Astronomy Picture of the Day Search Results for "Mercury"

APOD: 2025 April 18 - Comet C/2025 F2 SWAN
Explanation: In late March, the comet now designated C/2025 F2 SWAN was found independently by citizen scientists Vladimir Bezugly, Michael Mattiazzo, and Rob Matson while examining publicly available image data from the Solar Wind ANisotropies (SWAN) camera on the sun-staring SOHO spacecraft. Comet SWAN's coma, its greenish color a signature of diatomic carbon molecules fluorescing in sunlight, is at lower left in this telescopic image. SWAN's faint ion tail extends nearly two degrees toward the upper right across the field of view. The interplanetary scene was captured in clear but moonlit skies from June Lake, California on April 14. Seen against background of stars toward the constellation Andromeda, the comet was then some 10 light-minutes from our fair planet. Now a target for binoculars and small telescopes in northern hemisphere morning skies this comet SWAN is headed for a perihelion, its closest approach to the Sun, on May 1. That will bring this visitor from the distant Oort cloud almost as close to the Sun as the orbit of inner planet Mercury.

APOD: 2025 March 31 – Parker: The Solar System from Near the Sun
Explanation: If you watch long enough, a comet will appear. Before then, you will see our Solar System from inside the orbit of Mercury as recorded by NASA's Parker Solar Probe looping around the Sun. The video captures coronal streamers into the solar wind, a small Coronal Mass Ejection, and planets including, in order of appearance, Mercury, Venus, Saturn, Earth, Mars, and Jupiter. Between the emergence of Earth and Mars, Comet Tsuchinshan-ATLAS appears with a distinctive tail. The continuous fleeting streaks are high energy particles from the Sun impacting Parker's sideways looking camera. The featured time-lapse video was taken last year during Encounter 21, Parker's 21st close approach to the Sun. Studying data and images from Parker are delivering a better understanding of the dynamic Sun's effects on Earth's space weather as well as humanity's power grids, spacecraft, and space-faring astronauts.

APOD: 2024 November 23 - Interplanetary Earth
Explanation: In an interplanetary first, on July 19, 2013 Earth was photographed on the same day from two other worlds of the Solar System, innermost planet Mercury and ringed gas giant Saturn. Pictured on the left, Earth is the pale blue dot just below the rings of Saturn, as captured by the robotic Cassini spacecraft then orbiting the outermost gas giant. On that same day people across planet Earth snapped many of their own pictures of Saturn. On the right, the Earth-Moon system is seen against the dark background of space as captured by the sunward MESSENGER spacecraft, then in Mercury orbit. MESSENGER took its image as part of a search for small natural satellites of Mercury, moons that would be expected to be quite dim. In the MESSENGER image, the brighter Earth and Moon are both overexposed and shine brightly with reflected sunlight. Destined not to return to their home world, both Cassini and MESSENGER have since retired from their missions of Solar System exploration.

APOD: 2024 September 16 – Mercurys Vivaldi Crater from BepiColombo
Explanation: Why does this large crater on Mercury have two rings and a smooth floor? No one is sure. The unusual feature called Vivaldi Crater spans 215 kilometers and was imaged again in great detail by ESA's and JAXA's robotic BepiColombo spacecraft on a flyby earlier this month. A large circular feature on a rocky planet or moon is usually caused by either an impact by a small asteroid or a comet fragment, or a volcanic eruption. In the case of Vivaldi, it is possible that both occurred -- a heavy strike that caused a smooth internal lava flow. Double-ringed craters are rare, and the cause of the inner rings remains a topic of research. The speed-slowing gravity-assisted flyby of Mercury by BepiColombo was in preparation for the spacecraft entering orbit around the Solar System's innermost planet in 2026.

APOD: 2024 April 17 – Total Eclipse and Comets
Explanation: Not one, but two comets appeared near the Sun during last week's total solar eclipse. The expected comet was Comet 12P/Pons-Brooks, but it was disappointingly dimmer than many had hoped. However, relatively unknown Comet SOHO-5008 also appeared in long duration camera exposures. This comet was the 5008th comet identified on images taken by ESA & NASA's Sun-orbiting SOHO spacecraft. Likely much smaller, Comet SOHO-5008 was a sungrazer which disintegrated within hours as it passed too near the Sun. The featured image is not only unusual for capturing two comets during an eclipse, but one of the rare times that a sungrazing comet has been photographed from the Earth's surface. Also visible in the image is the sprawling corona of our Sun and the planets Mercury (left) and Venus (right). Of these planets and comets, only Venus was easily visible to millions of people in the dark shadow of the Moon that crossed North America on April 8.

APOD: 2024 February 19 – Looking Sideways from the Parker Solar Probe
Explanation: What's happening near the Sun? To help find out, NASA launched the robotic Parker Solar Probe (PSP) to investigate regions closer to the Sun than ever before. The PSP's looping orbit brings it nearer to the Sun each time around -- every few months. The featured time-lapse video shows the view looking sideways from behind PSP's Sun shield during its 16th approach to the Sun last year -- from well within the orbit of Mercury. The PSP's Wide Field Imager for Solar Probe (WISPR) cameras took the images over eleven days, but they are digitally compressed here into about one minute video. The waving of the solar corona is visible, as is a coronal mass ejection, with stars, planets, and even the central band of our Milky Way Galaxy streaming by in the background as the PSP orbits the Sun. PSP has found the solar neighborhood to be surprisingly complex and to include switchbacks -- times when the Sun's magnetic field briefly reverses itself.

APOD: 2024 January 12 - Good Morning Moon
Explanation: Yesterday, the Moon was New. But on January 9, early morning risers around planet Earth were treated to the sight of an old Moon, low in the east as the sky grew bright before dawn. Above the city of Saarburg in Rhineland-Palatinate, western Germany, this simple snapshot found the waning Moon's sunlit crescent just before sunrise. But also never wandering far from the Sun in Earth's sky, inner planets Venus and Mercury shared the cold morning skyview. In the foreground are the historic city's tower and castle with ruins from the 10th century.

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 October 27 - Encke and the Tadpoles
Explanation: History's second known periodic comet is Comet Encke (2P/Encke). As it swings through the inner Solar System, Encke's orbit takes it from an aphelion, its greatest distance from the Sun, inside the orbit of Jupiter to a perihelion just inside the orbit of Mercury. Returning to its perihelion every 3.3 years, Encke has the shortest period of the Solar System's major comets. Comet Encke is also associated with (at least) two annual meteor showers on planet Earth, the North and South Taurids. Both showers are active in late October and early November. Their two separate radiants lie near bright star Aldebaran in the head-strong constellation Taurus. A faint comet, Encke was captured in this telescopic field of view imaged on the morning of August 24. Then, Encke's pretty greenish coma was close on the sky to the young, embedded star cluster and light-years long, tadpole-shaped star-forming clouds in emission nebula IC 410. Now near bright star Spica in Virgo Comet Encke passed its 2023 perihelion only five days ago, on October 22.

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

APOD: 2023 August 3 - The Falcon and the Redstone
Explanation: In a photo from the early hours of July 29 (UTC), a Redstone rocket and Mercury capsule are on display at Cape Canaveral Launch Complex 5. Beyond the Redstone, the 8 minute long exposure has captured the arcing launch streak of a SpaceX Falcon Heavy rocket. The Falcon's heavy communications satellite payload, at a record setting 9 metric tons, is bound for geosynchronous orbit some 22,000 miles above planet Earth. The historic launch of a Redstone rocket carried astronaut Alan Shepard on a suborbital spaceflight in May 1961 to an altitude of about 116 miles. Near the top of the frame, this Falcon rocket's two reusable side boosters separate and execute brief entry burns. They returned to land side by side at Canaveral's Landing Zone 1 and 2 in the distance.

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

APOD: 2022 August 9 - Leaving Earth
Explanation: What it would look like to leave planet Earth? Such an event was recorded visually in great detail by the MESSENGER spacecraft as it swung back past the Earth in 2005 on its way in toward the planet Mercury. Earth can be seen rotating in this time-lapse video, as it recedes into the distance. The sunlit half of Earth is so bright that background stars are not visible. The robotic MESSENGER spacecraft orbit around Mercury from 2011 to 2015 has conducted the first complete map of the surface. On occasion, MESSENGER has continued to peer back at its home world. MESSENGER is one of the few things created on the Earth that will never return. At the end of its mission MESSENGER crashed into Mercury's surface.

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

APOD: 2022 June 28 - Mercury from Passing BepiColombo
Explanation: Which part of the Moon is this? No part -- because this is the planet Mercury. Mercury's old surface is heavily cratered like that of Earth's Moon. Mercury, while only slightly larger than Luna, is much denser and more massive than any Solar System moon because it is made mostly of iron. In fact, our Earth is the only planet more dense. Because Mercury rotates exactly three times for every two orbits around the Sun, and because Mercury's orbit is so elliptical, visitors on Mercury could see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun causes it to be visible only for a short time just after sunset or just before sunrise. The featured image was captured last week by ESA and JAXA's passing BepiColombo spacecraft as it sheds energy and prepares to orbit the innermost planet starting in 2025.

APOD: 2022 June 17 - Good Morning Planets from Chile
Explanation: On June 15, innermost planet Mercury had wandered about as far from the Sun as it ever gets in planet Earth's sky. Near the eastern horizon just before sunrise it stands over distant Andes mountain peaks in this predawn snapshot from the valley of Rio Hurtado in Chile. June's other morning planets are arrayed above it, as all the naked-eye planets of the Solar System stretch in a line along the ecliptic in the single wide-field view. Tilted toward the north, the Solar System's ecliptic plane arcs steeply through southern hemisphere skies. Northern hemisphere early morning risers will see the lineup of planets along the ecliptic at a shallower angle tilting toward the south. From both hemispheres June's beautiful morning planetary display finds the visible planets in order of their increasing distance from the Sun.

APOD: 2022 May 3 - Mercury's Sodium Tail
Explanation: That's no comet. Below the Pleiades star cluster is actually a planet: Mercury. Long exposures of our Solar System's innermost planet may reveal something unexpected: a tail. Mercury's thin atmosphere contains small amounts of sodium that glow when excited by light from the Sun. Sunlight also liberates these atoms from Mercury's surface and pushes them away. The yellow glow from sodium, in particular, is relatively bright. Pictured, Mercury and its sodium tail are visible in a deep image taken last week from La Palma, Spain through a filter that primarily transmits yellow light emitted by sodium. First predicted in the 1980s, Mercury's tail was first discovered in 2001. Many tail details were revealed in multiple observations by NASA's robotic MESSENGER spacecraft that orbited Mercury between 2011 and 2015. Tails, of course, are usually associated with comets.

APOD: 2021 December 5 - Total Solar Eclipse Below the Bottom of the World
Explanation: Yesterday there was a total solar eclipse visible only at the end of the Earth. To capture the unusual phenomenon, airplanes took flight below the clouded seascape of Southern Ocean. The featured image shows one relatively spectacular capture where the bright spot is the outer corona of the Sun and the eclipsing Moon is seen as the dark spot in the center. A wing and engine of the airplane are visible across the left and bottom of the image, while another airplane observing the eclipse is visible on the far left. The dark area of the sky surrounding the eclipsed Sun is called a shadow cone. It is dark because you are looking down a long corridor of air shadowed by the Moon. A careful inspection of the eclipsed Sun will reveal the planet Mercury just to the right. The next total solar eclipse shadow will cross parts of Australia and Indonesia in April of 2023, while the one after that will cross North America in April of 2024.

APOD: 2021 July 10 - Mercury and the Da Vinci Glow
Explanation: On July 8th early morning risers saw Mercury near an old Moon low on the eastern horizon. On that date bright planet, faint glow of lunar night side, and sunlit crescent were captured in this predawn skyscape from Tenerife's Teide National Park in the Canary Islands. Never far from the Sun in planet Earth's sky, the fleeting inner planet shines near its brightest in the morning twilight scene. Mercury lies just below the zeta star of the constellation Taurus, Zeta Tauri, near the tip of the celestial bull's horn. Of course the Moon's ashen glow is earthshine, earthlight reflected from the Moon's night side. A description of earthshine, in terms of sunlight reflected by Earth's oceans illuminating the Moon's dark surface, was written over 500 years ago by Leonardo da Vinci. Waiting for the coming dawn in the foreground are the Teide Observatory's sentinels of the Sun, also known as (large domes left to right) the THEMIS, VTT, and GREGOR solar telescopes.

APOD: 2021 July 6 - Saturn and Six Moons
Explanation: How many moons does Saturn have? So far 82 have been confirmed, the smallest being only a fraction of a kilometer across. Six of its largest satellites can be seen here in a composite image with 13 short exposure of the bright planet, and 13 long exposures of the brightest of its faint moons, taken over two weeks last month. Larger than Earth's Moon and even slightly larger than Mercury,Saturn's largest moon Titan has a diameter of 5,150 kilometers and was captured making nearly a complete orbit around its ringed parent planet. Saturn's first known natural satellite, Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens, in contrast with several newly discovered moons announced in 2019. The trail on the far right belongs to Iapetus, Saturn's third largest moon. The radius of painted Iapetus' orbit is so large that only a portion of it was captured here. Saturn leads Jupiter across the night sky this month, rising soon after sunset toward the southeast, and remaining visible until dawn.

APOD: 2021 May 7 - Mercury-Redstone 3 Launch
Explanation: Sixty years ago, near the dawn of the space age, NASA controllers "lit the candle" and sent Mercury astronaut Alan Shepard arcing into space atop a Redstone rocket. His cramped space capsule was dubbed Freedom 7. Broadcast live to a global television audience, the historic Mercury-Redstone 3 (MR-3) spacecraft was launched from Cape Canaveral Florida at 9:34 a.m. Eastern Time on May 5, 1961. The flight of Freedom 7, the first space flight by an American, followed less than a month after the first human venture into space by Soviet Cosmonaut Yuri Gagarin. The 15 minute sub-orbital flight achieved an altitude of 116 miles and a maximum speed of 5,134 miles per hour. As Shepard looked back near the peak of Freedom 7's trajectory, he could see the outlines of the west coast of Florida, Lake Okeechobe in central Florida, the Gulf of Mexico, and the Bahamas. Shepard would later view planet Earth from a more distant perspective and walk on the Moon as commander of the Apollo 14 mission.

APOD: 2020 November 14 - Venus, Mercury, and the Waning Moon
Explanation: Yesterday, early morning risers around planet Earth were treated to a waning Moon low in the east as the sky grew bright before dawn. From the Island of Ortigia, Syracuse, Sicily, Italy this simple snapshot found the slender sunlit crescent just before sunrise. Never wandering far from the Sun in Earth's sky, inner planets Venus and Mercury shared the calm seaside view. Also in the frame, right of the line-up of Luna and planets, is bright star Spica, alpha star of the constellation Virgo and one of the 20 brightest stars in Earth's night. Tomorrow the Moon will be New. The dark lunar disk means mostly dark nights for planet Earth in the coming week and a good chance to watch the annual Leonid Meteor Shower.

APOD: 2020 July 8 - Mercury's Sodium Tail
Explanation: What is that fuzzy streak extending from Mercury? Long exposures of our Solar System's innermost planet may reveal something unexpected: a tail. Mercury's thin atmosphere contains small amounts of sodium that glow when excited by light from the Sun. Sunlight also liberates these molecules from Mercury's surface and pushes them away. The yellow glow from sodium, in particular, is relatively bright. Pictured, Mercury and its sodium tail are visible in a deep image taken in late May from Italy through a filter that primarily transmits yellow light emitted by sodium. First predicted in the 1980s, Mercury's tail was first discovered in 2001. Many tail details were revealed in multiple observations by NASA's robotic MESSENGER spacecraft that orbited Mercury between 2011 and 2015. Tails are usually associated with comets. The tails of Comet NEOWISE are currently visible with the unaided eye in the morning sky.

APOD: 2020 May 30 - Green Flashes: Sun, Moon, Venus, Mercury
Explanation: Follow a sunset on a clear day against a distant horizon and you might glimpse green just as the Sun disappears from view. The green flash is caused by refraction of light rays traveling to the eye over a long path through the atmosphere. Shorter wavelengths refract more strongly than longer redder wavelengths and the separation of colors lends a green hue to the last visible vestige of the solar disk. It's harder to see a green flash from the Moon, not to mention the diminutive disks of Venus and Mercury. But a telescope or telephoto lens and camera can help catch this tantalizing result of atmospheric refraction when the celestial bodies are near the horizon. From Sicily, the top panels were recorded on March 18, 2019 for the Sun and May 8, 2020 for the Moon. Also from the Mediterranean island, the bottom panels were shot during the twilight apparition of Venus and Mercury near the western horizon on May 24.

