is evident vertically on the asteroid's surface, most probably caused by a chance collision with a stream of meteors. Space scientists will now study the data taken by Rosetta of asteroid Steins in an effort to better understand its composition, origin, and why the asteroid reflects light so well. As the Earth-bound scientists toil, Rosetta itself will continue to zoom across our Solar System, next swooping again by the Earth in 2009 November, flying by asteroid 21 Lutetia in 2010 July, and finally landing on comet Churyumov-Gerasimenko in 2014 November.

APOD: 2008 April 14 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning color image from the Mars Reconnaissance Orbiter, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. In 100 million years or so Phobos will likely be shattered by stress caused by the relentless tidal forces, the debris forming a decaying ring around Mars.

APOD: 2008 January 30 - Asteroid 2007 TU24 Passes the Earth
Explanation: Asteroid 2007 TU24 passed by the Earth yesterday, posing no danger. The space rock, estimated to be about 250 meters across, coasted by just outside the orbit of Earth's Moon. The passing was not very unusual -- small rocks strike Earth daily, and in 2003 a rock the size of a bus passed inside the orbit of the Moon, being detected only after passing. TU24 was notable partly because it was so large. Were TU24 to have struck land, it might have caused a magnitude seven earthquake and left a city-sized crater. A perhaps larger danger would have occurred were TU24 to have struck the ocean and raised a large tsunami. This radar image was taken two days ago. The Arecibo Radio Telescope in Puerto Rico broadcast radar that was reflected by the asteroid and then recorded by the Byrd Radio Telescope in Green Bank, West Virginia. The resulting image shows TU24 to have an oblong and irregular shape. TU24 was discovered only three months ago, indicating that other potentially hazardous asteroids might lurk in our Solar System currently undetected. Objects like TU24 are hard to detect because they are so faint and move so fast. Humanity's ability to scan the sky to detect, catalog, and analyze such objects has increased notably in recent years.

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.

APOD: 2007 November 3 - Golden Comet Holmes
Explanation: Surprising Comet Holmes remains easily visible as a round, fuzzy cloud in the northern constellation Perseus. Skywatchers with telescopes, binoculars, or those that just decide to look up can enjoy the solar system's latest prodigy as it glides about 150 million kilometers from Earth, beyond the orbit of Mars. Still expanding, Holmes now appears to be about 1/3 the size of the Full Moon, and many observers report a yellowish tint to the dusty coma. A golden color does dominate this telescopic view recorded on November 1, showing variations across the coma's bright central region. But where's the comet's tail? Like any good comet, Holmes' tail would tend to point away from the Sun. That direction is nearly along our line-of-sight behind the comet, making its tail very difficult to see.

APOD: 2007 October 25 - Apogee Moon, Perigee Moon
Explanation: Tonight, those blessed with clear skies can enjoy a glorious Full Moon, (exact full phase at 0452 UT, October 26). In fact, the Moon will reach its full phase within a few hours of perigee, the closest point in its elliptical orbit, making it the largest Full Moon of 2007. On April 3, the Full Moon was within hours of apogee, the farthest point in the lunar orbit, corresponding to the smallest Full Moon of 2007. The difference in apparent size between the largest and smallest Full Moon is quite dramatic and similar to this side by side comparison of the lunar apogee/perigee apparitions from 2006. But seen in the sky many months apart, the change is difficult to notice. Skygazers should also enjoy the Moon on Saturday, October 27, as it encounters the lovely Pleiades star cluster. Because the Moon will be so bright, it will be easiest to spot the Pleiades stars near the Moon with binoculars or a small telescope.

APOD: 2007 April 5 - Asteroid and Galaxy
Explanation: Apollo class asteroid 2006 VV2 flashed past planet Earth in late March, approaching to within 3.4 million kilometers or about 8.8 times the Earth-Moon distance. Due to the proximity of its orbit to Earth and its estimated diameter of over 1 kilometer, 2006 VV2 is classified as a Potentially Hazardous Asteroid. Telescopes large and small were trained on the much anticipated flyby, the closest for a known asteroid of comparable size until the year 2036. This composite telescopic view is from a series of images recorded over a period of about an hour on Mar. 28 from Vado, New Mexico. The asteroid begins near the center of the field and tracks down and to the left, apparently passing very near galaxy M81. Of course, along with its companion galaxy M82 on the right, M81 is really 12 million light years away, compared to the asteroid's range of a mere 15 light seconds.

APOD: 2007 March 30 - Three Galaxies and a Comet
Explanation: Diffuse starlight and dark nebulae along the southern Milky Way arc over the horizon and sprawl diagonally through this gorgeous nightscape. The breath-taking mosaic spans a wide 100 degrees, with the rugged terrain of the Patagonia, Argentina region in the foreground. Along with the insider's view of our own galaxy, the image features our outside perspective on two irregular satellite galaxies - the Large and Small Magellanic Clouds. Recorded on January 28, the scene also captures the broad tail and bright coma of Comet McNaught, The Great Comet of 2007.

APOD: 2007 March 4 - Triton: Neptunes Largest Moon
Explanation: In October of 1846, William Lassell was observing the newly discovered planet Neptune. He was attempting to confirm his observation, made just the previous week, that Neptune had a ring. But this time he discovered that Neptune had a satellite as well. Lassell soon proved that the ring was a product of his new telescope's distortion, but the satellite Triton remained. The above picture of Triton was taken in 1989 by the only spacecraft ever to pass Triton: Voyager 2. Voyager 2 found fascinating terrain, a thin atmosphere, and even evidence for ice volcanoes on this world of peculiar orbit and spin. Ironically, Voyager 2 also confirmed the existence of complete thin rings around Neptune - but these would have been quite invisible to Lassell!

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.

APOD: 2006 December 28 - Moon Over Andromeda
Explanation: The Great Spiral Galaxy in Andromeda (aka M31), a mere 2.5 million light-years distant, is the closest large spiral to our own Milky Way. Andromeda is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can't appreciate the galaxy's impressive extent in planet Earth's sky. This entertaining composite image compares the angular size of the nearby galaxy to a brighter, more familiar celestial sight. In it, a deep exposure of Andromeda, tracing beautiful blue star clusters in spiral arms far beyond the bright yellow core, is combined with a typical view of a nearly full Moon. Shown at the same angular scale, the Moon covers about 1/2 degree on the sky, while the galaxy is clearly several times that size. The deep Andromeda exposure also includes two bright satellite galaxies, M32 and M110 (bottom).

APOD: 2006 December 3 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The larger moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning color image from the Mars Express spacecraft, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

APOD: 2006 August 17 - Comet Dust over Colorado
Explanation: The rock formation in the foreground of this night view was recorded on August 10, illuminated by light from a waning gibbous Moon. Even though the sky above also scatters the bright moonlight, a brilliant meteor was captured as it flashed across the scene during the 30 second long exposure. Of course, the meteor was part of the annual rain of dust from periodic Comet Swift-Tuttle known as the Perseid Meteor Shower. Leaving trails that point back to a radiant in the constellation Perseus, the ancient dust particles are vaporized as they enter the atmosphere at about 60 kilometers per second, their visible streaks beginning at altitudes of around 100 kilometers. And though it looks like the knuckles of a giant hand, the curious rock formation can be found in Colorado National Monument park, USA, planet Earth.

APOD: 2006 August 13 - The Comet and the Galaxy
Explanation: The Moon almost ruined this photograph. During late March and early April 1997, Comet Hale-Bopp passed nearly in front of the Andromeda Galaxy. Here the Great Comet of 1997 and the Great Galaxy in Andromeda were photographed together on 1997 March 24th. The problem was the brightness of the Moon. The Moon was full that night and so bright that long exposures meant to capture the tails of Hale-Bopp and the disk of M31 would capture instead only moonlight reflected off the Earth's atmosphere. By the time the Moon would set, this opportunity would be gone. That's why this picture was taken during a total lunar eclipse.

