Astronomy Picture of the Day Search Results for "Neptune"

APOD: 2023 August 19 - Ringed Ice Giant Neptune
Explanation: Ringed ice giant Neptune lies near the center of this sharp near-infrared image from the James Webb Space Telescope. The dim and distant world is the farthest planet from the Sun, about 30 times farther away than planet Earth. But in the stunning Webb view, the planet's dark and ghostly appearance is due to atmospheric methane that absorbs infrared light. High altitude clouds that reach above most of Neptune's absorbing methane easily stand out in the image though. Coated with frozen nitrogen, Neptune's largest moon Triton is brighter than Neptune in reflected sunlight, seen at the upper left sporting the Webb telescope's characteristic diffraction spikes. Including Triton, seven of Neptune's 14 known moons can be identified in the field of view. Neptune's faint rings are striking in this space-based planetary portrait. Details of the complex ring system are seen here for the first time since Neptune was visited by the Voyager 2 spacecraft in August 1989.

APOD: 2023 June 25 – Lightning on Jupiter
Explanation: Does lightning occur only on Earth? No. Spacecraft in our Solar System have detected lightning on other planets, including Mars, Jupiter and Saturn, and lightning is likely on Venus, Uranus, and Neptune. Lightning is a sudden rush of electrically charged particles from one location to another. On Earth, drafts of colliding ice and water droplets usually create lightning-generating charge separation, but what happens on Jupiter? Images and data from NASA's Jupiter-orbiting Juno spacecraft bolster previous speculation that Jovian lightning is also created in clouds containing water and ice. In the featured Juno photograph, an optical flash was captured in a large cloud vortex near Jupiter's north pole. During the next few months, Juno will perform several close sweeps over Jupiter's night side, likely allowing the robotic probe to capture more data and images of Jovian lightning.

APOD: 2023 May 27 - Crescent Neptune and Triton
Explanation: Gliding through the outer Solar System, in 1989 the Voyager 2 spacecraft looked toward the Sun to find this view of most distant planet Neptune and its moon Triton together in a crescent phase. The elegant image of ice-giant planet and largest moon was taken from behind just after Voyager's closest approach. It could not have been taken from Earth because the most distant planet never shows a crescent phase to sunward eyes. Heading for the heliopause and beyond, the spacecraft's parting vantage point also robs Neptune of its familiar blue hue.

APOD: 2023 May 11 - Fomalhaut's Dusty Debris Disk
Explanation: Fomalhaut is a bright star, a 25 light-year voyage from planet Earth in the direction of the constellation Piscis Austrinus. Astronomers first noticed Fomalhaut's excess infrared emission in the 1980s. Space and ground-based telescopes have since identified the infrared emission's source as a disk of dusty debris, evidence for a planetary system surrounding the hot, young star. But this sharp infrared image from the James Webb Space Telescope's MIRI camera reveals details of Fomalhaut's debris disk never before seen, including a large dust cloud in the outer ring that is possible evidence for colliding bodies, and an inner dust disk and gap likely shaped and maintained by embedded but unseen planets. An image scale bar in au or astronomical units, the average Earth-Sun distance, appears at the lower left. Fomalhaut's outer circumstellar dust ring lies at about twice the distance of our own Solar System's Kuiper Belt of small icy bodies and debris beyond the orbit of Neptune.

APOD: 2023 January 2 – After Sunset Planet Parade
Explanation: Look up tonight and see a whole bunch of planets. Just after sunset, looking west, planets Venus, Saturn, Jupiter and Mars will all be simultaneously visible. Listed west to east, this planetary lineup will have Venus nearest the horizon, but setting shortly after the Sun. It doesn't matter where on Earth you live because this early evening planet parade will be visible through clear skies all around the globe. Taken late last month, the featured image captured all of these planets and more: the Moon and planet Mercury were also simultaneously visible. Below visibility were the planets Neptune and Uranus, making this a nearly all-planet panorama. In the foreground are hills around the small village of Gökçeören, Kaş, Turkey, near the Mediterranean coast. Bright stars Altair, Fomalhaut, and Aldebaran are also prominent, as well as the Pleiades star cluster. Venus will rise higher in the sky at sunset as January continues, but Saturn will descend.

APOD: 2022 September 23 - Ringed Ice Giant Neptune
Explanation: Ringed, ice giant Neptune lies near the center of this sharp near-infrared image from the James Webb Space Telescope. The dim and distant world is the farthest planet from the Sun, about 30 times farther away than planet Earth. But in the stunning Webb view the planet's dark and ghostly appearance is due to atmospheric methane that absorbs infrared light. High altitude clouds that reach above most of Neptune's absorbing methane easily stand out in the image though. Coated with frozen nitrogen, Neptune's largest moon Triton is brighter than Neptune in reflected sunlight and is seen at upper left sporting the Webb's characteristic diffraction spikes. Including Triton, seven of Neptune's 14 known moons can be identified in the field of view. Neptune's faint rings are striking in this new space-based planetary portrait. Details of the complex ring system are seen here for the first time since Neptune was visited by the Voyager 2 spacecraft in August 1989.

APOD: 2022 July 1 - The Solar System's Planet Trails
Explanation: Stars trail through a clear morning sky in this postcard from a rotating planet. The timelapse image is constructed from consecutive exposures made over nearly three hours with a camera fixed to a tripod beside the Forbidden City in Beijing, China on June 24. Arcing above the eastern horizon after the series of exposures began, a waning crescent Moon left the brightest streak and watery reflection. On that date the planets of the Solar System were also lined up along the ecliptic and left their own trails before sunrise. Saturn was first to rise on that morning and the ringed planet's trail starts close to the top right edge, almost out of the frame. Innermost planet Mercury rose only just before the Sun though. It left the shortest trail, visible against the twilight near the horizon at the far left. Uranus and Neptune are faint and hard to find, but mingled with the star trails the Solar System's planet trails are all labeled in the scene.

APOD: 2022 June 29 - Solar System Family Portrait
Explanation: Yes, but have you ever seen all of the planets at once? A rare roll-call of planets has been occurring in the morning sky for much of June. The featured fisheye all-sky image, taken a few mornings ago near the town of San Pedro de Atacama in Chile, caught not only the entire planet parade, but the Moon between Mars and Venus. In order, left to right along the ecliptic plane, members of this Solar System family portrait are Earth, Saturn, Neptune, Jupiter, Mars, Uranus, Venus, Mercury, and Earth. To emphasize their locations, Neptune and Uranus have been artificially enhanced. The volcano just below Mercury is Licancabur. In July, Mercury will move into the Sun's glare but reappear a few days later on the evening side. Then, in August, Saturn will drift past the direction opposite the Sun and so become visible at dusk instead of dawn. The next time that all eight planets will be simultaneously visible in the evening sky will be in 2122.

APOD: 2022 June 25 - Planets of the Solar System
Explanation: Simultaneous images from four cameras were combined to construct this atmospheric predawn skyscape. The cooperative astro-panorama captures all the planets of the Solar System, just before sunrise on June 24. That foggy morning found innermost planet Mercury close to the horizon but just visible against the twilight, below and left of brilliant Venus. Along with the waning crescent Moon, the other bright naked-eye planets, Mars, Jupiter, and Saturn lie near the ecliptic, arcing up and to the right across the wide field of view. Binoculars would have been required to spot the much fainter planets Uranus and Neptune, though they also were along the ecliptic in the sky. In the foreground are excavations at an ancient Roman villa near Marina di San Nicola, Italy, planet Earth.

APOD: 2021 July 25 - Crescent Neptune and Triton
Explanation: Gliding silently through the outer Solar System, the Voyager 2 spacecraft camera captured Neptune and Triton together in crescent phase. The elegant picture of the ice-giant planet and its cloudy moon was taken from behind just after closest approach in 1989. It could not have been taken from Earth because Neptune never shows a crescent phase to sunward Earth. The unusual vantage point also robs Neptune of its familiar blue hue, as sunlight seen from here is scattered forward, and so is reddened like the setting Sun. Neptune is smaller but more massive than Uranus, has several dark rings, and emits more light than it receives from the Sun.

APOD: 2020 May 19 - Posters of the Solar System
Explanation: Would you like a NASA astronomy-exploration poster? You are just one page-print away. Any of the panels you see on the featured image can appear on your wall. Moreover, this NASA page has, typically, several more posters of each of the Solar System objects depicted. These posters highlight many of the places humanity, through NASA, has explored in the past 50 years, including our Sun, and planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Moons of Jupiter that have been posterized include Europa, Ganymede, Callisto, and Io, while moons of Saturn that can be framed include Enceladus and Titan. Images of Pluto, Ceres, comets and asteroids are also presented, while six deep space scenes -- well beyond our Solar System -- can also be prominently displayed. If you lack wall space or blank poster sheets don't despair -- you can still print many of these out as trading cards.

APOD: 2020 January 1 - Betelgeuse Imagined
Explanation: Why is Betelgeuse fading? No one knows. Betelgeuse, one of the brightest and most recognized stars in the night sky, is only half as bright as it used to be only five months ago. Such variability is likely just normal behavior for this famously variable supergiant, but the recent dimming has rekindled discussion on how long it may be before Betelgeuse does go supernova. Known for its red color, Betelgeuse is one of the few stars to be resolved by modern telescopes, although only barely. The featured artist's illustration imagines how Betelgeuse might look up close. Betelgeuse is thought to have a complex and tumultuous surface that frequently throws impressive flares. Were it to replace the Sun (not recommended), its surface would extend out near the orbit of Jupiter, while gas plumes would bubble out past Neptune. Since Betelgeuse is about 700 light years away, its eventual supernova will not endanger life on Earth even though its brightness may rival that of a full Moon. Astronomers -- both amateur and professional -- will surely continue to monitor Betelgeuse as this new decade unfolds.

