Astronomy Picture of the Day Search Results for "rings of saturn"

APOD: 2009 March 4 - Saturn in View
Explanation: Very good telescopic views of Saturn can be expected in the coming days as the ringed planet nears opposition on March 8th, its closest approach to Earth in 2009. Of course, opposition means opposite the Sun in planet Earth's sky - an arrangement that occurs almost yearly for Saturn. But while Saturn itself grows larger in telescopic images, Saturn's rings seem to be vanishing as their tilt to our line-of-sight decreases. In fact, the rings will be nearly invisible, edge-on from our perspective, by September 4. Recorded on February 28, this sharp image was made with the 1 meter telescope at Pic Du Midi, a mountain top observatory in the French Pyrenees. The rings are seen to be tilted nearly edge-on, but remarkable details are visible in the gas giant's cloud bands. The icy moon Tethys appears just beyond the rings at the lower left.

APOD: 2009 February 27 - Lulin and Saturn near Opposition
Explanation: Tracking through the constellation Leo on February 23rd, bright planet Saturn and Comet Lulin were both near opposition -- opposite the Sun in planet Earth's sky. They also passed within only 2 degrees of each other creating a dramatic celestial photo-op. Comet Lulin was near its closest approach to planet Earth at the time, at a distance of some 61 million kilometers, but was orbiting in the opposite direction. As a result it swept remarkably rapidly across the background of stars. This telephoto image captures both bright Saturn and greenish Lulin in the same field in a scene not too different from binocular views. Don't recognize ringed Saturn? The rings are presently tilted nearly edge-on to our view and the brighter planet is overexposed to record details of the fainter comet. At the upper right, Saturn is marked by multiple diffraction spikes created by the aperture blades in the telephoto lens.

APOD: 2009 January 11 - In the Shadow of Saturn
Explanation: In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn recently drifted in giant planet's shadow for about 12 hours and looked back toward the eclipsed Sun. Cassini saw a view unlike any other. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn, slightly scattering sunlight, in this exaggerated color image. Saturn's rings light up so much that new rings were discovered, although they are hard to see in the image. Seen in spectacular detail, however, is Saturn's E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus and the outermost ring visible above. Far in the distance, at the left, just above the bright main rings, is the almost ignorable pale blue dot of Earth.

APOD: 2009 January 1 - Welcome to the International Year of Astronomy
Explanation: Astronomers all over planet Earth invite you to experience the night sky as part of the International Year of Astronomy 2009. This year was picked by the International Astronomical Union and the United Nations Educational, Scientific, and Cultural Organization because it occurs 400 years after Galileo turned one of the first telescopes toward the heavens. Peering through that small window, Galileo discovered that the Moon has craters, Venus has phases, Jupiter has moons, and Saturn has rings. This year you can discover these and many modern wonders of the amazing overhead tapestry that is shared by all of humanity. If, like many others, you find the night sky wondrous and educational, be sure to attend an IYA2009 event in your area, and tell any schools and children that might be interested. Also, please feel free to explore the extensive IYA2009 web pages to find international media events that include blogs, webcasts and much much more.

APOD: 2008 November 19 - Unusual Auroras Over Saturns North Pole
Explanation: What's causing this unusual aurora over Saturn? No one is sure. Infrared images by the robotic Cassini spacecraft of the north pole of Saturn have uncovered aurora unlike any other seen previously in our Solar System. The strange aurora are shown in blue in the above image, while the underlying clouds are shown in red. The previously recorded, also-strange hexagon cloud patterns are visible in red below the aurora. These Saturnian aurora can cover the entire pole, while auroras around Earth and Jupiter are typically confined by magnetic fields to rings surrounding the magnetic poles. More normal auroral rings had been previously imaged around Saturn. The recently imaged strange auroras above Saturn's north pole can change their global patterns significantly in only a few minutes. The large and variable nature of these auroras indicate that charged particles streaming in from the Sun are experiencing some type of magnetism above Saturn that was previously unexpected.

APOD: 2008 October 20 - Moons, Rings, and Unexpected Colors on Saturn
Explanation: Why would Saturn show such strange colors? The robotic Cassini spacecraft currently orbiting Saturn has beamed back images showing that the northern hemisphere our Solar System's most spectacularly ringed planet has changed noticeably since Cassini arrived in 2004, now sporting unusual and unexpected colors. No one is sure why. Although the cause for many of Saturn's colors is unknown, the recent change in colors is thought to be related to the changing seasons. Pictured above, the unusual colors are visible just north of the dark ring shadows. The razor-thin plane of ring particles is visible nearly edge-on across the bottom of the image. The cloudy moon Titan looms large just above the rings, while close observation will reveal three other moons. Cassini arrived at Saturn in 2004, sending back data and images that have not only led to a deeper understanding of the Jovian world's atmosphere, moons, and rings, but also raised new mysteries.

APOD: 2008 July 20 - Crescent Rhea Occults Crescent Saturn
Explanation: Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn. Looking nearly back toward the Sun, the robot Cassini spacecraft now orbiting Saturn captured crescent phases of Saturn and its moon Rhea in color a few years ago. As striking as the above image is, it is but a single frame from a recently released 60-frame silent movie where Rhea can be seen gliding in front of its parent world. Since Cassini was nearly in the plane of Saturn's rings, the normally impressive rings are visible here only as a thin line across the image center. Although Cassini has now concluded its primary mission, its past successes and opportunistic location have prompted NASA to start a two-year Equinox Mission, further exploring not only Saturn's enigmatic moons Titan and Enceladus, but Saturn herself as her grand rings tilt right at the Sun in August 2009.

APOD: 2008 June 9 - Saturn's Rings from the Other Side
Explanation: What do Saturn's rings look like from the other side? From Earth, we usually see Saturn's rings from the same side of the ring plane that the Sun illuminates them. Geometrically, in the above picture taken in April by the robot Cassini spacecraft now orbiting Saturn, the Sun is behind the camera but on the other side of the ring plane. This vantage point, specifically 17 degrees above the ring plane, gives a breathtaking views of the most splendid ring system in the Solar System. Strangely, the rings have similarities to a photographic negative of a front view. The ring brightness as recorded from different angles indicates ring thickness and particle density of ring particles. Elsewhere, ring shadows can be seen on the sunlit face of Saturn, shown sporting numerous cloud structures in nearly true color.

APOD: 2008 March 24 - Saturn and Titan from Cassini
Explanation: Spectacular vistas of Saturn and its moon continue to be recorded by the Cassini spacecraft. Launched from Earth in 1997, robotic Cassini entered orbit around Saturn in 2004 and has revolutionized much of humanity's knowledge of Saturn, its expansive and complex rings, and it many old and battered moons. Soon after reaching Saturn, Cassini released the Huygen's probe which landed on Titan, Saturn's largest moon, and send back unprecedented pictures from below Titan's opaque cloud decks. Recent radar images of Titan from Cassini indicate flat regions that are likely lakes of liquid methane, indicating a complex weather system where it likely rains chemicals similar to gasoline. Pictured above, magnificent Saturn and enigmatic Titan were imaged together in true color by Cassini earlier this year.

APOD: 2007 December 29 - Saturn's Infrared Glow
Explanation: Known for its bright ring system and many moons, gas giant Saturn looks strange and unfamiliar in this false-color view from the Cassini spacecraft. In fact, in this Visual and Infrared Mapping Spectrometer (VIMS) mosaic the famous rings are almost invisible, seen edge-on cutting across picture center. The most striking contrast in the image is along the terminator or boundary between night and day. To the right (day side) blue-green hues are visible sunlight reflected from Saturn's cloud tops. But on the left (night side) in the absence of sunlight, the lantern-like glow of infrared radiation from the planet's warm interior silhouettes features at Saturn's deeper cloud levels.

APOD: 2007 December 18 - Unusual Silica Rich Soil Discovered on Mars
Explanation: You're rolling across Mars when you unexpectedly uncover some unusually light soil. You stop. You turn. You return to inspect the soil and find out it is almost purely silica -- the main ingredient in quartz and glass. Such soil has never been found on Mars before. What created this soil? Since you are the robotic rover Spirit currently rolling across Mars, you send the images and data back to Earth for analysis. Your scientist friends from the blue water planet say that such soil on Earth is usually created by either volcanic steam or a hot spring. The second hypothesis in particular indicates, once again, a wet past for part of Mars, as possibly hot water saturated with silica deposited the white soil. Intriguingly, on Earth, living microbes typically flourish under either condition. Pictured above, the uncovered light soil is visible on the right.

APOD: 2007 December 17 - Saturn's Ancient Rings
Explanation: How old are Saturn's rings? No one is quite sure. One possibility is that the rings formed relatively recently in our Solar System's history, perhaps only about 100 million years ago when a moon-sized object broke up near Saturn. Evidence for a young ring age includes a basic stability analysis for rings, and the fact that the rings are so bright and relatively unaffected by numerous small dark meteor impacts. New evidence, however, raises the possibility that some of Saturn's rings may be billions of years old and so almost as old as Saturn itself. Inspection of images by the Saturn-orbiting Cassini spacecraft indicates that some of Saturn's ring particles temporarily bunch and collide, effectively recycling ring particles by bringing fresh bright ices to the surface. Seen here, Saturn's rings were imaged in their true colors by the robotic Cassini in late October. Icy bright Tethys, a moon of Saturn likely brightened by a sandblasting rain of ice from sister moon Enceladus, is visible in front of the darker rings.