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.

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.

APOD: 2006 March 11 - Colors of Comet Pojmanski
Explanation: Comet Pojmanski flew by planet Earth last weekend on a surprise trip through the inner solar system. Then an easy binocular target for morning skygazers, Pojmanski ultimately showed off a long tail, but it also presented some lovely green-blue hues as gas molecules in its tenuous coma and tail fluoresced in the sunlight. Astronomers Adam Block and Jay GaBany recorded this colorful high-resolution view on March 3rd in the darkness just before twilight. The picture spans about one full moon on the sky. Comet Pojmanski (C/2006 A1) is outward bound and fading now, still visible in binoculars for northern hemisphere observers.

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.

APOD: 2006 February 12 - Phoebe: Comet Moon of Saturn
Explanation: Was Saturn's moon Phoebe once a comet? Images from the robotic Cassini spacecraft taken two years ago when entering the neighborhood of Saturn indicate that Phoebe may have originated in the outer Solar System. Phoebe's irregular surface, retrograde orbit, unusually dark surface, assortment of large and small craters, and low average density appear consistent with the hypothesis that Phoebe was once part of the Kuiper belt of icy comets beyond Neptune before being captured by Saturn. Visible in the above image of Phoebe are craters, streaks, and layered deposits of light and dark material. The image was taken from around 30,000 kilometers out from this 200-kilometer diameter moon. Two weeks after taking the above image, Cassini fired its engines to decelerate into orbit around Saturn.

APOD: 2005 September 19 - Approaching Asteroid Itokawa
Explanation: What are asteroids made of? To help find out, Japan's JAXA space agency launched the Hayabusa mission to rendezvous with asteroid Itokawa. Last week, the small robotic Hayabusa spacecraft arrived at asteroid Itokawa and stationed itself only 20 kilometers away. Although a long term goal is to find out how much ice, rock and trace elements reside on the asteroid's surface, a shorter term goal is to determine the mass of the asteroid by measuring the attraction of the drifting Hayabusa spacecraft. During the next few months, Hayabusa will also image and map asteroid Itokawa as it orbits the Sun. The above time-lapse image sequence was taken by Hayabusa upon final approach, showing the general oblong shape of the asteroid. In November, a small coffee-can sized robot dubbed MINERVA is scheduled for release and is expected to hop around the asteroid taking pictures. Also in November, Hayabusa will fire pellets into asteroid Itokawa and collect some of the debris in a return capsule. In December, Hayabusa will fire its rockets toward Earth and drop the return capsule to Earth in 2007 June.

APOD: 2005 August 24 - Epimetheus: A Small Moon of Saturn
Explanation: How did Epimetheus form? No one is yet sure. To help answer that question, this small moon has recently been imaged again in great detail by the robot spacecraft Cassini now orbiting Saturn. Epimetheus sometimes orbits Saturn in front of Janus, another small satellite, but sometimes behind. The above false-color image, taken during mid July, shows a surface covered with craters indicating great age. Epimetheus spans about 115 kilometers across. Epimetheus does not have enough surface gravity to restructure itself into a sphere.

APOD: 2005 April 23 - Eclipsed Moon in Infrared
Explanation: In September of 1996, the Midcourse Space Experiment (MSX) satellite had a spectacular view of a total lunar eclipse from Earth orbit. SPIRIT III, an on board infrared telescope, was used to repeatedly image the moon during the eclipse. Above is one of the images taken during the 70 minute totality, the Moon completely immersed in the Earth's shadow. Infrared light has wavelengths longer than visible light - humans can not see it but feel it as heat. So, the bright spots correspond to the warm areas on the lunar surface, and dark areas are cooler. The brightest spot below and left of center is the crater Tycho, while the dark region at the upper right is the Mare Crisium. Of course, this Sunday's lunar eclipse will not be a total, or even a partial one. Instead, the Moon will glide through the subtle outer portion of the Earth's shadow in a penumbral eclipse of the Moon.

APOD: 2004 November 19 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. The largest moon, Phobos, is indeed seen to be a cratered, asteroid-like object in this stunning new color image from the Mars Express spacecraft, recorded at a resolution of about seven meters per pixel. But Phobos orbits so close to Mars - about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon - that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

APOD: 2004 October 21 - Apogee Moon, Perigee Moon
Explanation: Why don't these pieces fit? This third quarter Moon (left) and first quarter Moon were both photographed during the last lunar cycle or lunation with the same telescope and camera. But, simply combining the pictures into one sharp, full surface view would clearly be a problem. In fact, on October 6th the Moon's third quarter phase happened to occur near lunar apogee, the farthest point in the Moon's orbit. On September 21st, the first quarter phase fell close to lunar perigee, the Moon's closest approach to planet Earth. Viewed two weeks apart, the resulting difference in apparent sizes would not be noticed by casual skygazers, but the simultaneous side by side comparison makes it hard to ignore. Skygazers will likely notice the Moon next week though, as it slides through Earth's shadow during October 27th's total lunar eclipse.

APOD: 2004 October 2 - Toutatis Nears Planet Earth
Explanation: On Wednesday, September 29, asteroid Toutatis came within one million miles of Earth -- the closest predicted aproach of a sizable asteroid or comet to our fair planet in this century. Coming within one million miles or about 4 times the Earth-Moon distance, Earth would have appeared to be nearly the size of the full moon in the asteroid's sky. In Earth's sky, Toutatis appeared only as a faint, starlike, but rapidly moving object. Even so, asteroid 4179 Toutatis was still bright enough to see in small telescopes. Astronomers John Chumack, observing near Dayton Ohio, and Juergen Wolf from near Palo Alto, California, offer these composite images showing the progress of the asteroid (seen as a series of dots) against a background of distant stars. Their multiple exposures span a two hour period on two different days about a week before the asteroid's record close approach, which tracked through night skies south of the celestial equator.

APOD: 2004 October 1 - Earth Nears Asteroid Toutatis
Explanation: On Wednesday, September 29, the Earth came within one million miles of the asteroid Toutatis -- the closest predicted aproach of our fair planet to a sizable asteroid or comet in this century. Coming within one million miles or about 4 times the Earth-Moon distance, Earth would appear to be nearly the size of the full moon in the asteroid's sky, as suggested in this illustration. In Earth's sky, Toutatis appeared only as a faint object rapidly moving against a background of stars. Also known as Earth-crossing asteroid 4179, Toutatis is in an eccentric 4 year orbit which moves it from the asteroid belt between Mars and Jupiter to just inside Earth's orbit. When the Earth passed near it in 1992 Toutatis was imaged by radar and seen to be two irregularly shaped lumps, perhaps joined by a narrow neck. This bizarre object is up to 1.5 miles wide, 3 miles long, and is tumbling through space. Studies of Toutatis and other Earth-crossing asteroids help reveal connections between the Solar System's meteorites, main-belt asteroids and comets. These asteroids also offer tantalizing targets for robotic exploration and, over time, represent potential collision hazards for planet Earth!

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.