APOD: 2019 October 16 - BHB2007: A Baby Binary Star in Formation
Explanation: How do binary stars form? To help find out, the Atacama Large Millimeter Array (ALMA) recently captured one of the highest resolution images yet taken of a binary star system in formation. Most stars are not alone -- they typically form as part of a multiple star systems where star each orbits a common center of gravity. The two bright spots in the featured image are small disks that surround the forming proto-stars in [BHB2007] 11, while the surrounding pretzel-shaped filaments are gas and dust that have been gravitationally pulled from a larger disk. The circumstellar filaments span roughly the radius of the orbit of Neptune. The BHB2007 system is a small part of the Pipe Nebula (also known as Barnard 59), a photogenic network of dust and gas that protrudes from Milky Way's spiral disk in the constellation of Ophiuchus. The binary star formation process should be complete within a few million years.

APOD: 2019 February 14 - Solar System Family Portrait
Explanation: On Valentine's Day in 1990, cruising four billion miles from the Sun, the Voyager 1 spacecraft looked back one last time to make this first ever Solar System family portrait. The complete portrait is a 60 frame mosaic made from a vantage point 32 degrees above the ecliptic plane. In it, Voyager's wide angle camera frames sweep through the inner Solar System at the left, linking up with gas giant Neptune, the Solar System's outermost planet, at the far right. Positions for Venus, Earth, Jupiter, Saturn, Uranus, and Neptune are indicated by letters, while the Sun is the bright spot near the center of the circle of frames. The inset frames for each of the planets are from Voyager's narrow field camera. Unseen in the portrait are Mercury, too close to the Sun to be detected, and Mars, unfortunately hidden by sunlight scattered in the camera's optical system. Closer to the Sun than Neptune at the time, small, faint Pluto's position was not covered.

APOD: 2018 July 28 - One Night, One Telescope, One Camera
Explanation: Taken on the same night, from the same place, with the same telescope and camera, these postcards from our Solar System are shown at the same scale to provide an interesting comparison of apparent sizes. Spanning about half a degree in planet Earth's sky, the Moon is a stitched mosaic of six images. The others are the result of digitally stacked frames or simple single exposures, with the real distances to the objects indicated along the bottom of each insert. Most of the Solar System's planets with their brighter moons, and Pluto were captured during the telescopic expedition, but elusive Mercury was missed because of clouds near the horizon. The International Space Station was successfully hunted, though. The night was July 21st. Telescope and camera were located at the Centro Astronomico de Tiedra Observatory in Spain.

APOD: 2017 February 25 - All Planets Panorama
Explanation: For 360 degrees, a view along the plane of the ecliptic is captured in this remarkable panorama, with seven planets in a starry sky. The mosaic was constructed using images taken during January 24-26, from Nacpan Beach, El Nido in Palawan, Philippines. It covers the eastern horizon (left) in dark early morning hours and the western horizon in evening skies. While the ecliptic runs along the middle traced by a faint band of zodiacal light, the Milky Way also cuts at angles through the frame. Clouds and the Moon join fleeting planet Mercury in the east. Yellowish Saturn, bright star Antares, and Jupiter lie near the ecliptic farther right. Hugging the ecliptic near center are Leo's alpha star Regulus and star cluster M44. The evening planets gathered along the ecliptic above the western horizon, are faint Uranus, ruddy Mars, brilliant Venus, and even fainter Neptune. A well labeled version of the panorama can be viewed by sliding your cursor over the picture, or just following this link.

APOD: 2017 February 11 - Solar System Portrait
Explanation: On Valentine's Day in 1990, cruising four billion miles from the Sun, the Voyager 1 spacecraft looked back one last time to make this first ever Solar System family portrait. The complete portrait is a 60 frame mosaic made from a vantage point 32 degrees above the ecliptic plane. In it, Voyager's wide angle camera frames sweep through the inner Solar System at the left, linking up with gas giant Neptune, the Solar System's outermost planet, at the far right. Positions for Venus, Earth, Jupiter, Saturn, Uranus, and Neptune are indicated by letters, while the Sun is the bright spot near the center of the circle of frames. The inset frames for each of the planets are from Voyager's narrow field camera. Unseen in the portrait are Mercury, too close to the Sun to be detected, and Mars, unfortunately hidden by sunlight scattered in the camera's optical system. Closer to the Sun than Neptune at the time, small, faint Pluto's position was not covered.

APOD: 2017 January 13 - When Mars met Neptune
Explanation: On January 1, a Mars-assisted viewing opportunity allowed binocular-equipped skygazers to cross an ice giant off their life list. Remarkably, the line-of-sight to the bright Red Planet could guide you to within 0.02 degrees of a faint, pale Neptune in Earth's night skies. Taken within 3 hours of their closest conjunction, these panels capture the odd couple's appearance in skies over Brisbane, Australia. A wide field view includes the new year's slender crescent moon near the western horizon and Venus as the brilliant evening star. Mars and Neptune are indicated at the upper right. The two inset magnified views were taken with the same telephoto lens and so do show the Mars-Neptune conjunction and the apparent size of the crescent moon at the same scale. This week Neptune hangs out near Venus on the western sky.

APOD: 2016 November 30 - Milky Way over Shipwreck
Explanation: What happened to this ship? It was carried aground by a giant storm that struck the coast of Argentina in 2002. The pictured abandoned boat, dubbed Naufragio del Chubasco, wrecked near the nearly abandoned town of Cabo Raso (population: 1). The rusting ship provides a picturesque but perhaps creepy foreground for the beautiful sky above. This sky is crowned by the grand arch of our Milky Way and features galaxies including the Large and Small Magellanic Clouds, stars including Canopus and Altair, planets including Mars and Neptune, and nebulas including the Lagoon, Carina, and the Coal Sack. The mosaic was composed from over 80 images taken in early September. A 360-degree interactive panoramic version of this image is also available. The adventurous astrophotographer reports that the creepiest part of taking this picture was not the abandoned ship, but the unusual prevalence of black and hairy caterpillars.

APOD: 2016 June 22 - Cirrus over Paris
Explanation: What's that over Paris? Cirrus. Typically, cirrus clouds appear white or gray when reflecting sunlight, can appear dark at sunset (or sunrise) against a better lit sky. Cirrus are among the highest types of clouds and are usually thin enough to see stars through. Cirrus clouds may form from moisture released above storm clouds and so may herald the arrival of a significant change in weather. Conversely, cirrus clouds have also been seen on Mars, Jupiter, Saturn, Titan, Uranus, and Neptune. The featured image was taken two days ago from a window in District 15, Paris, France, Earth. The brightly lit object on the lower right is, of course, the Eiffel Tower.

APOD: 2015 June 22 - New Horizons
Explanation: In three weeks, the robotic New Horizons spacecraft will reach Pluto. As the featured video makes clear, though, humanity has been on an unprecedented epoch of robotic exploration of our Solar System's planets for the past half century. The video highlights artistic illustrations of Mariner 2 flying by Venus in 1962, Mariner 4 flying past Mars in 1965, Pioneer 10 flying past Jupiter in 1973, Mariner 10 flying past Mercury in 1974, Pioneer 11 flying past Saturn in 1979, and Voyager 2 flying past Uranus in 1986 and then Neptune in 1989. Next is a hypothetical sequence depicting New Horizons flying past Pluto next month. Assuming things work as planned, dwarf planet Pluto will then become the farthest world yet explored by humans. Of course, these Pluto illustrations are only a guess. How Pluto and its moons will really look may be a mixture of familiar things, such as craters, and unfamiliar things, such as …

APOD: 2015 May 27 - Approaching Pluto
Explanation: Here comes Pluto. NASA's robotic New Horizons spacecraft is now beyond the orbit of Neptune and closing fast on the Solar System's most famous unexplored world. The featured time lapse video shows Pluto and Pluto's largest moon, Charon, orbiting their common center of mass in 13 frames taken from April 12 to April 18. Although blurry, images in the video now rival even the best images of Pluto yet taken from Earth. New Horizons remains on schedule to zoom past the distant dwarf planet on July 14.

APOD: 2015 February 15 - Two Hours Before Neptune
Explanation: Two hours before closest approach to Neptune in 1989, the Voyager 2 robot spacecraft snapped this picture. Clearly visible for the first time were long light-colored cirrus-type clouds floating high in Neptune's atmosphere. Shadows of these clouds can even be seen on lower cloud decks. Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible. Neptune's blue color therefore comes from smaller amounts of atmospheric methane, which preferentially absorbs red light. Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometers per hour. Speculation holds that diamonds may be created in the dense hot conditions that exist under the cloud tops of Uranus and Neptune. Twenty-six years later, NASA's New Horizons is poised to be the first spacecraft to zoom past Pluto this July.