APOD: 2020 May 29 - Mercury Meets Crescent Venus
Explanation: That's not a bright star and crescent Moon caught between branches of a eucalyptus tree. It's Venus in a crescent phase and Mercury. Near the western horizon after sunset, the two inner planets closely shared this telescopic field of view on May 22, seen from a balcony in Civitavecchia, Italy. Venus, the very bright celestial beacon, is wandering lower into the evening twilight. It grows larger in apparent size and shows a thinner crescent as it heads toward its inferior conjunction, positioned between Earth and Sun on June 3. Mercury, in a fuller phase, is climbing in the western sky though, reaching its maximum angular distance from the Sun on June 4 Still, this remarkably close pairing with brilliant Venus made Mercury, usually lost in bright twilight skies, easier to spot from planet Earth.

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

APOD: 2020 May 4 - Earth Flyby of BepiColombo
Explanation: What it would look like to approach planet Earth? Such an event was recorded visually in great detail by ESA's and JAXA's robotic BepiColombo spacecraft last month as it swung back past Earth on its journey in to the planet Mercury. Earth can be seen rotating on approach as it comes out from behind the spacecraft's high-gain antenna in this nearly 10-hour time-lapse video. The Earth is so bright that no background stars are visible. Launched in 2018, the robotic BepiColombo used the gravity of Earth to adjust its course, the first of nine planetary flybys over the next seven years -- but the only one involving Earth. Scheduled to enter orbit in 2025, BepiColombo will take images and data of the surface and magnetic field of Mercury in an effort to better understand the early evolution of our Solar System and its innermost planet.

APOD: 2020 February 11 - Launch of the Solar Orbiter
Explanation: How does weather on the Sun affect humanity? To help find out, the European Space Agency (ESA) and NASA have just launched the Solar Orbiter. This Sun-circling robotic spaceship will monitor the Sun's changing light, solar wind, and magnetic field not only from the usual perspective of Earth but also from above and below the Sun. Pictured, a long duration exposure of the launch of the Solar Orbiter shows the graceful arc of the bright engines of United Launch Alliance's Atlas V rocket as they lifted the satellite off the Earth. Over the next few years, the Solar Orbiter will use the gravity of Earth and Venus to veer out of the plane of the planets and closer to the Sun than Mercury. Violent weather on the Sun, including solar flares and coronal mass ejections, has shown the ability to interfere with power grids on the Earth and communications satellites in Earth orbit. The Solar Orbiter is expected to coordinate observations with the also Sun-orbiting Parker Solar Probe launched in 2018.

APOD: 2020 January 21 - Parker: Sounds of the Solar Wind
Explanation: What does the solar wind sound like? A wind of fast moving particles blows out from our Sun, and although space transmits sound poorly, particle impact and variable-field data from NASA's near-Sun Parker Solar Probe is being translated into sound. The disarming audio track of the featured video recounts several of these reverberations, including spooky-sounding Langmuir Waves (heard first), hurricane-sounding Whistler Mode Waves (heard next), and hard-to-describe Dispersive Chirping Waves (heard last). Also impressive is the video's time-lapse visual track which shows Parker's view to the side of its sun shield, and where the planets Earth, Jupiter, Mercury and Venus appear in succession, interspersed with bursts of powerful cosmic rays impacting the imager. The nature of the solar wind near Mercury is surprisingly different from near the Earth, and much study is underway to better understand the differences.

APOD: 2019 December 2 - Mercury Crosses a Quiet Sun
Explanation: What's that black dot crossing the Sun? The planet Mercury. Mercury usually passes over or under the Sun, as seen from Earth, but last month the Solar System's innermost planet appeared to go just about straight across the middle. Although witnessed by planet admirers across the globe, a particularly clear view was captured by the Solar Dynamics Observatory (SDO) in Earth orbit. The featured video was captured by the SDO's HMI instrument in a broad range of visible light, and compresses the 5 1/2 hour transit into about 13 seconds. The background Sun was unusually quiet -- even for being near Solar Minimum -- and showed no sunspots. The next solar transit by Mercury will occur in 2032.

APOD: 2019 November 28 - Moon and Planets at Twilight
Explanation: This week's ongoing conjunction of Venus and Jupiter may have whetted your appetite for skygazing. Tonight is the main course though. On November 28, a young crescent Moon will join them posing next to the two bright planets above the western horizon at twilight. Much like tonight's visual feast, this night skyscape shows a young lunar crescent and brilliant Venus in the western evening twilight on October 29. The celestial beacons are setting over distant mountains and the Minya monastery, Ganzi Tibetan Autonomous Prefecture, Sichuan, China, planet Earth. Then Mercury, not Jupiter, was a celestial companion to Venus and the Moon. The fleeting innermost planet is just visible here in the bright twilight, below and left of Venus and near the center of the frame. Tomorrow, November 29, the crescent Moon will also help you spot planet Saturn for dessert.

APOD: 2019 November 14 - Mercury and the Quiet Sun
Explanation: On November 11, 2019 the Sun was mostly quiet, experiencing a minimum in its 11 year cycle of activity. In fact, the only spot visible was actually planet Mercury, making a leisurely 5 1/2 hour transit in front of the calm solar disk. About 1/200th the apparent diameter of the Sun, the silhouette of the solar system's inner most planet is near center in this sharp, full Sun snapshot. Taken with a hydrogen alpha filter and safe solar telescope, the image also captures prominences around the solar limb, the glowing plasma trapped in arcing magnetic fields. Of course, only inner planets Mercury and Venus can transit the Sun to appear in silhouette when viewed from planet Earth. Following its transit in 2016, this was Mercury's 4th of 14 transits across the solar disk in the 21st century. The next transit of Mercury will be on November 13, 2032.

APOD: 2019 November 13 - Mercury in Silhouette
Explanation: The small, dark, round spot in this solar close up is planet Mercury. In the high resolution telescopic image, a colorized stack of 61 sharp video frames, a turbulent array of photospheric convection cells tile the bright solar surface. Mercury's more regular silhouette still stands out though. Of course, only inner planets Mercury and Venus can transit the Sun to appear in silhouette when viewed from planet Earth. For this November 11, 2019 transit of Mercury, the innermost planet's silhouette was a mere 1/200th the solar diameter. So even under clear daytime skies it was difficult to see without the aid of a safe solar telescope. Following its transit in 2016, this was Mercury's 4th of 14 transits across the solar disk in the 21st century. The next transit of Mercury will be on November 13, 2032.

APOD: 2019 November 10 - A Mercury Transit Sequence
Explanation: Tomorrow -- Monday -- Mercury will cross the face of the Sun, as seen from Earth. Called a transit, the last time this happened was in 2016. Because the plane of Mercury's orbit is not exactly coincident with the plane of Earth's orbit, Mercury usually appears to pass over or under the Sun. The featured time-lapse sequence, superimposed on a single frame, was taken from a balcony in Belgium shows the entire transit of 2003 May 7. That solar crossing lasted over five hours, so that the above 23 images were taken roughly 15 minutes apart. The north pole of the Sun, the Earth's orbit, and Mercury's orbit, although all different, all occur in directions slightly above the left of the image. Near the center and on the far right, sunspots are visible. After Monday, the next transit of Mercury will occur in 2032.

APOD: 2019 November 9 - Saturn the Giant
Explanation: On May 25, 1961 U.S. president John Kennedy announced the goal of landing astronauts on the Moon by the end of the decade. By November 9, 1967 this Saturn V rocket was ready for launch and the first full test of its capabilities on the Apollo 4 mission. Its development directed by rocket pioneer Wernher Von Braun, the three stage Saturn V stood over 36 stories tall. It had a cluster of five first stage engines fueled by liquid oxygen and kerosene which together were capable of producing 7.9 million pounds of thrust. Giant Saturn V rockets ultimately hurled nine Apollo missions to the Moon and back again with six landing on the lunar surface. The first landing mission, Apollo 11, achieved Kennedy's goal on July 20, 1969.

APOD: 2019 October 21 - A Mercury Transit Music Video from SDO
Explanation: What's that small black dot moving across the Sun? Mercury. Possibly the clearest view of Mercury crossing in front of the Sun in 2016 May was from Earth orbit. The Solar Dynamics Observatory obtained an uninterrupted vista recording it not only in optical light but also in bands of ultraviolet light. Featured here is a composite movie of the crossing set to music. Although the event might prove successful scientifically for better determining components of Mercury' ultra-thin atmosphere, the event surely proved successful culturally by involving people throughout the world in observing a rare astronomical phenomenon. Many spectacular images of this Mercury transit from around (and above) the globe were proudly displayed. The next transit of Mercury will take place in three weeks: on 2019 November 11.

APOD: 2019 October 12 - Interplanetary Earth
Explanation: In an interplanetary first, on July 19, 2013 Earth was photographed on the same day from two other worlds of the Solar System, innermost planet Mercury and ringed gas giant Saturn. Pictured on the left, Earth is the pale blue dot just below the rings of Saturn, as captured by the robotic Cassini spacecraft then orbiting the outermost gas giant. On that same day people across planet Earth snapped many of their own pictures of Saturn. On the right, the Earth-Moon system is seen against the dark background of space as captured by the robotic MESSENGER spacecraft, then in Mercury orbit. MESSENGER took its image as part of a search for small natural satellites of Mercury, moons that would be expected to be quite dim. In the MESSENGER image, the Earth (left) and Moon (right) are overexposed and shine brightly with reflected sunlight. Destined not to return to their home world, both Cassini and MESSENGER have since retired from their missions of Solar System exploration.

APOD: 2019 August 25 - Leaving Earth
Explanation: What it would look like to leave planet Earth? Such an event was recorded visually in great detail by the MESSENGER spacecraft as it swung back past the Earth in 2005 on its way in toward the planet Mercury. Earth can be seen rotating in this time-lapse video, as it recedes into the distance. The sunlit half of Earth is so bright that background stars are not visible. The robotic MESSENGER spacecraft is now in orbit around Mercury and has recently concluded the first complete map of the surface. On occasion, MESSENGER has continued to peer back at its home world. MESSENGER is one of the few things created on the Earth that has left and will never return -- at the end of its mission MESSENGER crashed into Mercury's surface.

APOD: 2019 April 28 - All of Mercury
Explanation: Only six years ago, the entire surface of planet Mercury was finally mapped. Detailed observations of the innermost planet's surprising crust began when the robotic MESSENGER spacecraft first passed Mercury in 2008 and continued until its controlled crash landing in 2015. Previously, much of the Mercury's surface was unknown as it is too far for Earth-bound telescopes to see clearly, while the Mariner 10 flybys in the 1970s observed only about half. The featured video is a compilation of thousands of images of Mercury rendered in exaggerated colors to better contrast different surface features. Visible on the rotating world are rays emanating from a northern impact that stretch across much of the planet, while about half-way through the video the light colored Caloris Basin rotates into view, a northern ancient impact feature that filled with lava. Recent analysis of MESSENGER data indicates that Mercury has a solid inner core.

APOD: 2018 March 31 - Twilight in a Western Sky
Explanation: A slender crescent Moon and inner planets Venus and Mercury never wander far from the Sun in planet Earth's skies. In the fading evening twilight of March 18, they line up near the western horizon in this atmospheric skyscape. While the celestial scene was enjoyed around the world, this photo captures the trio, with fainter Mercury at the far right, above the crags of Big Bend National Park in southwest Texas. Tonight the Moon will be full though, and rise opposite the Sun. Look for it high in the sky at midnight, near bright star Spica.

APOD: 2018 March 4 - Clouds, Birds, Moon, Venus
Explanation: Sometimes the sky above can become quite a show. In early September of 2010, for example, the Moon and Venus converged, creating quite a sight by itself for sky enthusiasts around the globe. From some locations, though, the sky was even more picturesque. In the featured image taken in Spain, a crescent Moon and the planet Venus, on the far right, were captured during sunset posing against a deep blue sky. In the foreground, dark storm clouds loom across the image bottom, while a white anvil cloud shape appears above. Black specks dot the frame, caused by a flock of birds taking flight. Very soon after this picture was taken, however, the birds passed by, the storm ended, and Venus and the Moon set. Bright Venus is again visible just after sunset this month (2018 March) and will appear quite near Mercury tonight and the rest of this week.

APOD: 2017 December 23 - Phaethon's Brood
Explanation: Based on its well-measured orbit, 3200 Phaethon (sounds like FAY-eh-thon) is recognized as the source of the meteroid stream responsible for the annual Geminid meteor shower. Even though most meteor showers' parents are comets, 3200 Phaethon is a known and closely tracked near-Earth asteroid with a 1.4 year orbital period. Rocky and sun-baked, its perihelion or closest approach to the Sun is well within the orbit of innermost planet Mercury. In this telescopic field of view, the asteroid's rapid motion against faint background stars of the heroic constellation Perseus left a short trail during the two minute total exposure time. The parallel streaks of its meteoric children flashed much more quickly across the scene. The family portrait was recorded near the Geminid meteor shower's very active peak on December 13. That was just before 3200 Phaethon's historic December 16 closest approach to planet Earth.

APOD: 2017 December 11 - Mercury Visualized from MESSENGER
Explanation: What would it be like to fly over the planet Mercury? Images and data taken from NASA's robotic MESSENGER spacecraft that orbited Mercury from 2011 to 2015 have been digitally combined to envision a virtual flight that highlights much of the hot planet's surface. In general, the Solar System's innermost world appears similar to Earth's Moon as it is covered by a heavily cratered gray terrain. MESSENGER discovered much about Mercury including that shadows near its poles likely host water ice. The featured video opens as Mercury is viewed from the Sun-facing side and concludes with the virtual spacecraft retreating into Mercury's night. Mercury actually rotates so slowly that it only completes three rotations for every two trips around the Sun. In 2018, Europe and Japan plan to launch BepiColombo to better map Mercury's surface and probe its magnetic field.

APOD: 2017 September 21 - A September Morning Sky
Explanation: The Moon, three planets, and a bright star gathered near the ecliptic plane in the September 18 morning sky over Veszprem Castle, Hungary. In this twilight skyscape, Mercury and Mars still shine close to the eastern horizon, soon to disappear in the glare of the Sun. Regulus, alpha star of the constellation Leo, is the bright point next to a waning crescent Moon, with brilliant Venus near the top of the frame. The beautiful morning conjunction of Moon, planets, and bright star could generally be followed by early morning risers all around planet Earth. But remarkably, the Moon also occulted, or passed directly in front of, Regulus and each of the three planets within 24 hours, all on September 18 UT. Visible from different locations, timing and watching the lunar occultations was much more difficult though, and mostly required viewing in daytime skies.

APOD: 2017 July 23 - Mercury as Revealed by MESSENGER
Explanation: Mercury had never been seen like this before. In 2008, the robotic MESSENGER spacecraft buzzed past Mercury for the second time and imaged terrain mapped previously only by comparatively crude radar. The featured image was recorded as MESSENGER looked back 90 minutes after passing, from an altitude of about 27,000 kilometers. Visible in the image, among many other newly imaged features, are unusually long rays that appear to run like meridians of longitude out from a young crater near the northern limb. MESSENGER entered orbit around Mercury in 2011 and finished its primary mission in 2012, but took detailed measurements until 2015, at which time it ran out of fuel and so was instructed to impact Mercury's surface.

APOD: 2016 October 8 - Moon, Mercury, and Twilight Radio
Explanation: Sharing dawn's twilight with the Moon on September 29, Mercury was about as far from the Sun as it can wander, the innermost planet close to its maximum elongation in planet Earth's skies. In this colorful scene fleeting Mercury is joined by a waning sunlit lunar crescent and earthlit lunar nightside, the New Moon in the Old Moon's arms. Below is the Italian Medicina Radio Astronomical Station near Bologna with a low row of antennae that is part of Italy's first radio telescope array dubbed the "Northern Cross", and a 32-meter-diameter parabolic dish. Of course, moonwatchers won't have to rise in early morning hours on October 8. After sunset the Moon will be high and bright in evening skies, at its first quarter phase for International Observe the Moon Night.

APOD: 2016 September 1 - Light at the End of the Road
Explanation: The bright light at the end of this country road is actually a remarkably close conjunction of two planets. After sunset on August 27 brilliant Venus and Jupiter almost appear as a single celestial beacon in the night skyscape taken near Lake Wivenhoe, Queensland, Australia. A spectacular vertical panorama from the southern hemisphere, it shows the central Milky Way near zenith, posed on top of a pillar of Zodiacal light along the ecliptic plane. Of course Mars and Saturn are near the ecliptic too, just below the galaxy's central bulge. Above and left of a tree on the horizon, fleeting planet Mercury also adds to the light at the end of the road.