APOD: 2004 July 31 - Tonight: A Blue Moon
Explanation: How often does a full moon occur twice in a single month? Exactly once in a Blue Moon. In fact, the modern usage of the term "Blue Moon" refers to the second Full Moon in a single month. Tonight's Blue Moon will be the first since November 2001. A Blue Moon typically occurs every few years. The reason for the rarity of the Blue Moon is that the 29.53 days between full moons is just slightly shorter than the number of days in the average month. Don't, however, expect the moon to look blue tonight! The term "Blue Moon" has recently been traced to an error in a magazine article in 1946. It is possible for the Moon to appear tinged by a blue hue, sometimes caused by fine dirt circulating in the Earth's atmosphere, possibly from a volcanic explosion. The above picture was taken not during a full moon but through a morning sky that appeared dark blue. The bright crescent is the only part directly exposed to sunlight - the rest of the Moon glows from sunlight reflected from the Earth. In this dramatic photo, however, the planet Jupiter is also visible along with its four largest moons.

APOD: 2004 June 30 - Phoebe: Comet Moon of Saturn
Explanation: Was Saturn's moon Phoebe once a comet? Images from the robotic Cassini spacecraft taken two weeks ago when entering the neighborhood of Saturn indicate that Phoebe may have originated in the outer Solar System. Phoebe's irregular surface, retrograde orbit, unusually dark surface, assortment of large and small craters, and low average density appear consistent with the hypothesis that Phoebe was once part of the Kuiper belt of icy comets beyond Neptune before being captured by Saturn. Visible in the above image of Phoebe are craters, streaks, and layered deposits of light and dark material. The image was taken from around 30,000 kilometers out from this 200-kilometer diameter moon. Late today, Cassini will begin to fire its engines to decelerate into orbit around Saturn.

APOD: 2004 June 19 - Ida and Dactyl: Asteroid and Moon
Explanation: This asteroid has a moon! The robot spacecraft Galileo destined to explore the Jovian system, encountered and photographed two asteroids during its long interplanetary voyage to Jupiter. The second asteroid it photographed, Ida, was discovered to have a moon which appears as a small dot to the right of Ida in this image from 1993. The tiny moon, named Dactyl, is about one mile across, while the potato shaped Ida measures about 36 miles long and 14 miles wide. Dactyl is the first moon of an asteroid ever discovered. The names Ida and Dactyl are from Greek mythology. Many other asteroids are now known to have moons.

APOD: 2004 April 23 - Comet C/2001 Q4 (NEAT)
Explanation: Inbound from the distant solar system, comet C/2001 Q4 will soon pass just inside planet Earth's orbit and should be one of two bright, naked-eye comets visible in southern skies in May. First picked up nearly three years ago by the Near Earth Asteroid Tracking (NEAT) project Q4 appears in both of these stunning telescopic views recorded only a few days ago, on April 18th (left) and 19th, from a site near Alcohuaz, Chile. Remarkable changes in the structure of the long, graceful tail can be seen by comparing the two photos, including the dramatic kink seen near the tail's midpoint on April 19th. The apparent motion of the comet sweeping across the sky is evident when you compare the position of the tail relative to background galaxy NGC 1313, visible as a smudge near the top of each image. Q4's closest approach to the Sun will be on May 15th while its closest encounter with planet Earth will be on May 7th (see animation by L. Koehn).

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).

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.

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.

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.

APOD: 2003 November 27 - The Long Shadow of the Moon
Explanation: The long shadow of the Moon fell across the continent of Antarctica on November 23rd, during the second solar eclipse of 2003. In this view from orbit, based on data from the MODIS instrument on board the Earth observing Aqua satellite, the Moon's shadow stretches for almost 500 kilometers. Recorded between 23:15 and 23:20 Universal Time, the shadow was cast by a lunar disk silhouetted by the Sun hanging only about 15 degrees above the antarctic horizon. Observers within the central dark portion of the oval-shaped shadow could view the totally eclipsed sun. Shadows of mountains and clouds are also visible over the Norwegian named Queen Maud Land, Antarctica with the South Pole just beyond the lower right corner of the image.

APOD: 2003 November 8 - Eclipsed Moon in Infrared
Explanation: The total lunar eclipse of September 1996 disappointed many observers in North America who were cursed with cloudy skies. However, the Midcourse Space Experiment (MSX) satellite had a spectacular view from Earth orbit and SPIRIT III, an on board infrared telescope, was used to repeatedly image the moon during the eclipse. Above is one of the images taken during the 70 minute totality, the Moon completely immersed in the Earth's shadow. Infrared light has wavelengths longer than visible light - humans can not see it but feel it as heat. The bright spots correspond to the warm areas on the lunar surface, dark areas are cooler. The brightest spot below and left of center is the crater Tycho, the dark region at the upper right is the Mare Crisium. The series of SPIRIT III images allow the determination of cooling rates for geologically different areas, exploring the physical properties of the Moon's surface.

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.

APOD: 2003 August 29 - The Mineral Moon
Explanation: Even if the Moon really were made of green cheese it probably wouldn't look this bizarre. Still, this mosaic of 53 images was recorded by the Jupiter-bound Galileo spacecraft as it passed near our own large natural satellite in 1992. The pictures were recorded through three spectral filters and combined in an exaggerated false-color scheme to explore the composition of the lunar surface as changes in mineral content produce subtle color differences in reflected light. Familiar to earthdwellers, the lunar near side is on the left, but the space-based view looks down on the Moon's north pole located in the upper half of the image near the shadow line. Blue to orange shades indicate volcanic lava flows. The dark blue Mare Tranquillitatis at the lower left is richer in titanium bearing minerals than the green and orange maria above it. Near the bottom of the image and to the right of Tranquillitatis is the dark oval-shaped Mare Crisium surrounded by shocking pink colors indicating material of the lunar highlands.

APOD: 2003 July 1 - Martian Moon Phobos from MGS
Explanation: Why is Phobos so dark? Phobos, the largest and innermost of two Martian moons, is the darkest moon in the entire Solar System. Its unusual orbit and color indicate that it may be a captured asteroid composed of a mixture of ice and dark rock. The above picture was captured recently by the robot spacecraft Mars Global Surveyor (MGS) orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From MGS images like this, Phobos has been determined to be covered by perhaps a meter of loose dust. Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all. Phobos' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years.

APOD: 2003 April 6 - Phobos: Doomed Moon of Mars
Explanation: This moon is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1978 Viking 1 orbiter image, the largest moon, Phobos, is indeed seen to be a heavily cratered asteroid-like object. About 17 miles across, Phobos really zips through the Martian sky. Actually rising above Mars' western horizon and setting in the east, it completes an orbit in less than 8 hours. But Phobos orbits so close to Mars, (about 5,800 kilometers above the surface compared to 400,000 kilometers for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

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.

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.

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.

APOD: 2002 December 9 - Moon Shadow Moves Over Africa
Explanation: When the Moon's shadow reached out and touched the Earth last week, the result was a solar eclipse. Such an eclipse is total only for observers located along a narrow path corresponding to the ground track of the shadow's dark central portion or umbra. For this eclipse, racing along at nearly 2000 kilometers per hour, the Moon's umbra obligingly crossed over land along regions of Africa and Australia. Totality lasted for about two minutes or less at a given location. Many nearby regions fell within the lighter but much wider outer shadow region, the penumbra, and witnessed a partial solar eclipse. The above movie follows the Moon's shadow as it crossed Africa during a similar eclipse in June 2001. Each frame is separated in time by about 20 minutes. The movie was created from frames taken by the orbiting European satellite MeteoSat-6.