APOD: 2015 February 14 - Solar System Portrait
Explanation: On another Valentine's Day 25 years ago, cruising four billion miles from the Sun, the Voyager 1 spacecraft looked back one last time to make this first ever Solar System family portrait. The complete portrait is a 60 frame mosaic made from a vantage point 32 degrees above the ecliptic plane. In it, Voyager's wide angle camera frames sweep through the inner Solar System at the left, linking up with gas giant Neptune, the Solar System's outermost planet, at the far right. Positions for Venus, Earth, Jupiter, Saturn, Uranus, and Neptune are indicated by letters, while the Sun is the bright spot near the center of the circle of frames. The inset frames for each of the planets are from Voyager's narrow field camera. Unseen in the portrait are Mercury, too close to the Sun to be detected, and Mars, unfortunately hidden by sunlight scattered in the camera's optical system. Closer to the Sun than Neptune at the time, small, faint Pluto's position was not covered.

APOD: 2014 August 26 - Flying Past Neptune's Moon Triton
Explanation: What would it look like to fly past Triton, the largest moon of planet Neptune? Only one spacecraft has ever done this -- and now, for the first time, images of this dramatic encounter have been gathered into a movie. On 1989 August 25, the Voyager 2 spacecraft shot through the Neptune system with cameras blazing. Triton is slightly smaller than Earth's Moon but has ice volcanoes and a surface rich in frozen nitrogen. The first sequence in the video shows Voyager's approach to Triton, which, despite its unusual green tint, appears in approximately true color. The mysterious terrain seen under the spacecraft soon changed from light to dark, with the terminator of night soon crossing underneath. After closest approach, Voyager pivoted to see the departing moon, now visible as a diminishing crescent. Next July, assuming all goes well, the robotic New Horizons spacecraft will make a similar flight past Pluto, an orb of similar size to Triton.

APOD: 2014 May 15 - Voyager's Neptune
Explanation: Cruising through the outer solar system, the Voyager 2 spacecraft made its closest approach to Neptune on August 25, 1989, the only spacecraft to visit the most distant ice giant planet. Based on the images recorded during its close encounter and in the following days, this inspired composited scene covers the dim outer planet, largest moon Triton, and faint system of rings. From just beyond Neptune's orbit, the interplanetary perspective looks back toward the Sun, capturing the planet and Triton as thin sunlit crescents. Cirrus clouds and a dark band circle Neptune's south polar region, with a cloudy vortex above the pole itself. Parts of the very faint ring system along with the three bright ring arcs were first imaged by Voyager during the fly-by, though the faintest segments are modeled in this composited picture. Spanning 7.5 degrees, the background starfield is composed from sky survey data centered on the constellation Camelopardalis, corresponding to the outbound Voyager's view of the magnificent Neptunian system.

APOD: 2014 January 16 - Despina, Moon of Neptune
Explanation: Despina is a tiny moon of Neptune. A mere 148 kilometers across, diminutive Despina was discovered in 1989, in images from the Voyager 2 spacecraft taken during its encounter with the solar system's most distant gas giant planet. But looking through the Voyager 2 data 20 years later, amateur image processor and philosophy professor Ted Stryk discovered something no one had recognized before -- images that show the shadow of Despina in transit across Neptune's blue cloud tops. His composite view of Despina and its shadow is composed of four archival frames taken on August 24, 1989, separated by nine minutes. Despina itself has been artificially brightened to make it easier to see. In ancient Greek mythology, Despina is a daughter of Poseidon, the Roman god Neptune.

APOD: 2013 September 10 - Extrasolar Super Earth Gliese 1214b Might Hold Water
Explanation: Might this distant planet hold water? Actually, given how close Gliese 1214b is to its parent star, any water, if it exists, would surely be in the form of steam. In the above artist's illustration, the super-Earth Gliese 1214b is imagined passing in front of its parent star, creating a mini-eclipse that alerted humanity to its presence. Gliese 1214b, also designated GJ 1214b, has been designated a super-Earth because it is larger than the Earth but smaller a planet like Neptune. The entire Gliese 1214 planetary system is of the closest known systems to our Sun, located only 42 light years away. The parent star, Gliese 1214 is a slightly smaller and cooler version of our Sun. Recent observations from the Subaru telescope in Hawaii found very little scattering of blue light from the parent star by the planet. This appears most consistent with a planet that has a watery atmosphere -- although it is still possible that the super-Earth has clouds so thick that little of any color of light was scattered. Detecting water on exoplanets is important partly because most lifeforms on Earth need water to survive.

APOD: 2013 April 14 - Crescent Neptune and Triton
Explanation: Gliding silently through the outer Solar System, the Voyager 2 spacecraft camera captured Neptune and Triton together in crescent phase in 1989. The elegant picture of the gas giant planet and its cloudy moon was taken from behind just after closest approach. It could not have been taken from Earth because Neptune never shows a crescent phase to sunward Earth. The unusual vantage point also robs Neptune of its familiar blue hue, as sunlight seen from here is scattered forward, and so is reddened like the setting Sun. Neptune is smaller but more massive than Uranus, has several dark rings, and emits more light than it receives from the Sun.

APOD: 2013 February 14 - Solar System Portrait
Explanation: On another Valentine's Day (February 14, 1990), cruising four billion miles from the Sun, the Voyager 1 spacecraft looked back to make this first ever family portrait of our Solar System. The complete portrait is a 60 frame mosaic made from a vantage point 32 degrees above the ecliptic plane. In it, Voyager's wide angle camera frames sweep through the inner Solar System at the left, linking up with gas giant Neptune, at the time the Solar System's outermost planet, at the far right. Positions for Venus, Earth, Jupiter, Saturn, Uranus, and Neptune are indicated by letters, while the Sun is the bright spot near the center of the circle of frames. The inset frames for each of the planets are from Voyager's narrow field camera. Unseen in the portrait are Mercury, too close to the Sun to be detected, and Mars, unfortunately hidden by sunlight scattered in the camera's optical system. Small, faint Pluto's position was not covered.

APOD: 2012 May 9 - Shuttle Enterprise Over New York
Explanation: What's that in the background? Two famous New York City icons stand tall in the above photo taken last week. On the left looms the Statue of Liberty, a universal symbol of freedom, while on the right rises the Empire State Building, now the second largest building in the city. What's unique about this once-in-a-lifetime photograph, though, is the third icon that appears to Lady Liberty's left. High in the air and far in the background flies the space shuttle Enterprise -- perched atop a 747 jet -- on the way to its new home. New Yorkers and visitors to the Big Apple can visit the test space shuttle at the Intrepid Sea, Air & Space Museum on the West Side of Manhattan starting July 19.

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

APOD: 2011 August 27 - Hurricane Irene Forms
Explanation: How does a hurricane form? Although a complete picture is still being researched, insight into this process might be gleaned by watching the above time lapse movie of the formation of Hurricane Irene, a large storm system currently threatening the eastern seaboard of the USA. Starting as a slight pressure difference visible as nondescript clouds on the lower right, Hurricane Irene is shown growing into large spiraling storm system of low pressure off the coast of South Carolina. A hurricane is powered by evaporating ocean water, and so typically gains strength over warm water and loses strength over land. Besides Earth, other planets that have hurricane-like storm systems include Venus, Saturn, Jupiter, Uranus, and Neptune. Much remains unknown about hurricanes and cyclones, including the exact path they will take.

APOD: 2011 July 14 - Neptune: Once Around
Explanation: Neptune rotates once on its axis in about 16 hours. So, spaced about 4 hours apart these 4 images of the solar system's most distant gas giant cover one Neptune day. Recorded by the Hubble Space Telescope in late June they combine exposures made with visible and near-infrared filters to show high-altitude clouds composed of methane ice crystals against the planet's normally blue cloud tops. Because Neptune's axis of rotation is tilted to its orbital plane by 29 degrees, compared to Earth's 23.5 degrees, Neptune experiences seasons analogous to Earth's. As early summer comes to Neptune's southern hemisphere and winter to the north, Hubble observations have shown cloud activity shifting to the northern hemisphere. In fact the progression of Neptune's seasons has come around once since its position was predicted by French mathematician Urbain Le Verrier and British mathematician John Couch Adams, and the planet was subsequently discovered by German astronomer Johann Galle on September 23, 1846. With an orbital period of approximately 165 years, this week on July 12, Neptune has been once around the Sun since its discovery date.

APOD: 2011 February 23 - The Solar System from MESSENGER
Explanation: If you looked out from the center of the Solar System, what would you see? Nearly such a view was taken recently from the MESSENGER spacecraft currently orbiting the Sun from the distance of Mercury. The Sun's planets all appear as points of light, with the closest and largest planets appearing the brightest. The planets all appear to orbit in the same direction and are (nearly) confined to the same great circle around the sky -- the ecliptic plane. Mercury, Venus, Earth, Mars, Jupiter, and Saturn are all visible in the above horizontally compressed image, while the positions of Uranus and Neptune are labeled even though they are too faint to make out. Pluto, which has had its planetary status recently called into question, is much too faint to see. Earth's Moon is visible, however, as are the Galilean moons of Jupiter. The above image is the reverse of one taken from the outside of the Solar System in 1990 by Voyager 1. MESSENGER, which has flown by Mercury three times now, is on schedule to enter orbit around the Solar System's innermost planet next month.

APOD: 2010 August 25 - HD 10180: Richest Yet Planetary System Discovered
Explanation: Do other rich planetary systems exist? Our Solar System has the most planets of any known star, most probably because it is so hard to detect planets around other stars. Sensitive measurements, though, have now uncovered a slight but complex wobble of the Sun-like star HD 10180 indicating that it has at least five planets and possibly more, making it the richest extra-solar planetary system yet known. HD 10180's planets were discovered in years of data using the sensitive HARPS spectrograph attached to the ESO's 3.6-meter telescope in La Silla, Chile. The planetary system appears quite different than our Solar System, since all of HD 10180's discovered planets have Neptune-like masses but orbit inside the distance of Mars. An artist's depiction of flying into this system is shown in the above video. In the future, more sensitive data taken over longer time periods may extend the star-wobble detection technique into the realm of uncovering more distant and more Earth-like planets.

