Astronomy Picture of the Day Search Results for "Pluto"

APOD: 2024 November 16 - Pluto at Night
Explanation: The night side of Pluto spans this shadowy scene. In the stunning spacebased perspective the Sun is 4.9 billion kilometers (almost 4.5 light-hours) behind the dim and distant world. It was captured by far flung New Horizons in July of 2015 when the spacecraft was at a range of some 21,000 kilometers from Pluto, about 19 minutes after its closest approach. A denizen of the Kuiper Belt in dramatic silhouette, the image also reveals Pluto's tenuous, surprisingly complex layers of hazy atmosphere. Near the top of the frame the crescent twilight landscape includes southern areas of nitrogen ice plains now formally known as Sputnik Planitia and rugged mountains of water-ice in the Norgay Montes.

APOD: 2024 July 13 - Solar System Family Portrait
Explanation: In 1990, cruising four billion miles from the Sun, the Voyager 1 spacecraft looked back 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 ice 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. In 2024 Voyager 1, NASA’s longest-running and most-distant spacecraft, is some 15 billion miles away, operating in interstellar space.

APOD: 2024 January 28 – Pluto in True Color
Explanation: What color is Pluto, really? It took some effort to figure out. Even given all of the images sent back to Earth when the robotic New Horizons spacecraft sped past Pluto in 2015, processing these multi-spectral frames to approximate what the human eye would see was challenging. The result featured here, released three years after the raw data was acquired by New Horizons, is the highest resolution true color image of Pluto ever taken. Visible in the image is the light-colored, heart-shaped, Tombaugh Regio, with the unexpectedly smooth Sputnik Planitia, made of frozen nitrogen, filling its western lobe. New Horizons found the dwarf planet to have a surprisingly complex surface composed of many regions having perceptibly different hues. In total, though, Pluto is mostly brown, with much of its muted color originating from small amounts of surface methane energized by ultraviolet light from the Sun.

APOD: 2023 November 28 – Ganymede from Juno
Explanation: What does the largest moon in the Solar System look like? Jupiter's moon Ganymede, larger than even Mercury and Pluto, has an icy surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges. The cause of the grooved terrain remains a topic of research, with a leading hypothesis relating it to shifting ice plates. Ganymede is thought to have an ocean layer that contains more water than Earth -- and might contain life. Like Earth's Moon, Ganymede keeps the same face towards its central planet, in this case Jupiter. The featured image was captured in 2021 by NASA's robotic Juno spacecraft when it passed by the immense moon. The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days. Juno continues to study the giant planet's high gravity, unusual magnetic field, and complex cloud structures.

APOD: 2023 August 18 - Northern Pluto
Explanation: Gaze across the frozen canyons of northern Pluto in this contrast enhanced color scene. The image data used to construct it was acquired in July 2015 by the New Horizons spacecraft as it made the first reconnaissance flight through the remote Pluto system six billion kilometers from the Sun. Now known as Lowell Regio, the region was named for Percival Lowell, founder of the Lowell Observatory. Also famous for his speculation that there were canals on Mars, Lowell started the search that ultimately led to Pluto's discovery in 1930 by Clyde Tombaugh. In this frame Pluto's North Pole is above and left of center. The pale bluish floor of the broad canyon on the left is about 70 kilometers (45 miles) wide, running vertically toward the south. Higher elevations take on a yellowish hue. New Horizon's measurements were used to determine that in addition to nitrogen ice, methane ice is abundant across Lowell Regio. So far, Pluto is the only Solar System world named by an 11-year-old girl.

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

APOD: 2023 April 16 – M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured here, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousands of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause and shape planetary nebulae.

APOD: 2022 December 31 - Moon over Makemake
Explanation: Makemake (sounds like MAH-kay MAH-kay), second brightest dwarf planet of the Kuiper belt, has a moon. Nicknamed MK2, Makemake's moon reflects sunlight with a charcoal-dark surface, about 1,300 times fainter than its parent body. Still, in 2016 it was spotted in Hubble Space Telescope observations intended to search for faint companions with the same technique used to find the small satellites of Pluto. Just as for Pluto and its satellites, further observations of Makemake and orbiting moon will measure the system's mass and density and allow a broader understanding of the distant worlds. About 160 kilometers (100 miles) across compared to Makemake's 1,400 kilometer diameter, MK2's relative size and contrast are shown in this artist's vision. An imagined scene of an unexplored frontier of the Solar System, it looks back from a spacecraft's vantage as the dim Sun shines along the Milky Way. Of course, the Sun is over 50 times farther from Makemake than it is from planet Earth.

APOD: 2022 March 26 - Pluto at Night
Explanation: The night side of Pluto spans this shadowy scene. In the stunning spacebased perspective the Sun is 4.9 billion kilometers (almost 4.5 light-hours) behind the dim and distant world. It was captured by far flung New Horizons in July of 2015 when the spacecraft was at a range of some 21,000 kilometers from Pluto, about 19 minutes after its closest approach. A denizen of the Kuiper Belt in dramatic silhouette, the image also reveals Pluto's tenuous, surprisingly complex layers of hazy atmosphere. Near the top of the frame the crescent twilight landscape includes southern areas of nitrogen ice plains now formally known as Sputnik Planitia and rugged mountains of water-ice in the Norgay Montes.

APOD: 2021 August 1 - Pluto in Enhanced Color
Explanation: Pluto is more colorful than we can see. Color data and high-resolution images of our Solar System's most famous dwarf planet, taken by the robotic New Horizons spacecraft during its flyby in 2015 July, have been digitally combined to give an enhanced-color view of this ancient world sporting an unexpectedly young surface. The featured enhanced color image is not only esthetically pretty but scientifically useful, making surface regions of differing chemical composition visually distinct. For example, the light-colored heart-shaped Tombaugh Regio on the lower right is clearly shown here to be divisible into two regions that are geologically different, with the leftmost lobe Sputnik Planitia also appearing unusually smooth. After Pluto, New Horizons continued on, shooting past asteroid Arrokoth in 2019 and has enough speed to escape our Solar System completely.

APOD: 2021 June 14 - Ganymede from Juno
Explanation: What does the largest moon in the Solar System look like? Jupiter's moon Ganymede, larger than even Mercury and Pluto, has an icy surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges. The cause of the grooved terrain remains a topic of research, with a leading hypothesis relating it to shifting ice plates. Ganymede is thought to have an ocean layer that contains more water than Earth -- and might contain life. Like Earth's Moon, Ganymede keeps the same face towards its central planet, in this case Jupiter. The featured image was captured last week by NASA's robotic Juno spacecraft as it passed only about 1000 kilometers above the immense moon. The close pass reduced Juno's orbital period around Jupiter from 53 days to 43 days. Juno continues to study the giant planet's high gravity, unusual magnetic field, and complex cloud structures.

APOD: 2021 January 15 - A Plutonian Landscape
Explanation: This shadowy landscape of majestic mountains and icy plains stretches toward the horizon on a small, distant world. It was captured from a range of about 18,000 kilometers when New Horizons looked back toward Pluto, 15 minutes after the spacecraft's closest approach on July 14, 2015. The dramatic, low-angle, near-twilight scene follows rugged mountains formally known as Norgay Montes from foreground left, and Hillary Montes along the horizon, giving way to smooth Sputnik Planum at right. Layers of Pluto's tenuous atmosphere are also revealed in the backlit view. With a strangely familiar appearance, the frigid terrain likely includes ices of nitrogen and carbon monoxide with water-ice mountains rising up to 3,500 meters (11,000 feet). That's comparable in height to the majestic mountains of planet Earth. The Plutonian landscape is 380 kilometers (230 miles) across.

APOD: 2020 September 13 - M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured here, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousands of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause and shape planetary nebulae.

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: 2019 November 18 - Passing Asteroid Arrokoth
Explanation: What would it look like to pass asteroid Arrokoth? The robotic New Horizons spacecraft zoomed past Arrokoth in January, 3.5 years after the spacecraft passed Pluto. If this object's name doesn't sound familiar, that may be because the distant, double-lobed, Kuiper-belt object was unofficially dubbed Ultima Thule until recently receiving its official name: 486958 Arrokoth. The featured black and white video animates images of Arrokoth taken by New Horizons at different angles as it zoomed by. The video clearly shows Arrokoth's two lobes, and even hints that the larger lobe is significantly flattened. New Horizons found that Arrokoth is different from any known asteroid in the inner Solar System and is likely composed of two joined planetesimals -- the building blocks of planets as they existed billions of years ago. New Horizons continues to speed out of our Solar System gaining about three additional Earth-Sun separations every year.

APOD: 2019 October 20 - Pluto at Night
Explanation: The night side of Pluto spans this shadowy scene, a stunning spacebased view with the Sun 4.9 billion kilometers (almost 4.5 light-hours) behind the dim and distant world. It was captured by far flung New Horizons in July of 2015. The spacecraft was at a range of some 21,000 kilometers from Pluto, about 19 minutes after its closest approach. A denizen of the Kuiper Belt in dramatic silhouette, the image also reveals Pluto's tenuous, surprisingly complex layers of hazy atmosphere. The crescent twilight landscape near the top of the frame includes southern areas of nitrogen ice plains now formally known as Sputnik Planitia and rugged mountains of water-ice in the Norgay Montes.

APOD: 2019 September 10 - Pluto in True Color
Explanation: What color is Pluto, really? It took some effort to figure out. Even given all of the images sent back to Earth when the robotic New Horizons spacecraft sped past Pluto in 2015, processing these multi-spectral frames to approximate what the human eye would see was challenging. The result featured here, released three years after the raw data was acquired by New Horizons, is the highest resolution true color image of Pluto ever taken. Visible in the image is the light-colored, heart-shaped, Tombaugh Regio, with the unexpectedly smooth Sputnik Planitia, made of frozen nitrogen, filling its western lobe. New Horizons found the dwarf-planet to have a surprisingly complex surface composed of many regions having perceptibly different hues. In total, though, Pluto is mostly brown, with much of its muted color originating from small amounts of surface methane energized by ultraviolet light from the Sun.

APOD: 2019 February 28 - Sharpest Ultima Thule
Explanation: On January 1, New Horizons swooped to within 3,500 kilometers of the Kuiper Belt world known as Ultima Thule. That's about 3 times closer than its July 2015 closest approach to Pluto. The spacecraft's unprecedented feat of navigational precision, supported by data from ground and space-based observing campaigns, was accomplished 6.6 billion kilometers (over 6 light-hours) from planet Earth. Six and a half minutes before closest approach to Ultima Thule it captured the nine frames used in this composite image. The most detailed picture possible of the farthest object ever explored, the image has a resolution of about 33 meters per pixel, revealing intriguing bright surface features and dark shadows near the terminator. A primitive Solar System object, Ultima Thule's two lobes combine to span just 30 kilometers. The larger lobe, referred to as Ultima, is recently understood to be flattened like a fluffy pancake, while the smaller, Thule, has a shape that resembles a dented walnut.

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: 2019 February 11 - New Data: Ultima Thule Surprisingly Flat
Explanation: Ultima Thule is not the object humanity thought that it was last month. When the robotic New Horizons spacecraft zoomed past the distant asteroid Ultima Thule (officially 2014 MU69) in early January, early images showed two circular lobes that when most simply extrapolated to 3D were thought to be, roughly, spheres. However, analyses of newly beamed-back images -- including many taken soon after closest approach -- shows eclipsed stars re-appearing sooner than expected. The only explanation possible is that this 30-km long Kuiper belt object has a different 3D shape than believed only a few weeks ago. Specifically, as shown in the featured illustration, it now appears that the larger lobe -- Ultima -- is more similar to a fluffy pancake than a sphere, while the smaller lobe -- Thule -- resembles a dented walnut. The remaining uncertainty in the outlines are shown by the dashed blue lines. The new shape information indicates that gravity -- which contracts more massive bodies into spheres -- played perhaps less of a role in contouring the lobes of Ultima Thule than previously thought. The New Horizons spacecraft continued on to Ultima Thule after passing Pluto in mid-2015. New data and images are still being received.

