Astronomy Picture of the Day Search Results for "Callisto"

APOD: 2023 August 8 – Moon Meets Jupiter
Explanation: What's that below the Moon? Jupiter -- and its largest moons. Many skygazers across planet Earth enjoyed the close conjunction of Earth's Moon passing nearly in front of Jupiter in mid-June. The featured image is a single exposure of the event taken from Morón de la Frontera, Spain. The sunlit lunar crescent on the left is overexposed, while the Moon's night side, on the right, is only faintly illuminated by Earthshine. Lined up diagonally below the Moon, left to right, are Jupiter's bright Galilean satellites: Callisto, Ganymede, Io (hard to see as it is very near to Jupiter), and Europa. In fact, Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller. NASA's robotic spacecraft Juno is currently orbiting Jupiter and made a close pass near Io only a week ago. If you look up in the night sky tonight, you will again see two of the brightest objects angularly close together -- because tonight is another Moon-Jupiter conjunction.

APOD: 2022 February 4 - Moons at Twilight
Explanation: Even though Jupiter was the only planet visible in the evening sky on February 2, it shared the twilight above the western horizon with the Solar System's brightest moons. In a single exposure made just after sunset, the Solar System's ruling gas giant is at the upper right in this telephoto field-of-view from Cancun, Mexico. The snapshot also captures our fair planet's own natural satellite in its young crescent phase. The Moon's disk looms large, its familiar face illuminated mostly by earthshine. But the four points of light lined-up with Jupiter are Jupiter's own large Galilean moons. Top to bottom are Ganymede, [Jupiter], Io, Europa, and Callisto. Ganymede, Io, and Callisto are physically larger than Earth's Moon while water world Europa is only slightly smaller.

APOD: 2021 August 21 - Triple Transit and Mutual Events
Explanation: These three panels feature the Solar System's ruling gas giant Jupiter on August 15 as seen from Cebu City, Philippines, planet Earth. On that date the well-timed telescopic views detail some remarkable performances, transits and mutual events, by Jupiter's Galilean moons. In the top panel, Io is just disappearing into Jupiter's shadow at the far right, but the three other large Jovian moons appear against the planet's banded disk. Brighter Europa and darker Ganymede are at the far left, also casting their two shadows on the gas giant's cloud tops. Callisto is below and right near the planet's edge, the three moons in a triple transit across the face of Jupiter. Moving to the middle panel, shadows of Europa and Ganymede are still visible near center but Ganymede has occulted or passed in front of Europa. The bottom panel captures a rare view of Jovian moons in eclipse while transiting Jupiter, Ganymede's shadow falling on Europa itself. From planet Earth's perspective, similar mutual events, when Galilean moons occult and eclipse each other, can be seen every six years or so when Jupiter is near its own equinox.

APOD: 2020 December 23 - Jupiter Meets Saturn: A Red Spotted Great Conjunction
Explanation: It was time for their close-up. Two days ago Jupiter and Saturn passed a tenth of a degree from each other in what is known a Great Conjunction. Although the two planets pass each other on the sky every 20 years, this was the closest pass in nearly four centuries. Taken early in day of the Great Conjunction, the featured multiple-exposure combination captures not only both giant planets in a single frame, but also Jupiter's four largest moons (left to right) Callisto, Ganymede, Io, and Europa -- and Saturn's largest moon Titan. If you look very closely, the clear Chilescope image even captures Jupiter's Great Red Spot. The now-separating planets can still be seen remarkably close -- within about a degree -- as they set just after the Sun, toward the west, each night for the remainder of the year.

APOD: 2020 September 10 - Jupiter's Swimming Storm
Explanation: A bright storm head with a long turbulent wake swims across Jupiter in these sharp telescopic images of the Solar System's ruling gas giant. Captured on August 26, 28, and September 1 (left to right) the storm approximately doubles in length during that period. Stretching along the jetstream of the planet's North Temperate Belt it travels eastward in successive frames, passing the Great Red Spot and whitish Oval BA, famous storms in Jupiter's southern hemisphere. Galilean moons Callisto and Io are caught in the middle frame. In fact, telescopic skygazers following Jupiter in planet Earth's night have reported dramatic fast moving storm outbreaks over the past few weeks in Jupiter's North Temperate Belt.