APOD: 2010 August 8 - Two Hours Before Neptune
Explanation: Two hours before closest approach to Neptune in 1989, the Voyager 2 robot spacecraft snapped this picture. Clearly visible for the first time were long light-colored cirrus-type clouds floating high in Neptune's atmosphere. Shadows of these clouds can even be seen on lower cloud decks. Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible. Neptune's blue color therefore comes from smaller amounts of atmospheric methane, which preferentially absorbs red light. Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometers per hour. Speculation holds that diamonds may be created in the dense hot conditions that exist under the cloud tops of Uranus and Neptune.

APOD: 2010 May 6 - Northern and Southern Owls
Explanation: Captured in colorful telescopic portraits, two cosmic owls glare back toward planet Earth in this intriguing comparison of planetary nebulae. On the left is M97 in the constellation Ursa Major, also known in the northern hemisphere as the Owl Nebula. On the right is its visual counterpart, the southern Owl Nebula in the constellation Hydra, cataloged as PLN 283+25.1. Both nebulae are remarkably symmetric, round, and similar in size, some 2 light-years across or about 2,000 times the diameter of Neptune's orbit. Planetary nebulae are produced during a final phase in the life of a sun-like star, an example of the fate that awaits the Sun when it runs out of nuclear fuel in another 5 billion years. Both images were made using narrowband filters and different color mappings. The image of the southern Owl also includes broadband data, bringing out the surrounding star field.

APOD: 2009 December 13 - Crescent Neptune and Triton
Explanation: Gliding silently through the outer Solar System, the Voyager 2 spacecraft camera captured Neptune and Triton together in crescent phase in 1989. The above picture of the gas giant planet and its cloudy moon was taken from behind just after closest approach. It could not have been taken from Earth because Neptune never shows a crescent phase to sunward Earth. The unusual vantage point also robs Neptune of its familiar blue hue, as sunlight seen from here is scattered forward, and so is reddened like the setting Sun. Neptune is smaller but more massive than Uranus, has several dark rings, and emits more light than it receives from the Sun.

APOD: 2009 September 3 - Despina, Moon of Neptune
Explanation: Despina is a tiny moon of Neptune. A mere 148 kilometers across, diminutive Despina was discovered in 1989, in images from the Voyager 2 spacecraft taken during its encounter with the solar system's most distant gas giant planet. But looking through the Voyager 2 data 20 years later, amateur image processor (and philosophy professor) Ted Stryk discovered something no one had recognized before -- images that show the shadow of Despina in transit across Neptune's blue cloud tops. His composite view of Despina and its shadow is composed of four archival frames taken on August 24, 1989, separated by nine minutes. Despina itself has been artificially brightened to make it easier to see. In ancient Greek mythology, Despina is a daughter of Poseidon (the Roman god Neptune).

APOD: 2009 May 15 - M97: The Owl Nebula
Explanation: The Owl Nebula is perched in the sky about 2,600 light-years away toward the bottom of the Big Dipper's bowl. Also cataloged as M97, the 97th object in Messier's well-known list, its round shape along with the placement of two large, dark "eyes" do suggest the face of a staring owl. One of the fainter objects in Messier's catalog, the Owl Nebula is a planetary nebula, the glowing gaseous envelope shed by a dying sun-like star as it runs out of nuclear fuel. In fact, the Owl Nebula offers an example of the fate of our Sun as it runs out of fuel in another 5 billion years. As we see it, the nebula spans over 2 light-years making it roughly 2,000 times the diameter of Neptune's orbit. Beautiful to look at, this color image shows impressive details within the cosmic owl. The composite includes images made through narrow-band filters for a total of 24 hours of exposure time.

APOD: 2009 March 25 - Orcus of the Outer Solar System
Explanation: A newly discovered object in the outer Solar System moves like an anti-Pluto. 90482 Orcus was first discovered in 2004 and is slightly smaller than Pluto, although still one of the largest Kuiper belt objects known. Orcus may one day have the same IAU designation as Pluto: a dwarf planet. Orcus and Pluto have similar orbits: each achieves nearly the same maximum and minimum distances from the Sun, each orbits on a similarly shaped ellipse, and each orbital ellipse is tilted toward the other planets' orbital ellipse by roughly the same angle. The great mass of Neptune causes each to circle the Sun twice for every three Neptune orbits. Orcus is like an anti-Pluto, however, because the two objects always remain across the Solar System from each other. Orcus can be found as the spot near the center of these discovery frames moving slightly down from the top. Until the end of next week, the discoverers of Orcus ask for your help in naming its newly discovered moon.

APOD: 2009 January 2 - Alpine Conjunction
Explanation: Did you see it? The last conjunction of Moon and bright planets in 2008 featured a young crescent Moon and brilliant Venus in the west after sunset on December 31st. Seen here in dark, clear, mountain air from Mönichkirchen, Austria, are the two celestial beacons that dominate planet Earth's night sky. That pair was hard to miss, but skygazers watching lower along the western horizon in early twilight might also have glimpsed a pairing of Jupiter and Mercury as they both wandered closer to the Sun in the sky at year's end. Still, while this single, 5 second long exposure seriously overexposes the Moon's sunlit crescent, it does capture another planet not visible to the unaided eye. The tiny pinprick of light just above the photographer's head in the picture is the distant planet Neptune.

APOD: 2008 November 17 - HR 8799: Discovery of a Multi planet Star System
Explanation: How common are planetary systems like our own Solar System? In the twelve years previous to 2008, over 300 candidate planetary systems have been found orbiting nearby stars. None, however, were directly imaged, few showed evidence for multiple planets, and many had a Jupiter-sized planet orbiting inside the orbit of Mercury. Last week, however, together with recent images of Fomalhaut b, the above picture was released showing one of first confirmed images of planets orbiting a distant Sun-like star. HR 8799 has a mass about 1.5 times that of our own Sun, and lies about 130 light years from the Sun -- a distance similar to many stars easily visible in the night sky. Pictured above, a 10-meter Keck telescope in Hawaii captured in infrared light three planets orbiting an artificially obscured central star. The 8-meter Gemini North telescope captured a similar image. Each planet likely contains several times the mass of Jupiter, but even the innermost planet, labelled d, has an orbital radius near the equivalent of the Sun- Neptune distance. Although the HR 8799 planetary system has significant differences with our Solar System, it is a clear demonstration that complex planetary systems exist, systems that could conceivable contain an Earth-like planet.

APOD: 2008 September 10 - The Anthe Arc around Saturn
Explanation: What created this unusual partial ring around Saturn? Discovered last year, the arc was captured in clear detail only two months ago by the Saturn-orbiting Cassini spacecraft. Since the arc occupies the same orbit as the small moon Anthe, a leading hypothesis holds that the arc was created by, and is replenished by, meteor impacts on Anthe. Similar arcs have been previously discovered, including an arc associated with the small Saturnian moon Methone, one arc related to Saturn's G ring, and several arcs orbiting Neptune. Pictured above, Anthe, only two kilometers across, is seen as the bright point near the top of the Anthe arc. The Anthe arc was imaged by the robotic space probe as it swooped to within 1.5 million kilometers of the small moon.

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: 2006 September 18 - Eris: The Largest Known Dwarf Planet
Explanation: Is Pluto the largest dwarf planet? No! Currently, the largest known dwarf planet is (136199) Eris, renamed last week from 2003 UB313. Eris is just slightly larger than Pluto, but orbits as far as twice Pluto's distance from the Sun. Eris is shown above in an image taken by a 10-meter Keck Telescope from Hawaii, USA. Like Pluto, Eris has a moon, which has been officially named by the International Astronomical Union as (136199) Eris I (Dysnomia). Dysnomia is visible above just to the right of Eris. Dwarf planets Pluto and Eris are trans-Neptunian objects that orbit in the Kuiper belt of objects past Neptune. Eris was discovered in 2003, and is likely composed of frozen water-ice and methane. Since Pluto's recent demotion by the IAU from planet to dwarf planet status, Pluto has recently also been given a new numeric designation: (134340) Pluto. Currently, the only other officially designated "dwarf planet" is (1) Ceres.

APOD: 2006 August 28 - Eight Planets and New Solar System Designations
Explanation: How many planets are in the Solar System? This popular question now has a new formal answer according the International Astronomical Union (IAU): eight. Last week, the IAU voted on a new definition for planet and Pluto did not make the cut. Rather, Pluto was re-classified as a dwarf planet and is considered as a prototype for a new category of trans-Neptunian objects. The eight planets now recognized by the IAU are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Solar System objects now classified as dwarf planets are: Ceres, Pluto, and the currently unnamed 2003 UB313. Planets, by the new IAU definition, must be in orbit around the sun, be nearly spherical, and must have cleared the neighborhood around their orbits. The demotion of Pluto to dwarf planet status is a source of continuing dissent and controversy in the astronomical community.