APOD: 2018 December 29 - New Horizons at Ultima Thule
Explanation: When we celebrate the start of 2019, on January 1 the New Horizons spacecraft will fly by Ultima Thule. A world of the Kuiper belt 6.5 billion kilometers from the Sun, the nickname Ultima Thule (catalog designation 2014 MU69) fittingly means "beyond the known world". Following its 2015 flyby of Pluto, New Horizons was targeted for this journey, attempting the most distant flyby for a spacecraft from Earth by approaching Ultima Thule to within about 3500 kilometers. The tiny world itself is about 30 kilometers in size. In 2017 and 2018, observing campaigns with Earth-based telescopes determined the shape of the object to be a contact binary or a close binary sytem as in this artist's illustration. New Horizons will image close up its unexplored surface in the dim light of the distant Sun.

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

APOD: 2017 October 17 - Haumea of the Outer Solar System
Explanation: One of the strangest objects in the outer Solar System has recently been found to have a ring. The object, named Haumea, is the fifth designated dwarf planet after Pluto, Ceres, Eris, and Makemake. Haumea's oblong shape makes it quite unusual. Along one direction, Haumea is significantly longer than Pluto, while in another direction Haumea has an extent very similar to Pluto, while in the third direction is much smaller. Haumea's orbit sometimes brings it closer to the Sun than Pluto, but usually Haumea is further away. Illustrated above, an artist visualizes Haumea as a cratered ellipsoid surrounded by a uniform ring. Originally discovered in 2003 and given the temporary designation of 2003 EL61, Haumea was renamed in 2008 by the IAU for a Hawaiian goddess. Besides the ring discovered this year, Haumea has two small moons discovered in 2005, named Hi'iaka and Namaka for daughters of the goddess.

APOD: 2017 October 5 - Pluto's Bladed Terrain
Explanation: Imaged during the New Horizons spacecraft flyby in July 2015, Pluto's bladed terrain is captured in this close-up of the distant world. The bizarre texture belongs to fields of skyscraper-sized, jagged landforms made almost entirely of methane ice, found at extreme altitudes near Pluto's equator. Casting dramatic shadows, the tall, knife-like ridges seem to have been formed by sublimation. By that process, condensed methane ice turns directly to methane gas without passing through a liquid phase during Pluto's warmer geological periods. On planet Earth, sublimation can also produce standing fields of knife-like ice sheets, found along the high plateau of the Andes mountain range. Known as penitentes, those bladed structures are made of water ice and at most a few meters tall.

APOD: 2017 August 14 - Charon Flyover from New Horizons
Explanation: What if you could fly over Pluto's moon Charon -- what might you see? The New Horizons spacecraft did just this in 2015 July as it zipped past Pluto and Charon with cameras blazing. The images recorded allowed for a digital reconstruction of much of Charon's surface, further enabling the creation of fictitious flights over Charon created from this data. One such fanciful, minute-long, time-lapse video is shown here with vertical heights and colors of surface features digitally enhanced. Your journey begins over a wide chasm that divides different types of Charon's landscapes, a chasm that might have formed when Charon froze through. You soon turn north and fly over a colorful depression dubbed Mordor that, one hypothesis holds, is an unusual remnant from an ancient impact. Your voyage continues over an alien landscape rich with never-before-seen craters, mountains, and crevices. The robotic New Horizons spacecraft has now been targeted at Kuiper Belt object 2014 MU 69, which it should zoom past on New Year's Day 2019.

APOD: 2017 July 31 - Pluto Flyover from New Horizons
Explanation: What if you could fly over Pluto -- what might you see? The New Horizons spacecraft did just this in 2015 July as it shot past the distant world at a speed of about 80,000 kilometers per hour. Recently, many images from this spectacular passage have been color enhanced, vertically scaled, and digitally combined into the featured two-minute time-lapse video. As your journey begins, light dawns on mountains thought to be composed of water ice but colored by frozen nitrogen. Soon, to your right, you see a flat sea of mostly solid nitrogen that has segmented into strange polygons that are thought to have bubbled up from a comparatively warm interior. Craters and ice mountains are common sights below. The video dims and ends over terrain dubbed bladed because it shows 500-meter high ridges separated by kilometer-sized gaps. Although the robotic New Horizons spacecraft has too much momentum ever to return to Pluto, it has now been targeted at Kuiper Belt object 2014 MU 69, which it should shoot past on New Year's Day 2019.

APOD: 2017 May 14 - Ganymede: The Largest Moon
Explanation: What does the largest moon in the Solar System look like? Jupiter's moon Ganymede, larger than even Mercury and Pluto, has an icy surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges. The large circular feature on the upper right, called Galileo Regio, is an ancient region of unknown origin. Ganymede is thought to have an ocean layer that contains more water than Earth and might contain life. Like Earth's Moon, Ganymede keeps the same face towards its central planet, in this case Jupiter. The featured image was taken about 20 years ago by NASA's Galileo probe, which ended its mission by diving into Jupiter's atmosphere in 2003. Currently, NASA's Juno spacecraft orbits Jupiter and is studying the giant planet's internal structure, among many other attributes.

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: 2016 November 22 - Plutos Sputnik Planum
Explanation: Is there an ocean below Sputnik Planum on Pluto? The unusually smooth 1000-km wide golden expanse, visible in the featured image from New Horizons, appears segmented into convection cells. But how was this region created? One hypothesis now holds the answer to be a great impact that stirred up an underground ocean of salt water roughly 100-kilometers thick. The featured image of Sputnik Planum, part of the larger heart-shaped Tombaugh Regio, was taken last July and shows true details in exaggerated colors. Although the robotic New Horizons spacecraft is off on a new adventure, continued computer-modeling of this surprising surface feature on Pluto is likely to lead to more refined speculations about what lies beneath.

APOD: 2016 July 24 - M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousands of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

APOD: 2016 June 26 - Jupiter's Clouds from New Horizons
Explanation: The New Horizons spacecraft took some stunning images of Jupiter on its way out to Pluto. Famous for its Great Red Spot, Jupiter is also known for its regular, equatorial cloud bands, visible through even modest sized telescopes. The featured image, horizontally compressed, was taken in 2007 near Jupiter's terminator and shows the Jovian giant's wide diversity of cloud patterns. On the far left are clouds closest to Jupiter's South Pole. Here turbulent whirlpools and swirls are seen in a dark region, dubbed a belt, that rings the planet. Even light colored regions, called zones, show tremendous structure, complete with complex wave patterns. The energy that drives these waves surely comes from below. New Horizons is the fastest space probe ever launched, has successfully complete its main flyby of Pluto in 2015, and is now heading further out and on track to flyby Kuiper belt object 2014 MU69 in 2019. In the near term, many space enthusiasts excitedly await Juno's arrival at Jupiter next Monday.

APOD: 2016 June 18 - Sputnik Planum vs. Krun Macula
Explanation: Pluto's pitted plains meet rugged highlands in this stunning view. On the left lies a southeastern extent of the bright region still informally known as Sputnik Planum. At right the edge of a dark region, informally Krun Macula, rises some 2.5 kilometers above the icy plains. Along the boundary, connected clusters of large pits form deep valleys, some over 40 kilometers long with shadowy floors. Nitrogen ice is likely responsible for the more reflective plains. The dark red color of the highlands is thought to be from complex compounds called tholins, a product of ultraviolet light induced chemical reactions with methane in Pluto's atmosphere. The enhanced color image includes portions of the highest and second highest resolution image data from the New Horizons July 2015 flyby of the distant world.

APOD: 2016 June 9 - Pluto at Night
Explanation: The night side of Pluto spans this shadowy scene. The spacebased view with the Sun behind the distant world was captured by New Horizons last July. The spacecraft was at a range of over 21,000 kilometers, about 19 minutes after its closest approach. A denizen of the Kuiper Belt in dramatic silhouette, the image also reveals Pluto's tenuous, surprisingly complex layers of hazy atmosphere. The crescent twilight landscape near the top of the frame includes southern areas of nitrogen ice plains informally known as Sputnik Planum and rugged mountains of water-ice in the Norgay Montes.

APOD: 2016 April 30 - Moon over Makemake
Explanation: Makemake, second brightest dwarf planet of the Kuiper belt, has a moon. Nicknamed MK2, Makemake's moon reflects sunlight with a charcoal-dark surface, about 1,300 times fainter than its parent body. Still, it was spotted in Hubble Space Telescope observations intended to search for faint companions with the same technique used to find the small satellites of Pluto. Just as for Pluto and its satellites, further observations of Makemake and orbiting moon will measure the system's mass and density and allow a broader understanding of the distant worlds. About 160 kilometers (100 miles) across compared to Makemake's 1,400 kilometer diameter, MK2's relative size and contrast are shown in this artist's vision. An imagined scene of an unexplored frontier of the Solar System, it looks back from a spacecraft's vantage as the dim Sun shines along the Milky Way. Of course, the Sun is over 50 times farther from Makemake than it is from planet Earth.

APOD: 2016 April 2 - Pluto's Bladed Terrain in 3D
Explanation: Get out your red/blue glasses and gaze across a mountainous region informally known as Tartarus Dorsa. This scene sprawls some 300 kilometers (about 180 miles) across the Plutonian landscape. The color anaglyph creates a stereo view by combining parts of two images taken about 14 minutes apart during the New Horizons historic flyby of Pluto last July. Along with shadows near the terminator, or line between Pluto's dim day and night, the 3D perspective emphasizes the alignment of narrow, steep ridges. The region's remarkable bladed landforms typically extend 500 meters high and are 3 to 5 kilometers apart. Referring to a part of Hades in ancient Greek mythology, Tartarus Dorsa borders Tombaugh Regio to the east.

APOD: 2016 February 27 - Northern Pluto
Explanation: Gaze across the frozen canyons of northern Pluto in this contrast enhanced color scene, imaged last July by the New Horizons spacecraft. Currently known as Lowell Regio, the region has been informally named for Percival Lowell, founder of the Lowell Observatory. Also famous for his speculation that there were canals on Mars, in 1906 Lowell started the search that ultimately led to Pluto's discovery. Pluto's North Pole itself is above and left of center in the the frame. The pale bluish floor of the broad canyon on the left is about 70 kilometers (45 miles) wide, running vertically toward the south. Higher elevations take on a yellowish hue. New Horizon's measurements have determined that in addition to nitrogen ice, methane ice is abundant across northern Pluto's Lowell Regio.

APOD: 2016 February 22 - Flying Over Pluto's Moon Charon
Explanation: Given some poetic license, there is now scientific evidence that hell has frozen over. To start, Greek mythology holds that Charon is the ferryman of the underworld. Next, recent analysis of data taken by the robotic New Horizons spacecraft that shot past Charon -- the namesake that is the largest moon of Pluto -- in July now indicates that the cause of the huge chasm that runs across the 1200-km moon was that a huge internal sea froze. And since water expands when it freezes, the already hardened outer crust could not contain it and cracked. To better picture the crack, a fanciful journey over some of Charon's has been digitally created from collected images. The featured video starts by showing the Dark Polar Deposit (dubbed Mordor) near Charon's north pole and then flies over the dwarf-planet-wide canyon. Last, the video shows a much-debated protuberance called Moated Mountain. Understanding the history of Pluto and Charon is helping humanity to better understand both the friendliest and more forbidding places in the early Solar System from which Earth formed and life somehow emerged.