APOD: 2020 September 2 - Jupiter and the Moons
Explanation: How many moons do you see? Many people would say one, referring to the Earth's Moon, prominent on the lower left. But take a closer look at the object on the upper right. That seeming-star is actually the planet Jupiter, and your closer look might reveal that it is not alone – it is surrounded by some of its largest moons. From left to right these Galilean Moons are Io, Ganymende, Europa and Callisto. These moons orbit the Jovian world just like the planets of our Solar System orbit the Sun, in a line when seen from the side. The featured single shot was captured from Cancun, Mexico last week as Luna, in its orbit around the Earth, glided past the distant planet. Even better views of Jupiter are currently being captured by NASA's Juno spacecraft, now in a looping orbit around the Solar System's largest planet. Earth's Moon will continue to pass nearly in front of both Jupiter and Saturn once a month (moon-th) as the two giant planets approach their own great conjunction in December.

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

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

APOD: 2019 May 23 - Moons Near Jupiter
Explanation: On May 20, a nearly Full Moon and Jupiter shared this telephoto field of view. Captured when a passing cloud bank dimmed the moonlight, the single exposure reveals the familiar face of our fair planet's own large natural satellite, along with bright Jupiter (lower right) and some of its Galilean moons. Lined up left to right the tiny pinpricks of light near Jupiter are Ganymede, Europa, [Jupiter] and Callisto. (That's not just dust on your screen ...) Closer and brighter, our own natural satellite appears to loom large. But Ganymede, and Callisto are physically larger than Earth's Moon, while water world Europa is only slightly smaller. In fact, of the Solar System's six largest planetary satellites, Saturn's moon Titan is missing from the scene and a fourth Galilean moon, Io, is hidden by our ruling gas giant.

APOD: 2018 May 3 - Opposite the Setting Sun
Explanation: On April 30, a Full Moon rose opposite the setting Sun. Its yellowish moonglow silhouettes a low tree-lined ridge along Lewis Mountain in this northeastern Alabama skyscape. Sharing the telephoto field-of-view opposite the Sun are Earth's grey shadow, the pinkish Belt of Venus, and bright planet Jupiter. Nearing its own 2018 opposition on May 8, Jupiter is flanked by tiny pinpricks of light, three of its large Galilean moons. Europa lies just below Jupiter, and Ganymede and Callisto are just above. Closer and brighter, our own natural satellite appears to loom large but the Moon is physically a little smaller than Ganymede and Callisto, and slightly larger than water world Europa. Sharp eyes will also spot the trails of two jets across the clear evening sky.

APOD: 2017 April 13 - Moons and Jupiter
Explanation: On April 10, a Full Moon and Jupiter shared this telephoto field of view. Both were near opposition, opposite the Sun in Earth's night sky. Captured when a passing cloud bank dimmmed the bright moonlight, the single exposure reveals the familiar face of our fair planet's own large natural satellite, along with a line up of the ruling gas giant's four Galilean moons. Labeled top to bottom, the tiny pinpricks of light above bright Jupiter are Callisto, Europa, Ganymede, and Io. Closer and brighter, our own natural satellite appears to loom large. But Callisto, Ganymede, and Io are physically larger than Earth's Moon, while water world Europa is only slightly smaller. In fact, of the Solar System's six largest planetary satellites, only Saturn's moon Titan is missing from the scene.

APOD: 2016 July 10 - Moon Meets Jupiter
Explanation: What's that next to the Moon? Jupiter -- and its four largest moons. Skygazers around planet Earth enjoyed the close encounter of planets and Moon in 2012 July 15's predawn skies. And while many saw bright Jupiter next to the slender, waning crescent, Europeans also had the opportunity to watch the ruling gas giant pass behind the lunar disk, occulted by the Moon as it slid through the night. Clouds threaten in this telescopic view from Montecassiano, Italy, but the frame still captures Jupiter after it emerged from the occultation along with all four of its large Galilean moons. The sunlit crescent is overexposed with the Moon's night side faintly illuminated by Earthshine. Lined up left to right beyond the dark lunar limb are Callisto, Ganymede, Jupiter, Io, and Europa. In fact, Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller. Last week, NASA's Juno became the second spacecraft ever to orbit Jupiter.