APOD: 2006 June 18 - Crescent Neptune and Triton
Explanation: Gliding silently through the outer Solar System, the Voyager 2 spacecraft camera captured Neptune and Triton together in crescent phase in 1989. The above picture of the gas giant planet and its cloudy moon was taken from behind just after closest approach. It could not have been taken from Earth because Neptune never shows a crescent phase to sunward Earth. The unusual vantage point also robs Neptune of its familiar blue hue, as sunlight seen from here is scattered forward, and so is reddened like the setting Sun. Neptune is smaller but more massive than Uranus, has several dark rings, and emits more light than it receives from the Sun.

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 November 3 - Possible Pluto Moons
Explanation: In 1930, tiny, icy world Pluto was discovered orbiting in the distant solar system. In 1978, its relatively large companion Charon was detected by ground-based observations. This year, the Hubble Space Telescope may well have detected two further members of the Pluto system. Provisionally designated S/2005 P1 and S/2005 P2, the two potential new moons are seen orbiting in a counterclockwise direction about 44,000 kilometers (27,000 miles) from Pluto in these deep Hubble images recorded only three days apart. The diminutive and faint companions are also apparently detected on Hubble images of Pluto from 2002, but this coming February follow-up observations are planned in an effort to confirm the discovery of the new moons. Compared to Pluto's and Charon's diameters of 2,300 and 1,300 kilometers respectively, these moons are estimated to be between 60 and 200 kilometers across. Well within the Kuiper Belt, an extensive region beyond the orbit of Neptune, the Pluto system could be the first quadruple Kuiper Belt object known.

APOD: 2004 September 1 - An Inner Neptune for 55 Cancri
Explanation: Is our Solar System unique? The discovery of a Neptune-mass planet in an sub-Mercury orbit around nearby Sun-like star 55 Cancri, announced yesterday along with the discovery of other similar systems, gives a new indication that planetary systems as complex as our own Solar System likely exist elsewhere. The planet, discovered in data from the Hobby-Eberly telescope in Texas, the Lick Observatory in California, and the orbiting Hubble Space Telescope, is one of four planets now known to orbit 55 Cancri -- the others being similar in mass to Jupiter. The finding involved noting subtle changes in the speed of the star caused by its orbiting planets. The above drawing depicts what this planet might look like, assuming a mass similar to Neptune, but a composition similar to Earth. The star 55 Cancri, only 40 light-years distant, is visible with binoculars towards the constellation of Cancer.

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 26 - Neptune: Still Springtime After All These Years
Explanation: In the 1960s spring came to the southern hemisphere of Neptune, the Solar System's outermost gas giant planet. Of course, since Neptune orbits the Sun once every 165 earth-years, it's still springtime for southern Neptune, where each season lasts over four decades. Astronomers have found that in recent years Neptune has been getting brighter as illustrated in this Hubble Space Telescope image made in 2002. Compared to Hubble pictures taken as early as 1996, the 2002 image shows a dramatic increase in reflective white cloud bands in Neptune's southern hemisphere. Neptune's equator is tilted 29 degrees from the plane of its orbit, about the same as Earth's 23.5 degree tilt, and Neptune's weather seems to be dramatically responding to the similar relative seasonal increase in sunlight -- even though sunlight is 900 times less intense for the distant gas giant than for planet Earth. Meanwhile, summer is really just around the corner, coming to Neptune's southern hemisphere in 2005.

APOD: 2004 February 14 - Solar System Portrait
Explanation: On another Valentine's Day (February 14, 1990), cruising four billion miles from the Sun, the Voyager 1 spacecraft looked back to make this first ever family portrait of our Solar System. The complete portrait is a 60 frame mosaic made from a vantage point 32 degrees above the ecliptic plane. Voyager's wide angle camera frames sweep through the inner Solar System (far left) linking up with gas giant Neptune, at the time the Solar System's outermost planet (scroll right). Positions for Venus, Earth, Jupiter, Saturn, Uranus, and Neptune are indicated by the corresponding letters while the Sun is the bright spot near the center of the circle of frames. The inset frames for each of the planets are from Voyager's narrow field camera. Unseen in the portrait are Mercury, too close to the Sun to be detected, and Mars, unfortunately hidden by sunlight scattered in the camera's optical system. Small, faint Pluto's position was not covered.

APOD: 2003 October 20 - Neptune and Triton from Palomar
Explanation: How's the weather on Neptune? Tracking major weather patterns on the Solar System's outermost gas giant can help in the understanding of global weather patterns here on Earth. Each summer for the past five years, Neptune has been imaged and major weather patterns studied. The latest picture, taken on September 15, is shown above in false color. Visible in pink near Neptune's lower right is a new storm dubbed Annabelle that is several times larger than her terrestrial sister Isabel, a concurrent storm system that occurred here on Earth. Although Isabel lasted a few weeks, no one knows how long Annabelle will endure. On the upper right is Neptune's largest moon Triton, an unusual moon that sports volcanoes that spew ice.

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 June 13 - Neptune: Still Springtime After All These Years
Explanation: In the 1960s spring came to the southern hemisphere of Neptune, the Solar System's outermost gas giant planet. Of course, since Neptune orbits the Sun once every 165 earth-years, it's still springtime for southern Neptune, where each season lasts over four decades. Astronomers have found that in recent years Neptune has been getting brighter, as illustrated in this Hubble Space Telescope image made in 2002. Compared to Hubble pictures taken as early as 1996, the 2002 image shows a dramatic increase in reflective white cloud bands in Neptune's southern hemisphere. Neptune's equator is tilted 29 degrees from the plane of its orbit, about the same as Earth's 23.5 degree tilt, and Neptune's weather seems to be dramatically responding to the similar relative seasonal increase in sunlight -- even though sunlight is 900 times less intense for the distant gas giant than for planet Earth. Meanwhile, summer is really just around the corner, coming to Neptune's southern hemisphere in 2005.

APOD: 2002 September 22 - Two Hours Before Neptune
Explanation: Two hours before closest approach to Neptune in 1989, the Voyager 2 robot spacecraft snapped this picture. Clearly visible for the first time were long light-colored cirrus-type clouds floating high in Neptune's atmosphere. Shadows of these clouds can even be seen on lower cloud decks. Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible. Neptune's blue color therefore comes from smaller amounts of atmospheric methane, which preferentially absorbs red light. Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometers per hour. Speculation holds that diamonds may be created in the dense hot conditions that exist under the clouds-tops of Uranus and Neptune.

APOD: 2002 June 24 - The Sun's Heliosphere and Heliopause
Explanation: Where does the Sun's influence end? Nobody is sure. Out past the orbits of Neptune and Pluto extends a region named the heliosphere where the Sun's magnetic field and particles from the Solar Wind continue to dominate. The surface where the Solar Wind drops below sound speed is called the termination shock and is depicted as the inner oval in the above computer-generated illustration. It is thought that this surface occurs as close as 75-90 AU -- so close that a Pioneer or Voyager spacecraft may soon glide through it as they exit the Solar System at about 3 AU/year. The actual contact sheet between the Sun's ions and the Galaxy's ions is called the heliopause and is thought to occur at about 110 AU. It is depicted above as the middle surface. The Sun's heliopause moves through the local interstellar medium much as a boat moves on water, pushing a bow shock out in front, thought to occur near 230 AU.

APOD: 2002 June 7 - Portrait of an Infant Solar System
Explanation: This infant solar system was discovered posing along the lonely outskirts of the Rho Ophiuchi dark cloud, a star forming region 500 light-years from Earth. Enlarged in an infrared false-color portrait from the European Southern Observatory's Antu telescope, the dark dusty disk of planet-forming material lies edge-on, neatly dividing two small nebulae which reflect light from a hidden, youthful central star. Enthusiastically nicknamed the "Flying Saucer", the circumstellar disk is about 300 astronomical units across (1 a.u. is the Earth-Sun distance) or about 5 times the diameter of Neptune's orbit. The twin reflection nebulae have clearly different colors for reasons which still remain a mystery, but the relatively isolated neighborhood of the natal solar system is a stroke of luck. Planets should be able to develop within the dusty disk free from the destructive influence of radiation and winds from any nearby massive hot stars usually found in young star clusters.

APOD: 2002 February 14 - Solar System Portrait
Explanation: On another Valentine's Day (February 14, 1990), cruising four billion miles from the Sun, the Voyager 1 spacecraft looked back to make this first ever family portrait of our Solar System. The complete portrait is a 60 frame mosaic made from a vantage point 32 degrees above the ecliptic plane. Voyager's wide angle camera frames sweep through the inner Solar System (far left) linking up with gas giant Neptune, at the time the Solar System's outermost planet (scroll right). Positions for Venus, Earth, Jupiter, Saturn, Uranus, and Neptune are indicated by the corresponding letters while the Sun is the bright spot near the center of the circle of frames. The inset frames for each of the planets are from Voyager's narrow field camera. Unseen in the portrait are Mercury, too close to the Sun to be detected, and Mars, unfortunately hidden by sunlight scattered in the camera's optical system. Small, faint Pluto's position was not covered.

APOD: 2001 December 1 - Neptune's Great Dark Spot: Gone But Not Forgotten
Explanation: When NASA's Voyager 2 spacecraft flew by distant Neptune in August of 1989, astronomers were shocked. Since Neptune receives only 3 percent the sunlight Jupiter does, they expected to find a dormant, dark, frigid planet. Instead, the Voyager images revealed evidence of a dynamic and turbulent world. One of the most spectacular discoveries was of the Great Dark Spot, shown here in close-up. Surprisingly, it was comparable in size and at the same relative southern latitude as Jupiter's Great Red Spot, appearing to be a similar rotating storm system. Winds near the spot were measured up to 1500 miles per hour, the strongest recorded on any planet. The Voyager data also revealed that the Great Dark Spot varied significantly in size during the brief flyby. When the Hubble Space Telescope viewed the planet in 1994, the spot had vanished -- only to be replaced by another dark spot in the planet's northern hemisphere!

