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Slightly Beneath Saturn's Ri
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| Title |
Slightly Beneath Saturn's Ring Plane |
| Explanation |
When orbiting Saturn, be sure to watch for breathtaking superpositions of moon [ http://antwrp.gsfc.nasa.gov/apod/ap051003.html ]s, rings [ http://antwrp.gsfc.nasa.gov/apod/ap040117.html ], and shadow [ http://antwrp.gsfc.nasa.gov/apod/ap040803.html ]s. One such picturesque vista [ http://photojournal.jpl.nasa.gov/catalog/PIA08146 ] was visible recently to the robot Cassini spacecraft [ http://en.wikipedia.org/wiki/Cassini-Huygens ] now orbiting Saturn. In late February, Cassini captured Rhea [ http://en.wikipedia.org/wiki/Rhea_%28moon%29 ], the second largest moon of Saturn, while looking up from slightly beneath Saturn's expansive ring [ http://antwrp.gsfc.nasa.gov/apod/ap051219.html ] plane. Signature dark gaps [ http://www.planetary.org/saturn/rings.html ] are visible in the nearly edge-on rings. A shadow of Saturn's F ring [ http://antwrp.gsfc.nasa.gov/apod/ap041217.html ] cuts across the cratered ice-moon. Cassini is scheduled [ http://saturn.jpl.nasa.gov/operations/saturn-tour.cfm ] to continue sending back images from the orbit of Saturn until at least 2008. |
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Ancient Craters on Saturn's
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| Title |
Ancient Craters on Saturn's Rhea |
| Explanation |
Saturn's ragged moon Rhea has one of the oldest surfaces known. Estimated as changing little in the past billion years, Rhea [ http://en.wikipedia.org/wiki/Rhea_%28moon%29 ] shows craters [ http://en.wikipedia.org/wiki/Impact_crater ] so old they no longer appear round ? their edges have become compromised by more recent cratering. Like Earth's Moon [ http://antwrp.gsfc.nasa.gov/apod/ap051113.html ], Rhea's rotation is locked on Saturn, and the above image [ http://photojournal.jpl.nasa.gov/catalog/PIA08173 ] shows part of Rhea's surface [ http://antwrp.gsfc.nasa.gov/apod/ap051213.html ] that always faces Saturn. Rhea's leading surface is more highly cratered than its trailing surface. Rhea [ http://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search?rhea ] is composed mostly of water-ice but is thought to have a small rocky core. The above image [ http://photojournal.jpl.nasa.gov/catalog/PIA08173 ] was taken by the robot Cassini spacecraft [ http://www.esa.int/esaMI/Cassini-Huygens/SEM9D2HHZTD_0.html ] now orbiting Saturn. Cassini swooped past Rhea two months ago and captured the above image from about 100,000 kilometers away. Rhea [ http://www.nineplanets.org/rhea.html ] spans 1,500 kilometers making it Saturn's second largest moon after Titan [ http://antwrp.gsfc.nasa.gov/apod/ap060215.html ]. Several surface features on Rhea [ http://antwrp.gsfc.nasa.gov/apod/ap050215.html ] remain unexplained including large light patch [ http://antwrp.gsfc.nasa.gov/apod/ap050530.html ]es. |
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Crescent Rhea Occults Cresce
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| Title |
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 [ http://en.wikipedia.org/wiki/Cassini-Huygens ] now orbiting Saturn captured crescent phases [ http://antwrp.gsfc.nasa.gov/apod/ap060618.html ] of Saturn [ http://www.nineplanets.org/saturn.html ] and its moon Rhea [ http://antwrp.gsfc.nasa.gov/apod/ap060530.html ] in color a few months ago. As striking as the above image [ http://photojournal.jpl.nasa.gov/catalog/PIA07806 ] 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 [ http://pds-rings.seti.org/saturn/ ], the normally impressive rings are visible here only as a thin line [ http://antwrp.gsfc.nasa.gov/apod/ap051219.html ] 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 [ http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=670 ] investigating this complex and surprising system. |
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Iapetus: Saturn's Disappeari
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| Title |
Iapetus: Saturn's Disappearing Moon |
| Explanation |