APOD: 2016 March 3 - Moons and Jupiter
Explanation: Some of the Solar System's largest moons rose together on February 23. On that night, a twilight pairing of a waning gibbous Moon and Jupiter was captured in this sharp telescopic field of view. The composite of short and long exposures reveals the familiar face of our fair planet's own large natural satellite, along with a line up of the ruling gas giant's four Galilean moons. Left to right, the tiny pinpricks of light are Callisto, Io, Ganymede, [Jupiter], and Europa. Closer and brighter, our own natural satellite appears to loom large. But Callisto, Io, and Ganymede are actually larger than Earth's Moon, while water world Europa is only slightly smaller. In fact, of the Solar System's six largest planetary satellites, only Saturn's moon Titan is missing from the scene. (Editor's note: Composite corrected for orientation and field of view posted on March 7.)

APOD: 2015 February 6 - Jupiter Triple-Moon Conjunction
Explanation: Our solar system's ruling giant planet Jupiter and 3 of its 4 large Galilean moons are captured in this single Hubble snapshot from January 24. Crossing in front of Jupiter's banded cloud tops Europa, Callisto, and Io are framed from lower left to upper right in a rare triple-moon conjunction. Distinguishable by colors alone icy Europa is almost white, Callisto's ancient cratered surface looks dark brown, and volcanic Io appears yellowish. The transiting moons and moon shadows can be identified by sliding your cursor over the image, or following this link. Remarkably, two small, inner Jovian moons, Amalthea and Thebe, along with their shadows, can also be found in the sharp Hubble view. The Galilean moons have diameters of 3,000 to 5,000 kilometers or so, comparable in size to Earth's moon. But odd-shaped Amalthea and Thebe are only about 260 and 100 kilometers across respectively.

APOD: 2014 November 26 - Io and Callisto Mutual Event
Explanation: A 24 minute sequence from top to bottom, this intriguing series of telescopic frames tracks the occultation of Io by Callisto, two of Jupiter's Galilean moons, from San Pietro Polesine, Italy, planet Earth. A challenging observational project using a small telescope, the two contrasting Jovian worlds are both slightly larger than Earth's Moon. In fact, bright, volcanic Io and dark, cratered Callisto are about 3,640 and 4,820 kilometers in diameter respectively. With Earth itself now crossing near the orbital plane of Jupiter's moons, astronomers are enjoying a season of Galilean moon mutual events ranging from eclipses to occultations. The series of orbital plane crossings produce a mutual event season every 5 to 6 years.

APOD: 2013 November 2 - Jupiter's Triple Shadow Transit
Explanation: This webcam and telescope image of banded gas giant Jupiter shows the transit of three shadows cast by Jupiter's moons in progress, captured in Belgian skies on October 12 at 0528 UT. Such a three shadow transit is a relatively rare event, even for a large planet with many moons. Visible in the frame are the three Galilean moons responsible, Callisto at the far left edge, Io closest to Jupiter's disk, and Europa below and just left of Io. Of their shadows on the sunlit Jovian cloud tops, Callisto casts the most elongated one near the planet's south polar region at the bottom. Io's shadow is above and right of Jupiter's Great Red Spot. Of course viewed from Jupiter's perspective, these shadow crossings could be seen as solar eclipses, analogous to the Moon's shadow crossing the sunlit face of planet Earth.

APOD: 2013 February 25 - Fly Me to the Moons
Explanation: Sometimes the Moon is a busy direction. Last week, for example, our very Moon passed in front of the planet Jupiter. While capturing this unusual spectacle from New South Wales, Australia, a quick-thinking astrophotographer realized that a nearby plane might itself pass in front of the Moon, and so quickly reset his camera to take a continuous series of short duration shots. As hoped, for a brief instant, that airplane, the Moon, and Jupiter were all visible in a single exposure, which is shown above. But the project was not complete -- a longer exposure was then taken to bring up three of the Jupiter's own moons: Io, Callisto, and Europa (from left to right). Unfortunately, this triple spectacle soon disappeared. Less than a second later, the plane flew away from the Moon. A few seconds after that, the Moon moved to cover all of Jupiter. A few minutes after that, Jupiter reappeared on the other side of the Moon, and even a few minutes after that the Moon moved completely away from Jupiter. Although hard to catch, planes cross in front of the Moon quite frequently, but the Moon won't eclipse Jupiter again for another three years.