APOD: 2001 October 18 - Pluto: New Horizons
Explanation: Pluto's horizon spans the foreground in this artist's vision, gazing sunward across that distant and not yet explored world. Titled New Horizons, the painting also depicts Pluto's companion, Charon, as a darkened, ghostly apparition with a luminous crescent against a starry background. Beyond Charon, the diminished Sun is immersed in a flattened cloud of zodiacal dust. Here, Pluto's ruddy colors are based on existing astronomical observations while imagined but scientifically tenable details provided by the artist include high atmospheric cirrus and dark plumes from surface vents, in analogy to Neptune's large moon Triton explored by the Voyager 2 spacecraft in 1989. Craters suggest bombardment by Kuiper Belt objects, a newly understood population of outer solar system bodies likely related to the Pluto-Charon system. NASA is now considering a future robotic reconnaissance mission to Pluto-Charon and the Kuiper Belt which could reach the distant worlds late in the next decade.

APOD: 2001 August 26 - Uranus: The Tilted Planet
Explanation: Uranus is the third largest planet in our Solar System after Jupiter and Saturn. Uranus is composed mostly of rock and ices, but with a thick hydrogen and helium atmosphere. The blue hue of Uranus' atmosphere arises from the small amount of methane which preferentially absorbs red light. This picture was snapped by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Uranus. Uranus has many moons and a ring system. Uranus, like Venus, has a rotation axis that is greatly tilted and sometimes points near the Sun. It remains an astronomical mystery why Uranus' axis is so tilted. Uranus and Neptune are quite similar: Uranus is slightly larger but less massive.

APOD: 2001 August 21 - Dark Spots on Neptune
Explanation: Neptune has spots. The Solar System's outermost gas giant shows a nearly uniform blue hue created by small amounts of methane drifting in a thick atmosphere of nearly colorless hydrogen and helium. Dark spots do appear, however, that are anti-cyclones: large high-pressure systems that swirl in Neptune's cold cloud tops. Two dark spots are visible in the above picture taken by the robot Voyager 2 spacecraft in 1989: an Earth-sized Great Dark Spot located on the far left, and Dark Spot 2 located near bottom. A bright cloud dubbed Scooter accompanies the Great Dark Spot. Recent computer simulations indicate that scooters are methane clouds that might commonly be found near dark spots. Subsequent images of Neptune by the Hubble Space Telescope in 1994 indicated that both of these dark spots had dissipated, but another had been created.

APOD: 2001 June 19 - Crescent Neptune and Triton
Explanation: Gliding silently through the outer Solar System, the Voyager 2 spacecraft camera captured Neptune and Triton together in crescent phase in 1989. The above picture of the gas giant planet and its cloudy moon was taken from behind just after closest approach. It could not have been taken from Earth because Neptune never shows a crescent phase to sunward Earth. The unusual vantage point also robs Neptune of its familiar blue hue, as sunlight seen from here is scattered forward, and so is reddened like the setting Sun. Neptune is smaller but more massive than Uranus, has several dark rings, and emits more light than it receives from the Sun.

APOD: 2000 October 6 - X-Rays From Sirius B
Explanation: In visible light Sirius A (Alpha Canis Majoris) is the brightest star in the night sky, a closely watched celestial beacon throughout recorded history. Part of a binary star system only 8 light-years away, it was known in modern times to have a small companion star, Sirius B. Sirius B is much dimmer and appears so close to the brilliant Sirius A that it was not actually sighted until 1862, during Alvan Clark's testing of a large, well made optical refracting telescope. For orbiting x-ray telescopes, the Sirius situation is exactly reversed, though. A smaller but hotter Sirius B appears as the overwhelmingly intense x-ray source in this Chandra Observatory x-ray image (lines radiating from Sirius B are image artifacts). The fainter source seen at the position of Sirius A may be largely due to ultraviolet light from the star leaking into the x-ray detector. With a surface temperature of 25,000 kelvins, the mass of the Sun, and a radius just less than Earth's, Sirius B is the closest known white dwarf star. Can you guess what makes Sirius B like Neptune, the Sun's most distant gas giant planet? While still unseen, the presence of both celestial bodies was detected based on their gravitational influence alone ... making them early examples of dark matter.

APOD: 2000 February 18 - Neptune through Adaptive Optics
Explanation: From the Earth's surface, Neptune usually appears as a fuzzy blotch. The blurring effects of the Earth's atmosphere deny clearer images. By distorting mirrors in the telescope itself in time with the changing atmosphere, however, these effects can be greatly reduced. Many of the world's largest telescopes are now implementing these "rubber mirror" adaptive optics (AO) systems to bring out the finest details that these telescopes can resolve. Recently the 10-meter Keck II telescope in Hawaii came on-line with AO capability. The above image of Neptune in three infrared colors demonstrates the clarity of the new technique - as compared to an image of Neptune from Keck II without AO.

APOD: October 25, 1999 - Neptune in Infrared
Explanation: Neptune has never looked so clear in infrared light. Neptune is the eighth most distant planet from the Sun, thirty times the Earth-Sun distance. Neptune is the fourth largest planet, almost four times Earth's diameter. Surprisingly, Neptune radiates about twice as much energy as it receives from the Sun. A fascinating feature of the above photograph is that it was taken far from distant Neptune, through the Earth's normally blurry atmosphere. The great clarity of this recently released image was made possible by "rubber mirror" adaptive optics technology. Here, mirrors in the new Palomar High Angular Resolution Observer (PHARO) instrument connected to the 200-inch Hale Telescope flex to remove the effects of turbulence in the Earth's atmosphere.

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: October 5, 1999 - Two Hours Before Neptune
Explanation: Two hours before closest approach to Neptune in 1989, the Voyager 2 robot spacecraft snapped this picture. Clearly visible for the first time were long light-colored cirrus-type clouds floating high in Neptune's atmosphere. Shadows of these clouds can even be seen on lower cloud decks. Most of Neptune's atmosphere is made of hydrogen and helium, which is invisible. Neptune's blue color therefore comes from smaller amounts of atmospheric methane, which preferentially absorbs red light. Neptune has the fastest winds in the Solar System, with gusts reaching 2000 kilometers per hour. Recent speculation holds that diamonds may be created in the dense hot conditions that exist under the clouds-tops of Uranus and Neptune.

APOD: July 10, 1999 - Southern Neptune
Explanation: Neptune, the Solar System's outermost gas giant planet, is 30 times farther from the Sun than Earth. Twelve years after a 1977 launch, Voyager 2 flew by Neptune and found surprising activity on a planet that receives only 3 percent as much sunlight as Jupiter. In its brief but tantalizing close-up glimpse of this dim and distant world, the robot spacecraft recorded pulses of radio emission, zonal cloud bands, and large scale storm systems with up to 1500 mile per hour winds - the strongest measured on any planet. This mosaic of 5 Voyager images shows Neptune's Southern Hemisphere. Cloud bands and the Earth-sized, late "Great Dark Spot" with trailing white clouds located at about 22 degrees southern latitude are clearly visible. The distance from the Great Dark Spot feature to Neptune's South Pole (image center) is about 17,000 miles.

APOD: May 5, 1999 - A Solar System Portrait
Explanation: As the Voyager 1 spacecraft headed out of our Solar System, it looked back and took a parting family portrait of the Sun and planets. From beyond Pluto, our Solar System looks like a bright star surrounded by faint dots. In the above picture, the Sun is so bright it is blocked out for contrast. The innermost dots visible, labeled E and V for Earth and Venus, are particularly hard to discern. Gas giants Jupiter (J) and Saturn (S) are much more noticeable. The outermost planets visible are Uranus (U) and Neptune (N). Each planet is shown labeled and digitally enhanced in an inset image. Voyager 1 is only one of four human-made objects to leave our Solar System, the other three being Voyager 2, and Pioneer 10 and Pioneer 11.

APOD: February 13, 1999 - Pluto: The Frozen Planet
Explanation: This portrait of Pluto and its companion Charon was taken by the Hubble Space Telescope in 1994. Pluto is usually the most distant planet from the Sun but because of its eccentric orbit Pluto crossed inside of Neptune's orbit in 1979. On Thursday, February 11th, it crossed back out, recovering its status as the most distant of nine planets. Pluto is still considered to be a planet, although very little is known about it compared to other planets. Pluto is smaller than any other planet and even smaller than several other planet's moons. Pluto is probably composed of frozen rock and ice, much like Neptune's moon Triton. Pluto has not yet been visited by a spacecraft, but a mission is being planned for the next decade.

APOD: February 7, 1999 - Titan: Saturns Smog Moon
Explanation: The largest moon of Saturn is a rare wonder. Titan is the only one of Saturn's moons with an atmosphere, and one of only two moons in the Solar System with this distinction (Neptune's Triton is the other). Titan's thick cloudy atmosphere is mostly nitrogen, like Earth's, but contains much higher percentages of "smog-like" chemicals such as methane and ethane. The smog may be so thick that it actually rains "gasoline-like" liquids. The organic nature of some of the chemicals found in Titan's atmosphere cause some to speculate that Titan may harbor life! Because of its thick cloud cover, however, Titan's actual surface properties remain mysterious. Voyager 1 flew by in 1980 taking the above picture, and more recently much has been learned from observations by the Hubble Space Telescope. The Cassini mission launched in 1997 will map Titan's surface in 2004, helping to solve some of its mysteries.