APOD: 2007 October 24 - Ring Scan
Explanation: Scroll right and cruise above the thin, icy rings of Saturn. This high resolution scan is a mosaic of images presented in natural color and recorded in May, over about 2.5 hours as the Cassini spacecraft passed above the unlit side of the rings. The rings themselves are seen to be composed of many individual ringlets. To help track your progress, the rings are labeled below, along with the distance from the center of the gas giant in kilometers. Major ring gaps are labeled above. The alphabetical designation of Saturn's rings is historical and related to their order of discovery; rings A and B are the bright rings separated by the Cassini division. In order of increasing distance from Saturn, the seven main rings run D,C,B,A,F,G,E. (Faint, outer rings G and E are not imaged here.)

APOD: 2007 October 23 - Crescent Saturn
Explanation: Saturn never shows a crescent phase -- from Earth. But when viewed from beyond, the majestic giant planet can show an unfamiliar diminutive sliver. This image of crescent Saturn in natural color was taken by the robotic Cassini spacecraft in May. The image captures Saturn's majestic rings from the side of the ring plane opposite the Sun -- the unilluminated side -- another vista not visible from Earth. Pictured are many of Saturn's photogenic wonders, including the subtle colors of cloud bands, the complex shadows of the rings on the planet, the shadow of the planet on the rings, and the moons Mimas (2 o'clock), Janus (4 o'clock), and Pandora (8 o'clock). As Saturn moves towards equinox in 2009, the ring shadows are becoming smaller and moving toward the equator. During equinox, the rings will be nearly invisible from Earth and project only an extremely thin shadow line onto the planet.

APOD: 2007 June 27 - Neon Saturn
Explanation: If seen in the right light, Saturn glows like a neon sign. Although Saturn has comparatively little of the element neon, a composite image false-colored in three bands of infrared light highlights features of the giant ringed planet like a glowing sign. At the most blue band of the infrared light featured, false-colored blue in the above image, Saturn itself appears dark but Saturn's thin rings brightly reflect light from our Sun. Conversely, Saturn's B ring is so thick that little reflected light makes it through, creating a dark band between Saturn's A and C rings. At the most red band of the infrared, false-colored red above, Saturn emits a surprisingly detailed thermal glow, indicating planet-wide bands, huge hurricane-like storms, and a strange hexagon-shaped cloud system around the North Pole. In the middle infrared band, false-colored green, the sunlit side of Saturn's atmosphere reflects brightly. The above image was obtained in late February by the robotic Cassini spacecraft orbiting about 1.6 million kilometers out from Saturn.

APOD: 2007 April 10 - Saturn from Below
Explanation: Swooping below Saturn, the Cassini spacecraft spied several strange wonders. Visible in the distance are some of the many complex rings that orbit the Solar System's second largest planet. In the foreground looms the gigantic world itself, covered with white dots that are clouds high in Saturn's thick atmosphere. Saturn's atmosphere is so thick that only clouds are visible. At the very South Pole of Saturn lies a huge vortex that is a hurricane-like storm showing no sign of dissipating. The robotic Cassini spacecraft took the above image in January from about one million kilometers out, resolving details about 50 kilometers across.

APOD: 2007 April 6 - Four Years of Saturn
Explanation: Saturn and its magnificent ring system can offer even casual astronomers the most memorable of telescopic sights. Wandering between Leo and Cancer this month, a bright Saturn is well placed for viewing in evening skies. But from our earthbound perspective, the tilt of Saturn's rings does change with time. In 1995 and 1996 the broad rings were edge-on and nearly invisible, gradually opening to a spectacular maximum tilt of about 27 degrees by 2003. This frame from a series of Saturn images beginning a year later, in 2004, and ending just last month shows the steady decrease in apparent tilt as the rings head toward another edge-on presentation in 2009. Saturn's south pole is toward the bottom. Click on the picture to view the sharp, color gif movie.

APOD: 2007 March 16 - Eclipsing the Rings
Explanation: The March 3rd total lunar eclipse was widely viewed by denizens of planet Earth. But only a day before, well placed observers could also watch a lunar occultation of Saturn as the planet passed behind the nearly Full Moon. From Selsey, UK, astronomer Pete Lawrence actually saw Saturn graze the lunar limb, the Moon's bright surface dramatically eclipsing a substantial part of the gas giant's spectacular rings. In this summary view of the grazing occultation, south is up and Saturn's position is shown every 90 seconds in a composite of images constructed from video frames. The frames were all recorded near the occultation event, then combined and adjusted to compensate for the large difference in brightness between Saturn and the lunar surface.

APOD: 2007 March 6 - Saturn from Above
Explanation: This image of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. In fact, this image mosaic was taken in January by the robotic Cassini spacecraft now orbiting Saturn. The beautiful rings of Saturn are seen in full expanse, while cloud details are visible near the night-day terminator divide.

APOD: 2007 January 31 - Movie: Cassini Crosses Saturn's Ring Plane
Explanation: What would the rings of Saturn look like if you passed right through the ring plane? To find out, NASA aimed cameras from the Cassini spacecraft right at Saturn's rings as the robotic explorer passed from the sunlit side of the rings to the shadowed side. Resulting images from a vantage point outside the rings and most moons, but inside the orbit of Titan, have been gathered together in the above time-lapse movie. The dramatic movie demonstrates that ring particle density and reflectivity makes some parts of the shadowed side nearly the photographic negative of the sunlit side, but nearly empty regions remain continually dark. Visible also are Saturn-orbiting moons Enceladus, Mimas, Janus, Epimetheus, Prometheus, and Pandora. The extreme thinness of Saturn's rings can be appreciated from frames taken near the crossing time.

APOD: 2006 November 27 - Mysterious Spokes in Saturn's Rings
Explanation: What causes the mysterious spokes in Saturn's rings? Visible on the left of the above image as ghostlike impressions, spokes were first discovered by the Voyager spacecraft that buzzed by Saturn in the early 1980s. Their existence was unexpected, and no genesis hypothesis has ever become accepted. Oddly, the spokes were conspicuously absent from initial images sent back by the robot Cassini spacecraft now orbiting Saturn. Analyses of archived Voyager images have led to the conclusions that the transient spokes, which may form and dissipate over a few hours, are composed of electrically charged sheets of small dust-sized particles. Some recent images from Cassini like that shown above have now finally shown the enigmatic spokes superposed on Saturn's B ring. Hypotheses for spoke creation include small meteors impacting the rings and electron beams from Saturnian atmospheric lightning spraying the rings. Observations of the puzzling spokes, as well as creative origin speculations, are ongoing.

APOD: 2006 October 16 - In the Shadow of Saturn
Explanation: In the shadow of Saturn, unexpected wonders appear. The robotic Cassini spacecraft now orbiting Saturn recently drifted in giant planet's shadow for about 12 hours and looked back toward the eclipsed Sun. Cassini saw a view unlike any other. First, the night side of Saturn is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn and slightly scattering sunlight, in the above exaggerated color image. Saturn's rings light up so much that new rings were discovered, although they are hard to see in the above image. Visible in spectacular detail, however, is Saturn's E ring, the ring created by the newly discovered ice-fountains of the moon Enceladus, and the outermost ring visible above. Far in the distance, visible on the image left just above the bright main rings, is the almost ignorable pale blue dot of Earth.

APOD: 2006 October 12 - Saturn's Infrared Glow
Explanation: Known for its bright ring system and many moons, gas giant Saturn looks strange and unfamiliar in this false-color view from the Cassini spacecraft. In fact, in this Visual and Infrared Mapping Spectrometer (VIMS) mosaic the famous rings are almost invisible, seen edge-on cutting across picture center. The most striking contrast in the image is along the terminator or boundary between night and day. To the right (day side) blue-green hues are visible sunlight reflected from Saturn's cloud tops. But on the left (night side) in the absence of sunlight, the lantern-like glow of infrared radiation from the planet's warm interior silhouettes features at Saturn's deeper cloud levels. The thermal infrared glow is also apparent in the broad bands of ring shadows draped across the northern hemisphere of Saturn.

APOD: 2006 September 12 - Saturn at Night
Explanation: This is what Saturn looks like at night. In contrast to the human-made lights that cause the nighttime side of Earth to glow faintly, Saturn's faint nighttime glow is primarily caused by sunlight reflecting off of its own majestic rings. The above image of Saturn at night was captured in July by the Cassini spacecraft now orbiting Saturn. The above image was taken when the Sun was far in front of the spacecraft. From this vantage point, the northern hemisphere of nighttime Saturn, visible on the left, appears eerily dark. Sunlit rings are visible ahead, but are abruptly cut off by Saturn's shadow. In Saturn's southern hemisphere, visible on the right, the dim reflected glow from the sunlit rings is most apparent. Imprinted on this diffuse glow, though, are thin black stripes not discernable to any Earth telescope -- the silhouetted C ring of Saturn. Cassini has been orbiting Saturn since 2004 and its mission is scheduled to continue until 2008.

APOD: 2006 July 11 - Crescent Rhea Occults Crescent Saturn
Explanation: Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn. Looking nearly back toward the Sun, the robot Cassini spacecraft now orbiting Saturn captured crescent phases of Saturn and its moon Rhea in color a few months ago. As striking as the above image is, it is but a single frame from a recently released 60-frame silent movie where Rhea can be seen gliding in front of its parent world. Since Cassini was nearly in the plane of Saturn's rings, the normally impressive rings are visible here only as a thin line across the image center. Cassini has now passed the official half-way mark of its mission around Saturn, but is well situated to complete another two years investigating this complex and surprising system.

APOD: 2006 May 3 - Saturn in Blue and Gold
Explanation: Why is Saturn partly blue? The above picture of Saturn approximates what a human would see if hovering close to the giant ringed world. The above picture was taken in mid-March by the robot Cassini spacecraft now orbiting Saturn. Here Saturn's majestic rings appear directly only as a thin vertical line. The rings show their complex structure in the dark shadows they create on the image left. Saturn's fountain moon Enceladus, only about 500 kilometers across, is seen as the bump in the plane of the rings. The northern hemisphere of Saturn can appear partly blue for the same reason that Earth's skies can appear blue -- molecules in the cloudless portions of both planet's atmospheres are better at scattering blue light than red. When looking deep into Saturn's clouds, however, the natural gold hue of Saturn's clouds becomes dominant. It is not known why southern Saturn does not show the same blue hue -- one hypothesis holds that clouds are higher there. It is also not known why Saturn's clouds are colored gold.