APOD: 2012 July 20 - Moon Meets Jupiter
Explanation: Skygazers around planet Earth enjoyed the close encounter of planets and Moon in July 15's predawn skies. And while many saw bright Jupiter next to the slender, waning crescent, Europeans also had the opportunity to watch the ruling gas giant pass behind the lunar disk, occulted by the Moon as it slid through the night. Clouds threaten in this telescopic view from Montecassiano, Italy, but the frame still captures Jupiter after it emerged from the occultation along with all four of its large Galilean moons. The sunlit crescent is overexposed with the Moon's night side faintly illuminated by Earthshine. Lined up left to right beyond the dark lunar limb are Callisto, Ganymede, Jupiter, Io, and Europa. In fact, Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller.

APOD: 2012 March 2 - Jupiter Unplugged
Explanation: Five hand drawn sketches of Jupiter were used to create this beautifully detailed flat map of the ruling gas giant's turbulent cloud tops. Made with colored pencils at the eyepiece of a 16 inch diameter telescope, the original drawings are about 5 inches (12.5 cm) in diameter. The drawn planisphere map dimensions are 16x8 inches (40x20 cm). Observing on different dates in November and December of 2011, astronomical artist Fred Burgeot has relied on Jupiter's rotation to cover the planet's complete circumference. Digital animator Pascal Chauvet has also translated Burgeot's drawings into an intriguing video (vimeo), synthesizing a telescopic view of the rotating planet with a tilt and phase appropriate for the observing dates. The video includes the Galilean moons moving along their orbits, beginning with Ganymede and Io casting shadows as they glide in front of Jupiter, followed by Europa and Callisto passing behind the planet's banded disk.

APOD: 2010 October 8 - Two Planet Opposition
Explanation: In late September, two planets were opposite the Sun in Earth's sky, Jupiter and Uranus. Consequently closest to Earth, at a distance of only 33 light-minutes and 2.65 light-hours respectively, both were good targets for telescopic observers. Recorded on September 27, this well-planned composite of consecutive multiple exposures captured both gas giants in their remarkable celestial line-up accompanied by their brighter moons. The faint greenish disk of distant planet Uranus is near the upper left corner. Of the tilted planet's 5 larger moons, two can be spotted just above and left of the planet's disk. Both discovered by 18th century British astronomer Sir William Herschel and later named for characters in Shakespeare's A Midsummer Night's Dream, Oberon is farthest left, with Titania closer in. At the right side of the frame is ruling gas giant Jupiter, flanked along a line by all four of its Galilean satellites. Farthest from Jupiter is Callisto, with Europa and Io all left of the planet's disk, while Ganymede stands alone at the right.

APOD: 2004 December 9 - Jupiter and the Moon's Shadowed Horizon
Explanation: Early Tuesday morning, December 7th, June Croft thought the southeastern sky above Atmore, Alabama, USA was beautiful. Watching the Moon rise through gossamer clouds, she noted, " ... the crescent Moon looked like it was held in the sky by a star just off its shadowed horizon." What was that star? Bright Jupiter of course, and some watched as the Moon actually occulted or passed in front of the Solar System's reigning gas giant planet. For astronomer Jimmy Westlake in Colorado, Jupiter was already hidden at moonrise that morning, but later he was able to record this lovely image, not unlike the view that inspired Croft. Seen through gossamer clouds, Jupiter along with large Jovian satellites Ganymede and Callisto (bottom to top) has emerged from behind the crescent Moon's shadowed horizon.

APOD: 2004 November 11 - Pastel Planet, Triple Eclipse
Explanation: This false-color image of banded gas giant Jupiter shows a triple eclipse in progress on March 28 - a relatively rare event, even for a large planet with many moons. Captured by the Hubble Space Telescope's near-infrared camera are shadows of Jupiter's moons Ganymede (left edge), Callisto (right edge) and Io, three black spots crossing the sunlit Jovian cloud tops. In fact, Io itself is visible as a white spot near picture center with a bluish Ganymede above and to the right, but Callisto is off the right hand edge of the scene. Viewed from Jupiter's perspective, these shadow crossings would be seen as solar eclipses, analogous to the Moon's shadow crossing the sunlit face of planet Earth. Historically, timing the eclipses of Jupiter's moons allowed astronomer Ole Roemer to make the first accurate measurement of the speed of light in 1676.