APOD: 2002 November 20 - Leonids vs The Moon
Explanation: Beautiful and bright, the 2002 Leonid meteors battled against glaring moonlight. This winning example, from Tuesday morning skies above Laughlin, Nevada, USA, finds an undaunted Leonid streaking between the familiar constellation of Orion (left) and an overexposed full Moon. As anticipated, the Leonid shower packed a double punch on November 19 with planet Earth plunging through two dense clouds of meteroids, dusty debris left by the passage of comet Tempel-Tuttle. Some European observers reported 10 or so meteors a minute during the first peak near 4:00 Universal Time while North American skygazers witnessed slightly lower rates near the second peak around 10:30 UT. Overall, observed rates were much lower than last year's Leonid meteor storm, but for many the sky was still filled with a rewarding spectacle of bright meteors. And that performance may be a fond farewell for years to come. The annual Leonid meteor shower will not likely approach even these rates again until the end of this century.

APOD: 2002 November 16 - Tempel-Tuttle: The Leonid Comet
Explanation: Star trails streak this composite time exposure of comet Tempel-Tuttle recorded by Tim Puckett on January 26, 1998. Then passing through the inner solar system on its 33 year orbit around the Sun, Tempel-Tuttle brightened unexpectedly, but binoculars or small telescopes were still required to visually observe it. Tempel-Tuttle is also called "the Leonid Comet" as the yearly Leonid meteor shower results when the Earth crosses this comet's orbital plane and encounters a trail of cometary dust. So, while not rivaling spectacular naked-eye comets like Hyakutake or Hale-Bopp, Tempel-Tuttle still puts on a show. The Earth is now approaching relatively dense regions of Tempel-Tuttle's orbiting debris trail, so in the next few days, skywatchers will be searching for leonid meteors. An extremely active meteor shower is expected to be visible over Europe and North America in the early morning hours of Tuesday, November 19, despite interference from a glaring full moon.

APOD: 2002 November 13 - Asteroid Annefrank
Explanation: NASA's interplanetary probe STARDUST, on its way to Comet Wild 2 in January 2004, passed asteroid 5535 Annefrank earlier this month. Annefrank, named for a holocaust victim who kept a famous diary, is a member of the main asteroid belt between Mars and Jupiter. Just prior to closest approach at about 3300 km distant, the robot spacecraft took the above picture. Although images obtained were not detailed enough to resolve much of the asteroid's surface, the size, reflectivity, and general shape of the asteroid were recorded. At 6 kilometers across, asteroid Annefrank turned out to be larger -- and darker -- than expected.

APOD: 2002 September 19 - Asteroid 1998 KY26
Explanation: A day is just under 11 minutes long on 1998 KY26, a 30 meter wide, fast-spinning, water-rich asteroid. This computer simulated view of its lumpy surface has a resolution of about 3 meters and is based on radar and optical observations (click on the image for a series of surface views). The observations were made shortly after the discovery of the diminutive world which passed within about 800,000 kilometers of Earth, or about 2 times the Earth-Moon distance, in June of 1998. Around 10 million asteroids of similar size may exist in orbits that also come near Earth's, but little is known about them. However, spinning so fast, tiny 1998 KY26 can not be a loose conglomerate held together by gravity alone. Instead it is likely a monolithic chunk fragmented from a larger asteroid. As the radar and optical data suggest 1998 KY26 has a high water content, this relatively accessible asteroid could be a literal oasis for future space explorers.

APOD: 2002 August 17 - Asteroid 2002 NY40
Explanation: Asteroid 2002 NY40 will fly by planet Earth early in the morning August 18 Universal Time (late in the evening August 17 Eastern Daylight Time). Approaching to within about 530,000 kilometers or 1.3 times the Earth-Moon distance 2002 NY40 will definitely not be close enough to pose any danger of collision. But it will be close enough and just bright enough for experienced skygazers to see this 800 meter wide space rock in a small telescope or binoculars as it glides quickly through northern skies past the bright star Vega. It will also be close enough to ping with radar, and asteroid hunters using the large Arecibo radio telescope in Puerto Rico expect to determine the three dimensional outline of 2002 NY40. Similar investigations of other near Earth asteroids have revealed some surprising shapes. In this five minute time exposure, recorded at Cerro Tololo Inter-American Observatory on August 14, 2002 NY40 shows itself as a long smudge as it moves against a background of faint stars in the constellation Aquarius.

APOD: 2002 July 6 - Io: Moon Over Jupiter
Explanation: How big is the Jovian moon Io? The most volcanic body in the Solar System, Io (usually pronounced "EYE-oh") is 3,600 kilometers in diameter, about the size of planet Earth's single large natural satellite. Gliding past Jupiter at the turn of the millennium, the Cassini spacecraft captured this awe inspiring view of active Io with the largest gas giant as a backdrop, offering a stunning demonstration of the ruling planet's relative size. Although in the picture Io appears to be located just in front of the swirling Jovian clouds, Io hurtles around its orbit once every 42 hours at a distance of 420,000 kilometers or so from the center of Jupiter. That puts it nearly 350,000 kilometers above Jupiter's cloud tops, roughly equivalent to the distance between Earth and Moon. The Cassini spacecraft itself was about 10 million kilometers from Jupiter when recording the image data.

APOD: 2002 June 30 - Ida and Dactyl: Asteroid and Moon
Explanation: This asteroid has a moon! The robot spacecraft Galileo currently exploring the Jovian system, encountered and photographed two asteroids during its long journey to Jupiter. The second asteroid it photographed, Ida, was discovered to have a moon which appears as a small dot to the right of Ida in this picture. The tiny moon, named Dactyl, is about one mile across, while the potato shaped Ida measures about 36 miles long and 14 miles wide. Dactyl is the first moon of an asteroid ever discovered. The names Ida and Dactyl are based on characters in Greek mythology. Other asteroids are now known to have moons.

APOD: 2002 April 19 - The Old Moon in the New Moon's Arms
Explanation: Also known as the Moon's "ashen glow" or "the old Moon in the new Moon's arms", Earthshine is Earthlight reflected from the Moon's night side. This dramatic image of Earthshine and a young crescent Moon was taken by astrophotographer and APOD translator Laurent Laveder from the remote Pic du Midi Observatory on planet Earth. But the view from the Moon would have been stunning too. When the Moon appears in Earth's sky as a slender crescent, a dazzlingly bright, nearly full Earth would be seen from the lunar surface. The Earth's brightness, due to reflected sunlight, is strongly influenced by cloud cover and recent studies of Earthshine indicate that it is more pronounced during April and May. A description of Earthshine, in terms of sunlight reflected by Earth's oceans in turn illuminating the Moon's dark surface, was written 500 years ago by Leonardo da Vinci.

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.

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.

APOD: 2002 February 4 - Comet LINEAR WM1 Shines in the South
Explanation: A new comet has brightened unexpectedly and is currently visible to unaided observers of southern skies. Comet C/2000 WM1 (LINEAR) is now reported by some observers to be at third magnitude, making it brighter -- although more diffuse -- than most visible stars. A dust tail as long as 3 degrees has also been reported. Pictured above is the center of Comet LINEAR (WM1) taken the morning of February 1 from 300 km north of Sydney, Australia. A bright coma and the start of the dust tail are visible despite a bright, nearly full Moon. The comet has now passed its closest approach to the Sun (January) and the Earth (December) and will move toward northern skies as it fades.

APOD: 2001 December 24 - Asteroid 1998 WT24 Passes Near Earth
Explanation: Last week, an asteroid approached unusually close to the Earth. Passing well outside the orbit of our Moon, Asteroid 1998 WT24 posed no danger, but became bright enough to see with binoculars and to track with radar. Pictured above, the kilometer-sized asteroid was imaged crossing the sky on December 14, two days before closest approach. Every few years, an asteroid will actually pass inside the orbit of the Moon. Large impact features on the Earth are testaments to asteroids or comets that actually impacted the Earth in the distant past. Astronomers continue to discover, track, and study potentially hazardous asteroids with a goal of making planet Earth a safer place.