APOD: 2016 August 6 - Las Campanas Moon and Mercury
Explanation: Last Thursday the view toward sunset from the 2.5 kilometer summit of Cerro Las Campanas in the remote Chilean Andes was amazing. Bright but fading Mercury stood very close to a two day old Moon. Both a sunlit lunar crescent and earthlit lunar nightside are captured with the fleeting innermost planet in this breathtaking mountainscape. Even below the conjunction of Moon and Mercury, a close pairing of brilliant Venus and bright star Regulus hangs in the sky, still above the colorful western horizon. Of course amazing skies above Las Campanas are not unexpected. The region is currently home to the twin Magellan telescopes of the Las Campanas Observatory and the summit location is the site of the future Giant Magellan Telescope.

APOD: 2016 July 17 - Mercury on the Horizon
Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year, informed skygazers with a little determination can usually pick Mercury out from a site with an unobscured horizon. Above, a lot of determination has been combined with a little digital manipulation to show Mercury's successive positions during March of 2000. Each picture was taken from the same location in Spain when the Sun itself was 10 degrees below the horizon and superposed on the single most photogenic sunset. Currently, Mercury is rising higher above the horizon with each passing sunset, and just now is angularly very close to the brighter planet Venus.

APOD: 2016 May 20 - 3D Mercury Transit
Explanation: On May 9, innermost planet Mercury crossed IN FRONT of the Sun. Though pictures project the event in only two dimensions, a remarkable three dimensional perspective on the transit is possible by free viewing this stereo pair. The images were made 23 minutes apart and rotated so that Mercury's position shifts horizontally between the two. As a result, Mercury's orbital motion produced an exaggerated parallax simulating binocular vision. Between the two exposures, the appropriately named planet's speedy 47.4 kilometer per second orbital velocity actually carried it over 65,000 kilometers. Taken first, the left image is intended for the right eye, so a cross-eyed view is needed to see Mercury's tiny silhouette suspended in the foreground. Try it. Merging the text below the images helps.

APOD: 2016 May 13 - ISS and Mercury Too
Explanation: Transits of Mercury are relatively rare. Monday's leisurely 7.5 hour long event was only the 3rd of 14 Mercury transits in the 21st century. If you're willing to travel, transits of the International Space Station can be more frequent though, and much quicker. This sharp video frame composite was taken from a well-chosen location in Philadelphia, USA. It follows the space station, moving from upper right to lower left, as it crossed the Sun's disk in 0.6 seconds. Mercury too is included as the small, round, almost stationary silhouette just below center. In apparent size, the International Space Station looms larger from low Earth orbit, about 450 kilometers from Philadelphia. Mercury was about 84 million kilometers away. (Editor's note: The stunning video includes another double transit, Mercury and a Pilatus PC12 aircraft. Even quicker than the ISS to cross the Sun, the aircraft was about 1 kilometer away.)

APOD: 2016 May 12 - A Transit of Mercury
Explanation: On May 9, the diminutive disk of Mercury spent about seven and a half hours crossing in front of the Sun as viewed from the general vicinity of Earth. It was the third of 14 transits of the Solar System's innermost planet in the 21st century. Captured from Fulham, London, England, planet Earth the tiny silhouette shares the enormous solar disk with prominences, filaments, and active regions in this sharp image. But Mercury's round disk (left of center) appears to be the only dark spot, despite the planet-sized sunspots scattered across the Sun. Made with an H-alpha filter that narrowly transmits the red light from hydrogen atoms, the image emphasizes the chromosphere, stretching above the photosphere or normally visible solar surface. In H-alpha pictures of the chromosphere, normally dark sunspot regions are dominated by bright splotches called plages.

APOD: 2016 May 11 - A Mercury Transit Music Video from SDO
Explanation: What's that small black dot moving across the Sun? Mercury. Possibly the clearest view of Mercury crossing in front of the Sun earlier this week was from Earth orbit. The Solar Dynamics Observatory obtained an uninterrupted vista recording it not only in optical light but also in bands of ultraviolet light. Featured here is a composite movie of the crossing set to music. Although the event might prove successful scientifically for better determining components of Mercury' ultra-thin atmosphere, the event surely proved successful culturally by involving people throughout the world in observing a rare astronomical phenomenon. Many spectacular images of this Mercury transit from around (and above) the globe are being proudly displayed.

APOD: 2016 May 9 - Webb Telescope Mirror Rises after Assembly
Explanation: Move over Hubble -- here comes the James Webb Space Telescope (JWST). JWST promises to be the new most powerful telescope in space. In the last month, the 18-segment gold-plated primary mirror for JWST was unveiled. In the featured time-lapse video taken last week, the 6.5-meter diameter mirror was raised to a vertical position. The dramatic 30-second sequence shows NASA engineers monitoring the test as room lights glint brightly off the mirror's highly reflective surface. The beryllium mirrors have been coated with a thin film of gold to make them more reflective to infrared light. The science goals of JWST include studying the workings of the early universe and the properties of planets orbiting nearby stars. Because of the mirror's great size, it will be folded for launch and then, assuming all goes as planned, dramatically unfolded again in space. The JWST, a joint mission of the space agencies of the USA, Europe, and Canada, is currently scheduled to be launched in late 2018.

APOD: 2016 May 8 - Mercurys Transit: An Unusual Spot on the Sun
Explanation: What's that dot on the Sun? If you look closely, it is almost perfectly round. The dot is the result of an unusual type of solar eclipse that occurred in 2006. Usually it is the Earth's Moon that eclipses the Sun. This time, the planet Mercury took a turn. Like the approach to New Moon before a solar eclipse, the phase of Mercury became a continually thinner crescent as the planet progressed toward an alignment with the Sun. Eventually the phase of Mercury dropped to zero and the dark spot of Mercury crossed our parent star. The situation could technically be labeled a Mercurian annular eclipse with an extraordinarily large ring of fire. From above the cratered planes of the night side of Mercury, the Earth appeared in its fullest phase. Hours later, as Mercury continued in its orbit, a slight crescent phase appeared again. This was ten years ago -- the next Mercurian solar eclipse will occur tomorrow.

APOD: 2016 May 4 - A Mercury Transit Sequence
Explanation: This coming Monday, Mercury will cross the face of the Sun, as seen from Earth. Called a transit, the last time this happened was in 2006. Because the plane of Mercury's orbit is not exactly coincident with the plane of Earth's orbit, Mercury usually appears to pass over or under the Sun. The above time-lapse sequence, superimposed on a single frame, was taken from a balcony in Belgium shows the entire transit of 2003 May 7. The solar crossing lasted over five hours, so that the above 23 images were taken roughly 15 minutes apart. The north pole of the Sun, the Earth's orbit, and Mercury's orbit, although all different, all occur in directions slightly above the left of the image. Near the center and on the far right, sunspots are visible. After Monday, the next transit of Mercury will occur in 2019.

APOD: 2016 April 15 - Mercury and Crescent Moon Set
Explanation: Innermost planet Mercury and a thin crescent Moon are never found far from the Sun in planet Earth's skies. Taken near dusk on April 8, this colorful evening skyscape shows them both setting toward the western horizon just after the Sun. The broad Tagus River and city lights of Lisbon, Portugal run through the foreground under the serene twilight sky. Near perigee or closest approach to Earth, the Moon's bright, slender crescent represents about 3 percent of the lunar disk in sunlight. Of course as seen from the Moon, a nearly full Earth would light up the lunar night, and that strong perigee earthshine makes the rest of the lunar disk visible in this scene. Bright Mercury stays well above the western horizon at sunset for northern skywatchers in the coming days. The fleeting planet reaches maximum elongation, or angular distance from the Sun, on April 18. But Mercury will swing back toward the Sun and actually cross the solar disk on May 9, the first transit of Mercury since November 8, 2006.

APOD: 2016 February 6 - Five Planets at Castell de Burriac
Explanation: February's five planet line-up stretches across a clear sky in this predawn scene. A hilltop Castell de Burriac looms in the foreground, overlooking the town of Cabrera de Mar near Barcelona, Spain, planet Earth. The mosaicked, panoramic image looks south. It merges three different exposure times to record a bright Last Quarter Moon, planets, seaside city lights, and dark castle ruins. Seen on February 1st the Moon was near Mars on the sky. But this week early morning risers have watched it move on, passing near Saturn and finally Venus and Mercury, sliding along near the ecliptic toward the dawn, approaching the February 7 New Moon.

APOD: 2016 January 30 - A Five Planet Dawn
Explanation: As January closes and in the coming days of February, early morning risers can spot the five naked-eye planets before dawn. Though some might claim to see six planets, in this seaside panoramic view all five celestial wanderers were found above the horizon along with a bright waning gibbous Moon on January 27. Nearly aligned along the plane of the ecliptic, but not along a line with the Sun, the five planets are spread well over 100 degrees across the sky. Just arriving on the predawn scene, fleeting Mercury stands above the southeastern horizon in the golden light of the approaching sunrise.

APOD: 2015 May 5 - Gravitational Anomalies of Mercury
Explanation: What's that under the surface of Mercury? The robotic MESSENGER spacecraft that had been orbiting planet Mercury for the past four years had been transmitting its data back to Earth with radio waves of very precise energy. The planet's gravity, however, slightly changed this energy when measured on Earth, which enabled the reconstruction of a gravity map of unprecedented precision. Here gravitational anomalies are shown in false-color, superposed on an image of the planet's cratered surface. Red hues indicate areas of slightly higher gravity, which in turn indicates areas that must have unusually dense matter under the surface. The central area is Caloris Basin, a huge impact feature measuring about 1,500 kilometers across. Last week, after completing its mission and running low on fuel, MESSENGER was purposely crashed onto Mercury's surface.

APOD: 2015 May 1 - MESSENGER's Last Day on Mercury
Explanation: The first to orbit Mercury, the MESSENGER spacecraft came to rest on this region of Mercury's surface yesterday. Constructed from MESSENGER image and laser altimeter data, the scene looks north over the northeastern rim of the broad, lava filled Shakespeare basin. The large, 48 kilometer (30 mile) wide crater Janacek is near the upper left edge. Terrain height is color coded with red regions about 3 kilometers above blue ones. MESSENGER'S final orbit was predicted to end near the center, with the spacecraft impacting the surface at nearly 4 kilometers per second (over 8,700 miles per hour) and creating a new crater about 16 meters (52 feet) in diameter. The impact on the far side of Mercury was not observed by telescopes, but confirmed when no signal was detected from the spacecraft given time to emerge from behind the planet. Launched in 2004, the MErcury Surface, Space ENvironment, GEochemisty and Ranging spacecraft completed over 4,000 orbits after reaching the Solar System's innermost planet in 2011.

APOD: 2015 April 18 - The Great Crater Hokusai
Explanation: One of the largest young craters on Mercury, 114 kilometer (71 mile) diameter Hokusai crater's bright rays are known to extend across much of the planet. But this mosaic of oblique views focuses on Hokusai close up, its sunlit central peaks, terraced crater walls, and frozen sea of impact melt on the crater's floor. The images were captured by the MESSENGER spacecraft. The first to orbit Mercury, since 2011 MESSENGER has conducted scientific explorations, including extensive imaging of the Solar System's innermost planet. Now running out of propellant and unable to counter orbital perturbations caused by the Sun's gravity, MESSENGER is predicted to impact the surface of Mercury on April 30.

APOD: 2015 March 5 - Enhanced Color Caloris
Explanation: The sprawling Caloris basin on Mercury is one of the solar system's largest impact basins, created during the early history of the solar system by the impact of a large asteroid-sized body. The multi-featured, fractured basin spans about 1,500 kilometers in this enhanced color mosaic based on image data from the Mercury-orbiting MESSENGER spacecraft. Mercury's youngest large impact basin, Caloris was subsequently filled in by lavas that appear orange in the mosaic. Craters made after the flooding have excavated material from beneath the surface lavas. Seen as contrasting blue hues, they likely offer a glimpse of the original basin floor material. Analysis of these craters suggests the thickness of the covering volcanic lava to be 2.5-3.5 kilometers. Orange splotches around the basin's perimeter are thought to be volcanic vents.

APOD: 2015 January 15 - Venus and Mercury at Sunset
Explanation: Inner planets Venus and Mercury can never wander far from the Sun in Earth's sky. This week you've probably seen them both gathered near the western horizon just after sunset, a close conjunction of bright celestial beacons in the fading twilight. The pair are framed in this early evening skyview captured on January 13 from the ruins of Szarvasko Castle in northwestern Hungary. Above the silhouette of the landscape's prominent volcanic hill Venus is much the brighter, separated from Mercury by little more than the width of two Full Moons. On Friday, planet Earth's early morning risers will also be treated to a close conjunction, when Saturn meets an old crescent Moon near the southeastern horizon at dawn.

APOD: 2014 August 24 - Mercury's Transit: An Unusual Spot on the Sun
Explanation: What's that dot on the Sun? If you look closely, it is almost perfectly round. The dot is the result of an unusual type of solar eclipse that occurred in 2006. Usually it is the Earth's Moon that eclipses the Sun. This time, the planet Mercury took a turn. Like the approach to New Moon before a solar eclipse, the phase of Mercury became a continually thinner crescent as the planet progressed toward an alignment with the Sun. Eventually the phase of Mercury dropped to zero and the dark spot of Mercury crossed our parent star. The situation could technically be labeled a Mercurian annular eclipse with an extraordinarily large ring of fire. From above the cratered planes of the night side of Mercury, the Earth appeared in its fullest phase. Hours later, as Mercury continued in its orbit, a slight crescent phase appeared again. The next Mercurian solar eclipse will occur in 2016.

APOD: 2013 November 23 - Comet ISON from STEREO
Explanation: Still intact, on November 21 Comet ISON (C/2012 S1) swept into this animated field of view (left) from the HI-1 camera on the STEREO-A spacecraft. The camera has also captured periodic Comet Encke, Mercury, and Earth, with the Sun cropped out of the frame at the right, the source of the billowing solar wind. From STEREO's perspective in interplanetary space, planet Earth is actually the most distant of the group, seen in its orbit beyond the Sun. Mercury is closest, but both planets are still so bright they create sharp vertical lines in the camera's detector. Both comets clearly sport substantial tails, but ISON is closer to the camera and will continue to move more rapidly through the field. Cameras on STEREO and SOHO spacecraft will be able to follow Comet ISON as it falls towards its close encounter with the Sun on November 28, even as ISON gets more difficult to see in the bright dawn skies of planet Earth.

APOD: 2013 August 5 - Leaving Earth
Explanation: What it would look like to leave planet Earth? Such an event was recorded visually in great detail by the MESSENGER spacecraft as it swung back past the Earth, eight years ago, on its way in toward the planet Mercury. Earth can be seen rotating in this time-lapse video, as it recedes into the distance. The sunlit half of Earth is so bright that background stars are not visible. The robotic MESSENGER spacecraft is now in orbit around Mercury and has recently concluded the first complete map of the surface. On occasion, MESSENGER has continued to peer back at its home world. MESSENGER is one of the few things created on the Earth that has left and will never return -- at the end of its mission MESSENGER will be crashed into Mercury's surface.

APOD: 2013 July 23 - Two Views of Earth
Explanation: In a cross-Solar System interplanetary first, our Earth was photographed during the same day from both Mercury and Saturn. Pictured on the left, Earth is the pale blue dot just below the rings of Saturn, as captured by the robotic Cassini spacecraft now orbiting the gas giant. Pictured on the right, the Earth-Moon system is seen against a dark background, as captured by the robotic MESSENGER spacecraft now orbiting Mercury. In the MESSENGER image, the Earth (left) and Moon (right) shine brightly with reflected sunlight. MESSENGER took the overexposed image last Friday as part of a search for small natural satellites of the innermost planet, moons that would be expected to be quite dim. During this same day, humans across planet Earth snapped many of their own pictures of Saturn.

APOD: 2013 June 13 - Four Planet Sunset
Explanation: You can see four planets in this serene sunset image, created from a series of stacked digital exposures captured near dusk on May 25. The composite picture follows the trail of three of them, Jupiter, Venus, and Mercury (left to right) dropping toward the western horizon, gathered close in last month's remarkable triple planetary conjunction. Similar in brightness to planet Mercury, the star Elnath (Beta Tauri) is also tracked across the scene, leaving its dotted trail still farther to the right. Of course, in the foreground are the still, shallow waters of Alikes salt lake, reflecting the striking colors of sunset over Kos Island, Greece, planet Earth. For now, Jupiter has wandered into the glare of the setting Sun, but Mercury and Venus remain low in the west at twilight.