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

APOD: 2003 February 27 - When Moons and Shadows Dance
Explanation: It's no wonder Jupiter is a favorite target for even modest earthbound telescopes. The most massive planet in the solar system with four of the largest moons also boasts the famous Great Red Spot, a giant hurricane-like storm system over three hundred years old. Recorded on December 15, 2002 between 7:19 and 8:40 UT, over a thousand digital images were processed and stacked to create this spectacular 21 frame animation of the Jovian system. South is up and as the Great Red Spot tracks across the face of Jupiter, innermost Galilean moon Io enters the scene at the far right. Io occults (passes in front of) the edge of the more sedately orbiting Ganymede with Io's shadow moving quickly across the gas giant's cloud tops, just below the Red Spot. While the moon Callisto is outside the field of view, its large, dark shadow is also visible crossing the Jovian disk at the upper left. Viewed from Earth, the orbits of the Galilean moons presently lie nearly edge-on, offering many chances to observe similar dances of Jupiter's moons.

APOD: 2002 January 20 - Callisto Full Face
Explanation: Callisto's surface shows its age. While probably formed at the same time as Io, the difference between the surfaces of these two moons of Jupiter could hardly be greater. Io's surface is young, shows practically no impact craters, and is continually being repaved by the lava exploding from its many large volcanoes. Callisto's surface is old, shows the highest density of impact craters in the Solar System, and harbors no volcanoes or even any large mountains. Callisto's surface is one large ice-field, laced with cracks and craters from billions of years of collisions with interplanetary debris. The above image was taken in 2001 May and is, so far, the only complete global color image taken by the Jupiter-orbiting Galileo spacecraft.

APOD: 2001 August 28 - Jagged Hills on Jupiters Callisto
Explanation: Why does Jupiter's moon Callisto have unusual jagged hills? This mystery came to light after the robot spacecraft Galileo, in orbit around Jupiter since 1995, swooped past the dark moon in May. The resulting pictures were the highest resolution yet obtained for a Jovian moon: objects as small as 3 meters across are discernable. The strange landscapes pictured above show areas rich in bright sharp mounds about 100 meters tall. A likely formation hypothesis holds that these hills are the result of ejecta thrown billions of years ago during a violent impact. The lower inset region apparently has undergone an epoch of relatively high ice-erosion, where dark rock has filled in some of the inter-hill regions. NASA has recently cleared Galileo to continue swooping Jupiter's moons until 2003, when it will end its journey with a spectacular dive into Jupiter's atmosphere.

APOD: 2001 August 1 - Young Martian Terrain
Explanation: What caused the pits, ridges, and gullies on otherwise smooth Martian terrain? One hypothesis is water. The lack of craters at this mid-latitude location indicates that the terrain is quite young by geological standards, perhaps only 100,000 years old. Were the terrain since saturated by water ice, that ice would soon evaporate into the thin Martian air. Left over, however, might be fragile cake-like sand that can be broken up by wind into pits and ridges. Consequences of this hypothesis include that even the Martian equator undergoes epochs of relative wet and dry, and that future spacefarers might be able to find water (ice) in a relatively mild climate near the Martian equator. Pictured above is young-ridged terrain that also shows evidence of a downhill flow.

APOD: 2001 July 31 - Oceans Under Jupiters Callisto
Explanation: Why does Jupiter's moon Callisto alter the magnetic field of Jupiter in its vicinity? Callisto itself does not have a strong magnetic field. One possible answer is that Callisto harbors sub-surface oceans of electrically conducting salt-water. This hypothesis was bolstered recently by a new analysis of how Callisto creates and dissipates heat. Callisto is thought to create heat by the radioactive decay of internal rock -- a process that keeps the Earth's mantle molten. Callisto may not be able to dissipate this heat very efficiently, however, as it has thick layers of ice and rock on its surface. Perhaps this heat is enough to keep sub-surface water from freezing into ice. With this hypothesis, Callisto joins two other of Jupiter's moons, Europa and Ganymede, in candidates for sub-surface oceans. Callisto's oceans, however, might prove too hostile to support Earth-like life.