APOD: 2016 January 15 - Wright Mons in Color
Explanation: Informally named Wright Mons, a broad mountain about 150 kilometers across and 4 kilometers high with a wide, deep summit depression is featured in this inset image captured during the New Horizons flyby of Pluto in July 2015. Of course, broad mountains with summit craters are found elsewhere in the Solar System, like the large shield volcano Mauna Loa on planet Earth or giant Olympus Mons on Mars. New Horizons scientists note the striking similarity of Pluto's Wright Mons, and nearby Piccard Mons, to large shield volcanoes suggests the two could be giant cryovolcanoes that once erupted molten ice from the interior of the cold, distant world. In fact, found on a frozen dwarf planet Wright Mons could be the largest volcano in the outer Solar System. Since only one impact crater has been identified on its slopes, Wright Mons may well have been active late in Pluto's history. This highest resolution color image also reveals red material sparsely scattered around the region.

APOD: 2015 December 14 - Pluto: From Mountains to Plains
Explanation: What do the sharpest views ever of Pluto show? As the robotic New Horizons spacecraft moves into the outer Solar System, it is now sending back some of the highest resolution images from its historic encounter with Pluto in July. Featured here is one recently-received, high-resolution image. On the left is al-Idrisi Montes, mountainous highlands thought composed primarily of blocks of water ice. A sharp transitional shoreline leads to the ice plains, on the right, that compose part of the heart-shaped feature known as Sputnik Planum, which contains ices including solid nitrogen. Why the plains are textured with ice pits and segmented is currently unknown. The image was taken about 15 minutes before closest approach and shows an area about 30 kilometers across. The New Horizons spacecraft is next scheduled to fly past Kuiper Belt object 2014 MU 69 on New Year's Day 2019.

APOD: 2015 November 25 - Unusual Pits Discovered on Pluto
Explanation: Why are there unusual pits on Pluto? The indentations were discovered during the New Horizons spacecraft's flyby of the dwarf planet in July. The largest pits span a kilometer across and dip tens of meters into a lake of frozen nitrogen, a lake that sprawls across Sputnik Planum, part of the famous light-colored heart-shaped region named Tombaugh Regio. Although most pits in the Solar System are created by impact craters, these depressions look different -- many are similarly sized, densely packed, and aligned. Rather, it is thought that something has caused these specific areas of ice to sublimate and evaporate away. In fact, the lack of overlying impact craters indicates these pits formed relatively recently. Even though the robotic New Horizons is now off to a new destination, it continues to beam back to Earth new images and data from its dramatic encounter with Pluto.

APOD: 2015 November 14 - Wright Mons on Pluto
Explanation: Long shadows are cast by a low Sun across this rugged looking terrain. Captured by New Horizons, the scene is found just south of the southern tip Sputnik Planum, the informally named smooth, bright heart region of Pluto. Centered is a feature provisionally known as Wright Mons, a broad, tall mountain, about 150 kilometers across and 4 kilometers high, with a 56 kilometer wide, deep summit depression. Of course, broad mountains with central craters are found elsewhere in the Solar System, like Mauna Loa on planet Earth and Olympus Mons on Mars. In fact, New Horizons scientists announced the striking similarity of Pluto's Wright Mons, and nearby Piccard Mons, to large shield volcanoes strongly suggests the two could be giant cryovolcanoes that once erupted molten ice from the interior of the cold, distant world.

APOD: 2015 October 26 - Charon and the Small Moons of Pluto
Explanation: What do the moons of Pluto look like? Before a decade ago, only the largest moon Charon was known, but never imaged. As the robotic New Horizons spacecraft was prepared and launched, other moons were identified on Hubble images but remained only specks of light. Finally, this past summer, New Horizons swept right past Pluto, photographed Pluto and Charon in detail, and took the best images of Styx, Nix, Kerberos, and Hydra that it could. The featured image composite shows the results -- each moon is seen to have a distinct shape, while underlying complexity is only hinted. Even though not satisfyingly resolved, these images are likely to be the best available to humanity for some time. This is because the moons are too small and distant for contemporary Earth-based telescopes to resolve, and no new missions to the Pluto system are planned.

APOD: 2015 October 6 - Flying Past Pluto
Explanation: What would it look like to fly past Pluto? The robotic New Horizons spacecraft did just this in late July and continues to return stunning pictures of the dwarf planet. Some well-chosen flyby images have now been digitally sequenced to create the featured video. The animation begins by showing New Horizon's approach to the Pluto system, with Pluto and its largest moon Charon orbiting a common center of mass. As the spacecraft bears down on Pluto uniquely, surprising surface features are nearly resolved that, unfortunately, quickly rotate out of view. New Horizons then passes just above and near a large, fascinating, light-colored, heart-shaped, and unusually smooth region now known as Tombaugh Regio. The spacecraft then pivots to look back at Pluto's night side, seeing an encompassing atmospheric haze. Finally, Pluto fades away in a final sequence illustrated with the orbits of many of Pluto's smaller moons. Although humanity has no current plans to return to Pluto, the New Horizons spacecraft may well be directed next to fly past an asteroid currently known only as 2014 MU69.

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

APOD: 2015 September 25 - Pluto's Snakeskin Terrain
Explanation: A mountainous region informally known as Tartarus Dorsa sprawls some 530 kilometers (330 miles) across this Plutonian landscape. Recently downloaded from New Horizons, it combines blue, red, and infrared image data in an extended color view captured near the spacecraft's close approach to Pluto on July 14. Shadows near the terminator, the line between Pluto's dim day and night, emphasize a rough, scaly texture. The stunning image resolves details on the distant world about 1.3 kilometers (0.8 miles) across. Refering to a part of Hades in ancient Greek mythology, Tartarus Dorsa borders Tombaugh Regio to the east.

APOD: 2015 September 18 - A Plutonian Landscape
Explanation: This shadowy landscape of majestic mountains and icy plains stretches toward the horizon of a small, distant world. It was captured from a range of about 18,000 kilometers when New Horizons looked back toward Pluto, 15 minutes after the spacecraft's closest approach on July 14. The dramatic, low-angle, near-twilight scene follows rugged mountains still popularly known as Norgay Montes from foreground left, and Hillary Montes along the horizon, giving way to smooth Sputnik Planum at right. Layers of Pluto's tenuous atmosphere are also revealed in the backlit view. With a strangely familiar appearance, the frigid terrain likely includes ices of nitrogen and carbon monoxide with water-ice mountains rising up to 3,500 meters (11,000 feet). That's comparable in height to the majestic mountains of planet Earth. This Plutonian landscape is 380 kilometers (230 miles) across.

APOD: 2015 September 14 - Pluto from above Cthulhu Regio
Explanation: New high resolution images of Pluto are starting to arrive from the outer Solar System. The robotic New Horizons spacecraft, which zoomed by Pluto in July, has finished sending back some needed engineering data and is now transmitting selections from its tremendous storehouse of images of Pluto and its moons. The featured image, a digital composite, details a surprising terrain filled with craters, plains, landscape of unknown character, and landforms that resemble something on Earth but are quite unexpected on Pluto. The light area sprawling across the upper right has been dubbed Sputnik Planum and is being studied for its unusual smoothness, while the dark cratered area just under the spacecraft is known as Cthulhu Regio. So far, New Horizons has only shared a few percent of the images and data it took during its Pluto flyby, but will continue to send back new views of the dwarf planet even as it glides outward toward even more distant explorations.

APOD: 2015 August 31 - Pluto in Enhanced Color
Explanation: Pluto is more colorful than we can see. Color data and images of our Solar System's most famous dwarf planet, taken by the robotic New Horizons spacecraft during its flyby in July, have been digitally combined to give an enhanced view of this ancient world sporting an unexpectedly young surface. The featured enhanced color image is not only esthetically pretty but scientifically useful, making surface regions of differing chemical composition visually distinct. For example, the light-colored heart-shaped Tombaugh Regio on the lower right is clearly shown here to be divisible into two regions that are geologically different, with the leftmost lobe Sputnik Planum also appearing unusually smooth. New Horizons now continues on beyond Pluto, will continue to beam back more images and data, and will soon be directed to change course so that it can fly past asteroid 2014 MU69 in 2019 January.

APOD: 2015 August 6 - Stereo Pluto
Explanation: These two detailed, true color images of Pluto were captured during the historic New Horizons flyby last month. With slightly different perspectives on the now recognizeable surface features they are presented in this first high quality stereo pair intended for viewing by denizens of planet Earth. The left hand image (left eye) is a mosaic recorded when the spacecraft was about 450,000 kilometers from Pluto. The right single image was acquired earlier, a last full look before the spacecraft's closest approach. Despite a difference in resolution, the pair combine for a stunning 3D perception of the distant, underworldly terrain.

APOD: 2015 July 18 - Fly Over Pluto
Explanation: It took 9.5 years to get this close, but you can now take a virtual flight over Pluto in this animation of image data from the New Horizons spacecraft. The Plutonian terrain unfolding 48,000 miles (77,000 kilometers) below is identified as Norgay Montes, followed by Sputnik Planum. The icy mountains, informally named for one of the first two Mount Everest climbers Tenzing Norgay, reach up to 11,000 feet (3,500 meters) above the surface. The frozen, young, craterless plains are informally named for the Earth's first artificial satellite. Sputnik Planum is north of Norgay Montes, within Pluto's expansive, bright, heart-shaped feature provisionally known as Tombaugh Regio for Clyde Tombaugh, who discovered Pluto in 1930.

APOD: 2015 July 17 - Charon
Explanation: Icy world Charon is 1,200 kilometers across. That makes Pluto's largest moon only about 1/10th the size of planet Earth but a whopping 1/2 the diameter of Pluto itself. Charon is seen in unprecedented detail in this image from New Horizons. The image was captured late July 13 during the spacecraft's flight through the Plutonian system from a range of less than 500,000 kilometers. For reference, the distance separating Earth and Moon is less than 400,000 kilometers. Charonian terrain, described as surprising, youthful, and varied, includes a 1,000 kilometer swath of cliffs and troughs stretching below center, a 7 to 9 kilometer deep canyon cutting the curve of the upper right edge, and an enigmatic dark north polar region unofficially dubbed Mordor.

APOD: 2015 July 16 - 50 Miles on Pluto
Explanation: A 50 mile (80 kilometer) trip across Pluto would cover the distance indicated by the scale bar in this startling image. The close-up of the icy world's rugged equatorial terrain was captured when the New Horizons spacecraft was about 47,800 miles (77,000 kilometers) from the surface, 1.5 hours before its closest approach. Rising to an estimated 11,000 feet (3,500 meters) the mountains are likely composed of water ice. Suggesting surprising geological activity, they are also likely young with an estimated age of 100 million years or so based on the apparent absence of craters. The region pictured is near the base of Pluto's broad, bright, heart-shaped feature.

APOD: 2015 July 15 - Pluto Resolved
Explanation: New Horizons has survived its close encounter with Pluto and has resumed sending back images and data. The robotic spacecraft reported back on time, with all systems working, and with the expected volume of data stored. Featured here is the highest resolution image of Pluto taken before closest approach, an image that really brings Pluto into a satisfying focus. At first glance, Pluto is reddish and has several craters. Toward the image bottom is a surprisingly featureless light-covered region that resembles an iconic heart, and mountainous terrain appears on the lower right. This image, however, is only the beginning. As more images and data pour in today, during the coming week, and over the next year, humanity's understanding of Pluto and its moons will likely become revolutionized.