APOD: 2013 June 12 - All of Mercury
Explanation: For the first time, the entire surface of planet Mercury has been mapped. Detailed observations of the innermost planet's surprising crust have been ongoing since the robotic MESSENGER spacecraft first passed Mercury in 2008 and began orbiting in 2011. Previously, much of the Mercury's surface was unknown as it is too far for Earth-bound telescopes to see clearly, while the Mariner 10 flybys in the 1970s observed only about half. The above video is a compilation of thousands of images of Mercury rendered in exaggerated colors to better contrast different surface features. Visible on the rotating world are rays emanating from a northern impact that stretch across much of the planet, while about half-way through the video the light colored Caloris Basin rotates into view, a northern ancient impact feature that filled with lava. MESSENGER has now successfully completed its primary and first extended missions.

APOD: 2013 March 1 - Colors of Mercury
Explanation: The colors of the solar system's innermost planet are enhanced in this tantalizing view, based on global image data from the Mercury-orbiting MESSENGER spacecraft. Human eyes would not discern the clear color differences but they are real none the less, indicating distinct chemical, mineralogical, and physical regions across the cratered surface. Notable at the upper right, Mercury's large, circular, tan colored feature known as the Caloris basin was created by an impacting comet or asteroid during the solar system's early years. The ancient basin was subsequently flooded with lava from volcanic activity, analogous to the formation of the lunar maria. Color contrasts also make the light blue and white young crater rays, material blasted out by recent impacts, easy to follow as they extend across a darker blue, low reflectance terrain.

APOD: 2013 February 19 - Mercury on the Horizon
Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an unobscured horizon. Above, a lot of determination has been combined with a little digital manipulation to show Mercury's successive positions during March of 2000. Each picture was taken from the same location in Spain when the Sun itself was 10 degrees below the horizon and superposed on the single most photogenic sunset. Currently, Mercury is visible in the western sky after sunset, but will disappear in the Sun's glare after a few days.

APOD: 2012 December 1 - Northern Mercury
Explanation: Innermost planet Mercury would probably not be a good location for an interplanetary winter olympics. But new results based on data from the Mercury orbiting MESSENGER spacecraft indicate that it does have substantial water ice in permanently shadowed regions within craters near its north pole. The possibility of ice on Mercury has been entertained for years, inspired by the discovery of radar bright, hence highly reflective, regions near the north pole. Highlighted in yellow in this map based on projected MESSENGER images, radar bright regions are seen to correspond with floors and walls of north polar impact craters. Farther from the pole the regions are concentrated on the north facing crater walls. MESSENGER's neutron spectroscopy and thermal models for the craters indicate material in these regions has a hydrogen content consistent with nearly pure water ice and is trapped in an area with temperatures that remain below 100 kelvins (-280 deg.F, -173 deg.C). In circumstances similar to permanent shadows in craters of the Moon, debris from comet impacts is thought to be the source of ice on Mercury.

APOD: 2012 May 27 - Mercury Spotting
Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk in 2003, as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This was the first of 14 transits of Mercury which will occur during the 21st century. Next week, however, an event much more rare but easier to spot will occur -- a transit of Venus across the Sun. Need help spotting Mercury? Just click on the picture.

APOD: 2012 April 26 - Morning, Moon, and Mercury
Explanation: Last week Mercury wandered far to the west of the Sun. As the solar system's innermost planet neared its greatest elongation or greatest angle from the Sun (for this apparition about 27 degrees) it was joined by an old crescent Moon. The conjunction was an engaging sight for early morning risers in the southern hemisphere. There the pair rose together in predawn skies, climbing high above the horizon along a steeply inclined ecliptic plane. This well composed sequence captures the rising Moon and Mercury above the city lights of Brisbane in Queensland, Australia. A stack of digital images, it consists of an exposure made every 3 minutes beginning at 4:15 am local time on April 19. Mercury's track is at the far right, separated from the Moon's path by about 8 degrees.

APOD: 2012 March 27 - Unusual Hollows Discovered on Planet Mercury
Explanation: What are those unusual features on planet Mercury? The slightly bluish tinge of features dubbed hollows has been exaggerated on the above image by the robotic MESSENGER spacecraft currently orbiting Mercury. The rounded depressions appear different than impact craters and nothing like them has been noted on Earth's Moon or anywhere else in the Solar System. The above image is a section of the floor of Raditladi impact basin about 40 kilometers wide that includes the mountains of the central peak. One progenitor hypothesis is that the hollows formed from the sublimation of material exposed and heated during the violent impact that created the Raditladi basin. NASA's MESSENGER is the first spacecraft ever to orbit Mercury, and is currently scheduled to explore the Solar System's innermost planet into 2013.

APOD: 2011 October 8 - MESSENGER's First Day
Explanation: One solar day on a planet is the length of time from noon to noon. A solar day lasts 24 hours on planet Earth. On Mercury a solar day is about 176 Earth days long. And during its first Mercury solar day in orbit the MESSENGER spacecraft has imaged nearly the entire surface of the innermost planet to generate a global monochrome map at 250 meters per pixel resolution and a 1 kilometer per pixel resolution color map. Examples of the maps, mosaics constructed from thousands of images made under uniform lighting conditions, are shown (monochrome at left), both centered along the planet's 75 degrees East longitude meridian. The MESSENGER spacecraft's second Mercury solar day will likely include more high resolution targeted observations of the planet's surface features. (Editor's note: Due to Mercury's 3:2 spin-orbit resonance, a Mercury solar day is 2 Mercury years long.)

APOD: 2011 July 2 - Moon and Venus at Dawn
Explanation: Brilliant Venus and a thin crescent Moon stood together above the eastern horizon just before sunrise on June 30. The lovely celestial pairing is captured in this colorful twilight skyview overlooking a reservoir near Izmir, Turkey. For some, the close conjunction could be viewed as a daylight occultation. While Venus is nearing the end of its latest performance as planet Earth's morning star, the old lunar cresent, about 24 hours from its New Moon phase, was also bidding farewell for now to the dawn. In fact, for the next two nights a young Moon can be spotted just after sunset. Look for a thin sunlit sliver close to the western horizon, not far from bright planet Mercury.

APOD: 2011 June 22 - MESSENGER's Degas View
Explanation: Now imaging inner planet Mercury from orbit, the MESSENGER spacecraft wide angle camera has returned this impressive color view of Degas Crater, with a full resolution of 90 meters per pixel. Named for the impressionist painter, the 52 kilometer diameter crater is also shown in an inset context image from the Mariner 10 flyby mission in the mid 1970s. In MESSENGER's view, the crater floor is seen to be filled with an intricate series of cracks, formed as the molten surface resulting from the impact cooled and contracted. Starkly bright, patchy deposits, suggesting compositional differences and freshly exposed material, standout around the crater's central peaks and walls. Details of similar bright deposits are seen in even higher resolution images from MESSENGER.

APOD: 2011 June 16 - Mercury's Surface in Exaggerated Color
Explanation: The robotic MESSENGER spacecraft recently completed over 100 orbits of Mercury. MESSENGER's cameras have recorded detailed pictures utilizing eight different colors across visible and near infrared light, exploring the surface composition and looking for clues to the history and evolution of the solar system's innermost planet. This sharp image combines three of the MESSENGER wide angle camera's colors, but in exaggerated fashion. Otherwise, to the unaided human eye, Mercury's surface colors would appear comparatively muted. The image is about 1,000 kilometers across and features as small as a single kilometer are discernible at the original resolution. Today, the MESSENGER project will release new images and science findings from the first spacecraft to orbit Mercury.

APOD: 2011 June 4 - Dawn's Grande Finale
Explanation: After more than a month, the lovely lineup of four naked-eye planets in dawn skies is coming to a close. Still, on May 31st a slender Moon joined the grouping along the eastern horizon for a final celestial performance, presented in this early morning scene from a beach near Buenos Aires, Argentina. A favorable view of the configuration in the southern hemisphere autumn, the photo was taken about 30 minutes before sunrise. In order from bottom to top, wandering Mercury, Venus, Mars, and Jupiter are stretched along the ecliptic plane. The Moon's sunlit crescent is sinking into the colorful twilight glow just left of Mercury. In dawns to come, Mars and Jupiter will continue to rise while Venus and Mercury sink toward the horizon, drawing closer to the rising Sun.

APOD: 2011 May 21 - Planets, Endeavour at Dawn
Explanation: When dawn broke over Kennedy Space Center on Monday, May 16, the space shuttle orbiter Endeavour still stood on pad 39A. Its final launch, on mission STS-134 to the International Space Station, was only hours away. Shining through the early morning twilight four planets were also poised above the eastern horizon, a moving scene captured here from across the Banana River at the center's Saturn V VIP viewing site. Scattered by planet Earth's dense atmosphere, floodlight beams play over the launch pad, glancing skyward toward the celestial beacons. Jupiter is highest, near the top of the frame, but even the solar sytem's ruling gas giant is outshone by brilliant Venus near picture center. Innermost planet Mercury is below Venus, to the right. Below and left, Mars almost fades into the twilight glow. The four planets continue to hug the eastern horizon at dawn throughout the month, while Endeavour is now scheduled to make its final approach to planet Earth on June 1.

APOD: 2011 May 7 - Dawn of the Planets
Explanation: This month, four of the five naked-eye planets gather along the eastern horizon near dawn. The celestial grouping is seen here just before sunrise on May 5, from a beach near Buenos Aires, Argentina. Starting near the top of the frame, the brightest beacon is Venus. Mercury is below and right of Venus and brilliant Jupiter is lower still, near image center. Below Jupiter, Mars is relatively faint and struggles the most to shine through a thin cloud bank and the warming twilight glow. Watch, and as the month progresses the tantalizing configuration will change, with Mars and Jupiter moving higher while Venus and Mercury wander through the sky closer to the rising sun.

APOD: 2011 May 5 - 50 Years Ago: Freedom 7 Flies
Explanation: Fifty years ago, near the dawn of the space age, NASA controllers "lit the candle" and sent Mercury astronaut Alan Shepard arcing into space atop a Redstone rocket. His cramped space capsule was dubbed Freedom 7. Broadcast live to a global television audience, the historic Mercury-Redstone 3 (MR-3) spacecraft was launched from Cape Canaveral Florida at 9:34 a.m. Eastern Time on May 5, 1961. The flight of Freedom 7 - the first space flight by an American - followed less than a month after the first human venture into space by Soviet Cosmonaut Yuri Gagarin. The 15 minute sub-orbital flight achieved an altitude of 116 miles and a maximum speed of 5,134 miles per hour. As Shepard looked back toward planet Earth near the peak of Freedom 7's trajectory, he could see the outlines of the west coast of Florida, Lake Okeechobe in central Florida, the Gulf of Mexico, and the Bahamas.

APOD: 2011 March 31 - MESSENGER at Mercury
Explanation: On March 17, the MESSENGER spacecraft became the first to orbit Mercury, the solar system's innermost planet. This is its first processed color image since entering Mercury orbit. Larger, denser, and with almost twice the surface gravity of Earth's moon, Mercury still looks moon-like at first glance. But in this view its terrain shows light blue and brown areas near craters and long bright rays of material streaking the surface. The prominent bright ray crater Debussy at the upper right is 80 kilometers (50 miles) in diameter. Terrain toward the bottom of the historic image extends to Mercury's south pole and includes a region not previously imaged from space.

APOD: 2011 March 18 - Mercury and Jupiter at Sunset
Explanation: When warm sunset hues begin to fade, two celestial beacons now shine in the evening twilight, Mercury and Jupiter. Wandering away from the Sun in planet Earth's sky, Mercury will offer good views this month as spring approaches in the northern hemisphere where the ecliptic plane makes a steep angle with the western horizon. But Jupiter will continue sinking lower in the sky after sunset. In fact, the normally elusive Mercury shines well above Jupiter and the orange sunset glow in this serene sky. Captured earlier this week from the island of of Frösön in northern Sweden, the scene looks across Lake Storsjön toward the village of Hallen and distant mountains. Of course, even better views of Mercury can be had by the MESSENGER spacecraft, now orbiting the Solar System's innermost planet!

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 September 1 - Earth and Moon from MESSENGER
Explanation: What does Earth look like from the planet Mercury? The robotic spacecraft MESSENGER found out as it looked toward the Earth during its closest approach to the Sun about three months ago. The Earth and Moon are visible as the double spot on the lower left of the above image. Now MESSENGER was not at Mercury when it took the above image, but at a location from which the view would be similar. From Mercury, both the Earth and its comparatively large moon will always appear as small circles of reflected sunlight and will never show a crescent phase. MESSENGER has zipped right by Mercury three times since being launched in 2004, and is scheduled to enter orbit around the innermost planet in March of 2011.

APOD: 2010 July 31 - Four Planet Sunset
Explanation: This mesmerizing sunset photo was taken from the summit of volcanic Mount Lawu, 3,265 meters above sea level, on July 21. The view looks west, toward the city lights of Surakarta (aka Solo), Central Java, Indonesia. Two other volcanic peaks, sharp Merapi (left) and Merbabu lie along the colorful horizon. Four planets shine in the twilight sky above them. Spread out near the plane of the ecliptic are Mercury, Venus, Mars, and Saturn, along with bright Regulus, alpha star of the constellation Leo. For help finding them, just put your cursor over the picture. In fact, these four planets still shine in western skies at sunset, with Venus, Mars, and Saturn grouped much more tightly this weekend and in early August. By August 12, a young crescent Moon will join the four planet sunset.

APOD: 2010 April 22 - Venus, Mercury, and Moon
Explanation: Earlier this month, Venus and Mercury climbed into the western twilight, entertaining skygazers around planet Earth in a lovely conjunction of evening stars. Combining 8 images spanning April 4 through April 15, this composite tracks their progress through skies above Portsmouth, UK. Each individual image was captured at 19:50 UT. The sequential path for both bright planets begins low and to the left. But while Venus continues to swing away from the setting Sun, moving higher above the western horizon, Mercury first rises then falls. Its highest point is from the image taken on April 11. Of course on April 15, Venus and Mercury were joined by a young crescent Moon.

APOD: 2010 April 12 - Mercury and Venus Over Paris
Explanation: Go outside tonight and see one of the more interesting planetary conjunctions of recent years. Just after sunset, the planets Mercury and Venus are visible quite near each other. Now Venus, being commonly discernible as one of the brightest objects in the sky, is frequently mistaken for an airplane. (Venus will set quite slowly, though.) Mercury, however, is dimmer and usually harder to find. Recently, though, Mercury can be found just to the right of Venus, appearing increasingly below the brighter planet over the next week. Pictured above, Venus and Mercury were imaged next to the famous Notre Dame Cathedral in Paris, France. A careful inspection of the image will further reveal that the bright object nearly below Venus is iconic Eiffel Tower.

APOD: 2010 April 9 - Discovery's Dawn
Explanation: On April 5, visitors to Kennedy Space Center saw these colorful clouds, twisting and drifting through dawn skies. Of course, the clouds were rocket engine plumes from the predawn launch of the space shuttle Discovery on the STS-131 mission to the International Space Station. Their layered colors are created as they reflect the reddened light from the still rising Sun. Fittingly, denizens of the space center's rocket garden are lit in the foreground. At the far left is a 1960s vintage multistage Atlas-Agena rocket. Together on the right, are Mercury-Redstone and Mercury-Atlas rockets.

APOD: 2010 April 7 - Venus and Mercury in the West
Explanation: In this twilight skyview, a windmill stands in silent witness to a lovely pairing of planets in the west. The picture was recorded on April 5 from Gallegos del Campo, Zamora, Spain. Venus (left) and Mercury (right) are near their much anticipated conjunction in the early evening sky. But even in the coming days, these two evening stars will remain close in the western sky at sunset. In fact, with brighter Venus as a marker, sky watchers will have an excellent guide for spotting Mercury nearby, a planet often hidden in the Sun's glare.

APOD: 2009 October 22 - Moon and Planets in the Morning
Explanation: Last Friday, a gathering of three bright planets and the Moon graced the morning sky. With Mercury, Venus, Saturn, and a narrow lunar crescent close to the eastern horizon in the dawn twilight, this picture of the beautiful conjunction was recorded near Noerdlingen, Germany. These planets are wandering apart now and Mercury is sinking closer toward the rising Sun. But if you also scan the rest of the sky this week you should be able to add Jupiter and Mars to your planet spotting list, as Mars rises around midnight and Jupiter shines brightly after sunset. In fact, if you want a better view of Jupiter than Galileo had, you might check out the 2009 International Year of Astronomy activities and events during these next few Galilean Nights (October 22-24).