APOD: 2001 January 16 - Europa Rotating
Explanation: Evidence has been mounting that beneath the vast planes of ice that cover Europa lies water -- liquid oceans that might be home to alien life. The smallest of Jupiter's Galilean Moons (which include Io, Ganymede, and Callisto), Europa's deep interior is composed of mostly of silicate rock. Upon close inspection, many surface cracks stop abruptly only to continue on somewhere else -- indicating surface plates that might be sliding. The above time-lapse sequence is a composite of images taken during the Voyager spacecraft flyby of the moon twenty years ago. Not all regions are resolved in high detail. The movie shows Europa during a complete rotation, which corresponds to a complete revolution around Jupiter since Europa always keeps the same face toward the giant planet. The cause of many of the surface colors on Europa also remains a topic of research.

APOD: 2001 January 2 - Jupiter, Europa, and Callisto
Explanation: As the robot Cassini spacecraft rounds Jupiter on its way toward Saturn, it has taken a sequence of images of the gas giant with its four largest moons. Previously released images have highlighted Ganymede and Io. Pictured above are the two remaining Galilean satellites: Europa and Callisto. Europa is the bright moon superposed near Jupiter's Great Red Spot, while Callisto is the dark moon near the frame edge. Callisto is so dark that it would be hard to see here if its brightness was not digitally enhanced. Recent evidence indicates that both moons hold salt-water seas under surface ice that might be home to extra-terrestrial life. By noting the times that moons disappeared and reappeared behind Jupiter in 1676, Ole Roemer was able to make the first accurate estimation of the speed of light.

APOD: 2000 December 18 - Oceans Under Jupiter's Ganymede
Explanation: The search for extraterrestrial life came back into our own Solar System last week with the announcement that there may be liquid oceans under the surface of Jupiter's moon Ganymede. Ganymede now joins Callisto and Europa as moons of Jupiter that may harbor seas of liquid water under layers of surface ice. The ocean hypothesis surfaced as an explanation for Ganymede's unusually strong magnetic field. Ganymede, the largest moon in the Solar System, also has the largest measured magnetic field of any moon. Some exobiologists hypothesize that life may be able to emerge in such an ocean, much as it did in the oceans of ancient Earth. Above, a frame from a computer simulation shows what it would look like to fly over the surface of Ganymede, as extrapolated from photographs of the grooved moon taken by the robot spacecraft Galileo currently orbiting Jupiter.

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

APOD: 2000 October 24 - Io Rotating
Explanation: The surface of Io is continually changing. Jupiter's moon is the home to many powerful volcanoes so active they are effectively turning the moon inside out. The above time-lapse sequence is a composite of images taken during two space missions that approached the violent moon: Voyager and Galileo. The sequence shows Io during a complete rotation, which corresponds to a complete revolution around Jupiter since Io always keeps the same face toward the giant planet. The rampant volcanism is thought to be caused by Jupiter's more distant Galilean Moons (Europa, Ganymede, and Callisto) pulling on Io and continually distorting its shape, causing internal frictional heating. Io is composed mostly of rock, with the yellow color originating from sulfur. The causes of many of the other colors remain a topic of research.

APOD: 2000 May 8 - Jupiters Moons Thebe, Amalthea, and Metis
Explanation: The robot spacecraft Galileo in orbit around Jupiter has recently photographed the inner moons of Jupiter in greater detail than ever before. These pictures of Thebe, Amalthea, and Metis are shown to scale, and reveal details as small as three kilometers across. Amalthea, by contrast, has a total length of about 200 kilometers. The moons are composed mostly of ice, are much smaller than Jupiter's more famous Galilean satellites (Io, Europa, Ganymede, and Callisto), and orbit between Io and Jupiter's rings. Thebe appears dominated by a huge impact crater 40 kilometers across. Astronomers are uncertain of the origin of the unusual white gash at the bottom of Amalthea.

APOD: November 7, 1998 - Globular Cluster 47 Tucanae
Explanation: Stars come in bunches. Of the over 200 globular star clusters that orbit the center of our Milky Way Galaxy, 47 Tucanae is the second brightest globular cluster (behind Omega Centauri). Known to some affectionately as 47 Tuc or NGC 104, it is only visible from the Southern Hemisphere. Light takes about 20,000 years to reach us from 47 Tuc which can be seen near the SMC in the constellation of Tucana. Red Giant stars are particularly easy to see in this picture. The dynamics of stars near the center of 47 Tuc are not well understood, particularly why there are so few binary systems there.