APOD: February 5, 1999 - HR 4796A: Not Saturn
Explanation: These are not false-color renderings of the latest observations of Saturn's magnificent rings. Instead, the panels show a strikingly similar system on a much larger scale - a ring around the young, Vega-like star, HR 4796A, located about 200 light-years from Earth. Probably composed of dusty debris ground from colliding planetesimals, this ring is confined to a zone less than 17 AU wide (1 AU equals the Earth-Sun distance) and girdles the star at a radius of about 70 AU, roughly twice the orbital radius of Neptune. In analogy with the relationship of Saturn's rings and moons, this circumstellar ring could be held in place by forces due to planets - shepherding planetary bodies or the gravitational influence of larger planets orbiting closer to the parent star. In any event, because the ring would not survive long without something to keep it there, astronomers consider its presence strong evidence for unseen planetary bodies around HR 4796A. The top panels show the false-color images at two infrared wavelengths from the Hubble Space Telescope's NICMOS instrument, and the bottom panels trace the corresponding image contours. At the center of each, the overwhelming light of HR 4796A has been masked to reveal the fainter circumstellar ring.

APOD: January 20, 1999 - Possible Planets And Infrared Dust
Explanation: These near-infrared Hubble images of dust surrounding young stars offer the latest tantalizing evidence for planets beyond our Solar System. At left, the dark gap seen in the dust disk is reminiscent of a similar large gap in Saturn's rings believed to be sculpted by orbiting moons. By analogy, the gap in the dust disk of HD 141569 may be a larger scale result of unseen orbiting planets. At right is a relatively thin stellar dust ring suggestive of planetary rings held in place by orbiting moons. On a much larger scale this ring around the star HR 4796A could also indicate the presence of orbiting planet-sized bodies too faint to be directly visible. For a distance comparison, the orbit of Neptune is drawn at the lower right of each picture. The overwhelmingly bright starlight at the center has been blocked out to reveal the dim dust features.

APOD: June 27, 1998 - Southern Neptune
Explanation: Neptune, the Solar System's outermost gas giant planet, is 30 times farther from the Sun than Earth. Twelve years after a 1977 launch, Voyager 2 flew by Neptune and found surprising activity on a planet that receives only 3 percent as much sunlight as Jupiter. In its brief but tantalizing close-up glimpse of this dim and distant world, the robot spacecraft recorded pulses of radio emission, zonal cloud bands, and large scale storm systems with up to 1500 mile per hour winds - the strongest measured on any planet. This mosaic of 5 Voyager images shows Neptune's Southern Hemisphere. Cloud bands and the Earth-sized, late "Great Dark Spot" with trailing white clouds located at about 22 degrees southern latitude are clearly visible. The distance from the Great Dark Spot feature to Neptune's South Pole (image center) is about 17,000 miles.

APOD: May 17, 1998 - Our Solar System from Voyager
Explanation: After taking spectacular pictures of our Solar System's outer planets, Voyager 1 looked back at six planets to take our Solar System's first family portrait. Here Venus, Earth, Jupiter, Saturn, Uranus, and Neptune, were all visible across the sky. Each, however, was now just a small speck of light, dimmer than many of the stars in the sky. Voyager 1 is only one of four human-made objects to leave our Solar System, the other three being Voyager 2, and Pioneer 10 and Pioneer 11.

APOD: February 21, 1998 - Neptune: Big Blue Giant
Explanation: This picture was taken by the Voyager 2 spacecraft in 1989 - the only spacecraft ever to visit Neptune. Neptune will be the farthest planet from the Sun until 1999, when the elliptical orbit of Pluto will cause it to once again resume this status. Neptune, like Uranus, is composed mostly of liquid water, methane and ammonia, is surrounded by a thick gas atmosphere of mostly hydrogen and helium, and has many moons and rings. Neptune's moon Triton is unlike any other and has active volcanoes. The nature of Triton's unusual orbit around Neptune is the focus of much discussion and speculation.

APOD: December 28, 1997 - Pluto: The Frozen Planet
Explanation: The Hubble Space Telescope imaged Pluto and its moon Charon in 1994. Pluto is usually the most distant planet from the Sun but because of its elliptic orbit Pluto crossed inside of Neptune's orbit in 1979 and will cross back out again in 1999. Compared to the other planets, very little is known about Pluto. Pluto is smaller than any other planet and even smaller than several other planet's moons. From Pluto, the Sun is just a tiny point of light. Pluto is probably composed of frozen rock and ice, much like Neptune's moon Triton. Pluto has not yet been visited by a spacecraft, but a mission is being planned for the next decade.

APOD: December 4, 1997 - A Sky Full Of Planets
Explanation: Look up tonight. Just after sunset, the crescent moon and all five "naked-eye" planets (Mercury, Venus, Mars, Jupiter, and Saturn) will be visible (depending on your latitude), lying near our solar system's ecliptic plane. Venus and Jupiter will shine brilliantly as the brightest "stars" in the sky, but Mercury will be near the horizon and hard to see. A pair of binoculars will also reveal Uranus and Neptune and observers with a telescope and a good site may even be able to glimpse faint Pluto just above the Western horizon in the fading twilight (not shown on the chart above). Enjoy this lovely spectacle any clear night through about December 8. A similar gathering is expected in May 2000 but the planets will be hidden from view by the solar glare. A night sky as full of planets as this one will occur again though ... in about 100 years.

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: November 15, 1997 - Uranus: The Tilted Planet
Explanation: Uranus is the third largest planet after Jupiter and Saturn. This picture was snapped by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Uranus. Uranus has many moons and a ring system. Uranus is composed mostly of rock and ices, but with a thick hydrogen and helium atmosphere. Uranus is peculiar in that its rotation axis is greatly tilted and sometimes points near the Sun. It remains an astronomical mystery why Uranus' axis is so tilted. Uranus and Neptune are very similar.

APOD: November 3, 1997 - Irregular Moons Discovered Around Uranus
Explanation: Where did these two irregular moons of Uranus originate? Last week two previously undiscovered moons of the distant gas planet were confirmed, the first in irregular orbits. All fifteen previously known moons of Uranus are 'regular', circling near the planet's equator. Most of these were discovered by the passing Voyager 2 spacecraft in 1986. These newly discovered moons are thought to be odd-shaped and about 100 km across. They are considered irregular, though, because they orbit in odd directions and far from Uranus. If Uranus' irregular moons have the same origin as those orbiting Jupiter, Saturn, and Neptune, then they were probably caught from orbits around the Sun. Moons like this are discovered by their motion. One of these moons is shown above as the circled point of light moving from left to right. (To stop the movie from repeating, click "stop" on most browsers.)

APOD: June 9, 1997 - An Auroral Ring on Jupiter
Explanation: Do other planets have aurora? Terrestrial and spacecraft observations have found evidence for aurora on Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. In the above false-color photograph, a good portion of an auroral ring was captured recently in optical light by the Galileo spacecraft in orbit around Jupiter. Auroral rings encircle a planet's magnetic pole, and result from charged particles spiraling down magnetic field lines. Although the surroundings near Jupiter are much different than Earth, the auroral rings appear similar.

APOD: May 29, 1997 - Southern Neptune
Explanation: Neptune, the Solar System's outermost gas giant planet, is 30 times farther from the Sun than Earth. Twelve years after a 1977 launch, Voyager 2 flew by Neptune and found surprising activity on a planet that receives only 3 percent as much sunlight as Jupiter. In its brief but tantalizing close-up glimpse of this dim and distant world, the robot spacecraft recorded pulses of radio emission, zonal cloud bands, and large scale storm systems with up to 1500 mile per hour winds - the strongest measured on any planet. This mosaic of 5 Voyager images shows Neptune's Southern Hemisphere. Cloud bands and the Earth-sized, late "Great Dark Spot" with trailing white clouds located at about 22 degrees southern latitude are clearly visible. The distance from the Great Dark Spot feature to Neptune's South Pole (image center) is about 17,000 miles.

APOD: May 26, 1997 - Old Faithful Meets Hale-Bopp
Explanation: As Comet Hale-Bopp leaves our Northern Skies, it provides us with yet another burst of joy. On May 11th the fading comet was photographed behind the famous "Old Faithful" water geyser of Yellowstone National Park, Wyoming, USA, Planet Earth. Perhaps more familiar to Earth Dwellers than the dark geysers on Neptune's moon Triton, the gas geysers on Jupiter's moon Io, and the dirty water geysers hypothesized on Jupiter's moon Europa, Earth's Old Faithful is also reliable - every 60-80 minutes it gushes a plume of water and steam high into the air. Comet Hale-Bopp will continue to be visible to observers in the Southern Hemisphere as it moves away from the Sun towards the outer Solar System.

APOD: May 12, 1997 - Lightning on Jupiter
Explanation: Does lightning occur only on Earth? Spacecraft in our Solar System have detected radio signals consistent with lightning on other planets, including Venus, Jupiter, Saturn, Uranus, and Neptune. In the above photograph, optical flashes from Jupiter were photographed recently by the Galileo orbiter. Each of the circled dots indicates lightning. The numbers label lines of latitude. The size of the largest spot is about 500 kilometers across and might be high clouds illuminated by several bright lightning strokes.