APOD: 2015 July 14 - New Horizons Passes Pluto and Charon
Explanation: Will the New Horizons spacecraft survive its closest approach to Pluto and return useful images and data? Humanity will know in a few hours. Regardless of how well it functions, New Horizon's rapid speed will take it whizzing past Pluto and its moons today, with the time of closest approach being at 11:50 UT (7:50 am EDT). To better take images and data, though, the robotic spacecraft was preprogrammed and taken intentionally out of contact with the Earth until about 1:00 am UT July 15, which corresponds to about 9:00 pm EDT on July 14. Therefore, much of mankind will be holding its breath through this day, hoping that the piano-sized spacecraft communicates again with ground stations on Earth. Hopefully, at that time, New Horizons will begin beaming back new and enlightening data about a world that has remained remote and mysterious since its discovery 85 years ago. Featured above is a New Horizons composite image of the moon Charon (left) and Pluto (right) taken 3 days ago, already showing both worlds in unprecedented detail.

APOD: 2015 July 13 - Last Look at Pluto's Charon Side
Explanation: Pluto surface is strange. As the robotic New Horizons barrels toward its closest approach to Pluto and its moons tomorrow, images already coming back show Pluto's surface to be curiouser and curiouser. The featured image, taken two days ago, shows the side of Pluto that always faces Pluto's largest moon Charon. Particularly noteworthy is the dark belt near the bottom that circles Pluto's equator. It is currently unclear whether regions in this dark belt are mountainous or flat, why boundaries are so sharply defined, and why the light regions seem to be nearly evenly spaced. As New Horizons will be flying past the other side of Pluto, this should be the best image of this distant landscape that humanity sees for a long time. Assuming the robotic spacecraft operates as hoped, images taken of the other side of Pluto, taken near closest approach, will be about 300 times more detailed.

APOD: 2015 July 12 - New Horizons Launch to Pluto
Explanation: Destination: Pluto. The New Horizons spacecraft roared off its launch pad at Cape Canaveral in Florida, USA in 2006 toward adventures in the distant Solar System. The craft is the fastest spaceships ever launched by humans, having passed the Moon only nine hours after launch, and Jupiter only a year later. After spending almost a decade crossing the Solar System, New Horizons will fly past Pluto on Tuesday. Pluto, officially a planet when New Horizons launched, has never been visited by a spacecraft or photographed up close. After Pluto, the robot spaceship will visit one or more Kuiper Belt Objects orbiting the Sun even farther out than Pluto. Pictured, the New Horizons craft launches into space atop a powerful Atlas V rocket.

APOD: 2015 July 11 - Geology on Pluto
Explanation: Pluto is coming into focus. As the robotic New Horizons spacecraft bears down on this unexplored world of the distant Solar System, new features on its surface are becoming evident. In the displayed image taken last Thursday and released yesterday, an unusual polygonal structure roughly 200 kilometers wide is visible on the left, while just below it relatively complex terrain runs diagonally across the dwarf planet. New Horizon's images and data on these structures will likely be studied for years to come in an effort to better understand the geologic history of Pluto and our Solar System. After suffering a troublesome glitch last week, New Horizons will make its historic flyby of Pluto and its moons on Tuesday.

APOD: 2015 July 9 - 5 Million Miles from Pluto
Explanation: An image snapped on July 7 by the New Horizons spacecraft while just under 5 million miles (8 million kilometers) from Pluto is combined with color data in this most detailed view yet of the Solar System's most famous world about to be explored. The region imaged includes the tip of an elongated dark area along Pluto's equator already dubbed "the whale". A bright heart-shaped region on the right is about 1,200 miles (2,000) kilometers across, possibly covered with a frost of frozen methane, nitrogen, and/or carbon monoxide. The view is centered near the area that will be seen during New Horizons much anticipated July 14 closest approach to a distance of about 7,750 miles (12,500 kilometers).

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 May 14 - Dwarf Planet, Bright Spot
Explanation: Now at Ceres, Dawn's camera recorded this closer view of the dwarf planet's northern hemisphere and one of its mysterious bright spots on May 4. A sunlit portrait of a small, dark world about 950 kilometers in diameter, the image is part of a planned sequence taken from the solar-powered spacecraft's 15-day long RC3 mapping orbit at a distance of 13,600 kilometers (8,400 miles). The animated sequence shows Ceres' rotation, its north pole at the top of the frame. Imaged by Hubble in 2004 and then by Dawn as it approached Ceres in 2015, the bright spot itself is revealed to be made up of smaller spots of reflective material that could be exposed ice glinting in the sunlight. On Saturday, Dawn's ion propulsion system was turned on to spiral the spacecraft into a closer 4,350-kilometer orbit by June 6. Of course another unexplored dwarf planet, Pluto, is expecting the arrival of a visitor from Earth, the New Horizons spacecraft, by mid-July.

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 March 31 - 2012 VP113: A New Furthest Known Orbit in the Solar System
Explanation: What object has the furthest known orbit in our Solar System? In terms of how close it will ever get to the Sun, the new answer is 2012 VP113, an object currently over twice the distance of Pluto from the Sun. Pictured above is a series of discovery images taken with the Dark Energy Camera attached to the NOAO's Blanco 4-meter Telescope in Chile in 2012 and released last week. The distant object, seen moving on the lower right, is thought to be a dwarf planet like Pluto. Previously, the furthest known dwarf planet was Sedna, discovered in 2003. Given how little of the sky was searched, it is likely that as many as 1,000 more objects like 2012 VP113 exist in the outer Solar System. 2012 VP113 is currently near its closest approach to the Sun, in about 2,000 years it will be over five times further. Some scientists hypothesize that the reason why objects like Sedna and 2012 VP113 have their present orbits is because they were gravitationally scattered there by a much larger object -- possibly a very distant undiscovered planet.

APOD: 2013 September 15 - M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

APOD: 2013 July 8 - Pluto's Newly Discovered Moons Receive Names
Explanation: Pluto's newly discovered moons now have names. Known previously as P4 and P5, the International Astronomical Union has now given the fourth and fifth discovered moons of Pluto the names Kerberos and Styx. The small moons were discovered in 2011 and 2012 by the Hubble Space Telescope in preparation for the close passing of the New Horizons spacecraft by Pluto in 2015. Kerberos is named for the many headed dog in Greek mythology that guards the entrance to the underworld, while Styx is named for the goddess who overlooks the mythological river that runs between the Earth and the underworld. Both monikers are related to the name of Pluto, who rules the mythical nether region. Because their reflectivity is unknown, the size of each moon is quite uncertain -- but each is crudely estimated to be about 20 kilometers in diameter. The robotic New Horizons spacecraft is on schedule to pass by Pluto in 2015 and provide the first clear images of the dwarf planet and its companions.

APOD: 2013 April 30 - Humanity Explores the Solar System
Explanation: What spacecraft is humanity currently using to explore our Solar System? Presently, every inner planet has at least one robotic explorer, while several others are monitoring our Sun, some are mapping Earth's Moon, a few are chasing asteroids and comets, one is orbiting Saturn, and several are even heading out into deep space. The above illustration gives more details, with the inner Solar System depicted on the upper right and the outer Solar System on the lower left. Given the present armada, our current epoch might become known as the time when humanity first probed its own star system. Sometimes widely separated spacecraft act together as an InterPlanetary Network to determine the direction of distant explosions by noting when each probe detects high energy photons. Future spacecraft milestones, as indicated along the bottom of the graphic, include Dawn reaching Ceres, the largest object in the asteroid belt, and New Horizons reaching Pluto, both in 2015.

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: 2013 January 10 - The Orion Bullets
Explanation: Cosmic bullets pierce the outskirts of the Orion Nebula some 1500 light-years distant in this sharp infrared close-up. Blasted out by energetic massive star formation the bullets, relatively dense, hot gas clouds about ten times the size of Pluto's orbit, are blue in the false color image. Glowing with the light of ionized iron atoms they travel at speeds of hundreds of kilometers per second, their passage traced by yellowish trails of the nebula's shock-heated hydrogen gas. The cone-shaped wakes are up to a fifth of a light-year long. The detailed image was created using the 8.1 meter Gemini South telescope in Chile with a newly commisioned adaptive optics system (GeMS). Achieving a larger field of view than previous generation adaptive optics, GeMS uses five laser generated guide stars to help compensate for the blurring effects of planet Earth's atmosphere.

APOD: 2012 December 26 - Makemake of the Outer Solar System
Explanation: Makemake is one of the largest objects known in the outer Solar System. Pronounced MAH-kay MAH-kay, this Kuiper belt object is about two-thirds the size of Pluto, orbits the Sun only slightly further out than Pluto, and appears only slightly dimmer than Pluto. Makemake, however, has an orbit much more tilted to the ecliptic plane of the planets than Pluto. Discovered by a team led by Mike Brown (Caltech) in 2005, the outer Solar System orb was officially named Makemake for the creator of humanity in the Rapa Nui mythology of Easter Island. In 2008, Makemake was classified as a dwarf planet under the subcategory plutoid, making Makemake the third cataloged plutoid after Pluto and Eris. Makemake is known to be a world somewhat red in appearance, with colors indicating it is likely covered with patchy areas of frozen methane. No images of Makemake's surface yet exist, but an artist's illustration of the distant world is shown above. Careful monitoring of the brightness drop of a distant star recently eclipsed by Makemake indicates that the dwarf planet has little atmosphere.

APOD: 2012 July 16 - Fifth Moon Discovered Orbiting Pluto
Explanation: A fifth moon has been discovered orbiting Pluto. The moon was discovered earlier this month in images taken by the Hubble Space Telescope in preparation for the New Horizons mission's scheduled flyby of Pluto in 2015. Pictured above, the moon is currently seen as only a small blip that moves around the dwarf planet as the entire system slowly orbits the Sun. The moon, given a temporary designation of S/2012 (134340) 1 or just P5 (as labeled), is estimated to span about 15 kilometers and is likely composed mostly of water-ice. Pluto remains the only famous Solar System body never visited by a human-built probe and so its origins and detailed appearance remain mostly unknown.

APOD: 2011 October 23 - Jupiter's Clouds from New Horizons
Explanation: The New Horizons spacecraft took some stunning images of Jupiter on its way out to Pluto. Famous for its Great Red Spot, Jupiter is also known for its regular, equatorial cloud bands, visible through even modest sized telescopes. The above image, horizontally compressed, was taken in 2007 near Jupiter's terminator and shows the Jovian giant's wide diversity of cloud patterns. On the far left are clouds closest to Jupiter's South Pole. Here turbulent whirlpools and swirls are seen in a dark region, dubbed a belt, that rings the planet. Even light colored regions, called zones, show tremendous structure, complete with complex wave patterns. The energy that drives these waves surely comes from below. New Horizons is the fastest space probe ever launched, has now passed the orbits of Saturn and Uranus and is on track to reach Pluto in 2015.

APOD: 2011 July 22 - Pluto's P4
Explanation: Nix and Hydra were first introduced to human eyes in Hubble Space Telescope images from May 2005, as Pluto's second and third known moons. Now Hubble images have revealed a fourth satellite for the icy, dwarf planet. Provisionally designated P4, it completes an orbit of Pluto in about 31 days. Presently Pluto's smallest and dimmest known moon, P4 is estimated to be 13 to 34 kilometers across. The newly discovered satellite was first spotted in Hubble observations from June 28, and later confirmed in a follow-up on July 3 and July 18. These two panels are composites of both the short and long exposures that include brighter Pluto itself along with Pluto's largest moon Charon. Camera noise and image artifacts also show up in the long exposure segments. The Hubble observations were made while searching for faint rings around the distant world in support of NASA's New Horizons mission, set to fly by the Pluto system in 2015.