APOD: November 6, 1998 - Cutaway Callisto: Ice, Rock, and Ocean
Explanation: Cruising past the moons of reigning gas giant Jupiter, Voyager and Galileo have returned tantalizing evidence for a liquid water ocean beneath the surface of Europa. Now researchers are reporting telltale indications that the battered Jovian moon Callisto may also harbor a subsurface ocean. This cutaway view of Callisto shows a whitish 200 kilometer thick band of ice just beneath the moon's surface. The hypothetical ocean - indicated by the underlying light blue stripe - is potentially a salty layer of liquid water up to 10 kilometers thick, while the rest of the interior is seen as a jumble of rock and ice. Why a salty subsurface ocean? Magnetic measurements made during Galileo flybys so far indicate Callisto's magnetic field is variable, analogous to results during Europa passes, and a plausible explanation is that Callisto too has a subsurface liquid layer. If the liquid were salt water it could easily carry electrical currents and produce the changing magnetic field.

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: May 11, 1998 - Callisto in True Color
Explanation: Callisto's surface has many stories to tell. The most distant of Jupiter's Galilean Moons, Callisto shows the highest density of impact craters in the Solar System, but harbors no volcanoes or even any large mountains. Callisto's surface is laced with cracks and craters from billions of years of collisions with interplanetary debris. This image shows Callisto's true colors, and was taken in November 1997 by the robot spacecraft Galileo currently orbiting Jupiter.

APOD: February 2, 1998 - A Triple Eclipse on Jupiter
Explanation: Part of Jupiter is missing. Actually, three parts appear to be missing. In reality though, the three dark spots seen in the above photograph are only shadows. The unusual alignment of three of Jupiter's moons between the Jovian giant and the Sun was imaged last November 10th. The shadows of Io, Callisto, and Ganymede move across Jupiter as these moons progress in their orbits. It was by noting the times of eclipse of Jupiter's moons in 1675 that Ole Roemer became the first person to measure the speed of light. When a shadow from Earth's Moon crosses the Earth's surface, the people inside the shadow see an eclipse of the Sun.

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

APOD: June 12, 1997 - Jupiter's Dry Spots
Explanation: Known for its spectacular images of Jupiter's moons, Io, Ganymede, Callisto, and Europa, the robot spacecraft Galileo has also aggressively explored the Jovian atmosphere. In December of 1995, Galileo's atmospheric probe descended into Jupiter's clouds and reported a surprising absence of water. It is now believed that the probe entered through one of Jupiter's dry spots, similar to the dark region in this image of the swirling Jovian cloud deck. The smallest features visible here are tens of miles in size. These dry regions appear to correspond to locations where winds converge creating downdrafts. The downdrafts generate local cloudless clearings through which Jupiter's deeper warmer layers can be glimpsed. Just as the dark areas are extremely dry, the surroundings are full of moisture. The contrast is analogous to the desert and tropics of Earth.

APOD: February 18, 1997 - A Big Cliff on Jupiter's Callisto
Explanation: Callisto's surface is not without fault. In fact, an explorer crossing the surface of this large moon of Jupiter would need climbing equipment to pass this large, recently discovered fault. The above picture was released last week and was taken in November 1996 by the robot spacecraft Galileo currently orbiting Jupiter. As the Sun illuminates Callisto's surface from the left, the unusual cliff or scarp stands out by the shadow it casts to the right. This cliff and others were probably formed when a large object collided with Callisto early in its history. Of the many visible craters in the above photograph, the smallest visible is about a football field across, while the largest is more than a kilometer.

APOD: December 9, 1996 - Callisto Full Face
Explanation: Callisto's surface shows its age. While probably formed at the same time as Io, the difference between the surfaces of these two moons of Jupiter could hardly be greater. Io's surface is young, shows practically no impact craters, and is continually being repaved by the lava exploding from its many large volcanoes. Callisto's surface is old, shows the highest density of impact craters in the Solar System, and harbors no volcanoes or even any large mountains. Callisto's surface is one large ice-field, laced with cracks and craters from billions of years of collisions with interplanetary debris. The high-resolution vertical band in the above mosaic was taken by the robot spacecraft Galileo currently orbiting Jupiter. The rest of the mosaic was compiled from pictures taken by the Voyager spacecraft which passed the Callisto in 1979.