APOD: 2006 April 5 - Slightly Beneath Saturn's Ring Plane
Explanation: When orbiting Saturn, be sure to watch for breathtaking superpositions of moons, rings, and shadows. One such picturesque vista was visible recently to the robot Cassini spacecraft now orbiting Saturn. In late February, Cassini captured Rhea, the second largest moon of Saturn, while looking up from slightly beneath Saturn's expansive ring plane. Signature dark gaps are visible in the nearly edge-on rings. A shadow of Saturn's F ring cuts across the cratered ice-moon. Cassini is scheduled to continue sending back images from the orbit of Saturn until at least 2008.

APOD: 2006 March 22 - Enceladus Near Saturn
Explanation: Some images of Saturn appear surreal. Earlier this year, the robot spacecraft Cassini now orbiting Saturn took this surreal image of the gas giant Saturn, its majestic rings, and its enigmatic world Enceladus all in one frame. Enceladus, recently found to emit jets of ice from possible underground seas, appears white as its surface is covered with relatively clean water-ice. Below Enceladus are the rings of Saturn, seen nearly edge on. Compared to Enceladus, Saturn's rings show their comparatively high density of dirt with their golden-brown color in this natural color image. The planet Saturn, in the background, appears relatively featureless with the exception of thin ring shadows visible on the upper left. The terminator between night and day is seen vertically across the face of this distant world.

APOD: 2005 December 31 - A Year at Saturn
Explanation: Arriving at Saturn in July of 2004, the Cassini spacecraft has now spent a year and a half exploring the magnificent rings and moons of the distant gas giant. The year 2005 began with Cassini's Huygens probe landing on Saturn's large moon Titan. Cassini's continuing series of close flybys also revealed details and discoveries across the surface of the smog shrouded moon. In fact, with a ringside seat throughout 2005, Cassini's cameras have made spectacular pictures of Titan along with Saturn's other moons and rings almost common place. But often, Saturn itself provided the most dramatic backdrop. In this view, Saturn's moon Dione lies in front of edge-on rings and the gas giant's cloud tops draped with broad ring shadows. Dione is 1,118 kilometers across and lies about 300,000 kilometers from the ring's edge.

APOD: 2005 December 19 - Thin Rings Around Polarized Saturn
Explanation: How thin are the rings of Saturn? Brightness measurements from different angles have shown Saturn's rings to be about one kilometer thick, making them many times thinner, in relative proportion, than a razor blade. This thinness sometimes appears in dramatic fashion during an image taken nearly along the ring plane. The robot Cassini spacecraft now orbiting Saturn has now captured another shot that dramatically highlights the ring's thinness. The above artistic looking image was taken early last month in infrared polarized light. If alone in space, the unlit part of Saturn would be much darker. Reflection of light off of moons like Enceladus (pictured) and the billions of small particles in Saturn's rings, however, gives the giant space orb an unusual glow, an effect highlighted in polarized light.

APOD: 2005 October 26 - 4500 Kilometers Above Dione
Explanation: What does the surface of Saturn's moon Dione look like? To find out, the robot Cassini spacecraft currently orbiting Saturn flew right past the fourth largest moon of the giant planet earlier this month. Pictured above is an image taken about 4,500 kilometers above Dione's icy surface, spanning about 23 kilometers. Fractures, grooves, and craters in Dione's ice and rock are visible. In many cases, surface features are caused by unknown processes and can only be described. Many of the craters have bright walls but dark floors, indicating that fresher ice is brighter. Nearly parallel grooves run from the upper right to the lower left. Fractures sometimes across the bottom of craters, indicating a relatively recent formation. The lip of a 60-kilometer wide crater runs from the middle left to the upper center of the image, while the crater's center is visible on the lower right. Images like this will continue to be studied to better understand Dione as well as Saturn's complex system of rings and moons.

APOD: 2005 October 21 - Ringside
Explanation: Orbiting in the plane of Saturn's rings, Dione and the other icy saturnian moons have a perpetual ringside view of the gorgeous gas giant planet. Of course, while passing through the ring plane the Cassini spacecraft also shares their stunning perspective. The rings themselves can be seen slicing across the bottom of this Cassini snapshot. Remarkably thin, the bright rings still cast arcing shadows across the planet's cloud tops. Pale Dione, in the foreground, is about 1,100 kilometers across and orbits over 300,000 kilometers from the visible outer edge of the A ring.

APOD: 2005 October 10 - The Swirling Storms of Saturn
Explanation: Storms larger than hurricanes continually dot the upper atmosphere of the planet Saturn. A view of many storms occurring simultaneously was captured in July by the robot Cassini spacecraft now orbiting Saturn. An image of unusually high detail was made possible at that time when Cassini isolated a very specific color of polarized infrared light. The numerous white and dark spots visible above are the swirling storm systems. On Saturn, storms like these typically last for months and have even been seen merging. Bands of clouds that circle the entire planet are also clearly visible. Saturn's complex and majestic ring system is seen both in the foreground and the background. The above image has been digitally shortened along the vertical.

APOD: 2005 July 22 - Tethys, Rings, and Shadows
Explanation: Seen from ice moon Tethys, rings and shadows would play across fantastic views of the Saturnian system. Haven't dropped in on Tethys lately? Then this gorgeous ringscape from the Cassini spacecraft will have to do for now. Caught in sunlight just below and left of picture center, Tethys itself is about 1,000 kilometers in diameter and orbits not quite five saturn-radii from the center of the gas giant planet. At that distance (around 300,000 kilometers) it is well outside Saturn's main bright rings, but Tethys is still one of five major moons that find themselves within the boundaries of the faint and tenuous outer E ring. Discovered in the 1980s, two very small moons Telesto and Calypso are locked in stable locations along Tethys' orbit. Telesto precedes and Calypso follows Tethys as the trio circles Saturn.

APOD: 2005 July 11 - Sunrise Over Kilimanjaro
Explanation: Is the Roof of Africa on fire? A group hiking at 6 am near the top of Mt. Kilimanjaro watched the rising sun peak above the clouds and the horizon light up red. Don't worry -- in this case the highest volcano in Africa is not even erupting. The spectacular sunrise colors are caused by light scattering off the atmosphere and small cloud particles. If all of the scattered light that makes the sky blue were added back into the scene, the sunrise would appear Sun-colored and not so red. A similar light scattering effect involving small airborne dust particles causes sunsets on Mars to be red and has been used to determine the sizes of particles in the rings of Saturn. During this trek in 2000 November, a group of about 30 reached the Kilomanjaro summit after a six-day climb.

APOD: 2005 June 22 - Saturn's Rings from the Other Side
Explanation: What do Saturn's rings look like from the other side? From Earth, we usually see Saturn's rings from the same side of the ring plane that the Sun illuminates them. Geometrically, in the above picture taken in April by the robot Cassini spacecraft now orbiting Saturn, the Sun is behind the camera but on the other side of the ring plane. Such a vantage point gives a breathtaking views of the most splendid ring system in the Solar System. Strangely, the rings have similarities to a photographic negative of a front view. For example, the dark band in the middle is actually the normally bright B-ring. The ring brightness as recorded from different angles indicates ring thickness and particle density of ring particles. Images like these are also interesting for what they do not show: spokes. The unexpected shadowy regions once recorded by the Voyager missions when they passed Saturn in the early 1980s are not, so far, being seen by Cassini. Extra credit: Can you spot the small moon (Prometheus) among the rings?

APOD: 2005 June 6 - Saturn: Dirty Rings and a Clean Moon
Explanation: Eating surface ice from Enceladus might be healthier than eating ice from Saturn's rings -- it certainly appears cleaner. From their apparent densities and reflectance properties, both the rings of Saturn and its shiniest moon, Enceladus, are thought to be composed predominantly of water ice. For reasons that are not yet understood, however, many of Saturn's ring particles have become partly coated with some sort of relatively dark dust, while the surface of Enceladus appears comparatively bright and clean. The contrast between the two can be seen in the above image taken last month by the robot Cassini spacecraft now in orbit around Saturn. Bright Enceladus shines in the background in contrast to the darker foreground rings. The reason why Enceladus is so bright is currently unknown but might involve bringing fresh water to its surface with water volcanoes.

APOD: 2005 May 25 - Particle Sizes in Saturns Rings
Explanation: What size particles compose Saturn's rings? To help find out, the robot Cassini spacecraft now orbiting Saturn broadcast radio waves of three different wavelengths right through the rings to Earth earlier this month. The experiment was sensitive to ring particle sizes because ring particles much larger than a broadcast radio wavelength will reflect those radio waves away. Three different wavelengths were used: approximately 1 centimeter, 3.5 centimeters, and 13 centimeters. The results are coded into the above false-color digitally reconstructed image. In the above image, the color purple indicates regions populated predominantly by ring particles larger than 5 centimeters, while the color green indicates regions with a significant population of small ring particles less than even 1 centimeter. The white center of Saturn's B-ring indicates that the density of ring particles was too high to make a good determination. Other radio observations indicate that some ring particles can be as large as several meters across. The impressive nature and clarity of the above sharp image may help determine clues about the origin of Saturn's beautiful but enigmatic ring system.