APOD: 2009 October 7 - A Double Ringed Basin on Mercury
Explanation: What caused the smooth floor inside the double ringed basin on Mercury? No one is sure. The unusual feature spans 160 kilometers and was imaged during the robotic MESSENGER spacecraft's swing past our Solar System's innermost planet last week. Double and multiple ringed basins, although rare, have also been imaged in years past on Mars, Venus, Earth, and Earth's Moon. Mercury itself has several doubles, including huge Caloris basin, Rembrandt basin, and enigmatic Raditladi basin. Most large multiple ringed basins on planets and moons are caused initially by a forceful impact by a single asteroid or comet fragment. One interesting feature of the above-imaged double ring is that the basin center appears much smoother than the region between the rings. Possibly, the internal floor was smoothed by later volcanic activity. Also possible, however, is that the floor was smoothed by melting and flowing of material upon impact. MESSENGER has now completed its last flyby of Mercury but will return and attempt to enter orbit in 2011 March.

APOD: 2009 August 30 - D. rad Bacteria: Candidate Astronauts
Explanation: These bacteria could survive on another planet. In an Earth lab, Deinococcus radiodurans (D. rad) survive extreme levels of radiation, extreme temperatures, dehydration, and exposure to genotoxic chemicals. Amazingly, they even have the ability to repair their own DNA, usually within 48 hours. Known as an extremophile, bacteria such as D. rad are of interest to NASA partly because they might be adaptable to help human astronauts survive on other worlds. A recent map of D. rad's DNA might allow biologists to augment their survival skills with the ability to produce medicine, clean water, and oxygen. Already they have been genetically engineered to help clean up spills of toxic mercury. Likely one of the oldest surviving life forms, D. rad was discovered by accident in the 1950s when scientists investigating food preservation techniques could not easily kill it. Pictured above, Deinococcus radiodurans grow quietly in a dish.

APOD: 2009 July 30 - 6 Minutes 42 Seconds
Explanation: The July 22nd total solar eclipse was the longest of the 21st century. From the point of maximum eclipse along the Moon's shadow track across the Pacific Ocean, the Moon completely blocked the Sun for a total of 6 minutes and 39 seconds. But from the deck of this cruise ship the duration of the total eclipse phase was extended to a whopping 6 minutes and 42 seconds by the ship's motion along the shadow track. This panoramic view of the scene shows the shimmering solar corona in a darkened daytime sky, with clouds silhouetted by a bright sky on the distant horizon, beyond the Moon's shadow. Mercury can be seen near the eclipsed Sun. Venus lies near the upper right edge of the frame.

APOD: 2009 July 6 - Unknown Dark Material on Mercury
Explanation: What is that strange material on Mercury? When flying by Mercury last October, the robotic MESSENGER spacecraft imaged much of the solar system's innermost planet in unprecedented detail. As common in science, new data bring new mysteries. Pictured above on the lower right, a large crater -- about 100 kilometers across -- has unusual dark material of unknown composition near its center. The material's darkness does not appear to be caused by shadows, as the Sun was near zenith when the image was taken. One origin hypothesis is that the dark material was uncovered from beneath Mercury's surface during the impact that created the surrounding crater. If so, the composition of the dark mound might be similar to the composition of some mysterious dark rings also recently discovered on Mercury. Alternatively, the dark material could be related to an unusual composition of the impacting rock. MESSENGER will buzz past Mercury again later this year before entering orbit in 2011.

APOD: 2009 May 27 - Volcanic Terrain on Mercury
Explanation: Why are many large craters on Mercury relatively smooth inside? Recent images from the robotic MESSENGER spacecraft that flew by Mercury last October show previously uncharted regions of Mercury that have large craters with an internal smoothness similar to the maria on Earth's own Moon. Therefore, like our Moon's maria, these craters on Mercury are thought to have been flooded by lava floes that are old but not as old as the surrounding more highly cratered surface. The above image mosaic of the western limb of Mercury was created by MESSENGER as it approached the Solar System's innermost planet last October. Old and heavily textured terrain runs across much of the image bottom, while across the middle left lies comparatively smooth impact basins where small craters may appear similar at first to protruding hills. MESSENGER will buzz past Mercury again later this year before entering orbit in 2011.

APOD: 2009 May 4 - Rembrandt Impact Basin on Mercury
Explanation: Why do portions of this huge crater on Mercury have so much iron? The unusual Rembrandt impact basin was discovered recently in images taken during the robotic MESSENGER spacecraft's 2008 October flyby of the Solar System's innermost planet. The unusual Rembrandt spans over 700 kilometers and at 4 billion years old is possibly the youngest large impact basin on the planet. Multicolored images of the crater floor, however, indicate reflections from areas containing unusually high amounts of iron and titanium. These elements indicate that some exposed materials have not been covered by more recent lava floes, and so might originate from an epoch of Mercury's formation. Data from Rembrandt and across Mercury are now being interpreted as indicating a relatively active and volcanic past for Mercury that includes surface tectonics. Close inspection of the above image will reveal rings of Mercury's Rembrandt impact basin circling around the image center. Mercury's limb is visible on the upper left, high cliffs and small craters are visible inside Rembrandt, and the terminator between night and day runs diagonally through the image. MESSENGER is on track to fly past Mercury again this September and enter orbit around Mercury in 2011.

APOD: 2009 April 30 - Framed by Clouds
Explanation: Last Sunday's fading evening twilight featured a young crescent Moon along the western horizon. The young Moon also shared the sky with the lovely Pleiades star cluster and wandering planet Mercury. Framed by clouds in this serene skyscape from Selsey, UK, a similar twilight scene was visible around the globe. Emerging from the cloud bank below the Pleiades, the narrow sunlit lunar crescent is overexposed. Still, the Moon's dim night side is impressively clear, illuminated by earthshine. Bright, innermost planet Mercury lies near the bottom of the field. Mercury will remain near the Pleiades, low in the west after sunset over the coming days, an ongoing conjunction of planet and star cluster that will offer skygazers some excellent binocular views.

APOD: 2009 February 26 - Moon, Mercury, Jupiter, Mars
Explanation: When the Moon rose in predawn skies on February 23rd, it sported a sunlit crescent. It also offered early morning risers a tantalizing view of earthshine, the dark portion of the lunar disk illuminated by sunlight reflected from the Earth. Of course, on that morning a remarkable conjunction with three wandering planets added an impressive touch to the celestial scene. Recorded just before sunrise, this serene skyview looks east toward a glowing horizon across Tuggerah Lake on the Central Coast of New South Wales, Australia. Along with the waning crescent Moon, the picture captures (top to bottom) bright Mercury, Jupiter, and Mars.

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

APOD: 2008 November 3 - A Spectacular Rayed Crater on Mercury
Explanation: Why does Mercury have so many rayed craters? No one is sure. The robotic MESSENGER spacecraft that is taking unprecedented images as it swoops past the innermost planet has provided dramatic confirmation that Mercury has more rayed craters than Earth's Moon. Pictured above, a particularly spectacular rayed crater spanning approximately 80 kilometers was imaged by MESSENGER during last month's flyby from about 20,000 kilometers up. The rays prevalence is a mystery because space weathering effects such as dust accumulation and solar wind attenuation should be greater on Mercury than on the Moon. Hypothesized solutions currently include the optical properties of Mercurian dust, and that Mercury's high mass and proximity to the Sun cause more violent impacts, thus typically raising more light material. MESSENGER will buzz past Mercury again next year before entering orbit in 2011.

APOD: 2008 October 8 - Mercury as Revealed by MESSENGER
Explanation: The planet Mercury has been known since history has been recorded, but parts of the Solar System's innermost planet have never been seen like this before. Two days ago the robotic MESSENGER spacecraft buzzed past Mercury for the second time and imaged terrain mapped previously only by comparatively crude radar. The above image was recorded as MESSENGER looked back 90 minutes after passing, from an altitude of about 27,000 kilometers. Visible in the above image, among many other newly imaged features, are unusually long rays that appear to run like meridians of longitude out from a young crater near the northern limb. MESSENGER is scheduled to fly past Mercury once more before firing its thrusters to enter orbit in 2011.

APOD: 2008 September 30 - Planets Ahoy
Explanation: Can you spot the Solar System's four rocky planets? In the above image taken on September 20, all of them were visible in a single glance, but some of them may be different than you think. Pictured above, the brightest and highest object in the sky is the planet Venus. The object lowest in the sky is the planet Mars, while the object furthest to the left is the planet Mercury. The last remaining point of light is . . . the bright star Spica, which leaves the question -- where is the fourth rocky planet? That would be Earth, specifically part of Australia, visible across the entire bottom of the image.

APOD: 2008 September 12 - Planets over Perth
Explanation: A bright trio of terrestrial planets was joined by a young Moon on September 1st, in planet Earth's early evening skies. In this view of the celestial gathering from Perth, Western Australia, the Moon's sunlit crescent is nearly horizontal at Perth's southern latitude of about 32 degrees. Venus, then Mercury, and finally Mars shine above colorful city lights on the far shore of the Swan River. The six unlit towers on the left surround a large cricket stadium. For now, the planetary trio still lingers low in the west just after sunset. But in the coming days Venus will move farther from the Sun, climbing higher after sunset, while Mercury and Mars will steadily sink into the glare along the western horizon.

APOD: 2008 July 10 - Enhanced Color Caloris
Explanation: The sprawling Caloris basin on Mercury is one of the solar system's largest impact basins. Created during the early history of the solar system by the impact of a large asteroid-sized body, the basin spans about 1,500 kilometers and is seen in yellowish hues in this enhanced color mosaic. The image data is from the January 14th flyby of the MESSENGER spacecraft, captured with the MDIS instrument. Orange splotches around the basin's perimeter are now thought to be volcanic vents, new evidence that Mercury's smooth plains are indeed lava flows. Other discoveries at Mercury by NASA's MESSENGER mission include evidence that Mercury, like planet Earth, has a global magnetic field generated by a dynamo process in its large core, and that Mercury's surface has contracted significantly as its core cooled.

APOD: 2008 May 21 - A Dangerous Sunrise on Gliese 876d
Explanation: On planet Gliese 876d, sunrises might be dangerous. Although nobody really knows what conditions are like on this close-in planet orbiting variable red dwarf star Gliese 876, the above artistic illustration gives one impression. With an orbit well inside Mercury and a mass several times that of Earth, Gliese 876d might rotate so slowly that dramatic differences exist between night and day. Gliese 876d is imagined above showing significant volcanism, possibly caused by gravitational tides flexing and internally heating the planet, and possibly more volatile during the day. The rising red dwarf star shows expected stellar magnetic activity which includes dramatic and violent prominences. In the sky above, a hypothetical moon has its thin atmosphere blown away by the red dwarf's stellar wind. Gliese 876d excites the imagination partly because it is one of the few extrasolar planets known to be close to the habitable zone of its parent star.

APOD: 2008 May 9 - Moon Meets Mercury
Explanation: On Tuesday, May 6, while standing on planet Earth and sweeping your binoculars along the western horizon just after sunset, you might have encountered this arresting skyscape. The view features a slender crescent Moon and bright planet Mercury separated on the sky by only about 2 degrees. Cradled in the sunlit lunar crescent, the night side of the Moon is faintly illuminated by earthshine -- sunlight reflected from planet Earth. Of course, the clouds in silhouette and fading twilight colors are common elements in pictures of the sky after sunset, but much less often seen is inner planet Mercury, usually hiding close to the Sun in Earth's sky. Still, the coming week will be a good time to spot Mercury near the western horizon about 30 minutes after sunset. As for the Moon, tonight and tomorrow night the crescent Moon will wander close to Mars in the early evening sky.

APOD: 2008 March 19 - Mercury in Accentuated Color
Explanation: The colors of Mercury are subtle but beautiful. At first glance, our Solar System's innermost planet appears simply black and white, but images that include infrared colors normally beyond human vision accentuate a world of detail. One such image, shown above, was acquired by the robotic MESSENGER spacecraft that swung by Mercury in mid-January. Here, most generally, the hot world itself acquires a slightly more brown hue. Many craters that appear on top of other craters -- and so surely have formed more recently -- appear here as bright with bright rays that include a slightly blue tint, indicating that soil upended during the impact was light in color. A few craters, such as some in the huge Caloris Basin impact feature visible on the upper right, appear unexpectedly to be ringed with a dark material, the nature of which is being researched. MESSENGER continues to glide through the inner Solar System and will pass Mercury again this October and next September, before entering orbit around the desolate world in 2011.

APOD: 2008 March 10 - Planets Align Over Australian Radio Telescope Array
Explanation: Last week, Mercury, Venus, and the Moon all appeared close together in Earth's sky. This picturesque conjunction was caught on camera behind elements of the Australia Telescope Compact Array (ATCA) near the town of Narrabri in rural New South Wales. The ATCA consists of six radio telescopes in total, each one larger than a house. Together they form one of the highest resolution measurement devices in the world. Impressive planetary conjunctions occur every few years. Involving the brightest objects in the night sky, this alignment was easy to spot just before sunrise. In the picture, taken on the morning of March 6, Mercury is the highest of the three bright celestial beacons.

APOD: 2008 February 4 - A Spider Shaped Crater on Mercury
Explanation: Why does this crater on Mercury look like a spider? When the robotic MESSENGER spacecraft glided by the planet Mercury last month, it was able to image portions of the Sun's closest planet that had never been seen before. When imaging the center of Mercury's extremely large Caloris Basin, MESSENGER found a crater, pictured above, with a set of unusual rays emanating out from its center. A crater with such troughs has never been seen before anywhere in our Solar System. What isn't clear is the relation of the crater to the radial troughs. Perhaps the crater created the radial rays, or perhaps the two features appear only by a chance superposition -- the topic is sure to be one of future research. MESSENGER is scheduled to fly past Mercury twice more before firing its thrusters to enter orbit in 2011.

APOD: 2008 January 27 - Mercury on the Horizon
Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an unobscured horizon. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during March of 2000. Each picture was taken from the same location in Spain when the Sun itself was 10 degrees below the horizon and superposed on the single most photogenic sunset. Mercury is currently visible in the western sky after sunset, but will disappear in the Sun's glare after a few days.

APOD: 2008 January 26 - Crescent Mercury in Color
Explanation: Hard to spot against the twilight glow near planet Earth's horizon, a crescent Mercury was imaged close up by the MESSENGER spacecraft early last week. Colors in this remarkable picture were created using data recorded through infrared, red, and violet filters. The combination enhances color differences otherwise not visible to the eye across the innermost planet's cratered surface. In this view, light bluish material seems to surround relatively new craters, contrasting with the mostly drab, brown terrain. Mercury itself is 4,880 kilometers in diameter. The full resolution image shows features as small as 10 kilometers across.

APOD: 2008 January 21 - Mercury's Horizon from MESSENGER
Explanation: What would it look like to fly past Mercury? Just such an adventure was experienced last week by the MESSENGER spacecraft during its first flyby of the strange moon-like world nearest the Sun. Pictured above is the limb of Mercury seen by MESSENGER upon approach, from about 1 1/2 Earth diameters away. Visible on the hot and barren planet are many craters, many appeared to be more shallow than similarly sized craters on the Moon. The comparatively high gravity of Mercury helps flatten tall structures like high crater walls. MESSENGER was able to take over 1,000 images of Mercury which will be beamed back to Earth for planetary geologists to study. The robotic MESSENGER spacecraft is scheduled to fly past Mercury twice more before firing its thrusters to enter orbit in 2011.

APOD: 2008 January 16 - MESSENGER Passes Mercury
Explanation: Two days ago, the MESSENGER spacecraft became only the second spacecraft in human history to swoop past Mercury. The last spacecraft to visit the Sun's closest planet was Mariner 10 over 35 years ago. Mariner 10 was not able to photograph Mercury's entire surface, and the images it did send back raised many questions. Therefore, much about planet Mercury remains unknown. This week's flyby of MESSENGER was only the first of three flybys. Over the next few years MESSENGER will swing past twice more and finally enter Mercury's orbit in 2011. MESSENGER is currently moving too fast to enter orbit around Mercury now. The above image was taken two days ago during MESSENGER's flyby and shows part of Mercury's surface that has never been imaged in detail before. Many more detailed images of Mercury are expected to be sent back over the next few days. The data acquired by MESSENGER will hopefully help scientists better understand how Mercury's surface was formed, and why it is so dense.

APOD: 2008 January 12 - Mercury Chases the Sunset
Explanation: This colorful view of the western sky at sunset features last Wednesday's slender crescent Moon. Of course, when the Moon is in its crescent phase it can never be far from the Sun in the sky. Also always close to the Sun in Earth's sky is innermost planet Mercury, seen here below and right of center against the bright orange glow along the horizon. Mercury is usually difficult to glimpse because of overwhelming sunlight, but increasingly better views of the small planet after sunset will be possible as it wanders farther east of the Sun in the coming days. On January 14th, NASA's MESSENGER spacecraft will have a good view too, as it makes its first Mercury flyby.