APOD: 2001 December 15 - Ganymede: Torn Comet Crater Chain
Explanation: This striking line of 13 closely spaced craters on Jupiter's moon Ganymede was photographed by the Galileo spacecraft in 1997. The picture covers an area about 120 miles wide and the chain of craters cuts across a sharp boundary between dark and light terrain. What caused this crater chain? Remarkably, the exploration of the Solar System, has shown that crater chains like this one are not unique, though they were considered mysterious until a dramatic object lesson was offered by comet Shoemaker-Levy 9. In 1994 many denizens of planet Earth watched as huge pieces of this torn comet slammed into Jupiter itself in a spectacular series of sequential impacts. It is very likely that similar torn comets from the early history of the Solar System are responsible for this and other crater chains.

APOD: 2001 September 15 - Eclipsed Moon in Infrared
Explanation: The total lunar eclipse of September 1996 disappointed many observers in North America who were cursed with cloudy skies. However, the Midcourse Space Experiment (MSX) satellite had a spectacular view from Earth orbit and SPIRIT III, an onboard infrared telescope, was used to repeatedly image the moon during the eclipse. Above is one of the images taken during the 70 minute totality, the Moon completely immersed in the Earth's shadow. Infrared light has wavelengths longer than visible light - human's can not see it but feel it as heat. The bright spots correspond to the warm areas on the lunar surface, dark areas are cooler. The brightest spot below and left of center is the crater Tycho, the dark region at the upper right is the Mare Crisium. The series of SPIRIT III images allow the determination of cooling rates for geologically different areas, exploring the physical properties of the Moon's surface.

APOD: 2001 August 18 - Phobos: Doomed Moon of Mars
Explanation: Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the main asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1978 Viking 1 orbiter image, the largest moon, Phobos, is indeed seen to be a heavily cratered asteroid-like object. About 17 miles across, Phobos really zips through the Martian sky. Actually rising above Mars' western horizon and setting in the east, it completes an orbit in less than 8 hours. But Phobos is doomed. Phobos orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it will likely crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

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.

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.

APOD: 2001 April 20 - Io: Moon Over Jupiter
Explanation: How big is the Jovian moon Io? The most volcanic body in the Solar System, Io (usually pronounced "EYE-oh") is 3,600 kilometers in diameter, about the size of planet Earth's single large natural satellite. Gliding past Jupiter at the turn of the millennium, the Cassini spacecraft captured this awe inspiring view of active Io with the largest gas giant as a backdrop, offering a stunning demonstration of the ruling planet's relative size. Although in the picture Io appears to be located just in front of the swirling Jovian clouds, Io hurtles around its orbit once every 42 hours at a distance of 420,000 kilometers or so from the center of Jupiter. That puts it nearly 350,000 kilometers above Jupiter's cloud tops, roughly equivalent to the distance between Earth and Moon. The Cassini spacecraft itself was about 10 million kilometers from Jupiter when recording the image data.

APOD: 2001 March 8 - Bright Venus
Explanation: Have you seen a bright evening star in the western sky lately? That's no star, that's planet Venus the second "rock" from the Sun. Blazing at -4.6 magnitude, Venus, after the Sun and Moon, is the third brightest celestial body in planet Earth's sky. Venus is closer to the Sun than Earth and as Venus orbits the Sun it is seen to go through phases similar to the Moon. But unlike the Moon, as Venus waxes and wanes its distance from Earth and hence its apparent size changes drastically. This causes Venus to look brighter as it looms large in its crescent phases than when it is smaller and nearly full. Taken on January 28th, this dramatic picture finds a crescent Venus near its brightest to the right of a crescent Moon. The brilliant rivals seem poised above a satellite dish of the Scripps Satellite Oceanography Facility. Closer to the horizon, just below and to the right of the satellite dish, Mercury pierces the twilight glow.

APOD: 2001 February 13 - NEAR Spacecraft Survives Landing on Asteroid Eros
Explanation: Yesterday NEAR-Shoemaker became the first spacecraft to land on an asteroid and send signals back from its surface. Since the robot spacecraft was not designed for such a contingency, the success of the landing on asteroid 433 Eros was not assured. Shown above is the last picture taken by NEAR-Shoemaker before its touchdown. The streaking on the lower part of the image was caused by the loss of telemetry as the satellite impacted the surface. The image was taken 130 meters above the surface and spans 6 meters across. Rocks as small as a human hand are visible. As engineers continue to try to communicate with the beached car-sized spacecraft, scientists will work to understand features visible in the highest resolution photographs ever taken of an asteroid.

APOD: 2000 November 1 - Double Asteroid 90 Antiope
Explanation: This eight-frame animation is based on the first ever images of a double asteroid! Formerly thought to be a single enormous chunk of rock, asteroid 90 Antiope resides in the solar system's main asteroid belt between Mars and Jupiter. Now, these premier images reveal Antiope to actually consist of two 50 mile wide asteroids separated by about 100 miles. Like weights on each end of an elastic string, the pair mutually orbit their center of mass, or balance point in the space between them, once every 16.5 hours. Binary asteroids and asteroids with moons are believed to be rare, but observations of their orbits allow a direct determination of asteroid masses and densities. Surprisingly, Antiope and known asteroid-moon systems are found to have densities closer to ice than rock, despite their relatively dark and unreflective surfaces. These sharp images were made at the Keck Observatory atop the Hawaiian volcano Mauna Kea using newly developed adaptive optics technology to overcome the blurring effect of Earth's atmosphere.

APOD: 2000 September 9 - X Ray Moon and X Ray Star
Explanation: An x-ray star winks out behind the Moon in these before (left) and after views of a lunar occultation of the galactic x-ray source designated GX5-1. The false color images were made using data from the ROSAT (ROentgen SATellite), orbiting observatory. They show high energy x-rays in yellow (mostly from GX5-1), and lower energy x-rays in red (the Moon reflecting x-rays from the Sun). GX5-1 is a binary system consisting of a neutron star and a companion star in mutual orbit about the system's center of mass. The gas in the companion star's outer envelope falls toward the neutron star and accumulates in a disk around it. This disk material swirls deeper in to the neutron star's gravitational well, and is finally dumped onto its surface - in the process creating tremendous temperatures and generating the high energy x-rays.

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.

APOD: 2000 March 8 - Nearer To Asteroid Eros
Explanation: As the robot spacecraft NEAR lowers itself toward asteroid 433 Eros, more surface details are becoming visible. Last week's maneuvers brought NEAR to within 204 kilometers of the floating mountain's surface. With increased resolution, NEAR's camera then documented Eros' unusual shape, craters large and small, boulders, and mysterious grooves similar to asteroid Gaspra and Martian moon Phobos. If you could stand on Eros, you would still be too small to be visible on this recent image, which shows features as small as 20 meters across. However, you would feel gravity only 1/1000 that on Earth, so that you could easily jump over even this large 5 kilometer wide crater.

APOD: 2000 February 14 - An Unexpected Asteroid Valentine
Explanation: Maybe the Moon owns our hearts, but this won't stop the occasional asteroid from sending us a valentine. Friday, to the surprise of many, the NEAR mission on approach to asteroid 433 EROS photographed what appears to be a heart-shaped depression. After NEAR reaches the asteroid today, NASA plans a series of maneuvers to make the robot spacecraft the first ever to orbit an asteroid. More detailed pictures will soon be taken of the 33 kilometer long asteroid EROS and this 5 kilometer long depression. Most likely, fortuitous lighting and viewing angles accentuate the apparent heart shape -- but don't tell the Moon.