APOD: 2011 May 6 - Farther Along
Explanation: What is humanity's most distant spacecraft? Launched in 1977, Voyager 1 now holds that distinction at 17.5 billion kilometers from the Sun. That corresponds to 16 light-hours or 117 Astronomical Units (AU). This graphic shows the position of Voyager 1 relative to the outer solar system (top and side views) along with other distant spacecraft contenders. Next most distant, Pioneer 10 is about 15.4 billion kilometers from the Sun, though on the opposite side of the solar system from Voyager 1. Voyager 2 and Pioneer 11, both also well beyond the orbit of Pluto, are 14.2 billion and 12.4 billion kilometers from the Sun respectively. Still outbound for Pluto, the New Horizons spacecraft is presently 3 billion kilometers from the Sun and will encounter the Pluto system in July of 2015. All these spacecraft have used sling-shot style gravity assist maneuvers to increase their speeds through the outer solar system. Voyager 1 is moving the fastest, escaping the solar system at about 17 kilometers per second. Still operational, both Voyagers are headed towards the outer boundary of the solar system, in search of the heliopause and the beginning of interstellar space.

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 July 8 - Dim World, Dark Nebula
Explanation: Dim, distant, dwarf planet Pluto can be hard to spot, especially in recent months as it wanders through the crowded starfields of Sagittarius and the central Milky Way. But fortunately for backyard Pluto hunters, it crossed in front of a dark nebula in early July. The diminutive world is marked with two short lines near the center of this skyscape recorded from New Mexico Skies on July 5. Pluto stands out only because obscuring dark nebula Barnard 92 (B92) blocks the background of the Milky Way's congeries of faint, innumerable stars. Another of astronomer E. E. Barnard's cataloged dark markings on the sky, B93, is easy to pick out just left of B92. Prominent at the lower left is open star cluster NGC 6603. In fact, Pluto, dark nebulae, and star cluster all lie within a portion of M24, also known as the Sagittarius Star Cloud, filling most of the frame.

APOD: 2010 January 24 - Watch Jupiter Rotate
Explanation: What would it be like to coast by Jupiter and watch it rotate? This was just the experience of the New Horizons spacecraft as it approached and flew by Jupiter in 2007. Clicking on the image will bring up a movie of what the robotic spacecraft saw. Visible above in the extensive atmosphere of the Solar System's largest planet are bands and belts of light and dark clouds, as well as giant rotating storm systems seen as ovals. Other movies compiled by New Horizons and other passing spacecraft have captured the clouds swirling and moving relative to themselves. Jupiter has a diameter of about eleven times that of our Earth, and rotates once in about 10 hours. The robotic New Horizons spacecraft, launched four years ago last week, continues to speed toward the outer Solar System and has recently passed the halfway point between Earth and Pluto. New Horizons will reach Pluto in 2015.

APOD: 2009 September 20 - Ganymede Enhanced
Explanation: What does the largest moon in the Solar System look like? Ganymede, larger than even Mercury and Pluto, has a surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges. Like Earth's Moon, Ganymede keeps the same face towards its central planet, in this case Jupiter. In this historic and detailed image mosaic taken by the Galileo spacecraft that orbited Jupiter from 1995 to 2003, the colors of this planet-sized moon have been enhanced to increase surface contrasts. The violet shades extending from the top and bottom are likely due to frost particles in Ganymede's polar regions. Possible future missions to Jupiter are being proposed that can search Europa and Ganymede for deep oceans that may harbor elements thought important for supporting life.

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: 2008 September 23 - Haumea of the Outer Solar System
Explanation: One of the strangest objects in the outer Solar System was classified as a dwarf planet last week and given the name Haumea. This designation makes Haumea the fifth designated dwarf planet after Pluto, Ceres, Eris, and Makemake. Haumea's smooth but oblong shape make it extremely unusual. Along one direction, Haumea is significantly longer than Pluto, while in another direction Haumea has an extent very similar to Pluto, while in the third direction is much smaller. Haumea's orbit sometimes brings it closer to the Sun than Pluto, but usually Haumea is further away. Illustrated above, an artist visualizes Haumea as a nearly featureless ellipsoid. Quite possibly, however, Haumea has interesting craters and surface features that currently remain unknown. Originally discovered in 2003 and given the temporary designation of 2003 EL61, Haumea was recently renamed by the IAU for a Hawaiian goddess. Haumea has two small moons discovered in 2005, recently renamed Hi'iaka and Namaka for daughters of the goddess.

APOD: 2008 July 16 - Makemake of the Outer Solar System
Explanation: Recently discovered Makemake is one of the largest objects known in the outer Solar System. Pronounced MAH-kay MAH-kay, this Kuiper belt object is only slightly smaller than Pluto, orbits the Sun only slightly further out than Pluto, and appears only slightly dimmer than Pluto. Makemake, however, has an orbit much more tilted to the ecliptic plane of the planets than Pluto. Designated 2005 FY9 soon after its discovery by a team led by Mike Brown (Caltech) in 2005, the outer Solar System orb was recently renamed Makemake for the creator of humanity in the Rapa Nui mythology of Easter Island. Additionally, Makemake has been recently classified as a dwarf planet under the new subcategory plutoid, making Makemake the third cataloged plutoid after Pluto and Eris. Makemake is known to be a world somewhat red in appearance, with spectra indicating it is likely covered with frozen methane. Since no images of Makemake's surface yet exist, an artist's illustration originally meant to depict Sedna has been boldly co-opted above to now illustrate Makemake. A hypothetical moon is visualized above nearly in the direction of our distant Sun.

APOD: 2008 April 13 - Curious Cometary Knots in the Helix Nebula
Explanation: What causes unusual knots of gas and dust in planetary nebulas? Seen also in the Ring Nebula, the Dumbbell Nebula and NGC 2392, the knots' existence was not initially predicted and their origins are still not well understood. Pictured above is a fascinating image of the Helix Nebula by the Hubble Space Telescope showing tremendous detail of its mysterious gaseous knots. The above cometary knots have masses similar to the Earth but have radii typically several times the orbit of Pluto. One hypothesis for the fragmentation and evolution of the knots includes existing gas being driven out by a less dense but highly energetic stellar wind of the central evolving star. The Helix Nebula is the closest example of a planetary nebula created at the end of the life of a Sun-like star. The Helix Nebula, given a technical designation of NGC 7293, lies about 700 light-years away towards the constellation of Aquarius.

APOD: 2008 January 8 - A Jupiter-Io Montage from New Horizons
Explanation: As the New Horizons spacecraft sweeps through the Solar System, it is taking breathtaking images of the planets. In February of last year, New Horizons passed Jupiter and the ever-active Jovian moon Io. In this montage, Jupiter was captured in three bands of infrared light making the Great Red Spot look white. Complex hurricane-like ovals, swirls, and planet-ringing bands are visible in Jupiter's complex atmosphere. Io is digitally superposed in natural color. Fortuitously, a plume was emanating from Io's volcano Tvashtar. Frost and sulfuric lava cover the volcanic moon, while red-glowing lava is visible beneath the blue sunlight-scattering plume. The robotic New Horizons spacecraft is on track to arrive at Pluto in 2015.

APOD: 2007 October 15 - Jupiter's Clouds from New Horizons
Explanation: The New Horizons spacecraft took some stunning images of Jupiter earlier this year while on the way out to Pluto. Famous for its Great Red Spot, Jupiter is also known for its regular, equatorial cloud bands, visible through even modest sized telescopes. The above image was taken near Jupiter's terminator, and shows that the Jovian giant possibly has the widest diversity of cloud patterns in our Solar System. On the far left are clouds closest to Jupiter's south pole. Here turbulent whirlpools and swirls are seen in a dark region, dubbed a belt, that rings the planet. Even light colored regions, called zones, show tremendous structure, complete with complex wave patterns. The energy that drives these waves likely comes from below. New Horizons is the fastest space probe ever launched, and is zipping through the Solar System on track to reach Pluto in 2015.

APOD: 2007 July 30 - The Four Suns of HD 98800
Explanation: How would it look to have four suns in the sky? Planets of the HD 98800 system, if they exist, would experience such a view. HD 98800 is a multiple star system about 150 light years from Earth -- right in our section of the Milky Way Galaxy. For years it has been known that HD 98800 consists of two pairs of double stars, with one pair surrounded by a disk of dust. The star pairs are located about 50 AU from each other -- in comparison just outside the orbit of Pluto. Recent data from the Earth-trailing Spitzer Space Telescope in infrared light, however, indicate that the dust disk has gaps that appear consistent with being cleared by planets orbiting in the disk. If so, one planet appears to be orbiting at a distance similar to Mars of our own Solar System. Pictured above is an artist's drawing of how the HD 98800 system might appear to a nearby observer.

APOD: 2007 June 19 - Eris: More Massive than Pluto
Explanation: Eris, a dwarf planet currently orbiting the Sun at about twice Pluto's distance, has been measured to have about 27 percent more mass than Pluto. The mass was calculated by timing the orbit of Eris' moon Dysnomia. Images taken with a ground-based Keck telescope, when combined with existing images taken by Hubble Space Telescope, show that Dysnomia has a nearly circular orbit lasting about 16 days. Cataloged as 2003 UB313 only a year ago, infrared images also showed previously that Eris is actually larger in diameter than Pluto. The plane of Eris' orbit is well out of the plane of the Solar System's planets. In the above drawing, a scientific artist has imagined Eris and Dysnomia orbiting our distant Sun. No space missions are currently planned to Eris, although the robotic New Horizons spacecraft bound for Pluto has recently passed Jupiter.

APOD: 2007 May 7 - Europa Rising
Explanation: When passing Jupiter on your way to Pluto, what should you look for? NASA pondered just this question recently, and the response from one space enthusiast was to capture the above breathtaking moonrise. The unusual vista was then actually captured by the New Horizons spacecraft in February just after it buzzed past Jupiter on its way to Pluto and the outer Solar System. Visible above is the cracked surface of Europa's expansive ice fields, visible just behind a jumble of Jupiter's swirling clouds. Europa is one of the largest moons of Jupiter and a possible host to sub-surface liquid oceans that are real candidates for containing extra-terrestrial life. During the Jupiter flyby, New Horizons also carried out scientific observations of Jupiter's cloud tops and comparative images of Io's volcanoes and its continually changing surface.

APOD: 2007 April 4 - New Horizons at Io
Explanation: Spewed from a volcano, a complex plume rises over 300 kilometers above the horizon of Jupiter's moon Io in this image from cameras onboard the New Horizons spacecraft. The volcano, Tvashtar, is marked by the bright glow (about 1 o'clock) at the moon's edge, beyond the terminator or night/day shadow line. The shadow of Io cuts across the plume itself. Also capturing stunning details on the dayside surface, the high resolution image was recorded when the spacecraft was 2.3 million kilometers from Io. Later it was combined with lower resolution color data by astro-imager Sean Walker to produce this sharp portrait of the solar system's most active moon. Outward bound at almost 23 kilometers per second, the New Horizons spacecraft should cross the orbit of Saturn in June next year, and is ultimately destined to encounter Pluto in 2015.