Iapetus [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/iapetus.html ] has an unusual surface, one half of which is very dark, the other half very light. This caused it's discoverer Cassini [ http://seds.lpl.arizona.edu/nineplanets/nineplanets/help.html#cassini ] to remark that Iapetus [ http://www.c3.lanl.gov/~cjhamil/SolarSystem/iapetus.html ] could only be seen when on one side of Saturn but not the other. The reason for the difference between hemispheres is presently unknown. Iapetus is the third largest of Saturn [ http://antwrp.gsfc.nasa.gov/apod/ap950705.html ]'s moon behind Titan [ http://antwrp.gsfc.nasa.gov/apod/ap950923.html ] and Rhea [ http://antwrp.gsfc.nasa.gov/apod/ap951014.html ]. Iapetus is composed predominantly of water ice. |
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Rhea: Saturn's Second Larges
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| Title |
Rhea: Saturn's Second Largest Moon |
| Explanation |
Rhea [ http://www.nineplanets.org/rhea.html ] is the second largest moon of Saturn [ http://pds.jpl.nasa.gov/planets/choices/saturn1.htm ], behind Titan [ http://www.nineplanets.org/titan.html ], and the largest without an atmosphere. It is composed mostly of water ice, but has a small rocky core. Rhea [ http://www.solarviews.com/eng/rhea.htm ]'s rotation and orbit are locked together (just like Earth's Moon [ http://antwrp.gsfc.nasa.gov/apod/ap010218.html ]) so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than its trailing surface. The above photograph [ http://www.solarviews.com/cap/sat/rhea2.htm ] was taken with the Voyager 1 spacecraft [ http://voyager.jpl.nasa.gov/spacecraft/index.html ] in 1980. NASA's Cassini spacecraft [ http://saturn.jpl.nasa.gov/spacecraft/ ] is currently on route to Saturn and will arrive in 2004. |
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Saturn, Rings, and Two Moons
| Title |
Saturn, Rings, and Two Moons |
| Explanation |
NASA's robot spacecraft Voyager [ http://antwrp.gsfc.nasa.gov/apod/ap960629.html ] 2 made this image [ http://nssdc.gsfc.nasa.gov/photo_gallery/caption/saturn.txt ] of Saturn [ http://www.seds.org/nineplanets/nineplanets/saturn.html ] as it began to explore the Saturn system [ http://www.jpl.nasa.gov/cassini/Images/astro/23209.html ] in 1981. Saturn's famous rings [ http://ringside.arc.nasa.gov/www/saturn/saturn.html ] are visible along with two of its moons, Rhea [ http://bang.lanl.gov/solarsys/rhea.htm ] and Dione [ http://antwrp.gsfc.nasa.gov/apod/ap951009.html ] 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 [ http://newproducts.jpl.nasa.gov/saturn/back.html ]. A robot spacecraft named Cassini [ http://www.jpl.nasa.gov/cassini/ ] was launched [ http://antwrp.gsfc.nasa.gov/apod/ap971016.html ] last October and is expected to rendezvous [ http://www.jpl.nasa.gov/cassini/Mission/tour.html ] with the giant gas planet in 2004. |
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Saturn's Moon Rhea from Cass
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| Title |
Saturn's Moon Rhea from Cassini |
| Explanation |
Each moon of Saturn seems to come with its own mystery [ http://antwrp.gsfc.nasa.gov/apod/ap050201.html ]. Rhea, Saturn's second largest moon behind Titan [ http://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search?titan ], shows unusual wisps, visible above as light colored streaks. Higher resolution images of similar wisps on Dione indicate that they might be made of long braided fractures. Rhea [ http://www.nineplanets.org/rhea.html ] is composed mostly of water ice [ http://www.mtu.edu/carnival/2005/statues.html ], but likely has a small rocky core [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003PEPI..136..201A ]. Rhea's rotation and orbit are locked together, just like Earth's Moon [ http://antwrp.gsfc.nasa.gov/apod/ap040829.html ], so that one side always faces Saturn [ http://en.wikipedia.org/wiki/Saturn_%28planet%29 ]. A consequence of this is that one side always leads the other. Rhea [ http://antwrp.gsfc.nasa.gov/apod/ap030608.html ]'s leading surface is much more heavily cratered [ http://antwrp.gsfc.nasa.gov/apod/ap990711.html ] than the trailing surface, pictured above. The above image [ http://photojournal.jpl.nasa.gov/catalog/PIA06578 ] in natural color was taken last month by the Cassini robot spacecraft [ http://saturn.jpl.nasa.gov/overview/mission.cfm ] in orbit around Saturn. |
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A Great White Spot on Rhea