APOD: December 22, 1999 - Perigee Moon, Apogee Moon
Explanation: Tonight, those blessed with clear skies can enjoy a glorious full moon, the last full moon of the "Y1.9K"s. In fact, tonight's moon will be a full-perigee-solstice moon, reaching its full phase and perigee (the closest point in its orbit) on the solstice, the first day of northern hemisphere winter. Anticipation of this celestial event has generated a flood of e-mail chasing the rumor that the full-perigee-solstice moon will be intensely bright. However, calculations show it will only be a little brighter than usual, and certainly not dramatically so. Why would it be brighter? Mostly because the moon appears bigger and brighter at perigee. The above illustration, based on Galileo spacecraft images, shows the approximate difference in apparent size between a full moon at perigee (left) and a full moon at apogee, the farthest point in the lunar orbit. The Earth and moon are also a few percent closer to the sun during northern winter, but the combined effects will cause tonight's full moon to appear only about 20% brighter than an average full moon. That difference is not even likely to be noticeable to the human eye.

APOD: October 14, 1999 - Moon Over Eugenia
Explanation: Eugenia is an asteroid with a moon! This animation was constructed from infrared discovery images of the Eugenia-moon system taken in November 1998 using the Canada-France-Hawaii Telescope (CFHT). Main belt asteroid Eugenia, represented here as a central white patch, is a mere 215 kilometers in diameter. Its moon, seen at 5 separate positions around a clockwise orbit, is estimated to be 13 kilometers wide. An adaptive optics system was used with the CFHT, located atop Mauna Kea, Hawaii, to counteract the blurring effect of Earth's atmosphere making possible this premier discovery from a ground-based telescope. Only one other asteroid-moon system is known. Dactyl, moon of the asteroid Ida, was discovered by the Galileo spacecraft during a 1993 flyby.

Eugenia's moon has a nearly circular orbit with a radius of 1,190 kilometers which it completes once in 4.7 days. The orbit appears oval-shaped because it is tilted at a 45 degree angle to the line-of-sight. Knowing the moon's orbit allows astronomers to calculate the asteroid's mass. Combining mass and size determines the asteroid's density, which in this case gives a surprising result - Eugenia is found to have a density only 20% greater than water. The low density suggests that Eugenia itself is a porous "rubble pile" of rocks or composed mostly of water-ice with only a little additional rocky material.

APOD: October 10, 1999 - Triton: Neptune's Largest Moon
Explanation: One hundred and fifty three years ago, on October 10th, 1846, William Lassell was observing the newly discovered planet Neptune. He was attempting to confirm his observation, made just the previous week, that Neptune had a ring. But this time he discovered that Neptune had a satellite as well. Lassell soon proved that the ring was a product of his new telescope's distortion, but the satellite Triton remained. The above picture of Triton was taken in 1989 by the only spacecraft ever to pass Triton: Voyager 2. Voyager 2 found fascinating terrain, a thin atmosphere, and even evidence for ice volcanoes on this world of peculiar orbit and spin. Ironically, Voyager 2 also confirmed the existence of complete thin rings around Neptune - but these would have been quite invisible to Lassell!

APOD: August 7, 1999 - Ida and Dactyl: Asteroid and Moon
Explanation: This asteroid has a moon! The robot spacecraft Galileo currently exploring the Jovian system, encountered and photographed two asteroids during its long journey to Jupiter. The second asteroid it photographed, Ida, was discovered to have a moon which appears as a small dot to the right of Ida in this picture. The tiny moon, named Dactyl, is about one mile across, while the potato shaped Ida measures about 36 miles long and 14 miles wide. Dactyl is the first moon of an asteroid ever discovered. The names Ida and Dactyl are based on characters in Greek mythology. Do other asteroids have moons?

APOD: March 14, 1999 - The Comet and the Galaxy
Explanation: The Moon almost ruined this photograph. During late March and early April 1997, Comet Hale-Bopp passed nearly in front of the Andromeda Galaxy. Here the Great Comet of 1997 and the Great Galaxy in Andromeda were photographed together on 1997 March 24th. The problem was the brightness of the Moon. The Moon was full that night and so bright that long exposures meant to capture the tails of Hale-Bopp and the disk of M31 would capture instead only moonlight reflected off the Earth's atmosphere. By the time the Moon would set, this opportunity would be gone. That's why this picture was taken during a lunar eclipse.

APOD: August 5, 1998 - Ganymede: Torn Comet Crater Chain
Explanation: This remarkable line of 13 closely spaced craters on Jupiter's moon Ganymede was photographed by the Galileo spacecraft in 1997. The picture covers an area about 120 miles wide and the chain of craters cuts across a sharp boundary between dark and light terrain. What caused this crater chain? During the exploration of the Solar System, crater chains like this one have been discovered in several places and were considered mysterious until a dramatic object lesson was offered by comet Shoemaker-Levy 9. In 1994 many denizens of planet Earth watched as huge pieces of this torn comet slammed into Jupiter itself in a spectacular series of sequential impacts. It is very likely that similar torn comets from the early history of the Solar System are responsible for this and other crater chains.

APOD: July 9, 1998 - Hale-Bopp: The Crowd Pleaser Comet
Explanation: In 1997, the bright comet Hale-Bopp may have become the most viewed comet in history -- visible even to casual skywatchers in light polluted cities around the globe. In this picture, taken by photographer Joe Orman on the evening of May 8, 1997, Hale-Bopp easily competes with near twilight skies and a shining, over-exposed, crescent moon above Mobile, Arizona, USA. Where is Hale-Bopp now? Still visible to telescopic observers in the Southern Hemisphere, the comet is outbound, presently about 537 million miles from the Sun. (Jupiter orbits at about 480 million miles.) The long lead time provided by the early discovery of Hale-Bopp has allowed extensive observing campaigns producing a bonanza of information about this primordial chunk of our Solar System.

APOD: May 31, 1998 - Phobos: Doomed Moon of Mars
Explanation: Phobos is doomed. Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1977 Viking orbiter image, the largest moon, Phobos, is seen to be a heavily cratered asteroid-like object. It is about 17 miles across and zips through the Martian sky completing an orbit in less than 8 hours. Phobos orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it should crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

APOD: March 13, 1998 - Asteroids
Explanation: No asteroid or comet is known to be on a collision course with Earth. The asteroid designated 1997 XF11 had been predicted to come uncomfortably close, but new estimates place its passing beyond the orbit of the Moon. This earth-approaching asteroid was discovered by SpaceWatch astronomer Jim Scotti in December of last year. Orbital computations using new observations suggested that it would pass within 30,000 miles of the Earth's center on October 26, 2028 - a very near miss considering that the radius of the Earth itself is about 4,000 miles. However, more recent and further refined calculations based on both new and archival data indicate that the closest approach will be 600,000 miles in 2028. Imaged by NASA spacecraft, the three potato-shaped objects above are large main-belt asteroids orbiting between Mars and Jupiter. Shown to the same scale from left to right are Mathilde, Gaspra, and Ida. Mathilde has dimensions of about 37 by 29 miles. The asteroid 1997 XF11 is much smaller, probably a mile wide, yet the impact of an asteroid of this size could have catastrophic effects. Over the last two decades, teams of astronomers have just begun to catalog and track near-earth objects.

APOD: March 12, 1998 - Moon Shadow
Explanation: When the Moon's shadow reached out and touched the Earth last month, the result was a Solar Eclipse. Such an eclipse is total only for observers located along a narrow path corresponding to the ground track of the shadow's dark central portion or "umbra". For this eclipse, racing along at nearly 1,200 miles per hour, the Moon's umbra obligingly crossed over land along pleasant tropical locales in South America and the Caribbean islands. Totality lasted for about 3 minutes or less at a given location. Sizable fractions of North and South America fell within the lighter but much wider outer shadow region, the "penumbra", and witnessed a partial eclipse. This movie gif follows the Moon's shadow from Central and South America eastward across the Atlantic - as seen from the vantage point of an orbiting Geostationary Operational Environmental Satellite (GOES), 22,000 miles above Earth. The three frame animation spans about 2 hours. GOES satellite images are a corner stone of U.S. weather monitoring and forecast operations.