APOD: 2007 March 12 - Watch Jupiter Rotate
Explanation: What would it be like to coast by Jupiter and watch it rotate? This was just the experience of the New Horizons spacecraft as it approached and flew by Jupiter earlier this year. Clicking on the image will bring up a movie of what the robotic spacecraft saw. Visible above in the extensive atmosphere of the Solar System's largest planet are bands and belts of light and dark clouds, as well as giant rotating storm systems seen as ovals. Other movies compiled by New Horizons and other passing spacecraft have captured the clouds swirling and moving relative to themselves. Jupiter has a diameter of about eleven times that of our Earth, and rotates once in about 10 hours. The robotic New Horizons spacecraft continues to speed toward the outer Solar System where it is expected to approach Pluto in 2015.

APOD: 2007 March 7 - New Horizons Spacecraft Passes Jupiter
Explanation: A new spacecraft is headed for the outer Solar System. Named New Horizons, this robotic explorer passed Jupiter last week after being launched only in early 2006. New Horizons is being pulled by Jupiter's gravity to a greater speed toward its next target: Pluto in 2015. During its encounter with Jupiter, New Horizons was able to capture new images of many Jovian moons, Jupiter's complex and ever-changing atmosphere, and Jupiter's Little Red Spot, pictured above. Formed over the past few years from several smaller storms, Jupiter's Little Red Spot survived a near miss with Jupiter's better-known Great Red Spot last year. The above image of Jupiter covers over twice the diameter of the Earth.

APOD: 2006 October 21 - Tombaugh 4
Explanation: Clyde Tombaugh discovered planet Pluto in 1930 while surveying the skies with the 13-inch Lawrence Lowell Telescope. But the skilled and careful astronomer also went on to discover star clusters, comets, asteroids, and clusters of galaxies. For example, pictured is galactic or open star cluster Tombaugh 4 in the northern constellation Cassiopeia. Published in 1941, Tombaugh's description, based on his photographic images from the Lowell 13-inch, indicates the cluster is small and faint, and comprised of about 30 stars. Using the apparent brightness of the cluster stars he estimated the distance to be 20 to 30 thousand light-years, making Tombaugh 4 over 10 light-years in diameter. This deep color image, made with a modern ccd camera and another 13-inch telescope, includes the region's foreground stars and faint nebulosities.

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 September 3 - Pluto in True Color
Explanation: Pluto is mostly brown. The above picture captures the true colors of Pluto as well as the highest surface resolution so far recovered. Although no spacecraft has yet visited this distant world, the New Horizons spacecraft launched early this year is expected to reach Pluto in 2015. Pluto recent reclassification, by the International Astronomical Union, from planet to dwarf planet remains a topic of much debate. The above map was created by tracking brightness changes from Earth of Pluto during times when it was being partially eclipsed by its moon Charon. The map therefore shows the hemisphere of Pluto that faces Charon. Pluto's brown color is thought dominated by frozen methane deposits metamorphosed by faint but energetic sunlight. The dark band below Pluto's equator is seen to have rather complex coloring, however, indicating that some unknown mechanisms may have affected Pluto's surface.

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 24 - Nix and Hydra
Explanation: Discovered in mid-2005, Pluto's small moons were provisionally designated S/2005 P1 and S/2005 P2. They have now been officially christened Nix and Hydra. Compared to Pluto and its large moon Charon, at 2,360 and 1,210 kilometers in diameter respectively, Nix (inner moon) and Hydra (outer moon) are tiny, estimated to be only 40 to 160 kilometers across. Pluto and Charon are bright enough to create diffraction spikes in this Hubble Space Telescope image, but Nix and Hydra are some 5,000 times fainter than Pluto and appear only as small points of light. Still, their new names are appropriate for the distant Pluto system. In mythology, Nix was the goddess of darkness and night and the mother of Charon, while Hydra was a nine headed monster and is now orbiting the solar system's ninth planet. Of course Nix and Hydra also share initials with the pluto-bound spacecraft New Horizons.

APOD: 2006 February 7 - UB313: Larger than Pluto
Explanation: What do you call an outer Solar System object that is larger than Pluto? Nobody is yet sure. The question arose recently when 2003 UB313, an object currently twice as far out as Pluto and not in the plane with the rest of the planets, was verified recently to be 30 percent wider than Pluto. UB313's size was measured by a noting its distance from the Sun and how much infrared light it emits. Previous size estimates were based only on visible light and greatly affected by how reflective the object is. Whether 2003 UB313 is officially declared a planet will be answered shortly by the International Astronomical Union. In the above picture, a scientific artist has imagined UB313 in its distant orbit around the Sun coupled with a hypothetical moon.

APOD: 2006 January 24 - New Horizons Launches to Pluto
Explanation: Destination: Pluto. The New Horizons spacecraft roared off its launch pad at Cape Canaveral in Florida, USA last week toward adventures in the distant Solar System. The craft is one of the fastest spaceships ever launched by humans, having passed the Moon only nine hours after launch and is on track to buzz Jupiter in early 2007. Even traveling over 75,000 kilometers per hour, the New Horizons craft will not arrive at Pluto until 2015. Pluto is the only remaining planet that has never been visited by a spacecraft or photographed up close. After Pluto, the robot spaceship will visit one or more Kuiper Belt Objects orbiting the Sun even further out than Pluto. Pictured, the New Horizons craft launches into space atop a powerful Atlas V rocket.

APOD: 2005 December 26 - SN 1006: Supernova Remnant in X Rays
Explanation: This huge puff ball was once a star. One thousand years ago, in the year 1006, a new star was recorded in the sky that today we know was really an existing star exploding. The resulting expanding gas from the supernova is still visible with telescopes today, continues to expand, and now spans over 70 light years. SN 1006 glows in every type of light. The above image of SN 1006 was captured by the orbiting Chandra Observatory in X-ray light. Even today, not everything about the SN 1006 is understood, for example why particle shocks that produce the bright blue filaments are only visible at some locations. SN 1006 is thought to have once been a white dwarf that exploded when gas being dumped onto it by its binary star companion caused it to go over the Chandrasekhar limit. Foreground stars are visible that have nothing to do with the supernova.

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: 2005 October 23 - At the Center of the Milky Way
Explanation: At the center of our Milky Way Galaxy lies a black hole with over 2 million times the mass of the Sun. Once a controversial claim, this astounding conclusion is now virtually inescapable and based on observations of stars orbiting very near the galactic center. Using one of the Paranal Observatory's very large telescopes and a sophisticated infrared camera, astronomers patiently followed the orbit of a particular star, designated S2, as it came within about 17 light-hours of the center of the Milky Way (about 3 times the radius of Pluto's orbit). Their results convincingly show that S2 is moving under the influence of the enormous gravity of an unseen object that must be extremely compact -- a supermassive black hole. This deep near-infrared image shows the crowded inner 2 light-years of the Milky Way with the exact position of the galactic center indicated by arrows. The ability to track stars so close to the galactic center can accurately measure the black hole's mass and perhaps even provide an unprecedented test of Einstein's theory of gravity as astronomers watch a star orbit a supermassive black hole.

APOD: 2005 August 1 - 2003 UB 313: A Tenth Planet?
Explanation: Has a tenth planet been discovered? A newly discovered object, designated 2003 UB₃₁₃ and located more than twice the distance of Pluto, is expected to be at least as large as Pluto and probably larger, given current measurements. 2003 UB₃₁₃'s dimness and highly tilted orbit (44 degrees) prevented it from being discovered sooner. Many astronomers speculate that numerous other icy objects larger than Pluto likely exist in the Kuiper Belt of the far distant Solar System. If so, and if some are found closer in than 2003 UB₃₁₃, it may be premature to call 2003 UB₃₁₃ the tenth planet. Illustrated above is an artist's drawing showing how 2003 UB₃₁₃ might look. The unusually bright star on the right is the Sun. Much of the world eagerly await the decision by the International Astronomical Union on whether 2003 UB₃₁₃ will be designated a planet or given a name without subscripts.

APOD: 2005 July 31 - Solar System Object Larger than Pluto Discovered
Explanation: Is that a tenth planet? A faint, slowly moving dot discovered by computer shows clear signs of being a deep Solar System object at least as large as Pluto. The object, designated 2003 UB₃₁₃, is currently situated nearly 100 times the Earth-Sun distance -- over twice the average Pluto-Sun distance. That far out, the only way a single round object could be as bright as 2003 UB₃₁₃ would be if it is at least as large as Pluto and completely reflective. Since 2003 UB₃₁₃ is surely not completely reflective, it could be substantially larger. One of the discovery frames is shown above digitally expanded and artificially brightened. 2003 UB₃₁₃ was identified initially on frames taken by the automated 1.2-meter Samuel Oschin Telescope at Palomar Observatory in California, USA.

APOD: 2005 July 30 - M106 in Canes Venatici
Explanation: Close to the Great Bear (Ursa Major) and surrounded by the stars of the Hunting Dogs (Canes Venatici), this celestial nebula was discovered in 1781 by the metric French astronomer Pierre Mechain. Later, it was added to the catalog of his friend and colleague Charles Messier as M106. Modern deep telescopic views reveal it to be an island universe -- a spiral galaxy around 30 thousand light-years across located only about 21 million light-years beyond the stars of the Milky Way. Youthful blue star clusters and reddish stellar nurseries trace the striking spiral arms of M106. Seen so clearly in this beautiful image, the galaxy's bright core is also visible across the spectrum from radio to x-rays, making M106 a nearby example of the Seyfert class of active galaxies. The bright core of a Seyfert galaxy is believed to be powered by matter falling into a massive central black hole.

APOD: 2005 June 12 - M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

APOD: 2004 August 27- The Sedna Scenario
Explanation: The discovery of Sedna (aka 2003 VB12), the most distant known object orbiting the Sun, presents a mystery. Pluto's orbit averages about 40 AU in radius, where an AU (Astronomical Unit) is the Earth-Sun distance. But the closest point in Sedna's eccentric orbit scarcely comes within 75 AU, while its farthest point extends to nearly 1,000 AU. So how did something as large as Sedna get so far out there? Exploring the problem with computer simulations, astronomers Alessandro Morbidelli and Harold Levison suggest that while Sedna was not formed in its current location, it was also not moved there by encounters with other solar system objects. Instead, they find it more likely that Sedna resides in its present orbit because of an encounter with another star. In one scenario, objects like Sedna are yanked out of closer orbits by the gravitational pull of a Sun-sized star passing near the solar system during its formative years. Alternatively Sedna could have formed of material from another system entirely, captured during an early encounter with a much smaller star. Both Sedna-forming stellar encounter scenarios are consistent with idea that the Sun itself was born in an ancient, dense, cluster of stars.

APOD: 2004 May 14 - Zubenelgenubi and Friends
Explanation: Moderately bright Zubenelgenubi is the star just off the upper right hand limb of an eclipsed Moon in this telescopic view from Port Elizabeth, South Africa. Actually the second brightest star in the constellation Libra, Zubenelgenubi is fun to pronounce (try zoo-BEN-al-je-NEW-bee ...) and rewarding to spot in the night sky as it has a fainter companion star, seen here on the far right. Astronomer Francois du Toit reports that both stars were visible to the unaided eye on the night of May 4th, during the Moon's total eclipse phase. Orbiting a common center of gravity once every 200,000 years or so, the two stars are both larger and hotter than the Sun. About 77 light years away they are separated from each other by over 730 light hours -- about 140 times Pluto's average distance from the Sun. Zubenelgenubi was once considered the southern claw of the nearby arachnologically correct constellation Scorpius. What star was the northern claw? Zubeneschamali, of course.