APOD: 2005 May 23 - A Wavemaker Moon in Saturn's Rings
Explanation: What causes small waves in Saturn's rings? Observations of rings bordering the Keeler gap in Saturn's rings showed unusual waves. Such waves were first noticed last July and are shown above in clear detail. The picture is a digitally foreshortened image mosaic taken earlier this month by the robot Cassini spacecraft now orbiting Saturn. The rings, made of many small particles, were somehow not orbiting Saturn in their usual manner. Close inspection of the image shows the reason - a small moon is orbiting in the Keeler gap. The previously unknown moon is estimated to span about seven kilometers and appears to have the same brightness as nearby ring particles. The gravity of the small moon likely perturbs the orbits of ring particles that come near it, causing them to shimmy back and forth after the moon passes. Since inner particles orbit more quickly than outer particles, only the leading particles of the inner rings and the trailing particles of the outer rings show the wave effect.

APOD: 2005 May 4 - Cassini Spacecraft Crosses Saturns Ring Plane
Explanation: If this is Saturn, where are the rings? When Saturn's "appendages" disappeared in 1612, Galileo did not understand why. Later that century, it became understood that Saturn's unusual protrusions were rings and that when the Earth crosses the ring plane, the edge-on rings will appear to disappear. This is because Saturn's rings are confined to a plane many times thinner, in proportion, than a razor blade. In modern times, the robot Cassini spacecraft orbiting Saturn now also crosses Saturn's ring plane. A series of plane crossing images from late February was dug out of the vast online Cassini raw image archive by interested Spanish amateur Fernando Garcia Navarro. Pictured above, digitally cropped and set in representative colors, is the striking result. Saturn's thin ring plane appears in blue, bands and clouds in Saturn's upper atmosphere appear in gold, and dark shadows of the rings curve across the top of the gas giant planet. Moons appear as bumps in the rings.

APOD: 2005 April 18 - Saturnian Moon and Rings
Explanation: When can a robot produce art? When it glides past the rings of Saturn. As the robot spacecraft Cassini orbiting Saturn crossed outside the famous photogenic ring plane of the expansive planet, the rings were imaged from the outside, nearly edge on, and in the shadow of Saturn. From the upper left, ring features include the A ring, the Cassini gap, the B ring, and the darker C ring that includes the Titan gap and a gap yet unnamed. Last month when the above image was taken, the gliding spacecraft was about one million kilometers from foreground Enceladus, a small Saturnian moon only about 500 kilometers across. Cassini is scheduled to continue its 70 orbit tour of Saturn over the next three years, sending back images of the gas giant, its rings, and its moons that will be studied for decades to come.

APOD: 2005 April 13 - A Window to the Once Secret Sky
Explanation: If there were a window nearby to the distant universe -- would you look through it? Quite possibly, there is, in the form of a small telescope. A local skykeeper could be a relative or a stranger and is frequently proud to show off the sky free of charge. Through a window called an eyepiece, on a dark cloudless night, you can see clusters of stars, rings around Saturn, glowing nebulas of gas, craters on the Moon, and galaxies across the universe. The technology to create this window -- and the secret sky it reveals -- was unknown only 400 years ago. Modern sky opportunities may occur this Saturday, Astronomy Day, at local amateur astronomy clubs, universities, science centers, or planetariums. Pictured above is a small telescope being deployed at picturesque Hohe Wand, about 50 kilometers south of Vienna, Austria. The spin of the Earth is visible in the above photo as the long star trails.

APOD: 2005 February 22 - Persistent Saturnian Auroras
Explanation: Are Saturn's auroras like Earth's? To help answer this question, the Hubble Space Telescope and the Cassini spacecraft monitored Saturn's South Pole simultaneously as Cassini closed in on the gas giant in January 2004. Hubble snapped images in ultraviolet light, while Cassini recorded radio emissions and monitored the solar wind. Like on Earth, Saturn's auroras make total or partial rings around magnetic poles. Unlike on Earth, however, Saturn's auroras persist for days, as opposed to only minutes on Earth. Although surely created by charged particles entering the atmosphere, Saturn's auroras also appear to be more closely modulated by the solar wind than either Earth's or Jupiter's auroras. The above sequence shows three Hubble images of Saturn each taken two days apart.

APOD: 2005 February 19 - Saturnian Aurora
Explanation: Saturn's Rings are one of the most spectacular sights in the solar system. Still, this image from the Hubble Space Telescope offers a striking view of another kind of ring around Saturn - pole encircling rings of ultraviolet aurora. Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays were thought to be analogous to Earth's. But following the ebb and flow of Saturn's aurora, with the Hubble's cameras and instruments onboard the Cassini spacecraft, researchers are now reporting some surprising results. In this false-color image made in ultraviolet light, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules.

APOD: 2005 February 11 - Blue Saturn
Explanation: Serene blue hues highlight this view of Saturn's northern hemisphere from the Cassini spacecraft. The image has been adjusted to approximate the natural blue color of visible sunlight scattered by the gas giant's upper atmosphere. Saturn's famous rings cast the dark shadows stretching across the frame with infamous cratered moon Mimas lurking at the lower left. Orbiting beyond the main inner rings, Mimas itself is 400 kilometers across and lies nearly 200,000 kilometers, over 3 Saturn radii, from the center of the planet. Still, Mimas orbits within Saturn's faint and tenuous outer E ring.

APOD: 2005 February 10 - Red Saturn
Explanation: This strange, false-color image of otherwise familiar planet Saturn shows temperature changes based on thermal infrared emission in the gas giant's atmosphere and rings. Recorded from the Keck I telescope on Mauna Kea, the sharp, ground-based picture of Saturn's southern hemisphere is a mosaic of 35 images. Based on the effects of sunlight during the southern summer season, general warming trends were anticipated. But a surprising result of the infrared image data is the a clear indication of an abruptly warmer polar cap and bright hot spot at Saturn's south pole. The warm south pole and hot spot may be unique in the solar system and a further exploration of the region is planned using instruments on the Cassini spacecraft. So how hot is Saturn's hot spot? The upper tropospheric temperature is a sweltering 91 Kelvin (-296 degrees Fahrenheit) at the pole.

APOD: 2004 December 17 - Prometheus and the Rings of Saturn
Explanation: In Greek Mythology, Prometheus was known for stealing fire from the gods. Ironically, in a story for more modern times Prometheus may also become known for stealing, but this time for stealing icy particles from Saturn's rings. This eerie close-up from the Cassini spacecraft shows the 100 kilometer-long Saturnian moon Prometheus orbiting near the inner edge of Saturn's F ring. The dramatic view clearly resolves the potato-shaped moon and multiple strands of the narrow F ring. It also reveals a faint strand of material connecting Prometheus with the rings. One possibility is that the tiny moon's gravity is indeed drawing off particles from the rings and is influencing the formation of the gaps and kinks seen in the ring structure.

APOD: 2004 December 2 - Mimas, Rings, and Shadows
Explanation: Caught in sunlight, icy moon Mimas shines above a broad shadow across gas giant Saturn. In this remarkable image from the Cassini spacecraft, tiny Mimas is at the upper right. The broad shadow across the giant planet is cast by Saturn's dense B ring with intriguing threadlike shadows from Saturn's inner C ring arrayed below. While the B and C rings are otherwise not visible here, the very narrow outer F ring lies toward the bottom of the image as well as a section of the partly transparent A ring and its 300 kilometer wide Encke gap crisscrossing the ring shadows. Sunlight streaming through the much larger Cassini gap that separates the A and B rings is responsible for the bright band seen above Mimas. The Cassini gap itself is just off the bottom of this cropped view. Orbiting well beyond Saturn's F ring, Mimas is a mere 400 kilometers in diameter.

APOD: 2004 September 20 - Seeing Through Saturn's C Ring
Explanation: Are Saturn's rings transparent? The Cassini spacecraft that recently entered orbit around Saturn has confirmed that some of Saturn's rings are more transparent than others. Pictured above, Saturn's main A, B, and C rings can be seen, top to bottom, superposed against the gas giant planet. Although the B-ring across the top is opaque, Saturn's cloud tops can be clearly seen through the lower C-ring. The translucent nature of the C-ring likely indicates that it is less densely populated with ring particles than the B-ring. The above image was taken on July 30 while Cassini was over 7 million kilometers from Saturn.

APOD: 2004 July 23 - Saturns Rings in Natural Color
Explanation: What colors are Saturn's rings? Recent images from the Cassini spacecraft now orbiting Saturn confirm that different rings have slightly different colors. The above image shows their sometimes-subtle differences in brightness and color. The rings reflect sunlight and so, even if they were perfectly reflecting, would appear the color of the Sun. The ring particles are mostly light water-ice, although these particles can be shaded by an unknown type of darker dirt. Thinner and more isolated rings also naturally appear darker. The brightest section pictured above is Saturn's B ring.

APOD: 2004 July 21 - A Shadow on the Rings of Saturn
Explanation: This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. Rather, this picture was taken by the robot Cassini spacecraft that began orbiting Saturn earlier this month. The dark western limb of Saturn looms large on the image right, while complex concentrations of small ring particles reflect sunlight on the image left. Saturn's enigmatic F ring is visible around the outside, showing mysterious knots. The small moon Epimetheus, only about 100 kilometers across, can also been seen on the far left. Cassini is scheduled to drop a probe toward the largest moon Titan in December.

APOD: 2004 July 12 - Cassini Images Saturns A Ring
Explanation: What are Saturn's rings made of? In an effort to find out, the robot spacecraft Cassini that entered orbit around Saturn two weeks ago took several detailed images of the area surrounding Saturn's large A ring in ultraviolet light. In the above image, the bluer an area appears, the richer it is in water ice. Conversely, the redder an area appears, the richer it is in some sort of dirt. This and other images show that inner rings have more dirt than outer rings. Specifically, as shown above, the thin rings in the Cassini Division on the left have relatively high dirt content compared to the outer parts of Saturn's A ring, shown on the right. This dirt/ice trend could be a big clue to the ring's origin. The thin red band in the otherwise blue A ring is the Encke Gap. The exact composition of dirt remains unknown.