APOD: 2007 October 3 - Comet Encke s Tail Ripped off
Explanation: Swinging inside the orbit of Mercury, on April 20th periodic comet Encke encountered a blast from the Sun in the form of a Coronal Mass Ejection (CME). When CMEs, enormous clouds of energetic particles ejected from the Sun, slam into Earth's magnetosphere, they often trigger auroral displays. But in this case, the collison carried the tail of the comet away. The tail was likely ripped off by interacting magnetic fields rather than the mechanical pressure of the collision. Clicking on the two panel image will play a movie gif of the remarkable event as recorded by the Heliospheric Imager onboard the STEREO A spacecraft. In the movie, the time between frames is about 45 minutes, while the frames span about 14x20 million kilometers at the distance of the comet. Of course, similar collisions have happened before as the ancient comet loops through its 3.3 year solar orbit. So don't worry, Encke's tail grows back!

APOD: 2007 January 20 - SOHO: Comet McNaught Movie
Explanation: This frame from a spectacular time lapse movie shows Comet McNaught - the Great Comet of 2007 - sweeping through the inner solar system. The movie frames were recorded from January 12 through Jan 16 by a coronograph onboard the sun-staring SOHO spacecraft. Bright planet Mercury also glides dramatically through the field of view but the Sun itself remains fixed, hidden behind the coronograph's central occulting disk. The broad-tailed comet is so bright it almost overwhelms SOHO's sensitive camera designed to explore the fainter structures in the Sun's outer atmosphere. Comet McNaught's closest approach to the Sun (perihelion on January 12) was only 0.17 astronomical units, or about half the distance between the Sun and Mercury. (Note: To download the movie file, click on the picture.)

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

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

APOD: 2006 December 9 - Three Planets in Dawn Skies
Explanation: Three children of the Sun rise in the east in this peaceful dawn skyview recorded December 7th near Bolu, Turkey. Inner planet Mercury, fresh from its second transit of the 21st century, stands highest in the bright sky at the top right. Gas giant Jupiter lies below the cloud bank near picture center. A newsworthy Mars is also visible, right of Jupiter and just above the dark cloud bank. On Sunday, these planets will form a much tighter grouping before sunrise, while in the coming days the western sky after sunset will be ruled by brilliant planet Venus, also known as the evening star.

APOD: 2006 November 25 - 3D Mercury Transit
Explanation: Mercury is now visible shortly before dawn, the brightest "star" just above the eastern horizon. But almost two weeks ago Mercury actually crossed the face of the Sun for the second time in the 21st century. Viewed with red/blue glasses, this stereo anaglyph combines space-based images of the Sun and innermost planet in a just-for-fun 3D presentation of the Mercury transit. The solar disk image is from Hinode. (sounds like "hee-no-day", means sunrise). A sun-staring observatory, Hinode was launched from Uchinoura Space Center and viewed the transit from Earth orbit. Superimposed on Mercury's dark silhouette is a detailed image of the planet's rugged surface based on data from the Mariner 10 probe that flew by Mercury in 1974 and 1975.

APOD: 2006 November 17 - Hand Drawn Transit
Explanation: The sight of Mercury's tiny round disk drifting slowly across the face of the Sun inspired and entertained many denizens of planet Earth last week. In fact, artist and astronomer Mark Seibold viewed both the 1999 and 2006 transits of the solar system's innermost planet through solar filtered telescopes and composed this rendering of Mercury "hovering in the photosphere" near the edge of an enormous solar disk. The original work is a 23 by 17 inch pastel sketch. While the artist's hand is creatively superimposed, Seibold concentrated on offering an impression of Mercury's silhouette, surrounded by shadings reflecting his visual experience that are not easily captured in photographic exposures. Of course, before the age of cameras drawings were more widely used to record telescopic observations of sunspots and planetary transits.

APOD: 2006 November 16 - Children of the Sun
Explanation: For a moment, planets Jupiter, Venus, Mars, and Mercury all posed near their parent star in this Sun-centered view, recorded on November 11. The picture, from a coronograph onboard the space-based SOlar Heliospheric Observatory, spans 15 degrees with the Sun's size and position indicated by the white circle. Background stars are also visible as the otherwise overwhelming sunlight is blocked by the coronograph's occulting disk. But the planets themselves, in particular Jupiter and Venus, are still bright enough to cause significant horizontal streaks in the image. Mercury is actually moving most rapidly (left to right) through the field and days earlier was seen to cross in front of the solar disk. So what's that bright double star to the left of Mars? Zubenelgenubi, of course.

APOD: 2006 November 14 - Mercurys Transit: An Unusual Spot on the Sun
Explanation: What's that dot on the Sun? If you look closely, it is almost perfectly round. The dot is the result of an unusual type of solar eclipse that occurred last week. Usually it is the Earth's Moon that eclipses the Sun. Last week, for the first time in over three years, the planet Mercury took a turn. Like the approach to New Moon before a solar eclipse, the phase of Mercury became a continually thinner crescent as the planet progressed toward an alignment with the Sun. Eventually the phase of Mercury dropped to zero and the dark spot of Mercury crossed our parent star. The situation could technically be labeled a Mercurian annular eclipse with an extraordinarily large ring of fire. From above the cratered planes of the night side of Mercury, the Earth appeared in its fullest phase. Hours later, as Mercury continued in its orbit, a slight crescent phase appeared again. The next Mercurian solar eclipse will occur in 2016.

APOD: 2006 November 10 - Mercury and the Chromosphere
Explanation: Enjoying Wednesday's transit of Mercury from Dallas, Texas, astronomer Phil Jones recorded this detailed image of the Sun. Along with a silhouette of the innermost planet, a network of cells and dark filaments can be seen against a bright solar disk with spicules and prominences along the Sun's edge. The composited image was taken through a telescope equiped with an H-alpha filter that narrowly transmits only the red light from hydrogen atoms. Such images emphasize the solar chromosphere, the region of the Sun's atmosphere immediately above its photosphere or normally visible surface. Left of center, the tiny disk of Mercury seems to be imitating a small sunspot that looks a little too round. But in H-alpha pictures, sunspot regions are usually dominated by bright splotches (called plages) on the solar chromosphere.

APOD: 2006 November 8 - Simulated Transit of Mercury
Explanation: Mercury, the solar system's innermost planet, will spend about five hours crossing in front of the Sun today - beginning at 1912 UT (2:12pm EST), November 8. Specially equipped telescopes are highly recommended to safely spot the planet's diminutive silhouette however, as Mercury should appear about 200 times smaller than the enormous solar disk. This simulated view is based on a filtered solar image recorded on November 3rd. It shows active regions and the Mercury transit across the Sun at six positions from lower left to middle right. Depending on your location, the Sun may not be above the horizon during the entire transit, but webcasts of the event are planned - including one using images from the sun-staring SOHO spacecraft. This is the second of 14 transits of Mercury during the 21st century. The next similar event will be a transit of Venus in June of 2012.

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 29 - Old Moon and Sister Stars
Explanation: An old crescent Moon shares the eastern sky over Menton, France with the sister stars of the Pleiades cluster in this early morning skyscape recorded just last Friday, June 23rd. (Bright Venus was also near the eastern horizon, but is not pictured here.) Astronomical images of the well-known Pleiades often show the cluster's alluring blue reflection nebulae, but they are washed out here by the bright moonlight. Still, while the crescent Moon is overexposed, surface features can be seen on the dim lunar night side illuminated by earthshine - light from sunlit planet Earth. Of course, you can spot a young crescent Moon in the early evening sky tonight. Having left the Pleiades behind, a lovely lunar crescent now appears in the west, lining up with planets Mars, Saturn, and Mercury along the solar system's ecliptic plane.

APOD: 2006 June 17 - Saturn, Mars, and the Beehive Cluster
Explanation: Grab a pair of binoculars and check out Saturn and Mars in the early evening sky tonight! Looking west shortly after sunset, your view could be similar to this one - recorded on June 14. But while this picture shows the two bright planets (Saturn at left) separated by around 1.5 degrees and neatly flanking M44, the Beehive Star Cluster, tonight should find those planets even closer together. In fact, Saturn and Mars are scheduled to achieve their closest alignment near sunset, approaching to within about half a degree. The Beehive will still stand out in the distant starry background. Still got those binoculars in hand? You might as well look for Mercury and Jupiter too.

APOD: 2005 August 17 - Planets over Paranal
Explanation: Very bright planets and very large telescopes are part of this sunset view of Paranal Observatory. The observatory's four, massive 8.2 meter telescope units are situated on top of the 2,600 meter high mountain, Cerro Paranal, in the dry Atacama Desert in northern Chile. The individual unit telescopes can be used separately or in combination and are named Antu, Kueyen, Melipal, and Yepun. Together they are fittingly known as the European Southern Observatory's Very Large Telescope. Of course, the very bright planets are Venus (near center), joined by Mercury (below) and Saturn (left) in late June's western evening skies.

APOD: 2005 July 15 - Reflections on the Inner Solar System
Explanation: Only Mars is missing from this reflective view of the major rocky bodies of the inner solar system. Captured on July 8th, the serene, twilight picture looks out over the Flat Tops Wilderness area from near Toponas, Colorado, USA and includes planets Mercury, Venus, Earth, and Earth's large natural satellite, the Moon. The Moon is in a young crescent phase about three degrees above bright planet Venus. Forest fires contribute to a layer of smoke in Earth's sky that almost hides planet Mercury, still visible very near the horizon. Just a week earlier Venus and Mercury were joined by Saturn, forming a notable grouping in the west also enjoyed by skygazers across planet Earth.

APOD: 2005 July 2 - Three Planets by the Sea
Explanation: On Tuesday, June 28th, the setting Sun flooded the horizon with a beautiful warm light in this view from the beach beside the pier at Brighton in Adelaide, South Australia. The Sun also illuminated three planets gathered in the western sky, Mercury, Venus, and Saturn. From this perspective Mercury is at the highest point in the celestial triangle, brilliant Venus is just below, and Saturn stands farther to the left and below the close pair. Of course, the planets only appear close together on the sky but are actually quite far apart in space. The orbits of Mercury and Venus are both interior to Earth's orbit, while gas giant Saturn lies in the outer solar system, over nine astronomical units from the Sun. Late next week, Venus and Mercury will share western skies with the young crescent Moon.

APOD: 2005 June 30 - Three Planets from Mt Hamilton
Explanation: Venus, Mercury, and Saturn wandered close together in western evening skies last week. On Saturnday, June 25, astronomer R. Jay GaBany recorded this snapshot of their eye-catching planetary conjunction, from historic Lick Observatory on Mt. Hamilton, California, USA. The view looks toward the Pacific shortly after sunset with the lights of San Jose and the southern San Francisco Bay area in the foreground. Of course, Venus is the brightest of the trio. Mercury is nearby on the right and Saturn is below and left, closest to the horizon. Farther to the right of the planetary triangle are Pollux and Castor, twin stars of Gemini, with Regulus, bright star of the constellation Leo, at the very upper left corner of the picture. In the coming days, Venus and Mercury remain close, while Saturn continues to drop below them, toward the horizon.

APOD: 2005 June 24 - Planets in the West
Explanation: This weekend three planets will grace the western sky, forming a lovely trio easily visible shortly after sunset. Saturday evening in particular will find Saturn, Venus, and Mercury all within a 2 degree circle (about the size of your thumb held at arm's length) above the western horizon. Recorded last Sunday, June 19, this image shows the same three planets arrayed along the ecliptic plane above a Colorado Rocky Mountain skyline. Venus is easiest to pick out of the twilight, the brightest celestial beacon below picture center, with Saturn above and to the left of Venus, and Mercury closest to the horizon, right of prominent Pinnacle Peak. By Saturday, the wandering planets will draw even closer together. For help spotting the planets here, put your cursor over the picture.

APOD: 2005 March 18 - Moon, Mercury, Monaco
Explanation: Low on the western horizon after sunset, a slender crescent Moon and wandering planet Mercury join the lights of Menton and Monaco along the French Riviera. Astronomer Vincent Jacques took advantage of this gorgeous photo opportunity a week ago on March 11, when the Moon and Mercury were separated in the sky by just three degrees. Of course, the Moon in a slender crescent phase is always seen near the horizon, as is Mercury - a bright planet which can be otherwise difficult to glimpse as it never strays far from the Sun in Earth's sky. In the coming days good views of Mercury will indeed be fleeting as the solar system's innermost planet is rapidly dropping closer to the glare of the setting Sun. But tonight a waxing Moon will join another bright planet wandering overhead through the evening sky, Saturn.

APOD: 2005 March 7 - Mercury Over Leeds
Explanation: Have you ever seen the planet Mercury? This week might be a good time. Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an clear horizon. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during March of 2004. Each picture was taken from the same location in Leeds, England exactly 33 minutes after sunset. Over the next two weeks, Mercury will again be well placed for viewing above the western horizon at sunset, but by the third week in March it will have faded and dropped into the twilight.

APOD: 2004 September 12 - Mercury: A Cratered Inferno
Explanation: Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. Mercury's diameter is about 4800 km, while the Moon's is slightly less at about 3500 km (compared with about 12,700 km for the Earth). But Mercury is unique in many ways. Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit. As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius. The place nearest the Sun in Mercury's orbit changes slightly each orbit - a fact used by Albert Einstein to help verify the correctness of his then newly discovered theory of gravity: General Relativity. The above picture was taken by the only spacecraft ever to pass Mercury: Mariner 10 in 1974. A new mission, Messenger, launched for Mercury last month and is scheduled to enter orbit around the Solar System's innermost planet in in 2011.

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

APOD: 2004 August 14 - Messenger Launch
Explanation: Streaking into the early morning sky on August 3rd, a Delta II rocket launches NASA's Messenger spacecraft on an interplanetary voyage to Mercury. Scheduled to become the first probe to orbit Mercury, Messenger will begin by looping through the inner Solar System in a series of close flybys of planet Earth and Venus. The flybys are designed as trajectory changing gravity assist encounters to ultimately achieve the goal of orbiting Mercury in 2011. Prior to entering orbit, Messenger will also flyby Mercury in 2008 and 2009 as the first spacecraft to visit the Solar System's innermost planet since Mariner 10 in the mid 1970s. This dramatic view of the Messenger launch was recorded from a pier in Jetty Park at the north end of Cocoa Beach about 2.5 miles from the Cape Canaveral launch site. So what's that erratic blue streak on the right? It's the reflection from a camera blurred in the time exposure.

APOD: 2004 June 8 - A Planet Transits the Sun
Explanation: Today an astronomical event will occur that no living person has ever seen: Venus will cross directly in front of the Sun. A Venus crossing, called a transit, last occurred in 1882 and was front-page news around the world. Today's transit will be visible in its entirety throughout Europe and most of Asia and Africa. The northeastern half of North America will see the Sun rise with the dark dot of Venus already superposed. Never look directly at the Sun, even when Venus is in front. Mercury's closer proximity to the Sun cause it to transit every few years. In fact, the above image mosaic of Mercury crossing the Sun is from two transits ago, in November 1999. Will anyone living see the next Venus transit? Surely yes since it occurs in 2012.

APOD: 2004 June 6 - Mercury Spotting
Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk on 2003 May 7, as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This was the first of 14 transits of Mercury which will occur during the 21st century, but the next similar event will be a much more rare transit of Venus this coming Tuesday. Need help spotting Mercury? Just click on the picture.

APOD: 2004 May 16 - Venus: Earth's Cloudy Twin
Explanation: This picture by the Galileo spacecraft shows just how cloudy Venus is. Venus is very similar to Earth in size and mass - and so is sometimes referred to as Earth's sister planet - but Venus has a quite different climate. Venus' thick clouds and closeness to the Sun (only Mercury is closer) make it the hottest planet - much hotter than the Earth. Humans could not survive there, and no life of any sort has ever been found. When Venus is visible it is usually the brightest object in the sky after the Sun and the Moon. More than 20 spacecraft have visited Venus including Venera 9, which landed on the surface, and Magellan, which used radar to peer through the clouds and make a map of the surface. This visible light picture of Venus was taken by the Galileo spacecraft that orbited Jupiter from 1995 to 2003. Many things about Venus remain unknown, including the cause of mysterious bursts of radio waves.