APOD: January 29, 1998 - The Earth-Moon System
Explanation: This evocative mosaic image of the Earth-Moon system was recorded by NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft earlier this month. The relative sizes shown are appropriate for viewing both the Earth and Moon from a distance of about 250,000 miles, although the apparent brightness of the Moon has been increased by about a factor of five for the sake of appearances. This space-based perspective is a unique one, the bland and somber Lunar Southern Hemisphere contrasting strongly with blue oceans, swirling clouds, and the bright icy white continent of Antarctica on planet Earth. Though its lack of atmosphere and oceans make it relatively dull looking, the Earth's moon is one of the largest moons in the solar system - even larger than the planet Pluto. During this recent flyby of the Earth-Moon system, the NEAR spacecraft used Earth's gravity to deflect it towards its ultimate destination, the Asteroid 433 Eros. It is scheduled to arrive at Eros in January 1999.

APOD: November 25, 1997 - The Comet and the Galaxy
Explanation: The Moon almost ruined this photograph. During late March and early April, Comet Hale-Bopp passed nearly in front of the Andromeda Galaxy. Here the Great Comet of 1997 and the Great Galaxy in Andromeda were photographed together on March 24th. The problem was the brightness of the Moon. The Moon was full that night and so bright that long exposures meant to capture the tails of Hale-Bopp and the disk of M31 would capture instead only moonlight reflected off the Earth's atmosphere. By the time the Moon would set, this opportunity would be gone. That's why this picture was taken during the lunar eclipse.

APOD: November 23, 1997 - Triton: Neptune's Largest Moon
Explanation: On October 10th, 1846, William Lassell was observing the newly discovered planet Neptune. He was attempting to confirm his observation, made just the previous week, that Neptune had a ring. But this time he discovered that Neptune had a satellite as well. Lassell soon proved that the ring was a product of his new telescope's distortion, but the satellite Triton remained. The above picture of Triton was taken in 1989 by the only spacecraft ever to pass Triton: Voyager 2. Voyager 2 found fascinating terrain, a thin atmosphere, and even evidence for ice volcanoes on this world of peculiar orbit and spin. Ironically, Voyager 2 also confirmed the existence of complete thin rings around Neptune - but these would have been quite invisible to Lassell!

APOD: September 10, 1997 - Comet Hale-Bopp Outbound
Explanation: Hale-Bopp, the Comet of the Century, is leaving the inner Solar System. Outbound at about 12 miles per second it is presently nearing the main asteroid belt between Mars and Jupiter. This false-color image represents a recent view from low Earth orbit showing the comet surrounded by its shrinking coma against a background of stars. It was constructed from a fraction of the data taken aboard the Space Shuttle Discovery in August by a small innovative telescopic camera known as the Southwest Ultraviolet Imaging System or SWUIS. SWUIS (sounds like "swiss") images will be particularly interesting to astronomers who wish to continue to follow the Great Comet's interaction with the Solar Wind. The once bright Hale-Bopp has faded below 4th magnitude but is still visible to Earthbound observers south of 35 degrees north latitude.

APOD: July 28, 1997 - Help Aldebaran Map the Moon
Explanation: Turn on your camcorder, go outside, and become an astronomer. How?. Tomorrow morning, our Moon will pass directly in front of Aldebaran, the brightest star in this picture and in entire constellation of Taurus. Aldebaran is visible to the left and below Comet Hale-Bopp in the above photograph, which was taken on April 30th in Tenerife, Spain. This occultation is valuable because disappearance times from different locations can be used to map the height of the lunar terrain at the occultation points. You can help by clicking here, where a site will detail how to tape a familiar cable channel and then take your still-running camcorder outside to tape the occultation of Aldebaran by the Moon. You can then donate your VCR tape to science by mailing it to this address. Leave yourself plenty of time for a practice run and be sure to check the weather before going to a lot of trouble!

APOD: June 29, 1997 - Ida and Dactyl: Asteroid and Moon
Explanation: This asteroid has a moon! The robot spacecraft Galileo currently exploring the Jovian system, encountered and photographed two asteroids during its long journey to Jupiter. The second asteroid it photographed, Ida, was discovered to have a moon which appears as a small dot to the right of Ida in this picture. The tiny moon, named Dactyl, is about one mile across, while the potato shaped Ida measures about 36 miles long and 14 miles wide. Dactyl is the first moon of an asteroid ever discovered. The names Ida and Dactyl are based on characters in Greek mythology. Do other asteroids have moons?

APOD: June 18, 1997 - Asteroid 3753: Earth's Curious Companion
Explanation: Earth is not alone. It orbits the Sun with a small companion: Asteroid 3753. First discovered in 1986 and designated 1986 OT, this five kilometer rock was recently found to orbit the Sun while executing a strange dance with the Earth. A portion of the asteroid's complex orbit is shown above. As the Earth orbits once, Asteroid 3753 follows the yellow line - while also orbiting the Sun. Each time around, however, the yellow kidney-bean traced by Asteroid 3753 shifts slightly - eventually going from trailing the Earth to leading the Earth. Every 385 years the cycle repeats. Because the plane of 3753's orbit is tilted when compared to the Earth's orbit, the two will never collide. In autumn 1997, Asteroid 3753 will pass below the Earth's South Pole at about 100 times the distance to the Moon. It will, however, be very faint - about 15th magnitude - 10,000 times fainter than the dimmest star without a telescope. Suggestions are being taken for a good name for this asteroid.

APOD: March 27, 1997 - Comet Country
Explanation: Moonlight illuminates the landscape and lends a painterly quality to this beautiful photograph of the Absaroka mountain range, located along the Montana-Wyoming border near the USA's Yellowstone National Park. In the foreground lies the North Fork of the Shoshone River. The snow-covered peak rising 10,500 feet on the left is Jim Mountain, while Comet Hale-Bopp graces the western sky. Inspired by painters of the American West, like Fredric Remington, Charles Russell, and George "Dee" Smith, photographer Dewey Vanderhoff of Cody, Wyoming has taken advantage of the double blessings of a bright Moon and a bright comet to produce this breathtaking image.

APOD: January 20, 1997 - Earth Nears Asteroid Toutatis
Explanation: On November 29, 1996 the Earth came within 3.3 million miles of the asteroid Toutatis. Above is a computer simulated picture of this spectacle from the surface of Toutatis (a 2.5 degree field of view looking toward Earth). In Earth's sky, Toutatis appeared only as a faint object moving against a background of stars. Also known as Earth-crossing asteroid 4179, Toutatis is in an eccentric 4 year orbit which moves it from the asteroid belt between Mars and Jupiter to just inside Earth's orbit. When the Earth passed near it in 1992 Toutatis was imaged by radar and seen to be two irregularly shaped lumps, perhaps joined by a narrow neck. This bizarre object is up to 1.5 miles wide, 2.9 miles long, and is tumbling through space. In the year 2004, on September 29, the Earth will pass very near Toutatis, closing to within a million miles (4 times the Earth-Moon distance) - the closest approach predicted for any asteroid or comet between now and 2060. Studies of Toutatis and other Earth-crossing asteroids help reveal connections between the Solar System's meteorites,main-belt asteroids and comets. These wayward asteroids also offer tantalizing targets for robotic exploration and, over time, represent potential collision hazards for planet Earth!