| Title |
A Great White Spot on Rhea |
| Explanation |
What caused this great white spot on the surface of Saturn's moon Rhea? The spot was first noticed last year by the robot Cassini spacecraft [ http://saturn.jpl.nasa.gov/spacecraft/index.cfm ] now orbiting Saturn [ http://www.solarviews.com/eng/saturn.htm ]. Cassini's flyby of Rhea [ http://www.nineplanets.org/rhea.html ] in April imaged [ http://photojournal.jpl.nasa.gov/catalog/PIA06648 ] in the spot in great detail. Astronomers hypothesize [ http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1526 ] that the light-colored spot is the result of a relatively recent impact [ http://www.spacegrant.hawaii.edu/class_acts/CratersTe.html ] on the surface of the icy moon. The impact that likely created the crater also splashed light-colored material from the interior onto the darker surface. Rhea [ http://en.wikipedia.org/wiki/Rhea_%28mythology%29 ] spans 1,500 kilometers across and is the second largest moon of Saturn after Titan [ http://antwrp.gsfc.nasa.gov/apod/ap050518.html ]. Rhea sports several other light colored surface features [ http://antwrp.gsfc.nasa.gov/apod/ap050215.html ] that are, as yet, not well understood. |
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620 Kilometers Above Rhea
| Title |
620 Kilometers Above Rhea |
| Explanation |
What does the surface of Saturn's moon Rhea look like? To help find out, the robot Cassini [ http://saturn.jpl.nasa.gov/overview/mission.cfm ] spacecraft currently orbiting Saturn [ http://www.nineplanets.org/saturn.html ] was directed to fly right past the second largest moon of the gas giant planet [ http://antwrp.gsfc.nasa.gov/apod/ap051010.html ] late last month. Pictured above [ http://photojournal.jpl.nasa.gov/catalog/PIA07765 ] is an image taken only 620 kilometers above Rhea [ http://www.nineplanets.org/rhea.html ]'s icy surface, spanning about 90 kilometers. The rim of an old crater crosses the middle of the image, with many smaller and younger craters [ http://en.wikipedia.org/wiki/Impact_crater ] scattered throughout. A linear [ http://en.wiktionary.org/wiki/linear ] depression -- possibly a tectonic fault [ http://www.extremescience.com/PlateTectonicsmap.htm ] -- is visible toward the right, crossing the likely loose material that composes Rhea's surface regolith [ http://antwrp.gsfc.nasa.gov/apod/ap020303.html ]. The origins of many features on Rhea [ http://www.pantheon.org/articles/r/rhea.html ] are currently unexplained [ http://antwrp.gsfc.nasa.gov/apod/ap050530.html ] and being researched [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005LPI....36.2243C ]. |
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The Moving Moons of Saturn
| Title |
The Moving Moons of Saturn |
| Explanation |
The moons of Saturn never stop. A space traveler orbiting the ringed giant planet [ http://en.wikipedia.org/wiki/Saturn ] would witness a continuing silent dance [ http://antwrp.gsfc.nasa.gov/apod/ap030227.html ] where Saturn's multiple moons [ http://csep10.phys.utk.edu/astr161/lect/saturn/moons.html ] pass near each other in numerous combinations. Like a miniature Solar System [ http://www.nineplanets.org/overview.html ], the innermost moons orbit [ http://csep10.phys.utk.edu/astr161/lect/history/newtonkepler.html ] Saturn the fastest. The above movie [ http://photojournal.jpl.nasa.gov/catalog/PIA07804 ] was centered on Saturn's moon Rhea [ http://antwrp.gsfc.nasa.gov/apod/ap060530.html ], so that the moons Mimas [ http://antwrp.gsfc.nasa.gov/apod/ap050308.html ] and Enceladus [ http://antwrp.gsfc.nasa.gov/apod/ap060310.html ] appear to glide by. At 1,500 kilometers across, Rhea [ http://en.wikipedia.org/wiki/Rhea_%28moon%29 ] is over three times larger than the comparably sized Mimas [ http://en.wikipedia.org/wiki/Mimas_%28moon%29 ] and Enceladus [ http://en.wikipedia.org/wiki/Enceladus_%28moon%29 ]. The Sun illuminates the scene from the lower right, giving all of the moons the same crescent phase [ http://antwrp.gsfc.nasa.gov/apod/ap051113.html ]. The above time lapse movie [ http://photojournal.jpl.nasa.gov/catalog/PIA07804 ] was created by the Saturn-orbiting robotic Cassini spacecraft [ http://saturn.jpl.nasa.gov/spacecraft/index.cfm ] over a period of about 40 minutes. |
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