APOD: 2004 July 5 - Cassini Images Density Waves in Saturns Rings
Explanation: What causes the patterns in Saturn's rings? The Cassini spacecraft just entering orbit around Saturn has started sending back spectacular images of Saturn's immense ring system in unprecedented detail. The physical cause for many of newly resolved ring structures is not always understood. The cause for the beautifully geometric type of ring structure shown above in Saturn's A ring, however, is hypothesized to be a spiral density wave. A small moon systematically perturbing the orbits of ring particles orbiting at slightly different distances causes such a density wave bunching. Also visible on the image right is a bending wave, a vertical wave in ring particles also caused by the gravity of a nearby moon. This close-up spans about 220 kilometers. Cassini is scheduled to take and send back images of the distant ringed Saturn and its unusual moons for the next four years.

APOD: 2004 July 4 - M57: The Ring Nebula
Explanation: Except for the rings of Saturn, the Ring Nebula (M57) is probably the most famous celestial band. This planetary nebula's simple, graceful appearance is thought to be due to perspective -- our view from planet Earth looking straight into what is actually a barrel-shaped cloud of gas shrugged off by a dying central star. Astronomers of the Hubble Heritage Project produced this strikingly sharp image from Hubble Space Telescope observations using natural appearing colors to indicate the temperature of the stellar gas shroud. Hot blue gas near the energizing central star gives way to progressively cooler green and yellow gas at greater distances with the coolest red gas along the outer boundary. Dark, elongated structures can also be seen near the nebula's edge. The Ring Nebula is about one light-year across and 2,000 light-years away in the northern constellation Lyra.

APOD: 2004 July 2 - The Encke Gap: A Moon Goes Here
Explanation: Yesterday, Cassini became the first spacecraft to enter orbit around gas giant Saturn, rocketing through a 25,000 kilometer wide gap in the distant planet's magnificent system of icy rings at about 15 kilometers per second. Turning to snap pictures, Cassini's narrow angle camera recorded this stunning close-up of a much smaller gap in the rings, the Encke Gap. A mere 300 kilometers wide, the Encke Gap is flanked by amazing structures within the rings -- scalloped edges and patterns of density waves are clear in the sharp image. While the rings of Saturn are likely debris from the breakup of a fair-sized icy moon, the Encke Gap itself is created by the repeated passage of a tiny moon. Only 20 kilometers wide that tiny moon, Pan, was also detected by Cassini's camera as the spacecraft approached the Saturnian system.

APOD: 2004 May 31 - 24 Million Kilometers to Saturn
Explanation: Next stop: Saturn. The Cassini-Huygens spacecraft is approaching Saturn and will fire its engines to break into orbit around the ringed giant on July 1. The robot spacecraft was launched in 1997 and rounded Jupiter in 2001. As Cassini orbits Saturn over the next four years, it will swoop past many of Saturn's moons for unprecedented close-ups and even drop a probe onto Titan. Pictured above, Cassini imaged Saturn two weeks ago as it closed to only 24 million kilometers out. Visible are complex cloud patterns, thousands of rings, a shadow angle not visible from Earth, and a moon (if you can find it).

APOD: 2004 April 30 - Eyeful of Saturn
Explanation: Now a bright speck of light wandering through Earth's night sky, magnificent planet Saturn lies nearly 1.5 billion kilometers from the Sun. But after an interplanetary voyage of seven years the planet's stunning rings nearly fill the field of the Cassini spacecraft's narrow angle camera in this image recorded on March 27. Tip to tip, the ring system spans about 270,000 kilometers. Named for discoverers, the large, easily visible gap in the rings is known as the Cassini division, while the narrower outer gap is the Encke division. Illuminated from below and to the right, the rings cast a shadow on Saturn's upper hemisphere, interrupted where sunlight streams through the Cassini division and creates a light blue streak. At the left, Saturn also casts a stark shadow across the planet girdling rings. On July 1, the Cassini spacecraft is scheduled to fire its main engine and enter Saturn orbit.

APOD: 2004 January 17 - Saturn: Lord of the Rings
Explanation: Born in 1564, Galileo used a telescope to explore the Solar System. In 1610, he became the first to be amazed by Saturn's rings, After nearly 400 years, Saturn's magnificent rings still offer one of the most stunning astronomical sights. Uniquely bright compared to the rings of the other gas giants, Saturn's ring system is around 250,000 kilometers wide but in places only a few tens of meters thick. Modern astronomers believe the rings are perhaps only a hundred million years young. Accumulating dust and dynamically interacting with Saturn's moons, the rings may eventually darken and sag toward the gas giant, losing their lustre over the next few hundred million years. Since Galileo, astronomers have subjected the entrancing rings to intense scrutiny to unlock their secrets. On December 31, 2003, Saturn made its closest approach to Earth for the next 29 years, a mere 1,200,000,000 kilometers. It will remain a tantalizing target for earthbound telescopes in the coming months.

APOD: 2003 December 10 - Cassini Approaches Saturn
Explanation: Cassini, a robot spacecraft launched in 1997 by NASA, is close enough now to resolve many rings and moons of its destination planet: Saturn. The spacecraft has now closed to within a single Earth-Sun separation from the ringed giant. Early last month, Cassini snapped the contrast-enhanced color composite pictured above. Many features of Saturn's rings and cloud-tops now show considerable detail. When arriving at Saturn in July 2004, the Cassini orbiter will begin to circle and study the Saturnian system. Several months later, a probe named Huygens will separate and attempt to land on the surface of Titan.

APOD: 2003 September 18 - Saturn by Three
Explanation: These three views of Saturn were recorded by the Hubble Space Telescope on March 7th of this year, as the southern hemisphere of the solar system's most gorgeous planet reached its maximum 27 degree tilt toward Earth. The images used to construct the false-color pictures were made through a combination of filters covering the electromagnetic spectrum from ultraviolet (top), to visible (middle) and infrared (bottom) wavelengths highlighting different features in the Saturnian atmospheric bands and rings. Well known for its bright ring system and large, mysterious moon Titan, gas giant Saturn is also a planet with a dynamic atmosphere and high-speed winds. In fact, in the 1980s, Voyager spacecraft measured equatorial winds of over 1,000 miles per hour. Giant storm systems, comparable in size to planet Earth itself, have been seen erupting in Saturn's cloud tops.

APOD: 2003 August 17 - Natural Saturn On The Cassini Cruise
Explanation: What could you see approaching Saturn aboard an interplanetary cruise ship? Your view would likely resemble this subtly shaded image of the gorgeous ringed gas giant. Processed by the Hubble Heritage project, the picture intentionally avoids overemphasizing color contrasts and presents a natural looking Saturn with cloud bands, storms, nearly edge-on rings, and the small round shadow of the moon Enceladus near the center of the planet's disk. Of course, seats were not available on the only ship currently en route, the Cassini spacecraft. Cassini flew by Jupiter at the turn of the millennium and is scheduled to arrive at Saturn in the year 2004. After an extended cruise to a world 1,400 million kilometers from the Sun, Cassini will tour the Saturnian system, conducting a remote, robotic exploration with software and instruments designed by denizens of planet Earth.

APOD: 2003 April 5 - The Seasons of Saturn
Explanation: Since Saturn's axis is tilted as it orbits the Sun, Saturn has seasons, like those of planet Earth ... but Saturn's seasons last for over seven years. So what season is it on Saturn now? Orbiting the equator, the tilt of the rings of Saturn provides quite a graphic seasonal display. In fact, this month, Saturn's rings will reach their most "open" angle after appearing nearly edge on in the mid-1990s. The ringed planet is also well placed in evening skies providing a grand view as summer comes to Saturn's southern hemisphere and winter to the north. The Hubble Space Telescope took the above sequence of images about a year apart, starting on the left in 1996 and ending on the right in 2000. Although they look solid, Saturn's Rings are likely less than 50 meters thick and consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: 2003 March 22 - M57: The Ring Nebula
Explanation: Except for the rings of Saturn, the Ring Nebula (M57) is probably the most famous celestial band. This planetary nebula's simple, graceful appearance is thought to be due to perspective -- our view from planet Earth looking straight into what is actually a barrel-shaped cloud of gas shrugged off by a dying central star. Astronomers of the Hubble Heritage Project produced this strikingly sharp image from Hubble Space Telescope observations using natural appearing colors to indicate the temperature of the stellar gas shroud. Hot blue gas near the energizing central star gives way to progressively cooler green and yellow gas at greater distances with the coolest red gas along the outer boundary. Dark, elongated structures can also be seen near the nebula's edge. The Ring Nebula is about one light-year across and 2,000 light-years away in the northern constellation Lyra.

APOD: 2003 February 22 - Infrared Saturn
Explanation: This delightfully detailed false-color image of Saturn was taken in January 1998 by the orbiting Hubble Space Telescope. The picture is a combination of three images from Hubble's NICMOS instrument and shows the lovely ringed planet in reflected infrared sunlight. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere. The bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn's many moons have also put in an appearance, Tethys just beyond the planet's disk at the upper right, and Dione at the lower left. Presently, Saturn shines brightly in evening skies as a pale yellow "star" near the constellation Orion.

APOD: 2003 January 15 - Ringed Planet Uranus
Explanation: Yes it does look like Saturn, but Saturn is only one of four giant ringed planets in our Solar System. And while Saturn has the brightest rings, this system of rings and moons actually belongs to planet Uranus, imaged here in near-infrared light by the Antu telescope at the ESO Paranal Observatory in Chile. Since gas giant Uranus' methane-laced atmosphere absorbs sunlight at near-infrared wavelengths the planet appears substantially darkened, improving the contrast between the otherwise relatively bright planet and the normally faint rings. In fact, the narrow Uranian rings are all but impossible to see in visible light with earthbound telescopes and were discovered only in 1977 as careful astronomers noticed the then unknown rings blocking light from background stars. The rings are thought to be younger than 100 million years and may be formed of debris from the collision of a small moon with a passing comet or asteroid-like object. With moons named for characters in Shakespeare's plays, the distant ringed world Uranus was last visited in 1986 by the Voyager 2 spacecraft.