APOD: 2004 April 25 - D rad Bacteria: Candidate Astronauts
Explanation: These bacteria could survive on another planet. In an Earth lab, Deinococcus radiodurans (D. rad) survive extreme levels of radiation, extreme temperatures, dehydration, and exposure to genotoxic chemicals. Amazingly, they even have the ability to repair their own DNA, usually with 48 hours. Known as an extremophile, bacteria such as D. rad are of interest to NASA partly because they might be adaptable to help human astronauts survive on other worlds. A recent map of D. rad's DNA might allow biologists to augment their survival skills with the ability to produce medicine, clean water, and oxygen. Already they have been genetically engineered to help clean up spills of toxic mercury. Likely one of the oldest surviving life forms, D. rad was discovered by accident in the 1950s when scientists investigating food preservation techniques could not easily kill it. Pictured above, Deinococcus radiodurans grow quietly in a dish.

APOD: 2004 April 2 - Mercury and Venus in the West
Explanation: Doing their part in the ongoing dance of the planets, Mercury and Venus both reached their greatest elongation or maximum apparent distance from the Sun only a few days ago, on March 29th. Eager to record their celestial accomplishment, astronomer Jimmy Westlake snapped this view of the two inner most planets shining in western twilight skies above Yampa, Colorado, USA. The picture was taken using a digital camera mounted on a tripod. Mercury is easily the brightest celestial object near the horizon, appearing to the right of the foreground structure and just above a thin cloud silhouetted by fading sunlight. Still, near the top of the picture brilliant Venus dominates the scene as the magnificent evening star. After climbing in western skies throughout the month of March, Venus lies just below the Pleiades star cluster. Tonight and tomorrow night, skygazers can spot Venus at the southern edge of the Pleiades.

APOD: 2004 March 26 - Moon and Planets Sky
Explanation: Look up into the sky tonight and without a telescope or binoculars you might have a view like this one of Moon, planets and stars. The lovely photo was taken on March 23rd, and captures the crescent Moon on the horizon with Venus above it. Both brilliant celestial bodies are over-exposed. Farther above Venus is the tinted glow of Mars with the Pleiades star cluster just to the red planet's right. The V-shaped arrangement of stars to the left of Mars is the Hydaes star cluster. Bright red giant Aldebaran, not itself a member of the Hyades cluster, marks the top left of the V. During the next week, all five naked-eye planets, Mercury, Venus, Mars, Jupiter, and Saturn, along with the Moon will grace the evening sky together - a lunar and planetary spectacle that can be enjoyed by skygazers around the world. But look just after sunset, low on the western horizon, to see Mercury before it sets. The next similar gathering of the planets will be in 2008.

APOD: 2003 May 27 - A Mercury Transit Sequence
Explanation: Earlier this month, the planet Mercury crossed the face of the Sun, as seen from Earth. Because the plane of Mercury's orbit is not exactly coincident with the plane of Earth's orbit, Mercury usually appears to pass over or under the Sun. The above time-lapse sequence, superimposed on a single frame, was taken from a balcony in Belgium on May 7 and shows the entire transit. The solar crossing lasted over five hours, so that the above 23 images were taken roughly 15 minutes apart. The north pole of the Sun, the Earth, Mercury's orbit, although all different, all occur in directions slightly above the left of the image. Near the center and on the far right, sunspots are visible.

APOD: 2003 May 13 - Mercury Transits the Sun
Explanation: How big is the Sun? The Sun is not only larger than any planet, it is larger than all of the planets put together. The Sun accounts for about 99.9 percent of all the mass in its Solar System. Merely stating the Sun's diameter is about 1,400,000 kilometers does not do it justice. Last week a chance to gain visual size perspective occurred when planet Mercury made a rare crossing in front to Sun. Mercury, a planet over a third of the diameter of our Earth, is the dark dot on the upper right. In comparison to the Sun, Mercury is so small it is initially hard to spot. Also visible on the Sun are dark circular sunspots, bright plages, and dark elongated prominences -- many of which are larger than Mercury. The above contrast-enhanced picture was captured last week from France.

APOD: 2003 May 8 - Mercury Spotting
Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk yesterday (Wednesday, May 7th), as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This is the first of 14 transits of Mercury which will occur during the 21st century, but the next similar event will be a transit of Venus in June of 2004. Need help spotting Mercury? Just click on the picture.

APOD: 2003 April 12 - Mercury on the Horizon
Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it never wanders far from the Sun in Earth's sky. If trailing the Sun, Mercury will be visible low on the horizon for only a short while after sunset. If leading the Sun, Mercury will be visible only shortly before sunrise. So at certain times of the year an informed skygazer with a little determination can usually pick Mercury out from a site with an unobscured horizon. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during March of 2000. Each picture was taken from the same location in Spain when the Sun itself was 10 degrees below the horizon and superposed on the single most photogenic sunset. By the middle of this month, Mercury will again be well placed for viewing above the western horizon at sunset, but by the end of April it will have faded and dropped into the twilight. On May 7th, Mercury will cross the Sun's disk.

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

APOD: 2003 February 16 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun causes it to be visible only for a short time just after sunset or just before sunrise.

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

APOD: 2002 July 16 - Outbound from Mercury
Explanation: After just passing Mercury, the robot spacecraft Mariner 10 looked back. The above picture is what it saw. Mercury, the closest planet to the Sun, is heavily cratered much like Earth's Moon. As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius in the half facing away from the Sun, to an unbearably hot 400 degrees Celsius in the half facing toward the Sun. Mercury is slightly larger than Earth's Moon and much denser. The Mariner 10 spacecraft swooped by Mercury three times in its journey around the inner Solar System in the mid-1970s. This outbound view has similarities to the inbound view. Nearly half of Mercury's surface has yet to be photographed in detail.

APOD: 2002 May 10 - Trailing Planets
Explanation: Positioning his camera and tripod on planet Earth, near Maricopa, Arizona, USA, astrophotographer Joe Orman created this trailing display of the ongoing sky-full-of-planets on May 3rd. He initially captured the grouping in a 20 second long time exposure recording the positions of the bright planets and stars. Covering the camera lens for five minutes, he then exposed the same frame for 45 minutes, tracing the gentle arcs of the celestial wanderers as the Earth's rotation carried them toward the western horizon. Of course these planets, Mercury, Venus, Mars, Jupiter, and Saturn all still dazzle in western skies near sunset, but sky gazers who want to see Mercury should look soon. Mercury starts the evening closest to the horizon - visible here above the wide bright trail left by Venus - and in the coming days Mercury will be the first to leave the evening sky entirely as it moves closer to the setting Sun. Tonight Venus and Mars will appear very close together, separated by only one third of a degree.

APOD: 2002 April 29 - Dusk of the Planets
Explanation: A great grouping of planets is now visible to the west just after sunset. Over the next two weeks, Mercury, Venus, Earth, Mars, Jupiter, and Saturn -- the innermost six planets of our Solar System -- can be seen in a single knowing glance. The image on the left captured them all in one frame. Connecting the planetary dots delineates the edge-on ecliptic, the plane in which the planets orbit the Sun. The shot was taken on April 23 near Chatsworth, New Jersey, USA, and even includes scattered light from the Sun and the Moon. Besides the planets, the Pleiades and Hyades open clusters of stars are visible.

APOD: 2002 April 18 - Planets in the West
Explanation: Have you seen any bright planets lately? Chances are if you've been outside under clear skies just after sunset, then you have. Now shining in the west as bright "stars" in the night sky, are all five planets of the solar system known to ancient astronomers - Mercury, Venus, Mars, Saturn, and Jupiter. Recorded from Holt, Michigan, USA about 40 minutes after sunset on April 14th, this digital image captures three of them, Venus, Mars, and Saturn, along with a young crescent Moon. Also indicated are the Pleiades star cluster and bright red giant star Aldebaran in Taurus. Mercury, setting, is lost in the trees and glow along the horizon, while Jupiter is off the top of this view. The coming weeks will see photo opportunities galore as all five planets gradually move closer together, posing after sunset with the Moon and stars in the western sky. Venus, Mars, and Saturn will form the closest trio, drawing within a 5 degree circle (about the apparent size of your fist with arm extended) above Aldebaran by May 3rd.

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

APOD: 2001 November 24 - Mariner's Mercury
Explanation: Mercury, the closest planet to the Sun, remains the most mysterious of the Solar System's inner planets. Hiding in the Sun's glare it is a difficult target for Earth bound observers. The only spacecraft to explore Mercury close-up was Mariner 10 which executed three flybys of Mercury in 1974 and 1975, surveying approximately 45 percent of its surface. Mariner 10 deftly manuevered to photograph part of the sunlit hemisphere during each approach, passed behind the planet, and continued to image the sun-facing side as the spacecraft receded. Its highest resolution photographs recorded features approximately a mile across. A reprocessing of the Mariner 10 data has resulted in this dramatic mosaic. Like the Earth's Moon, Mercury's surface shows the scars of impact cratering - the smooth vertical band and patches visible above represent regions where no image information is available.

APOD: 2001 August 19 - Mercury: A Cratered Inferno
Explanation: Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. Mercury's diameter is about 4800 km, while the Moon's is slightly less at about 3500 km (compared with about 12,700 km for the Earth). But Mercury is unique in many ways. Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit. As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius. The place nearest the Sun in Mercury's orbit changes slightly each orbit - a fact used by Albert Einstein to help verify the correctness of his then newly discovered theory of gravity: General Relativity. The above picture was taken by the only spacecraft ever to pass Mercury: Mariner 10 in 1974.

APOD: 2001 May 5 - Shepard Flies Freedom 7
Explanation: Forty years ago today (May 5, 1961), at the dawn of the space age, NASA controllers "lit the candle" and sent Alan Shepard arcing into space atop a Redstone rocket. The picture shows the pressure-suited Shepard before launch in his cramped space capsule dubbed "Freedom 7". Broadcast live to a global television audience, the flight of Freedom 7 - the first space flight by an American - followed less than a month after the first human venture into space by Soviet Cosmonaut Yuri Gagarin. Freedom 7's historic flight was suborbital, lasting only about 15 minutes, but during it Shepard demonstrated manual control of his capsule. Naval aviator Shepard was chosen as one of the original seven Mercury Program astronauts. He considered this first flight the greatest challenge and actively sought the assignment. Shepard's career as an astronaut spanned a remarkable period in human achievement and in 1971 he walked on the moon as commander of the Apollo 14 mission. A true pioneer and intrepid explorer, Alan Shepard died in 1998 at age 74.

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

APOD: 2001 February 9 - Nashville Four Planet Skyline
Explanation: So far this February, evening skies have been blessed with a glorious Moon and three bright planets; Venus, Jupiter, and Saturn. But just last week, on January 30th, an extreme wide-angle lens allowed astrophotographer Larry Koehn to capture this twilight view of Moon and four planets above Nashville, Tennessee, USA. These major solar system bodies lie along the ecliptic plane and so follow a diagonal line through the picture. Starting near the upper left corner is bright Jupiter, which takes on a slightly triangular shape due to the lens distortion. Just below and right of Jupiter is Saturn. Continuing along the diagonal toward the lower right is an overexposed, six day old Moon and brilliant Venus seemingly embedded in clouds. The fourth planet pictured is Mercury. Notoriously hard to see from planet Earth because it never wanders far from the Sun, Mercury is visible just above the lower right corner. The line from Jupiter to Mercury spans about 92 degrees across the Nashville sky.

APOD: 2000 December 16 - Degas Ray Crater on Mercury
Explanation: Like the Earth's Moon, Mercury is scarred with craters testifying to an intense bombardment during the early history of the Solar System. In 1974, the Mariner 10 spacecraft surveyed this innermost planet up close, producing the only detailed images of its tortured surface. In the above mosaic the bright rays emanating from the 45 kilometer wide Degas crater almost appear to be painted on. The rays consist of light colored material blasted out during the crater's formation. Craters older than Degas are covered by the ray material while younger craters are seen superimposed on the rays. Mercury's gravity and density are about twice that of Earth's Moon so such bright ray craters on the lunar surface tend to be much larger. NASA plans to launch MESSENGER to the least explored terrestrial planet in 2004.

APOD: 2000 March 20 - Mercury on the Horizon
Explanation: Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it is never seen far from the Sun, and so is only visible near sunrise or sunset. If trailing the Sun, Mercury will be visible for several minutes before it follows the Sun behind the Earth. If leading the Sun, Mercury will be visible for only several minutes before the Sun rises and hides it with increasing glare. An informed skygazer can usually pick Mercury out of a dark horizon glow with little more than determination. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during the middle of last month. Each picture was taken from the same location in Spain when the Sun was 10 degrees below the horizon and superposed on the single most photogenic sunset.

APOD: 2000 March 10 - Sky and Planets
Explanation: On February 10th, an evocative evening sky above Rocklin, California, USA inspired astrophotographer Steve Sumner to record this remarkable sight - five planets and the Moon. Near its first quarter phase, the bright Moon was intentionally overexposed but Saturn, Jupiter, Mars, and Mercury (and, of course, planet Earth's horizon) are all clearly visible in the deepening twilight. Notably absent in this grouping of naked-eye planets is Venus which is still putting in an early appearance as the morning star. This month, Mercury has joined Venus in the dawn twilight while Saturn, Jupiter, and Mars still shine brightly in the western sky at nightfall making another gorgeous close grouping with the crescent Moon.

APOD: 2000 February 13 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise.

APOD: 2000 January 28 - Astronomy From An F-18
Explanation: In an era of blossoming ground and space-based observatories, astronomers are also pushing the envelope with airborne instrumentation - successfully capturing an asteroid occultation from a high performance jet aircraft. This blinking animation represents two digitized frames from inflight data of asteroid number 308, Polyxo, passing in front of or occulting a faint star near the center of the field. The camera used, known as the SouthWest Ultraviolet Imaging System (SWUIS) -A, was mounted in the cockpit of a NASA F/A-18 jet (inset lower left). A former US Navy fighter aircraft, the F/A-18 was able to maneuver to the precise position to record the occultation while cruising above clouds and much of Earth's obscuring atmosphere. Using the SWUIS-A data to time the occultation will reveal the size of the asteroid which is otherwise too small to be imaged by even the orbiting Hubble Space Telescope. Future SWUIS-A airborne missions may include a hunt for Vulcanoids, a suspected population of small asteroids circling the Sun inside the orbit of Mercury.

APOD: December 10, 1999 - Spot The Planet
Explanation: OK, it's a picture of the Sun (duh!), but can you spot the planet? Of course, most of the spots you've spotted are sunspots, as large or larger than planet Earth itself. The sunspots are regions of strong surface magnetic fields which are dark in this picture only because they are relatively cool compared to their surroundings. Over the past few years, the number of sunspots has been steadily increasing as the Sun approaches the maximum in its 11 year activity cycle. But also visible in this photograph from November 15, is planet Mercury. At just over 1/3 Earth's size, Mercury is passing in front of the Sun, its silhouette briefly creating a diminutive dark spot drifting across an enormous solar disk. While "transits" of Mercury do occur 13 times a century, this one was additionally a very rare grazing transit of our Solar System's innermost planet. Spotted Mercury yet? Click on the picture for a hint.

APOD: November 19, 1999 - Mercury And The Sun
Explanation: Just days before the peak of the Leonid meteor shower, skywatchers were offered another astronomical treat as planet Mercury crossed the face of the Sun on November 15. Viewed from planet Earth, a transit of Mercury is not all that rare. The last occurred in 1993 and the next will happen in 2003. Enjoying a mercurial transit does require an appropriately filtered telescope, still the event can be dramatic as the diminutive well-done world drifts past the dominating solar disk. This slow loading gif animation is based on images recorded by the earth-orbiting TRACE satellite. The false-color TRACE images were made in ultraviolet light and tend to show the hot gas just above the Sun's visible surface. Mercury's disk is silhouetted against the seething plasma as it follows a trajectory near the edge of the Sun.

APOD: November 11, 1999 - Mercury And The Moon
Explanation: Mercury is the closest planet to the Sun and never moves far from our parent star in Earth's sky. Racing around its tight orbit, this well-done world is a little over 1/3 the diameter of Earth and is often lost to our view in the solar glare. But, just one day before the August 11 total solar eclipse, astronomer Tunc Tezel captured this fleeting view of a close conjunction of Mercury and the soon to be silhouetted Moon as seen from Turkey. Mercury at the lower right shines brightly in reflected sunlight while only a thin crescent of the almost new Moon is directly illuminated. The rest of the lunar nearside is faintly visible though, illuminated by light from an almost full Earth. On Monday, November 15th, Mercury will actually be seen to transit or pass across the disk of the Sun for well placed observers in the pacific hemisphere.