APOD: 2004 March 16 - Sedna of the Outer Solar System
Explanation: What is the most distant known object in our Solar System? A new answer to this centuries-old question was announced yesterday by NASA with the discovery of a dark red object dubbed Sedna. Although over twice the distance to Pluto, Sedna is near its closest approach to the Sun. Sedna's highly elliptical orbit will further displace it by 10 times, making it a candidate for the long-hypothesized Oort cloud of icy objects thought to extend to the Solar System's edge. Sedna is estimated to be about three-quarters the size of Pluto and therefore the largest Solar System object found since Pluto in 1930. Whether Sedna is ever designated a planet is at the discretion of the International Astronomical Union. The above drawing depicts how Sedna might look facing the distant Sun. The unexpectedly red color, the unusual orbit, and the origin of Sedna will surely be topic of much future research.

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: 2004 February 1 - M2-9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

APOD: 2003 December 4 - New Horizons at Jupiter
Explanation: Headed for the first close-up exploration of the Pluto-Charon system and the icy denizens of the Kuiper belt, NASA's New Horizons spacecraft is pictured here in an artist's vision of the robot probe outward bound. The dramatic scene depicts the 465 kilogram spacecraft about one year after a planned 2006 launch, following a flyby of gas giant Jupiter. While the Jupiter flyby will be used as a gravity assist maneuver to save fuel and cut travel time to the outer reaches of the Solar System, it will also provide an opportunity to test instruments and study the giant planet, its moons, and magnetic fields. The Sun is seen from eight hundred million kilometers away, with inner planets Earth, Venus, and Mercury aligned on the left. A dim crescent of outermost Galilean moon Callisto, orbiting Jupiter just inside of the spacecraft's trajectory, appears to the upper right of the fading Sun. Left of Jupiter itself is Europa and in the distant background are the faint, unresolved stars and dust clouds of the Milky Way. New Horizons' planned arrival at Pluto-Charon is in the summer of 2015.

APOD: 2003 September 9 - A Gemini Sky
Explanation: Where will Gemini take us tonight? It is dusk and Gemini North, one of the largest telescopes on planet Earth, prepares to peer into the distant universe. Gemini's flexible 8.1-mirror has taken already effectively taken humanity to distant stars, nebulas, galaxies, and quasars, telling us about the geometry, composition, and evolution of our universe. The above picture is actually a composite of over 40 images taken while the Gemini dome rotated, later adding an image of the star field taken from the same location. The Gemini dome is not transparent -- it only appears so because it rotated during the exposures of this image. The constellations of Scorpius and Sagittarius can be seen above the dome, as well as the sweeping band of our Milky Way Galaxy, including the direction toward the Galactic center. Gemini North's twin, Gemini South, resides in Cerro Pachn, Chile. This night, 2003 August 19, Gemini North took us only into the outer Solar System, observing Pluto in an effort to better determine the composition of its thin atmosphere.

APOD: 2002 October 18 - At the Center of the Milk Way
Explanation: At the center of our Milky Way Galaxy lies a black hole with over 2 million times the mass of the Sun. Once a controversial claim, this astounding conclusion is now virtually inescapable and based on observations of stars orbiting very near the galactic center. Using one of the Paranal Observatory's very large telescopes and the sophisticated infrared camera NACO, astronomers patiently followed the orbit of a particular star, designated S2, as it came within about 17 light-hours of the center of the Milky Way (17 light-hours is only about 3 times the radius of Pluto's orbit). Their results convincingly show that S2 is moving under the influence of the enormous gravity of an unseen object which must be extremely compact -- a supermassive black hole. This deep NACO near-infrared image shows the crowded inner 2 light-years of the Milky Way with the exact position of the galactic center indicated by arrows. NACO's ability to track stars so close to the galactic center can accurately measure the black hole's mass and perhaps even provide an unprecedented test of Einstein's theory of gravity as astronomers watch a star orbit a supermassive black hole.

APOD: 2002 October 9 - Quaoar: Large Asteroid in the Outer Solar System
Explanation: Asteroids almost as large as planets are still being discovered in our own Solar System. Recently an asteroid more than half the size of Pluto was found orbiting at a distance only a little further than the Solar System's most distant planet. The large asteroid moves relative to background stars in the discovery images shown above taken by the Oschin Telescope at Palomar, California, USA. Quaoar, the name suggested for the space rock by its discoverers, is one of several large asteroids discovered recently that roam in the distant Kuiper Belt. Quaoar's size was resolved by images from the Hubble Space Telescope. Quaoar is likely a cold world covered in ice from which the Sun appears only as a particularly bright star.

APOD: 2002 September 11 - Pluto and Charon Eclipse a Triple Star
Explanation: Occasionally, a planet in our Solar System will pass in front of a bright star. Since stars and planets take up so little space on the sky, such events are quite rare. Two months ago, however, Pluto and its large moon Charon passed in front of a comparatively bright triple star system known as P126. By noting how P126 A dimmed, the event was useful for studying Pluto's relatively unknown atmosphere. A Very Large Telescope in Chile using a deformable mirror to counter the blurring effect of Earth's atmosphere captured the above image.

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 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: 2002 January 6 - M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

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 October 15 - The Earth and Moon Planetary System
Explanation: How similar in size are the Earth and the Moon? A dramatic visual answer to this question is found by combining photographs taken by the Mariner 10 spacecraft that headed out toward Venus and Mercury in 1973. The Moon can be seen to have a diameter over one quarter that of Earth, relatively large compared to its planetary companion. In our Solar System, only Pluto and Charon are closer together in size. Striking features of the Earth visible to the passing spacecraft include blue oceans and white clouds, showing the Earth to be truly a water world.

APOD: 2001 August 30 - How Big Is 2001 KX76?
Explanation: Newly discovered minor planet 2001 KX76 is circled in the top panel above, a recent composite image from the European Southern Observatory's 2.2 meter telescope at La Silla, Chile. Though 2001 KX76 appears here as single point of light in an unremarkable star field, its orbit has been accurately measured by Astrovirtel, a newly operational "virtual telescope" capable of mining many years of archival data for previously unrecognized images of 2001 KX76. The results show this minor planet to be very distant, now orbiting just beyond Pluto and Charon in the realm of the Kuiper Belt. At its distance, apparent brightness, and assuming a reasonable surface reflectivity, 2001 KX76 would be 1,200 kilometers or more across -- larger than the largest main-belt asteroid, Ceres. In fact, the illustration in the bottom panel graphically compares this size estimate to Pluto, Charon, and the largest previously known Kuiper Belt objects, indicating the newfound minor planet is second only to Pluto in diameter. Along with other evidence, the comparison suggests that Pluto and Charon are closely related to Kuiper Belt worlds like 2001 KX76.

APOD: 2001 August 25 - Pioneer 10: The First 7 Billion Miles
Explanation: Q: What was made by humans and is 7.3 billion miles away? A: Pioneer 10 -- and 1997 was the 25th anniversary of its launch. Almost 11 light-hours distant, Pioneer 10 is presently about twice as far from the Sun as Pluto, and bound for interstellar space at 28,000 miles per hour. The distinction of being the first human artifact to venture beyond the known planets of the Solar System is just one in a long list of firsts for this spacefaring ambassador, including; the first spacecraft to travel through the asteroid belt and explore the outer Solar System, the first spacecraft to visit Jupiter, and the first to use a planet's gravity to change its course and to reach solar-system-escape velocity. Pioneer 10's mission is nearing an end. Now exploring the distant reaches of the heliosphere it will soon run out of sufficient electrical power to operate science instruments. However, the 570 lb. spacecraft will continue to coast and in 300,000 years or so it will pass within about 3 light years of nearby star Ross 248. Ross 248 is a faint red dwarf just over 10 light years distant in the constellation Taurus. (Note: In 1998 Voyager 1, launched 5 years later but traveling faster than Pioneer 10, became humanity's most distant spacecraft.)

APOD: 2001 March 19 - Pluto in True Color
Explanation: Pluto is mostly brown. The above picture captures the true colors of Pluto as well as the highest surface resolution so far recovered. No spacecraft has yet visited this most distant planet in our Solar System. The above map was created by tracking brightness changes from Earth of Pluto during times when it was being partially eclipsed by its moon Charon. The map therefore shows the hemisphere of Pluto that faces Charon. Pluto's brown color is thought dominated by frozen methane deposits metamorphosed by faint but energetic sunlight. The dark band below Pluto's equator is seen to have rather complex coloring, however, indicating that some unknown mechanisms may have affected Pluto's surface.

APOD: 2000 December 17 - M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

APOD: 2000 November 18 - Jupiter And Family
Explanation: This composite image features classic portraits of members of one of the Solar System's most prominent families - Jupiter and its four large "Galilean" moons. Starting from the top the moons are Io, Europa, Ganymede, and Callisto. The top-to-bottom order is also the order of increasing distance from Jupiter. These are big moons indeed which attend the largest planet. The smallest of the lot, Europa, is the size of Earth's moon while Ganymede is the largest moon in the Solar System. In fact, Ganymede with a diameter of 3,100 miles, is larger than the planets Mercury and Pluto. The swirling Great Red Spot appears at the edge of Jupiter. A hurricane-like storm system that has persisted for over 300 years, two to three earths could fit inside it. Battered Callisto's image was recorded during the 1979 flyby of Voyager. The other portraits were taken by the Galileo spacecraft which began exploring the Jovian system in 1995.

APOD: 2000 September 21 - XZ Tauri System Ejects Gas Bubble
Explanation: Why is the binary star system XZ Tauri emitting a hot bubble of expanding gas? Although astronomers can only presently speculate, the Hubble Space Telescope clearly documents this unusual behavior in three dramatic photographs over the past five years. Even without knowing why, the recently released sequence shows in unprecedented clarity the beginnings of a cooling zone -- a region where the expanding gas bubble cools off by emitting light as electrons and ions meet and recombine. The XZ Tauri star system is known to reside in the Taurus star forming region located about 500 light-years away. XZ Tau is composed of two very young stars separated by roughly the same distance as between our Sun and Pluto. The bubble has been expanding over the past thirty years and now extends to nearly fifteen times the binary separation.

APOD: 2000 June 20 - Ganymede: The Largest Moon in the Solar System
Explanation: If Ganymede orbited the Sun, it would be considered a planet. The reason is that Jupiter's moon Ganymede is not only the largest moon in the Solar System, it is larger than planets Mercury and Pluto. The robot spacecraft Galileo currently orbiting Jupiter has been able to zoom by Ganymede several times and snap many close-up pictures. Ganymede, shown above in its natural colors, sports a large oval dark region known as Galileo Regio. In general, the dark regions on Ganymede are heavily cratered, implying they are very old, while the light regions are younger and dominated by unusual grooves. The origin of the grooves is still under investigation.

APOD: 2000 April 9 - Mysterious Pluto and Charon
Explanation: Pluto is the only planet in our Solar System remaining unphotographed by a passing spacecraft. Distant Pluto and its moon Charon therefore remain somewhat mysterious. In addition to direct imaging by the Hubble Space Telescope, careful tracking of brightness changes that occur as each object eclipses the other have allowed astronomers to build up the above black & white surface maps. These maps depict the face of Pluto (left) that always faces Charon, and the face of Charon that always faces away from Pluto. The rectangular pixels are an artifact of the mapping software. The Pluto-Kuiper Express mission is tentatively planned for launch in 2004 and might encounter Pluto as early as 2012.

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: March 21, 1999 - M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

APOD: March 4, 1999 - Ganymede Mosaic
Explanation: Ganymede, one of the four Galilean moons of Jupiter, is the largest moon in the Solar System. With a diameter of 5,260 kilometers it is even larger than planets Mercury and Pluto and just over three quarters the size of Mars. Ganymede is locked in synchronous rotation with Jupiter. This detailed mosaic of images from the Galileo spacecraft shows the trailing hemisphere of this planet-sized moon. Speckled with bright young craters, Ganymede's surface shows a mixture of old, dark, cratered terrain and lighter regions laced with grooves and ridges. Ganymede's true colors tend toward subtle browns and grays, but this mosaic's colors have been enhanced to increase surface contrasts. The violet shades extending from the top and bottom are likely due to frost particles in Ganymede's polar regions.