APOD: 2003 January 10 - The Crab that Played with the Planet
Explanation: Wandering through the constellation Taurus, Saturn made its closest approach to planet Earth last month, tilting its lovely rings toward appreciative skygazers while rising high in midnight skies. On January 4th and 5th, Saturn also crossed in front of the high and far-off Crab Nebula (M1), a cosmic cloud of debris from a stellar explosion and first on the list of astronomer Charles Messier's celestial sights. But Saturn and the Crab made poor playmates, as light from the bright planet overwhelmed the the diffuse nebula, all but hiding the Crab during the transit. Taken on January 2nd, a few days before their closest encounter, this composite digital image illustrates the problem. The subtle nebula is just visible at the right, while on the left, light from a drastically over-exposed Saturn overflows its pixels. Composited into the image is a correctly exposed picture of ringed Saturn with the Saturnian moons labeled. The well-exposed Saturn image was also taken on January 2nd, but captured with an exposure lasting only a fraction of a second, in contrast with the tens of seconds of exposure time required to reveal the Crab.

APOD: 2002 November 4 - Cassini Approaches Saturn
Explanation: Cassini, a robot spacecraft launched in 1997 by NASA, is close enough now to resolve many rings and moons of its destination planet: Saturn. The spacecraft has closed to about two Earth-Sun separations from the ringed giant. Last month, Cassini snapped several images during an engineering test. These images have been combined into the contrast-enhanced color composite pictured above. Saturn's rings and cloud-tops are visible on the far right, while Titan, its largest moon, is visible as the speck on the lower left. When arriving at Saturn in July 2004, the Cassini orbiter will begin to circle and study the Saturnian system. Several months later, a probe named Huygens will separate and attempt to land on the surface of Titan.

APOD: 2002 September 7 - Stereo Saturn
Explanation: Get out your red/blue glasses and launch yourself into this stereo picture of Saturn! The picture is actually composed from two images recorded weeks apart by the Voyager 2 spacecraft during its visit to the Saturnian System in August of 1981. Traveling at about 35,000 miles per hour, the spacecraft's changing viewpoint from one image to the next produced this exaggerated but pleasing stereo effect. Saturn is the second largest planet in the Solar System, after Jupiter. Its spectacular ring system is so wide that it would span the space between the Earth and Moon. Although they look solid here, Saturn's rings consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: 2002 May 11 - Natural Saturn On The Cassini Cruise
Explanation: What could you see approaching Saturn aboard an interplanetary cruise ship? Your view would likely resemble this subtly shaded image of the gorgeous ringed gas giant. Processed by the Hubble Heritage project, the picture intentionally avoids overemphasizing color contrasts and presents a natural looking Saturn with cloud bands, storms, nearly edge-on rings, and the small round shadow of the moon Enceladus near the center of the planet's disk. Of course, seats were not available on the only ship currently enroute, the Cassini spacecraft. Cassini flew by Jupiter at the turn of the millennium and is scheduled to arrive at Saturn in the year 2004. After an extended cruise to a world 1,400 million kilometers from the Sun, Cassini will tour the Saturnian system, conducting a remote, robotic exploration with software and instruments designed by denizens of planet Earth.

APOD: 2002 February 15 - Saturn: Lord of the Rings
Explanation: Born on today's date in 1564, Galileo used a telescope to explore the Solar System. In 1610, he became the first to be amazed by Saturn's rings. After nearly 400 years, Saturn's magnificent rings still offer one of the most stunning astronomical sights. Uniquely bright compared to the rings of the other gas giants, Saturn's ring system is around 250,000 kilometers wide but in places only a few tens of meters thick. Modern astronomers believe the rings are perhaps only a hundred million years young. But accumulating dust and dynamically interacting with Saturn's moons, the rings may eventually darken and sag toward the gas giant, losing their lustre over the next few hundred million years. Since Galileo, astronomers have subjected the entrancing rings to intense scrutiny to unlock their secrets. Still mesmerized, some will take advantage of next week's (February 20th) favorable lunar occultation of Saturn to search for evidence of ring material outside the well known boundaries of the ring system. The presence of such a "lost" ring of Saturn was first hinted at in reports dating back to the early 20th century.

APOD: 2002 February 9 - Moon Over Mongolia
Explanation: Fighting clouds and the glow of city lights, a young Moon shines over the western horizon of Mongolia's capital, Ulaan-Baatar. The thin sunlit crescent is about 2 days old and strongly over exposed in this image taken on March 10, 1997. The night side of the Moon is also visible due to earthshine - sunlight reflected from the Earth to the Moon. Just below the Moon, bright Saturn shines through the clouds. Skygazers will have a chance to watch the Moon actually pass in front of the ringed planet in February, March, and April this year. In fact, an excellent lunar occultation of Saturn will be visible from parts of North America on February 20th as Saturn disappears behind the dark limb of a first quarter Moon. Some may even take this opportunity to search for Saturn's lost ring.

APOD: 2001 December 23 - Saturnian Aurora
Explanation: the second largest planet in the Solar System, Saturn's Rings are one of the most spectacular sights for earthbound telescopes. This image from the orbiting Hubble Space Telescope's STIS instrument, offers a striking view of another kind of ring around Saturn - pole encircling rings of ultraviolet aurora. Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. Energetic charged particles in the Solar Wind are funneled by the planet's magnetic field into polar regions where they interact with atmospheric gases. Following the ebb and flow of Saturn's aurora, researchers can remotely explore the planet's atmosphere and magnetic field. In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules. In 2004, NASA plans to begin making close-up studies of the Saturnian system with the Cassini Spacecraft.

APOD: 2001 August 17 - The 47 Ursae Majoris System
Explanation: Watching and waiting, astronomers have uncovered the presence of more than 70 planets orbiting stars other than the Sun. So far almost all these extrasolar planets have crazy elongated orbits, lie uncomfortably close to their parent stars, or are found in bizarre, inhospitable systems. Yet a reported new planet discovery indicates for the first time that a nearby sun-like star, 47 Ursae Majoris (47 UMa), has at least two planets in nearly circular orbits more reminiscent of Jupiter and Saturn in our own familiar Solar System. The planets are too distant and faint to be photographed directly. Still, 13 years of spectroscopic observations of 47 UMa have revealed the wobbling signature of a second planet intertwined with one previously known. In this artist's illustration, the worlds of 47 UMa hang over the rugged volcanic landscape of a hypothetical moon. The moon orbits the newly discovered planet, imagined here with Saturn-like rings, while the previously known planet is visible as a tiny crescent, close to the yellowish star. Closer still to 47 UMa is another tiny dot, a hypothetical Earth-like water world. About 51 light-years distant, 47 UMa can be found in planet Earth's sky near the Big Dipper.

APOD: 2001 July 29 - M57: The Ring Nebula
Explanation: Except for the rings of Saturn, the Ring Nebula (M57) is probably the most famous celestial band. This planetary nebula's simple, graceful appearance is thought to be due to perspective -- our view from planet Earth looking straight into what is actually a barrel-shaped cloud of gas shrugged off by a dying central star. Astronomers of the Hubble Heritage Project produced this strikingly sharp image from Hubble Space Telescope observations using natural appearing colors to indicate the temperature of the stellar gas shroud. Hot blue gas near the energizing central star gives way to progressively cooler green and yellow gas at greater distances with the coolest red gas along the outer boundary. Dark, elongated structures can also be seen near the nebula's edge. The Ring Nebula is about one light-year across and 2,000 light-years away in the northern constellation Lyra.

APOD: 2001 July 2 - The Seasons of Saturn
Explanation: Soon it will be winter in Saturn's northern hemisphere. Since Saturn is tilted in its orbit around the Sun, it has seasons just like the Earth. When a hemisphere is tilted so that the Sun passes more directly overhead, summer occurs. Half an orbit later -- about 15 (Earth) years for Saturn -- winter occurs. Since the rings of Saturn orbit the equator, they provide a quite graphic seasonal display. The Hubble Space Telescope took the above sequence of images about a year apart, starting on the lower left in 1996. Saturn's rings are less than 50 meters thick and are composed of pebble and boulder sized chunks of dusty water ice.

APOD: 2001 March 7 - Saturn At Night
Explanation: From a spectacular vantage point over 1.4 billion kilometers from the sun, the Voyager 1 spacecraft looked back toward the inner solar system to record this startling view of Saturn's nightside. The picture was taken on November 16, 1980, some four days after the robot spacecraft's closest approach to the gorgeous gas giant. The crescent planet casts a broad shadow across its bright rings while the translucent rings themselves can be seen to cast a shadow on Saturn's cloud tops. Since Earth is closer to the sun than Saturn, only Saturn's dayside is visible to Earth-bound telescopes which could never take a picture like this one. After this successful flyby two decades ago, Voyager 1 has continued outward bound and is presently humanity's most distant spacecraft. The next spacecraft to approach Saturn will be Cassini, on course to arrive in 2004.

APOD: 2000 September 17 - Saturnian Aurora
Explanation: Girdling the second largest planet in the Solar System, Saturn's Rings are one of the most spectacular sights for earthbound telescopes. This image from the orbiting Hubble Space Telescope's STIS instrument, offers a striking view of another kind of ring around Saturn - pole encircling rings of ultraviolet aurora. Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. Energetic charged particles in the Solar Wind are funneled by the planet's magnetic field into polar regions where they interact with atmospheric gases. Following the ebb and flow of Saturn's aurora, researchers can remotely explore the planet's atmosphere and magnetic field. In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules. In 2004, NASA plans to begin making close-up studies of the Saturnian system with the Cassini Spacecraft.