APOD: December 14, 1996 - Our Solar System from Voyager
Explanation: After taking spectacular pictures of our Solar System's outer planets, Voyager 1 looked back at six planets to take our Solar System's first family portrait. Here Venus, Earth, Jupiter, Saturn, Uranus, and Neptune, were all visible across the sky. Each, however, was now just a small speck of light, dimmer than many of the stars in the sky. Voyager 1 is only one of four human-made objects to leave our Solar System, the other three being Voyager 2, and Pioneer 10 and Pioneer 11.

APOD: October 28, 1996 - The Weather on Neptune
Explanation: Today's weather on Neptune will be typical. Highs in the upper atmosphere will be about -150 Centigrade, with winds ranging about 900 miles per hour near the equator. Much was learned recently about Neptune's weather by the release last week of time-lapse pictures of the most distant gas giant in our Solar System. These pictures show how the weather differs between the two hemispheres of Neptune. The overall blue color of Neptune is caused by methane gas preferentially absorbing red light, while the yellow spots are the highest clouds. It is still not known why Neptune's Great Dark Spot disappeared.

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: August 28, 1996 - NGC 5882: A Small Planetary Nebula
Explanation: Will most stars one day look like this? Pictured above is the planetary nebula NGC 5882, captured by the Hubble Space Telescope. Although planetary nebulae can appear similar to planets like Uranus and Neptune, they are actually gas clouds surrounding stars typically hundreds of light years away. Planetary nebula form when a typical star completes fusion in its core and ejects an outer envelope of gas - usually about 10 percent of the star's initial mass. This gas shell dims in about 50,000 years - short compared to the lifetimes of stars. Therefore, although only about 1000 planetary nebula are known in our Galaxy, it is thought that most stars go through this phase. Green light is emitted when oxygen ions acquire electrons from the surrounding gas.

APOD: May 8, 1996 - Neptune's Great Dark Spot: Gone But Not Forgotten
Explanation: When NASA's Voyager 2 spacecraft flew by distant Neptune in August of 1989, astronomers were shocked. Since Neptune receives only 3 percent the sunlight Jupiter does, they expected to find a dormant, dark, frigid planet. Instead, the Voyager images revealed evidence of a dynamic and turbulent world. One of the most spectacular discoveries was of the Great Dark Spot, shown here in close-up. Surprisingly, it was comparable in size and at the same relative southern latitude as Jupiter's Great Red Spot, appearing to be a similar rotating storm system. Winds near the spot were measured up to 1500 miles per hour, the strongest recorded on any planet. The Voyager data also revealed that the Great Dark Spot varied significantly in size during the brief flyby. When the Hubble Space Telescope viewed the planet in 1994, the spot had vanished -- only to be replaced by another dark spot in the planet's northern hemisphere!

APOD: May 7, 1996 - The Clouds of Neptune
Explanation: These Hubble Space Telescope (HST) images reveal glimpses of the dynamic atmosphere of Neptune, the Solar System's most distant gas giant planet. The first close-up of Neptune's clouds was provided by NASA's Voyager 2 spacecraft during its August 1989 flyby, giving a tantalizingly brief look. From its vantage point in Earth orbit, HST can patiently watch in detail as the planet's massive weather systems change over time. These three pictures, taken during October and November 1994 when Neptune was only 2.8 billion miles distant, show the planet's characteristic aqua-blue colored atmosphere highlighted by pink, high altitude clouds and streaked with dark bands. The aqua-blue color is caused by gaseous methane which absorbs red light. Here, the high altitude clouds were intentionally given a pink tint to indicate that they were imaged in near infrared light - in true color images they would appear white.

APOD: April 30, 1996 - Uranus' Ring System
Explanation: The rings of Uranus are thin, narrow, and dark compared to other planetary ring systems. Brightened artificially by computer, the ring particles reflect as little light as charcoal, although they are really made of ice chucks darkened by rock. This false-color, infrared picture from the Hubble Space Telescope taken in July 1995 shows the rings in conjunction to the planet. The infrared light allows one to see detail in different layers of Uranus' atmosphere, which has been digitally enhanced with false color. Three other planets in our Solar System are known to have rings: Jupiter, Saturn, and Neptune. Four of Uranus' moons are visible outside the ring plane. The rings of Uranus were discovered from ground-based observations in 1977.

APOD: March 4, 1996 - Uranus' Largest Moon: Titania
Explanation: Titania's tortured terrain is a mix of valleys and craters. NASA's interplanetary robot spacecraft Voyager 2 passed this moon of Uranus in 1986 and took the above photograph. The photograph was then transmitted back to earth by radio. The valleys of Titania resemble those on Ariel indicate that Titania underwent some unknown tumultuous resurfacing event in its distant past. Although Titania is Uranus' largest moon, it is still much smaller than Triton - the largest moon of Uranus' sister planet Neptune. Titania is essentially a large dirty iceball that orbits Uranus - it is composed of about half water-ice and half rock. Titania was discovered by William Hershel in 1787.

APOD: November 4, 1995 - Neptune's Moon Proteus
Explanation: Proteus is the second largest moon of Neptune behind the mysterious Triton. Proteus was discovered only in 1982 by the Voyager 2 spacecraft. This is unusual since Neptune has a smaller moon - Nereid - which was discovered 33 years earlier from Earth. The reason Proteus was not discovered sooner is that its surface is very dark and it orbits much closer to Neptune. Proteus has an odd box-like shape and were it even slightly more massive, its own gravity would cause it to reform itself into a sphere.

APOD: September 23, 1995 - Titan: Saturn's Smog Moon
Explanation: The largest moon of Saturn is a rare wonder. Titan is the only one of Saturn's moons with an atmosphere, and one of only two moons in the Solar System with this distinction (Neptune's Triton is the other). Titan's thick cloudy atmosphere is mostly nitrogen, like Earth's, but contains much higher percentages of "smog-like" chemicals such as methane and ethane. The smog may be so thick that it actually rains "gasoline-like" liquids. The organic nature of some of the chemicals found in Titan's atmosphere cause some to speculate that Titan may harbor life! Because of its thick cloud cover, however, Titan's actual surface properties remain mysterious. Voyager 1 flew by in 1980 taking the above picture, and recently much has been learned from Hubble Space Telescope observations. The Cassini mission currently scheduled for launch in 1997 will map Titan's surface, helping to solve some of its mysteries.

APOD: August 19, 1995 - Our Solar System from Voyager
Explanation: After taking its spectacular pictures of the outer solar system planets, Voyager 1 looked back at six planets from the inner solar system. Here Venus, Earth, Jupiter, Saturn, Uranus, and Neptune, were all visible across the sky. Each, however, was now just a small speck of light, dimmer than many of the stars in the sky. Voyager 1 is only one of four human-made objects to leave our Solar System, the other three being Voyager 2, and Pioneer 10 and 11.

APOD: August 18, 1995 - Pluto: The Frozen Planet
Explanation: The Hubble Space Telescope imaged Pluto and its moon Charon in 1994. Pluto is usually the most distant planet from the Sun but because of its elliptic orbit Pluto crossed inside of Neptune's orbit in 1979 and will cross back out again in 1999. Compared to the other planets, very little is known about Pluto. Pluto is smaller than any other planet and even smaller than several other planet's moons. From Pluto, the Sun is just a tiny point of light. Pluto is probably composed of frozen rock and ice, much like Neptune's moon Triton. Pluto has not yet been visited by a spacecraft, but a mission is being planned for the next decade.

APOD: August 17, 1995 - Neptune: Big Blue Giant
Explanation: This picture was taken by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Neptune. Neptune will be the farthest planet from the Sun until 1999, when the elliptical orbit of Pluto will cause it to once again resume this status. Neptune, like Uranus, is composed mostly of liquid water, methane and ammonia, is surrounded by a thick gas atmosphere of mostly hydrogen and helium, and has many moons and rings. Neptune's moon Triton is unlike any other and has active volcanoes. The nature of Triton's unusual orbit around Neptune is the focus of much discussion and speculation.

APOD: August 16, 1995 - Uranus: The Tilted Planet
Explanation: This picture was snapped by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Uranus. Uranus is the third largest planet after Jupiter and Saturn. Uranus has many moons and a ring system. Uranus is composed mostly of liquid water, methane and ammonia, surrounded by a thick gas atmosphere of mostly hydrogen and helium. Uranus is peculiar in that its rotation axis is greatly tilted and sometimes points near the sun. It remains an astronomical mystery why Uranus' axis is so tilted. Uranus and Neptune are very similar.

APOD: August 5, 1995 - Geysers on Triton
Explanation: In August of 1989 NASA's Voyager 2 spacecraft passed by Neptune, the most distant of the solar system's gas giant planets. Its encounter with Neptune climaxed with its closest approach to Neptune's largest moon Triton. From a distance of about 24,000 miles the robot space probe surveyed Triton's surface, whose temperature averages nearly -400 degrees Fahrenheit, and discovered surprising evidence of a complex and active world. For example, the prominent dark streaks in this image seem to come from small volcanoes and may consist of nitrogen frost mixed with organic compounds ejected during geyser-like eruptions.

APOD: August 2, 1995 - Jupiter's Rings
Explanation: Astronomers using NASA's Voyager spacecraft to search for a ring system around Jupiter discovered these faint rings in 1979. Unlike Saturn's bright rings which are composed of chunks of rock and ice, Jupiter's rings appear to consist of fine particles of dust. One possibility is that the dust is produced by impacts with Jupiter's inner moons. This false color image has been computer enhanced.