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: December 27, 1998 - M2 9: Wings of a Butterfly Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

APOD: September 25, 1998 - Twin Proto Planetary Disks
Explanation: Sun-like stars are forming - and probably planets too - hidden inside Lynds 1551, an interstellar cloud of molecular gas and dust in the constellation Taurus. Using new receivers, coordinated radio telescopes at the Very Large Array near Socorro, New Mexico, USA, can now sharply image the dusty proto-planetary disks surrounding these young stars at radio wavelengths. Just announced, this exciting example shows a false-color radio picture of twin disks in a double star system! A yellow bar indicates the scale in astronomical units (AUs) where one AU is the average distance between the Earth and Sun. The stars (unseen near the center of each disk) are about 45 AUs apart, comparable to the radius of the orbit of Pluto. Similar proto-planetary disks have been seen around single stars, but these twin disks are much smaller, each limited in size by the gravity of the nearby companion star. In fact, if large planets form orbiting near the edges of these disks they may be ejected from the binary system.

APOD: July 8, 1998 - Mysterious Pluto and Charon
Explanation: Pluto is the only planet in our Solar System remaining unphotographed by a passing spacecraft. Distant Pluto and its moon Charon therefore remain somewhat mysterious. In addition to direct imaging by the Hubble Space Telescope, careful tracking of brightness changes that occur as each object eclipses the other have allowed astronomers to build up the above black & white surface maps. These maps depict the face of Pluto (left) that always faces Charon, and the face of Charon that always faces away from Pluto. The rectangular pixels are an artifact of the mapping software. The Pluto-Kuiper Express mission is tentatively planned for launch in 2003 and should encounter Pluto around the year 2012.

APOD: June 20, 1998 - Pioneer 10: The First 6 Billion Miles
Explanation: Q: What was made by humans and is 6.5 billion miles away? A: Pioneer 10 - and last year was the 25th anniversary of its launch. More than 9.5 light-hours distant, Pioneer 10 is presently about twice as far from the Sun as Pluto, bound for interstellar space at 28,000 miles per hour. The distinction of being the first human artifact to venture beyond the Solar System is just one in a long list of firsts for this spacefaring ambassador, including; the first spacecraft to travel through the asteroid belt and explore the outer Solar System, the first spacecraft to visit Jupiter, the first to use a planet's gravity to change its course and to reach solar-system-escape velocity, and the first spacecraft to pass beyond the known planets. Pioneer 10's mission is nearing an end - now exploring the distant reaches of the heliosphere it will soon run out of sufficient electrical power to operate science instruments. However, the 570 lb. spacecraft will continue to coast and in 30,000 years or so it will pass within about 3 light years of a nearby star known as Ross 248. Ross 248 is a faint red dwarf just over 10 light years distant in the constellation Taurus. (Note: This year Voyager 1, launched 21 years ago but traveling faster than Pioneer 10, became humanity's most distant spacecraft.)

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

APOD: 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 23, 1997 - M2-9: Wings of a Planetary Nebula
Explanation: Are stars better appreciated for their art after they die? Actually, stars usually create their most artistic displays as they die. In the case of low-mass stars like our Sun and M2-9 pictured above, the stars transform themselves from normal stars to white dwarfs by casting off their outer gaseous envelopes. The expended gas frequently forms an impressive display called a planetary nebula that fades gradually over thousand of years. M2-9, a butterfly planetary nebula 2100 light-years away shown in representative colors, has wings that tell a strange but incomplete tale. In the center, two stars orbit inside a gaseous disk 10 times the orbit of Pluto. The expelled envelope of the dying star breaks out from the disk creating the bipolar appearance. Much remains unknown about the physical processes that cause planetary nebulae.

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: September 29, 1997 - Jupiter And Family
Explanation: This composite image features classic portraits of members of one of the Solar System's most prominent families - Jupiter and its four large "Galilean" moons. Starting from the top the moons are Io, Europa, Ganymede, and Callisto. The top-to-bottom order is also the order of increasing distance from Jupiter. These are big moons indeed which attend the largest planet. The smallest of the lot, Europa, is the size of Earth's moon while Ganymede is the largest moon in the Solar System. In fact, Ganymede with a diameter of 3,100 miles, is larger than the planets Mercury and Pluto. The swirling Great Red Spot appears at the edge of Jupiter. A hurricane-like storm system that has persisted for over 300 years, two to three earths could fit inside it. Battered Callisto's image was recorded during the 1979 flyby of Voyager. The other portraits were taken by the Galileo spacecraft which began exploring the Jovian system in 1995.

APOD: March 3, 1997 - Pioneer 10: The First 6 Billion Miles
Explanation: Q: What was made by humans and is 6 billion miles away? A: Pioneer 10 - and yesterday was the 25th anniversary of its launch. More than 9 light hours distant, Pioneer 10 is presently about twice as far from the Sun as Pluto, bound for interstellar space at 28,000 miles per hour. The distinction of being the first human artifact to venture beyond the Solar System is just one in a long list of firsts for this spacefaring ambassador, including; the first spacecraft to travel through the asteroid belt and explore the outer Solar System, the first spacecraft to visit Jupiter, the first to use a planet's gravity to change its course and to reach solar-system-escape velocity, and the first spacecraft to pass beyond the known planets. Pioneer 10's mission is nearing an end - now exploring the distant reaches of the heliosphere it will soon run out of sufficient electrical power to operate science instruments. However, the 570 lb. spacecraft will continue to coast and in 30,000 years or so it will pass within about 3 light years of a nearby star. The star itself, a faint red dwarf known as Ross 248, is just over 10 light years distant in the constellation of Taurus.

APOD: February 4, 1997 - Clyde W. Tombaugh: 1906-1997
Explanation: Astronomer Clyde Tombaugh, discoverer of Pluto, died on January 17th. Inspiring many during his long and exceptional career, he had been living in Las Cruces, New Mexico with his wife of 60 years, Patsy. Today would have been his 91st birthday. He is pictured above in 1995 in his backyard with a telescope he knew well - a 9 inch Newtonian reflector he built in 1927 with discarded farm machinery and car parts. Using this telescope under the dark night skies of Western Kansas, he made drawings of Mars and Jupiter and submitted them to Lowell Observatory in 1928. Hired to work at Lowell in 1929, Tombaugh embarked on a systematic photographic search for the long sought Planet X with a newly constructed 13 inch astrograph. In 1930 Tombaugh triumphed in his struggle to find the 9th planet, discovering faint and distant Pluto orbiting at the edge of our Solar System. Founding father of New Mexico State University's Astronomy Department, he retired as professor emeritus in 1973 but continued to tour as a lecturer and promoter until failing health prevented it. Always an active stargazer, he was asked by the Smithsonian if they could have the telescope he used to make his 1928 drawings. His response: "I told them I was still using it."

APOD: April 16, 1996 - Cometary Knots in the Helix Nebula
Explanation: Four hundred fifty light-years from Earth, the wind from a dying, sun-like star produced a planetary nebula popularly known as the Helix. While exploring the Helix's gaseous envelope with the Hubble Space Telescope (HST), astronomers discovered indications of 1,000s of striking "cometary knots" like those shown above. So called because of their resemblence to comets, they are actually much larger - their heads are several billion miles across (roughly twice the size of the our solar system itself) while their tails, pointing radially away from the central star, stretch over 100 billion miles. Previously known from ground based observations, the sheer number of cometary knots found in this single nebula is astonishing. What caused them to form? Hot, fast moving shells of nebular gas overrunning cooler, denser, slower shells ejected by the star during an earlier expansion may produce these droplet-like condensations as the two shells intermix and fragment. An intriguing possibility is that instead of dissipating over time, these objects, could collapse and form pluto-like bodies. If so, these icy worlds created near the end of a star's life, would be numerous in our galaxy.

APOD: March 11, 1996 - Hubble Telescope Maps Pluto
Explanation: No spacecraft from Earth has yet explored Pluto but astronomers have found ways of mapping its surface. A stunning map of this distant, diminutive planet, the first based on direct images, was revealed late last week in a Hubble Space Telescope press release. Above are two opposite hemisphere views of the computer constructed map of Pluto's surface (north is up). The grid pattern is due to the computer technique used where each grid element is over 100 miles across. The map is based on Hubble images made when Pluto was a mere 3 billion miles distant. It shows strong brightness variations - confirming and substantially improving upon ground based observations. While the brightness variations may be due to surface features like craters and basins they are more likely caused by regions of nitrogen and methane frost. The frost regions should show "seasonal" changes which can be tracked in future Hubble observations. Yes, Pluto is a planet even though it is only 2/3 the size of Earth's Moon!

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

APOD: February 12, 1996 - Pluto Not Yet Explored
Explanation: Cold, distant, Pluto is the only planet in our Solar System which has not been visited by a spacecraft from Earth. The story goes that the legend "Pluto Not Yet Explored" on a US postal stamp depicting the tiny, mysterious world inspired a JPL employee to develop plans for a Pluto flyby. These plans evolved into the current "Pluto Express" mission intended for launch early in the next decade. The type of small, high-tech spacecraft proposed is depicted above in an artist's vision approaching Pluto's mottled surface. A tenuous, transient atmosphere is visible as blue haze beyond the bright limb while Pluto's companion Charon looms in the distance. Images and data from such a mission would be an incredible boon to those studying these bizarre, inaccessible worlds as evidence mounts that Pluto itself is only the largest of many small ice dwarf mini-planets. Some have dubbed the yet unexplored Pluto-Charon system the last "astronomers' planet". Note: Pluto's discoverer, astronomer Clyde Tombaugh, celebrated his 90th birthday on February 4.

APOD: September 4, 1995 - Ganymede: Moonquake World
Explanation: Ganymede probably undergoes frequent ground shaking events not unlike terrestrial earthquakes. Ganymede, the largest moon of Jupiter and the Solar System, has a thick outer coating of water ice. Passing Voyager spacecraft found a large number of cracks and grooves in the ice so it is thought that Ganymede, like the Earth, has large shifting surface masses called tectonic plates. Ganymede was discovered by Galileo and Marius in 1610, and is larger than the planets Mercury and Pluto. The NASA spacecraft Galileo is scheduled to arrive at Jupiter is December of 1995.

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 14, 1995 - Mercury: Closest Planet to the Sun
Explanation: This picture was compiled from images taken by the NASA spacecraft Mariner 10 which flew by the planet three times in 1974. Mercury is the closest planet to the Sun, the second hottest planet (Venus gets hotter), and the second smallest planet (Pluto is smaller). Mercury rotates so slowly that one day there - "day" meaning the normal time it takes from sunset to sunset - lasts 176 days on Earth. It is difficult to see Mercury not because it is dim but because it always appears near the Sun, and is therefore only visible for a short time just after sunset or just before sunrise. Mercury is made of rocky material like Earth. No one knows why Mercury has the magnetic field that it does.

APOD: June 29, 1995 - The Earth-Moon System
Explanation: A double planet? From 4 million miles away on December 16, 1992, NASA's robot spacecraft Galileo took this picture of the Earth-moon system. The bright, sunlit half of the Earth contrasts strongly with the darker subdued colors of the moon. Our moon is one of the largest moons in the solar system. It is even larger than the planet Pluto. In this picture, the Earth-moon system actually appears to be a double planet.