APOD: November 6, 1999 - X ray Transit of Mercury
Explanation: This sequence of false color X-ray images captures a rare event - the passage or transit of planet Mercury in front of the Sun. Mercury's small disk is silhouetted against the bright background of X-rays from the hot Solar Corona. It appears just to the right of center in the top frame and moves farther right as the sequence progresses toward the bottom. The dark notch is a coronal hole near the Solar South Pole, while a flaring coronal bright point can be seen to the left of the notch in the top frames. The frames were recorded on November 6, 1993 by the Soft X-ray Telescope on board the orbiting Yohkoh satellite. Transits of Mercury (and Venus) were historically used to discover the geometry of the solar system and to map planet Earth itself. The next transit of Mercury will occur on November 15.

APOD: September 18, 1999 - Mercury Astronauts and a Redstone
Explanation: Space suited project Mercury astronauts John H. Glenn, Virgil I. Grissom, and Alan B. Shepard Jr. (left to right) are posing in front of a Redstone rocket in this vintage 1961 NASA publicity photo. Project Mercury was the first U.S. program designed to put humans in space. It resulted in 6 flights using one-man capsules and Redstone and Atlas rockets. Shortly after the first U.S. manned flight on May 5, 1961, a suborbital flight piloted by Alan Shepard, President Kennedy announced the goal of a manned lunar landing by 1970. This goal was achieved by NASA's Apollo program and Shepard himself walked on the moon as commander of the Apollo 14 mission. Alan Shepard passed away in 1998. Virgil Grissom died in a tragic fire during an Apollo launch pad test in 1967. Senator John Glenn flew again on the 25th voyage of the Space Shuttle Discovery.

APOD: September 8, 1999 - A Superior Conjunction Of Mercury
Explanation: In astronomical parlance, an interior planet is at superior conjunction when it is located on the opposite side of the Sun from Earth. Mercury, the solar system's innermost planet, zips past this point in its orbit today. In fact, this recent picture from a solar coronagraph on board the the space-based SOHO observatory shows Mercury positioned very close to the Sun as seen from a near Earth vantage point. The coronagraph uses an internal occulting disk to block the intense solar glare which otherwise hides this sight from ground-based observers. The shadow of the occulting disk is at the center with the Sun's size and position indicated by the white circle. Mercury is the bright dot with a horizontal line (a digital artifact), while faint dots scattered throughout the field are stars. Bright regions of the sun's outer atmosphere are also visible. As Mercury continues in its orbit, on November 15 it will actually appear to cross the disk of the Sun as viewed from Earth.

APOD: May 6, 1999 - Liberty Bell 7
Explanation: Today, the space capsule Liberty Bell 7 rests about 3 miles below the surface of the Atlantic Ocean. But on July 21, 1961, astronaut Virgil I. "Gus" Grissom rode this tiny craft 118 miles above the Earth to become the second American in space. Grissom's flight was suborbital - like fellow Mercury astronaut Alan Shepard's first flight - however his capsule was different, with a window, a new manual spacecraft control system, and an explosive hatch. Unfortunately, after Grissom brought Liberty Bell 7 to a successful splash down in the planned area, the hatch blew prematurely and rough seas began to flood the capsule. While Grissom was able to get out, the military recovery helicopter could not lift the waterlogged spacecraft. This dramatic picture was taken from the helicopter shortly before Liberty Bell 7 was released and sank.

APOD: March 25, 1999 - March of the Planets
Explanation: This March stargazers have been treated to eye-catching formations of bright planets in western evening skies. On March 3rd, looking toward a beautiful sunset from a beach on the Hawaiian isle of Maui, photographer Rick Scott recorded this fleeting, four-planet "hockey stick" array. Mercury, closest to the horizon and immersed in fading sunlight, is easily visible between silhouetted clouds. To the left and up in the deepening blue is Jupiter with a brilliant Venus above and Saturn shining in the darkened sky near the top of the image. The planets are seen to lie close to the ecliptic - the apparent path of the sun - which is nearly perpendicular to the horizon for Hawaiian latitudes at this time of year.

APOD: January 2, 1999 - Mercury: A Cratered Inferno
Explanation: Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. Mercury's diameter is about 4800 km, while the Moon's is slightly less at about 3500 km (compared with about 12,700 km for the Earth). But Mercury is unique in many ways. Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit. As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius. The place nearest the Sun in Mercury's orbit changes slightly each orbit - a fact used by Albert Einstein to help verify the correctness of his then newly discovered theory of gravity: General Relativity. The above picture was taken by the only spacecraft ever to pass Mercury: Mariner 10 in 1974.

APOD: October 29, 1998 - John Glenn: Friendship 7 To Discovery
Explanation: Rehearsing for his historic flight on February 20, 1962, Mercury program astronaut John H. Glenn Jr. works in a cramped training capsule preparing for a few hours' voyage through space. Dubbed Friendship 7, his own snug spacecraft was launched by an Atlas rocket and carried Glenn three times around planet Earth at an altitude of about 120 miles, returning him safely to a "splashdown" in the Atlantic Ocean. The first American in orbit, Senator Glenn's remarkable return to space will be 36 years later as a payload specialist on the Space Shuttle Discovery mission STS-95. Discovery is a roomier craft which will carry a crew of 7 and an array of scientific payloads, such as the International Extreme Ultraviolet Hitchhiker. Scheduled for launch today at 2:00 PM Eastern Time, Discovery will orbit at an altitude of 320 miles and land after 8 days at Kennedy Space Center's shuttle landing facility. Godspeed the crew of STS-95 !

APOD: October 1, 1998 - Happy 40th Birthday NASA
Explanation: Happy Birthday, NASA! The National Aeronautics and Space Administration officially began operations on October 1, 1958, absorbing its forerunner organization the National Advisory Committee for Aeronautics, NACA. Its landmark achievements in human spaceflight include the Mercury and Gemini Projects culminating in the Apollo Project moon landings in the 1960s and early 1970s, Apollo-Soyuz and Skylab in the 1970s, and the Space Shuttle program of the 1980s and 1990s. (Pictured is the June 1998 launch of the Space Shuttle Discovery.) NASA's science programs have produced the robotic exploration of our Solar System, views of the Universe across the electromagnetic spectrum, and valuable meteorological and remote-sensing Earth observations. At birth, NASA's priorities were largely driven by the pressures and competitions of the Cold War. But looking back over 40 years, the sum of its accomplishments have produced needed new technologies and a vital new perspective on Planet Earth and the Cosmos.

APOD: September 6, 1998 - Mariner's Mercury
Explanation: Mercury, the closest planet to the Sun, remains the most mysterious of the Solar System's inner planets. Hiding in the Sun's glare it is a difficult target for Earth bound observers. The only spacecraft to explore Mercury close-up was Mariner 10 which executed 3 flybys of Mercury in 1974 and 1975, surveying approximately 45 percent of its surface. Mariner 10 deftly manuevered to photograph part of the sunlit hemisphere during each approach, passed behind the planet, and continued to image the sun-facing side as the spacecraft receded. Its highest resolution photographs recorded features approximately a mile across. A recent reprocessing of the Mariner 10 data has resulted in this dramatic mosaic. Like the Earth's Moon, Mercury's surface shows the scars of impact cratering - the smooth vertical band and patches visible above represent regions where no image information is available.

APOD: July 24, 1998 - Alan B. Shepard Jr. 1923 1998
Explanation: On another Friday (May 5, 1961), at the dawn of the space age, NASA controllers "lit the candle" and sent Alan B. Shepard Jr. arcing into space atop a Redstone rocket. The picture shows the pressure-suited Shepard before the launch in his cramped space capsule dubbed "Freedom 7" . This historic flight - the first spaceflight by an American - made Shepard a national hero. Born in East Derry, New Hampshire on November 18, 1923, Shepard graduated from the United States Naval Academy in 1944 and went on to train and serve as a Naval Aviator. Chosen as one of the original seven Mercury Program astronauts, he considered this first flight the greatest challenge and actively sought the assignment. Shepard's accomplishments in his career as an astronaut spanned a remarkable period in human achievement and in 1972 he walked on the moon as commander of the Apollo 14 mission. A true pioneer and intrepid explorer, Alan Shepard died Tuesday at age 74 after a lengthy illness.

APOD: July 2, 1998 - X-ray Transit of Mercury
Explanation: This sequence of false color X-ray images captures a rare event - the passage or transit of planet Mercury in front of the Sun. Mercury's small disk is silhouetted against the bright background of X-rays from the hot Solar Corona. It appears just to the right of center in the top frame and moves farther right as the sequence progresses toward the bottom. The dark notch is a coronal hole near the Solar South Pole, while a flaring coronal bright point can be seen to the left of the notch in the top frames. The frames were recorded on November 6, 1993 by the Soft X-ray Telescope on board the orbiting Yohkoh satellite. Transits of Mercury (and Venus) were historically used to discover the geometry of the solar system and to map planet Earth itself.

APOD: May 12, 1998 - Callisto Enhanced
Explanation: Callisto is half rock and half ice. This moon of Jupiter is approximately the size of the planet Mercury, making it the third largest moon in the Solar System, after Ganymede and Titan. Callisto's icy surface is billions of years old, lacks any sign of volcanic activity, and is densely covered with rifts and craters. These features are particularly apparent in this contrast-enhanced image taken by the Galileo spacecraft, and released last week. Visible near the image center is Valhalla, one of the largest impact craters in the Solar System, measuring about 4,000 kilometers across. The rings and size of Valhalla make its appearance similar to the Caloris Impact Basin on Mercury.

APOD: April 4, 1998 - Mercury Astronauts and a Redstone
Explanation: Space suited project Mercury astronauts John H. Glenn, Virgil I. Grissom, and Alan B. Shepard Jr. (left to right) are posing in front of a Redstone rocket in this vintage 1961 NASA publicity photo. Project Mercury was the first U.S. program designed to put humans in space. It resulted in 6 flights using one-man capsules and Redstone and Atlas rockets. Shortly after the first U.S. manned flight on May 5, 1961, a suborbital flight piloted by Alan Shepard, President Kennedy announced the goal of a manned lunar landing by 1970. This goal was achieved by NASA's Apollo program and Shepard himself walked on the moon as commander of the Apollo 14 mission. Virgil Grissom died in a tragic fire during an Apollo launch pad test in 1967. Senator John Glenn will fly again on the 25th voyage of the Space Shuttle Discovery.

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

APOD: November 30, 1997 - Mercury: A Cratered Inferno
Explanation: Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. Mercury's diameter is about 4800 km, while the Moon's is slightly less at about 3500 km (compared with about 12,700 km for the Earth). But Mercury is unique in many ways. Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit. As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius. The place nearest the Sun in Mercury's orbit changes slightly each orbit - a fact used by Albert Einstein to help verify the correctness of his then newly discovered theory of gravity: General Relativity. The above picture was taken by the only spacecraft ever to pass Mercury: Mariner 10 in 1974.

APOD: April 6, 1997 - Mercury Astronauts and a Redstone
Explanation: Space suited project Mercury astronauts John H. Glenn, Virgil I. Grissom, and Alan B. Shepard Jr. (left to right) are pictured here posing in front of a Redstone rocket in this vintage 1961 NASA publicity photo. Project Mercury was the first U.S. program designed to put humans in space. It resulted in 6 flights using one-man capsules and Redstone and Atlas rockets. Shortly after the first U.S. manned flight on May 5, 1961, a suborbital flight piloted by Alan Shepard, President Kennedy announced the goal of a manned lunar landing by 1970. This goal was achieved by NASA's Apollo program and Shepard himself walked on the moon as commander of the Apollo 14 mission.

APOD: January 12, 1997 - Mercury in Stereo: Craters Within Craters
Explanation: This Stereo image pair of craters on on Mercury was produced using data from NASA's robot explorer Mariner 10 which performed three close flybys of the Sun's closest companion, two in 1974 and one in 1975. However, the spacecraft was not equipped with a Stereo camera! Instead, the Stereo pair was created using two images of the same region each recorded from a slightly different angle. The image on the left was actually taken during the first flyby, the one on the right during the second. A crater within a crater is visible at the upper left, the outer one is about 70 miles in diameter. The embedded craters themselves are within the 230 mile wide Dostoevsky crater - a segment of Dostoevsky's rim runs through the lower half of the image. To get the 3D Stereo effect, your left eye should see only the left image and your right eye only the right one. (Try placing one edge of a piece of paper on the screen between the pictures and touching your nose to the other edge while viewing.)

APOD: December 17, 1996 - Mariner's Mercury
Explanation: Mercury, the closest planet to the Sun, remains the most mysterious of the Solar System's inner planets. Hiding in the Sun's glare it is a difficult target for Earth bound observers. The only spacecraft to explore Mercury close-up was Mariner 10 which executed 3 flybys of Mercury in 1974 and 1975, surveying approximately 45 percent of its surface. Mariner 10 deftly manuevered to photograph part of the sunlit hemisphere during each approach, passed behind the planet, and continued to image the sun-facing side as the spacecraft receded. Its highest resolution photographs recorded features approximately a mile across. A recent reprocessing of the Mariner 10 data has resulted in this dramatic mosaic. Like the Earth's Moon, Mercury's surface shows the scars of impact cratering - the smooth vertical band and patches visible above represent regions where no image information is available.

APOD: December 8, 1996 - Degas Ray Crater on Mercury
Explanation: Like the Earth's Moon, Mercury is scarred with craters, testifying to an intense bombardment during the early history of the Solar System. In 1974, the Mariner 10 spacecraft surveyed this innermost planet up close, producing the only detailed images of its tortured surface. In the above mosaic the bright rays emanating from the 27 mile wide Degas crater almost appear to be painted on. The rays consist of light colored material blasted out during the crater's formation. Numerous smaller, younger craters are seen superposed on the Degas crater itself.

APOD: September 23, 1996 - Venus: Earth's Cloudy Twin
Explanation: If Venus weren't so cloudy it would be more similar to Earth. This picture by the Galileo spacecraft shows just how cloudy Venus is. Venus is very similar to Earth in size and mass - and so is sometimes referred to as Earth's sister planet - but Venus has a quite different climate. Venus' thick clouds and closeness to the Sun (only Mercury is closer) make it the hottest planet - much hotter than the Earth. Humans could not survive there, and no life of any sort has ever been found. When Venus is visible it is usually the brightest object in the sky after the Sun and the Moon. More than 20 spacecraft have visited Venus including Venera 9, which landed on the surface, and Magellan, which used radar to peer through the clouds and make a map of the surface. This visible light picture of Venus was taken by the Galileo spacecraft now in orbit around Jupiter. Many things about Venus remain unknown, including the cause of mysterious bursts of radio waves.

APOD: September 13, 1996 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercuryrotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise.

APOD: September 12, 1996 - Mercury: A Cratered Inferno
Explanation: Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. Mercury's diameter is about 4800 km, while the Moon's is slightly less at about 3500 km (compared with about 12,700 km for the Earth). But Mercury is unique in many ways. Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit. As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius. The place nearest the Sun in Mercury's orbit changes slightly each orbit - a fact used by Albert Einstein to help verify the correctness of his then newly discovered theory of gravity: General Relativity. The above picture was taken by the only spacecraft ever to pass Mercury: Mariner 10 in 1974.

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

APOD: January 21, 1996 - Mercury's Faults
Explanation: The surface of the planet Mercury is not without fault. In this case, however, "fault" refers to unusual surface features that are the topic of much speculation. The above fault line is called Santa Maria Rupes, and runs through many prominent craters. The meandering feature is thought to be the result of huge forces of compression on Mercury's surface. Such rupes probably originate from large impacts and a general shrinking of Mercury's crust, which in turn causes parts of the crust to push above other parts.

APOD: January 20, 1996 - Mercury's Caloris Basin
Explanation: Mercury, the closest planet to the Sun, has a surface with so many craters it resembles the Earth's Moon. The largest surface feature on Mercury is the Caloris Basin, which resulted from a collision with an asteroid. The basin, which is more that 1000 kilometers across, is visible as the large circular feature at the bottom of the above photograph. Similar features, such as the Mare Orientale, are seen on the Moon. The Caloris Basin gets very hot because it is near the "sub-solar point" - the point on Mercury's surface that is directly under the Sun when Mercury is closest to the Sun.

APOD: January 7, 1996 - Mercury Astronauts and a Redstone
Explanation: Space suited project Mercury astronauts John H. Glenn, Virgil I. Grissom, and Alan B. Shepard Jr. (left to right) are pictured here posing in front of a Redstone rocket in this 1961 NASA publicity photo. Project Mercury was the first U.S. program designed to put humans in space. It resulted in 6 manned flights using one-man capsules and Redstone and Atlas rockets. Shortly after the first U.S. manned flight on May 5, 1961, a suborbital flight piloted by Alan Shepard, President Kennedy announced the goal of a manned lunar landing by 1970. This goal was achieved by NASA's Apollo program and Shepard himself walked on the moon as a member of the Apollo 14 mission.

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.