APOD: 2000 July 16 - M57: The Ring Nebula
Explanation: Except for the rings of Saturn, The Ring Nebula (M57) is probably the most famous celestial band. This planetary nebula's simple, graceful appearance is thought to be due to perspective -- our view from planet Earth looking straight into what is actually a barrel-shaped cloud of gas shrugged off by a dying central star. Astronomers of the Hubble Heritage Project produced this strikingly sharp image from Hubble Space Telescope observations using natural appearing colors to indicate the temperature of the stellar gas shroud. Hot blue gas near the energizing central star gives way to progressively cooler green and yellow gas at greater distances with the coolest red gas along the outer boundary. Dark, elongated structures can also be seen near the nebula's edge. The Ring Nebula is about one light-year across and 2,000 light-years away in the northern constellation Lyra.

APOD: 2000 June 24 - Sunlight Through Saturns Rings
Explanation: Normally, earth-bound astronomers view Saturn's spectacular ring system fully illuminated by reflected sunlight. However, this intriguing picture was made to take advantage of an unusual orientation, with the Sun actually illuminating the rings from below. The three bright ring features are visible because the rings themselves are not solid. Composed of many separate chunks of rocky, icy material, the rings allow the scattered sunlight to pass through them -- offering a dramatic demonstration that they are not continuous, uninterrupted bands of material. The picture is a false-color composite based on Hubble Space Telescope images recorded in November of 1995.

APOD: 2000 March 30 - Saturn-Sized Worlds Discovered
Explanation: The last decade saw the profound discovery of many worlds beyond our solar system, but none analogs of our home planet Earth. Exploiting precise observational techniques, astronomers inferred the presence of well over two dozen extrasolar planets, most nearly as massive as gas giant Jupiter or more, in close orbits around sun-like stars. Less massive planets must certainly exist, and yesterday preeminent planet-finders announced the further detection of two more new worlds -- each a potentially smaller, saturn-sized planet. The parent suns are 79 Ceti (constellation Cetus), at a distance of 117 light-years, and HD46375 (constellation Monoceros), 109 light-years away. With at least 70 percent the mass of Saturn, 79 Ceti's planet orbits on average 32.5 million miles from the star compared to 93 million miles for the Earth-Sun distance. This arresting artist's vision depicts the newly discovered world with rings and moons, known characteristics of giant planets in our solar system. HD46375's planet is at least 80 percent Saturn's mass, orbiting only 3.8 million miles from its parent star. While Saturn's mass is only one third of Jupiter's, it is still about 100 times that of Earth, and dramatic discoveries in the search for smaller planets are still to come.

APOD: 2000 March 4 - Saturn At Night
Explanation: From a spectacular vantage point over 1.4 billion kilometers from the sun, the Voyager 1 spacecraft looked back toward the inner solar system to record this startling view of Saturn's nightside. The picture was taken on November 16, 1980, some four days after the robot spacecraft's closest approach to the gorgeous gas giant. The crescent planet casts a broad shadow across its bright rings while the translucent rings themselves can be seen to cast a shadow on Saturn's cloud tops. Since Earth is closer to the sun than Saturn, only Saturn's dayside is visible to Earth-bound telescopes which could never take a picture like this one. After this successful flyby two decades ago, Voyager 1 has continued outward bound and is presently humanity's most distant spacecraft. The next spacecraft to approach Saturn will be Cassini, on course to arrive in 2004.

APOD: 2000 February 12 - Stereo Saturn
Explanation: Get out your red/blue glasses and launch yourself into this stereo picture of Saturn! The picture is actually composed from two images recorded weeks apart by the Voyager 2 spacecraft during its visit to the Saturnian System in August of 1981. Traveling at about 35,000 miles per hour, the spacecraft's changing viewpoint from one image to the next produced this exaggerated but pleasing stereo effect. Saturn is the second largest planet in the Solar System, after Jupiter. Its spectacular ring system is so wide that it would span the space between the Earth and Moon. Although they look solid here, Saturn's rings consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: 2000 January 29 - Natural Saturn On The Cassini Cruise
Explanation: What could you see approaching Saturn aboard an interplanetary cruise ship? Your view would likely resemble this subtly shaded image of the gorgeous ringed gas giant. Processed by the Hubble Heritage project, the picture intentionally avoids overemphasizing color contrasts and presents a natural looking Saturn with cloud bands, storms, nearly edge-on rings, and the small round shadow of the moon Enceladus near the center of the planet's disk. Of course, seats were not available on the only ship currently enroute - the Cassini spacecraft, launched in 1997 and scheduled to arrive at Saturn in the year 2004. After an extended cruise to a world 1,400 million kilometers from the Sun, Cassini will tour the Saturnian system, conducting a remote, robotic exploration with software and instruments designed by denizens of planet Earth. But where is Cassini now? Still about 980 million kilometers from Saturn, last Sunday the spacecraft flew by asteroid 2685 Masursky.

APOD: July 24, 1999 - Infrared Saturn
Explanation: This delightfully detailed false color image of Saturn was earmarked to celebrate the 8th anniversary of the orbiting Hubble Space Telescope. The picture is a combination of three images taken in January 1998 and shows the lovely ringed planet in reflected infrared light. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere, while the bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn's many moons have also put in an appearance, Tethys just beyond the planet's disk at the upper right, and Dione at the lower left.

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

APOD: January 30, 1999 - Stereo Saturn
Explanation: Get out your red/blue glasses and launch yourself into this stereo picture of Saturn! The picture is actually composed from two images recorded weeks apart by the Voyager 2 spacecraft during its visit to the Saturnian System in August of 1981. Traveling at about 35,000 miles per hour, the spacecraft's changing viewpoint from one image to the next produced this exaggerated but pleasing stereo effect. Saturn is the second largest planet in the Solar System, after Jupiter. Its spectacular ring system is so wide that it would span the space between the Earth and Moon. Although they look solid here, Saturn's Rings consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: January 23, 1999 - Saturnian Aurora
Explanation: Girdling the second largest planet in the Solar System, Saturn's Rings are one of the most spectacular sights for earthbound telescopes. This image from the orbiting Hubble Space Telescope's STIS instrument, offers a striking view of another kind of ring around Saturn - pole encircling rings of ultraviolet aurora. Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. Energetic charged particles in the Solar Wind are funneled by the planet's magnetic field into polar regions where they interact with atmospheric gases. Following the ebb and flow of Saturn's aurora, researchers can remotely explore the planet's atmosphere and magnetic field. In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules. In 2004, NASA plans to begin making close-up studies of the Saturnian system with the Cassini Spacecraft.

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

APOD: January 7, 1999 - The Ring
Explanation: Except for the Rings of Saturn, The Ring Nebula (M57) is probably the most famous celestial band. This planetary nebula's simple, graceful appearance is thought to be due to perspective -- our view from planet Earth looking straight into what is actually a barrel-shaped cloud of gas shrugged off by a dying central star. Astronomers of the Hubble Heritage Project produced this strikingly sharp image from Space Telescope observations using natural appearing colors to indicate the temperature of the stellar gas shroud. Hot blue gas near the energizing central star gives way to progressively cooler green and yellow gas at greater distances with the coolest red gas along the outer boundary. Dark, elongated structures can also be seen near the nebula's edge. The Ring Nebula is about one light-year across and 2,000 light-years away in the northern constellation Lyra.

APOD: November 5, 1998 - Natural Saturn On The Cassini Cruise
Explanation: What you could see approaching Saturn aboard an interplanetary cruise ship would closely resemble this subtly shaded view of the gorgeous ringed gas giant. Processed by the Hubble Heritage project, the picture intentionally avoids overemphasizing color contrasts and presents a natural looking Saturn with cloud bands, storms, nearly edge-on rings, and the small round shadow of the moon Enceladus near the center of the planet's disk. Of course, seats were not available on the only ship currently enroute - the Cassini spacecraft, launched just over a year ago and scheduled to arrive at Saturn in the year 2004. After an extended cruise to a world 1,400 million kilometers from the Sun, Cassini will tour the Saturnian system, conducting a remote, robotic exploration with software and instruments designed by denizens of planet Earth.

APOD: October 18, 1998 - Saturns Rings Seen Sideways
Explanation: Saturn's rings are actually very thin. This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth. Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops! Titan itself looks a brownish color because of its thick atmosphere. Four other moons of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet. Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.

APOD: September 2, 1998 - Saturn from Earth
Explanation: Saturn is the second largest planet in our Solar System. Saturn has been easily visible in the sky since history has been recorded. Galileo used one of the first telescopes in 1610 to discover Saturn's rings, which he first thought were moons. Maxwell showed in 1856 that Saturn's rings couldn't be a single solid, since Saturn's own gravity would break it up. Were Saturn's rings assembled into a single body, it would measure less than 100 kilometers across. The origin of Saturn's rings, and of unusual radial patterns that appear on them called spokes, are still unknown. The above representative-color picture was taken from Earth in infrared light. A robot spacecraft Cassini launched in 1997 will reach Saturn in 2004.

APOD: April 24, 1998 - Infrared Saturn
Explanation: This delightfully detailed false color image of Saturn has been earmarked to celebrate the 8th anniversary of the orbiting Hubble Space Telescope. The picture is a combination of three images taken in January of this year with the Hubble's new NICMOS instrument and shows the lovely ringed planet in reflected infrared light. Different colors indicated varying heights and compositions of cloud layers generally thought to consist of ammonia ice crystals. The eye-catching rings cast a shadow on Saturn's upper hemisphere, while the bright stripe seen within the left portion of the shadow is infrared sunlight streaming through the large gap in the rings known as the Cassini Division. Two of Saturn's many moons have also put in an appearance, Tethys just beyond the planet's disk at the upper right, and Dione at the lower left.