APOD: January 10, 1997 - Eclipsed Moon in Infrared
Explanation: Last September's total lunar eclipse disappointed many observers in the Eastern and Midwestern US who were cursed with cloudy skies. However, the Midcourse Space Experiment (MSX) satellite had a spectacular view from Earth orbit and SPIRIT III, an onboard imaging infrared telescope, was used to repeatedly image the moon during the eclipse. Above is one of the images taken during the 70 minute totality, the Moon completely immersed in the Earth's shadow. Infrared light has wavelengths longer than visible light - human's can not see it but feel it as heat. The bright spots correspond to the warm areas on the lunar surface, dark areas are cooler. The brightest spot below and left of center is the crater Tycho, the dark region at the upper right is the Mare Crisium. The series of SPIRIT III images allow the determination of cooling curves for geologically different areas, exploring the physical properties of the Moon's surface.

APOD: October 10, 1996 - Triton: Neptune's Largest Moon
Explanation: Today marks the 150^th anniversary of the discovery of Triton. On October 10 1846, William Lassell was observing the newly discovered planet Neptune. He was attempting to confirm his observation, made just the previous week, that Neptune had a ring. But this time he discovered that Neptune had a satellite as well. Lassell soon proved the ring was product of his new telescope's distortion, but the satellite Triton remained. The above picture of Triton was taken in 1989 by the only spacecraft ever to pass Triton: Voyager 2. Voyager 2 found fascinating terrain, a thin atmosphere, and even evidence for ice volcanoes on this world of peculiar orbit and spin. Ironically, Voyager 2 also confirmed the existence of complete thin rings around Neptune - but these would have been quite invisible to Lassell!

APOD: September 29, 1996 - The X-Ray Moon
Explanation: This X-Ray image of the Moon was made by the orbiting Roentgen Observatory Satellite (ROSAT) in 1990. It shows three distinct regions: a bright X-ray sky, a bright part of the Moon, and a relatively dark part of the Moon. The bright X-ray sky is due to the diffuse cosmic X-ray background. The bright lunar crescent shines because it reflects X-rays emitted by the Sun. The dark lunar face is in shadow and so stands stands out from the relatively bright background - but, surprisingly it is not completely dark! Where do those X-rays from? They are currently thought to result from energetic particles from the solar wind bombarding the lunar surface.

APOD: April 11, 1996 - Unexpected X-rays from Comet Hyakutake
Explanation: The first X-rays ever detected from a comet were discovered from Comet Hyakutake with the ROSAT satellite on March 27th. The discovery is particularly surprising because there was little previous indication that comets emit any significant X-radiation. As the comet passed the Earth in late March, repeated observations with ROSAT also showed that the X-ray brightness changed over just a few hours. The crescent shape of the X-ray emission is also enigmatic. One possible explanation is that X-rays emitted from the Sun are absorbed by water in the comet's coma causing fluorescence. Another possible explanation involves interaction with the solar wind - fast moving particles streaming away from the Sun.

APOD: April 8, 1996 - Uranus's Moon Oberon: Impact World
Explanation: Oberon is the most distant and second largest moon of Uranus. Discovered by William Herschel in 1787, the properties of the world remained relatively unknown until the robot spacecraft Voyager 2 passed it during its flyby of Uranus in January 1986. Compared to Uranus' moons Ariel, Titania, and Miranda, Oberon is heavily cratered, and in this way resembles Umbriel. Like all of Uranus' large moons, Oberon is composed of roughly half ice and half rock. Note that Oberon has at least one large mountain, visible on the limb at the lower left, that rises 6 km off the surface.

APOD: April 7, 1996 - Uranus's Moon Umbriel: A Mysterious Dark World
Explanation: Why is Umbriel so dark? This dark moon reflects only half the light of other Uranus' moons such as Ariel. And what is that bright ring at the top? Unfortunately, nobody yet knows. These questions presented themselves when Voyager 2 passed this satellite of Uranus in January 1986. Voyager found an old surface with unusually large craters, and determined Umbriel's composition to be about half ice and half rock. Umbriel is the fourth largest and third most distant of Uranus' five large moons. Umbriel was discovered in 1851 by William Lassell.

APOD: April 3, 1996 - A Lucky Lunar Eclipse
Explanation: Tonight's full moon would normally washout the spectacle of Comet Hyakutake's lovely tail, even for those far from light polluted skies. Except that tonight comet observers are in luck - the dance of the planets calls for a total lunar eclipse! Lunar eclipses are caused when the Moon passes through the Earth's shadow. Although dimmed, the eclipsed Moon may not appear completely dark. Sunlight scattered into the Earth's shadow after passing around the planet's edge and through its dusty atmosphere can make the Moon take on dramatic shades of red during totality as demonstrated in the above photo of the November 1993 lunar eclipse. Tonight, totality begins at 6:26 p.m. EST and lasts about an hour and a half. Weather permitting, the eclipse will be visible for all those comet and moon watchers lucky enough to be on the Earth's nightside.

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!

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).

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.

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!

APOD: March 3, 1996 - Uranus' Moon Ariel: Valley World
Explanation: What formed Ariel's valleys? This question presented itself when Voyager 2 passed this satellite of Uranus in January 1986. Speculation includes that heating caused by the ancient tides of Uranus caused moonquakes and massive shifting of the moon's surface. In any event, a huge network of sunken valleys was found to cover this frozen moon, and some unknown material now coats the bottoms of many of these channels. Ariel is the second closest to Uranus outside of Miranda, and is composed of roughly half water ice and half rock. Ariel was discovered by William Lassell in 1851.

APOD: October 2, 1995 - Phobos: Doomed Moon of Mars
Explanation: Mars, the red planet named for the Roman god of war, has two tiny moons, Phobos and Deimos, whose names are derived from the Greek for Fear and Panic. These Martian moons may well be captured asteroids originating in the asteroid belt between Mars and Jupiter or perhaps from even more distant reaches of the Solar System. In this 1977 Viking orbiter image, the largest moon, Phobos, is seen to be a heavily cratered asteroid-like object. It is about 17 miles across and zips through the Martian sky completing an orbit in less than 8 hours. Phobos is doomed. It orbits so close to Mars, (about 3,600 miles above the surface compared to 250,000 miles for our Moon) that gravitational tidal forces are dragging it down. In 100 million years or so it could crash into the surface or be shattered by stress caused by the relentless tidal forces, the debris forming a ring around Mars.

APOD: August 31, 1995 - X-Raying the Moon
Explanation: Above is a picture of the Moon taken in X-rays by the Roentgen Observatory Satellite ROSAT in 1990. This famous picture shows three distinct regions: a bright X-ray sky, a bright part of the Moon, and a relatively dark part of the Moon. The bright X-ray region is exemplary of the mysterious X-ray background that is seen everywhere on the sky. The bright lunar crescent shines because it reflects X-rays emitted by the Sun. The dark lunar face is surprising because it is not completely dark, and its slight emission is thought to result from energetic particles from the solar wind striking the Moon.

APOD: June 30, 1995 - Ida and Dactyl: Asteroid and Moon
Explanation: An asteroid with a moon! The robot spacecraft Galileo whose primary mission is to explore the Jupiter system, has encountered and photographed two asteroids during its long journey to Jupiter. The second asteroid it photographed, called Ida, was discovered to have a moon which appears as a small dot to the right of Ida in this picture. The tiny moon, named Dactyl, is about one mile across, while the potato shaped Ida measures about 36 miles long and 14 miles wide. Dactyl is the first moon of an asteroid ever discovered. The names Ida and Dactyl are based on characters in Greek mythology.