APOD: October 25, 1996 - A Flyby View of Ganymede
Explanation: This is what it would look like to fly over the surface of Jupiter's moon Ganymede. NASA's robot spacecraft Galileo recently approached only 6000 miles from this frozen ice-ball of a moon. The above image is a digital reconstruction from two images taken during this flyby. The blue color of the sky is artificial. Deep furrows cover Ganymede's surface, and several impact craters are also present. Galileo continues to orbit Jupiter and send back breathtaking photos of Ganymede, Io, Europa, Callisto, and, of course, Jupiter itself.

APOD: October 22, 1996 - The Cracked Ice Plains of Europa
Explanation: What caused the cracks in this giant ice-ball? Jupiter's moon Europa has smoothest surface in the solar system and is composed mostly of cracked water-ice. In the above false-colored picture released last week by the NASA team in charge of the Galileo mission, blue hues represent ice plains divided by dirty red and brown bands of mottled terrain. As the robot Galileo spacecraft orbits Jupiter, it sends back revealing pictures of Jupiter and its large moons including Europa, Io, Ganymede, and Callisto. The region of Europa highlighted above is known as Minos Linea. The cause for many of the cracks remains unknown but may involve shifting stresses from gravity and temperature variations. The new Galileo pictures have increased evidence that liquid oceans may indeed exist under these giant ice-sheets, a place possibly ripe for the development of life.

APOD: July 12, 1996 - Ancient Cratered Plains on Ganymede
Explanation: The largest moon in the Solar System shows regions that are ancient and battered. The high density of craters demonstrate that patches of Ganymede are indeed billions of years old. This photo is one of a series released by NASA two days ago from the Galileo spacecraft orbiting Jupiter. The large impact crater on the left is 19 kilometers across, while dark indentations in Ganymede's crusty surface-ice run diagonally. Ganymede is composed of half rock and half water-ice. The Galileo spacecraft will continue to orbit Jupiter over the next 16 months and send back data about Jupiter and its four largest moons: Io, Europa, Ganymede, and Callisto.

APOD: July 10, 1996 - Galileo Photographs Ganymede
Explanation: Ganymede's surface is slowly being pulled apart. This photo of Ganymede was released earlier today by the Galileo team at NASA. The Galileo Spacecraft arrived at Jupiter in December 1995. In late June, the spacecraft passed within 10,000 kilometers of Ganymede's icy surface, and took pictures showing complex surface details for the first time. The line-like features in this photo are sunlit ridges rising above Ganymede's ice-plains. The circular features are impact craters. Ganymede is the largest moon of Jupiter and hence the largest of the four Galilean satellites: Io, Europa, Ganymede, and Callisto.

APOD: November 3, 1995 - Jupiter's Moon Amalthea
Explanation: Amalthea is Jupiter's fifth largest moon, much smaller than the four Galilean satellites Io, Europa, Ganymede, and Callisto. The orbit of Amalthea is inside of these moons, and with its long axis always pointing toward Jupiter. It's dark surface color is probably due to sulfur being expelled from Io. Amalthea is not massive enough for its internal gravity to make it spherical. Amalthea was discovered in 1892 by Edward Barnard.

APOD: September 6, 1995 - Callisto: Dark Smashed Iceball
Explanation: Callisto is a dirty battered world, showing the most beaten surface of Jupiter's major moons. Made of a rocky core covered by fractured ice, Callisto's past collisions with large meteors are evident as large craters surrounded by concentric rings. The four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto were all discovered by Galileo and Marius in 1610 with early telescopes and are now known as the Galilean satellites. The NASA spacecraft Galileo is scheduled to arrive at Jupiter is December of 1995.

APOD: July 15, 1995 - The Crater Chain
Explanation: NASA's robot spaceprobe Voyager 1, took this closeup image of the surface of Jupiter's crater scarred moon Callisto in 1979. A mysterious chain of craters is seen to extend diagonally across the image (upper left to lower right). What could cause the craters to line up in such a regular fashion? Scientists were at a loss to explain this crater chain along with several other chain like features observed on Callisto's surface. Fifteen years later, with the discovery of Comet Shoemaker-Levy 9, also known as the "string of pearls" comet, the mystery was solved. Comets whose orbits stray too close to Jupiter are torn apart by the strong gravity. When the individual pieces, strung out along the orbital path of the comet hit an object like Callisto, the sequence of impacts produces a crater chain.