APOD: January 18, 1998 - Saturn, Rings, and Two Moons
Explanation: NASA's robot spacecraft Voyager 2 made this image of Saturn as it began to explore the Saturn system in 1981. Saturn's famous rings are visible along with two of its moons, Rhea and Dione which appear as faint dots on the right and lower right part of the picture. Astronomers believe that Saturn's moons play a fundamental role in sculpting its elaborate ring system. A robot spacecraft named Cassini was launched last October and is expected to rendezvous with the giant gas planet in 2004.

APOD: January 9, 1998 - Saturnian Aurora
Explanation: Girdling the second largest planet in the Solar System, Saturn's Rings are one of the most spectacular sights for earthbound telescopes. This recently released image, from the orbiting Hubble Space Telescope's STIS instrument, offers a striking view of another kind of ring around Saturn - pole encircling rings of ultraviolet aurora. Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. Energetic charged particles in the Solar Wind are funneled by the planet's magnetic field into polar regions where they interact with atmospheric gases. Following the ebb and flow of Saturn's aurora, researchers can remotely explore the planet's atmosphere and magnetic field. In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules. In 2004, NASA plans to begin making close-up studies of the Saturnian system with the Cassini Spacecraft.

APOD: October 29, 1997 - Stereo Saturn
Explanation: Get out your red/blue glasses and launch yourself into this stereo picture of Saturn! The picture is actually composed from two images recorded weeks apart by the Voyager 2 spacecraft during its visit to the Saturnian System in August of 1981. Traveling at about 35,000 miles per hour, the spacecraft's changing viewpoint from one image to the next produced this exaggerated but pleasing stereo effect. Saturn is the second largest planet in the Solar System, after Jupiter. Its spectacular ring system is so wide that it would span the space between the Earth and Moon. Although they look solid here, Saturn's Rings consist of individually orbiting bits of ice and rock ranging in size from grains of sand to barn-sized boulders.

APOD: September 21, 1997 - Looking Down on Saturn
Explanation: This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. In fact, this photo was taken by the Voyager 1 spacecraft as it flew by Saturn in November 1980. The next spacecraft to approach Saturn will be Cassini which is currently scheduled to be launched later this year and reach Saturn in 2004.

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

APOD: May 31, 1997 - Saturn with Moons Tethys and Dione
Explanation: Saturn and two of its larger moons - Tethys and Dione - were photographed by the Voyager 1 spacecraft which flew by the planet in November of 1980. This picture gives an indication of Saturn's extensive ring system, which can be seen casting a shadow on the planet, as does Tethys. Saturn's rings are composed of many chunks of ice ranging in size from a pebble to a car. The rings have several large gaps, the largest of which is clearly visible in the picture and is named the Cassini Division, after its discoverer. Saturn appears brighter than most stars in the sky, and its rings can be discerned with a small telescope. A new spacecraft - Cassini - will visit Saturn and is currently scheduled for launch later in 1997.

APOD: May 24, 1997 - Saturn's Rings Seen Sideways
Explanation: Saturn's rings are actually very thin. This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth. Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops! Titan itself looks a brownish color because of its thick atmosphere. Four other moon's of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet. Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.

APOD: March 12, 1997 - Saturn in Color
Explanation: Saturn is unusual but photogenic. The second largest planet in our Solar System, behind Jupiter, has been easily identifiable at night since history has been recorded. It was only with the invention of the telescope, however, that any evidence of its majestic ring system became apparent. Saturn itself is composed of mostly hydrogen and helium gas. Saturn's rings are composed of many ice chunks ranging in size from a penny to car. The above picture of Saturn is one of the earliest taken by the Hubble Space Telescope and is a digital reconstruction of three color images. The Cassini mission to Saturn is scheduled to be launched later this year and should reach Saturn in 2004.

APOD: February 5, 1997 - Running Red Rings Around Jupiter
Explanation: Jupiter has rings, too. Unlike Saturn's bright rings which are composed of chunks of ice, Jupiter's rings are darker and appear to consist of fine particles of rock. The six pictures above were taken in infrared light from the Infrared Telescope Facility in Hawaii in 1994, and cover a time span of two hours. Quite visible are Jupiter's rings, bands and spots in the outer atmosphere. Also visible in the photos, however, are two small Jovian moons. Metis, only 40 kilometers across, appears in the second picture as a dim spot on the rings to the right of Jupiter. Amalthea, much larger and brighter, appears in the third frame on the far left, and can be seen to pass across the face of Jupiter in frames four and five. The origin of Jupiter's rings remains unknown, although hypothesized to be created by material scattered from meteorite impacts onto Jupiter's moons.

APOD: July 17, 1996 - Looking Down on Saturn
Explanation: This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. In fact, this photo was taken by the Voyager 1 spacecraft as it flew by Saturn in November 1980. The next spacecraft to approach Saturn will be Cassini which is currently scheduled to be be launched next year and reach Saturn in 2004.

APOD: May 12, 1996 - Tracking Saturn's Moons
Explanation: These five pairs of Hubble Space Telescope images track some of Saturn's moons as they orbit the ringed planet. A pair of images was taken every 97 minutes on November 21, 1995 with the Wide Field Planetary Camera-2, the normally bright ring system appearing nearly edge-on. In the top pair, the large bright moon Dione hangs above center while the smaller moons Pandora, Prometheus, and Mimas (top right image) appear near the planet's disk close to the outer ring. By the second and third pair of images, moons Rhea and Epimetheus have joined the dance. During the Saturn ring plane crossings, the reduction in light from the edge-on rings provided an opportunity for astronomer's to explore Saturn's complex moon system and search for elusive undiscovered satellites.

APOD: May 11, 1996 - Sunlight Through Saturn's Rings
Explanation: Normally, Earth based astronomers view Saturn's spectacular ring system fully illuminated by reflected sunlight. However, this November 1995 Hubble Space Telescope composite image was made to take advantage of an unusual perspective, with the Sun actually illuminating the rings from below. The three bright ring features are visible because the rings themselves are not solid. Composed of many separate chunks of rocky, icy material, the rings allow the scattered sunlight to pass through them -- offering a dramatic demonstration that they are not continuous, uninterrupted bands of material.

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

APOD: April 29, 1996 - Saturn's Rings Seen Sideways
Explanation: Saturn's rings are actually very thin. This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth. Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops! Titan itself looks a brownish color because of its thick atmosphere. Four other moon's of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet. Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.

APOD: March 18, 1996 - Saturn with Moons Tethys and Dione
Explanation: Saturn and two of its larger moons - Tethys and Dione - were photographed by the Voyager 1 spacecraft which flew by the planet in November of 1980. This picture gives an indication of Saturn's extensive ring system, which can be seen casting a shadow on the planet, as does Tethys. Saturn's rings are composed of many chunks of ice ranging in size from a pebble to a car. The rings have several large gaps, the largest of which is clearly visible in the picture and is named the Cassini Division, after its discoverer. Saturn appears brighter than most stars in the sky, and its rings can be discerned with a small telescope. A new spacecraft - Cassini - will visit Saturn and is currently scheduled for launch in 1997.

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

APOD: August 1, 1995 - Crossing The Ring Plane
Explanation: "I do not know what to say in a case so surprising, so unlooked for and so novel." announced Galileo when Saturn's rings appeared to vanish in 1612. In fact, every 15 years Saturn's rings seem to almost disappear as viewed from the Earth. This happens just as the orbiting Earth crosses the plane of Saturn's rings. The edge on perspective temporarily robs astronomers of a spectacular sight, however, the ring plane crossing affords them the opportunity to measure the rings' thickness and search for undiscovered moons. In this image of Saturn, produced on May 22, 1995 by NASA's Hubble Space Telescope, two of Saturn's known moons are visible as star like objects to the left of the planet.

APOD: July 31, 1995 - Exploring Saturn's Rings
Explanation: By watching a star flicker and fade as it passed behind Saturn's rings, NASA's Voyager 2 spacecraft was able explore the ring system in amazing detail. Data produced by Voyager's instruments as the star Delta Scorpii was occulted by some of the outer rings was used to reconstruct this image which shows details almost 1000 times smaller than normally possible with Voyager's cameras.

APOD: July 30, 1995 - The Rings of Saturn
Explanation: Saturn's spectacular system of bright rings has been the subject of study and wonder since Galileo first turned his telescope on the ringed planet in 1610. To Galileo, the blurry image produced by his small telescope was confusing. Saturn appeared to him to have "ear-like" appendages which he thought might be large moons. Eventually, larger telescopes revealed the incredible truth - Saturn was surrounded by bright rings. The image above, made by NASA's Voyager 2 spacecraft, further reveals the intricate structure of the ring system. The image has been computer enhanced and color coded to bring out the subtle details.

APOD: July 6, 1995 - Saturn, Rings, and Two Moons
Explanation: This image of Saturn was made by NASA's robot spacecraft Voyager 2 as it began to explore the Saturn system in 1981. Saturn's famous rings are visible along with two of its moons, Rhea and Dione which appear as faint dots in the right and lower right part of the picture. Astronomers believe that Saturn's moons play a fundamental role in sculpting its elaborate ring system.

APOD: July 5, 1995 - The Night Side of Saturn
Explanation: This image of Saturn was made in November 1980 by the Voyager 1 spacecraft as it flew past the ringed gas giant planet. From a spectacular vantage point, looking back toward the inner solar system, the robot spacecraft recorded this view of the night side of Saturn casting a sharp shadow across the bright rings. No Earth based telescope could ever take a similar picture. Since Earth is closer to the sun than Saturn, only the day side of the planet is visible from the Earth.