|
||||
|
|
||||
|
|
||||
|
Chaos seen in movement of ring-herding moons of Saturn NASA/JPL NEWS RELEASE Posted: October 12, 2002 Scientists have a new explanation for weird movements of two small moons that shepherd one of Saturn's rings: Pandora, which keeps the narrow F ring from spreading outward, and Prometheus, which rides herd along the same ring's inner edge.
"Chaotic gravitational interactions between them can fully account for these discrepancies," said Dr. Nicole Rappaport of NASA's Jet Propulsion Laboratory, Pasadena, Calif. She and Prof. Peter Goldreich of the California Institute of Technology, Pasadena, reported the new explanation this week at the annual meeting of the American Astronomical Society's Division of Planetary Sciences in Birmingham, Ala. With chaotic interactions, a barely perceptible difference in starting conditions can make such a great difference in later positions that the movements are not fully predictable over time. The two moons give each other a gravitational kick each time Pandora passes inside Prometheus, about every 28 days. Because neither's orbit is quite circular, the distance between them on those occasions -- hence the strength of the kick -- varies. The perturbations lead to changes in motion that are not periodic or predictable, say Rappaport and Goldreich. They had predicted 20 years ago that Pandora's motion might be chaotic. "This is like a dream come true," Goldreich said of the observations that fit the prediction. For him, it's a recurring dream, since he and Dr. Scott Tremaine, then a post-doctoral fellow at Caltech, also predicted the very existence of shepherd moons -- confirmed later by Voyager -- as an explanation for the narrowness of the F ring. By the principle of action and reaction, the same transfer of momentum by which Pandora pushes F ring particles inward toward Saturn and Prometheus pushes them outward should also gradually push the two moons away from the ring. The effects of Saturn's wider A ring, which is just inside the F ring, are even more important in pushing the moons outward. The F ring would slowly widen. So, although the discovery of the shepherd moons fulfilled a scientific prediction explaining the current narrowness of the F ring, it led to more questions about the longer-term history and fate of the ring and shepherds. One theory among several has been that the ring must be quite young to still be so narrow, perhaps about 10 million-years-old. But in proportion to the age of Saturn itself, that would be as if the ring were created in the last three minutes of a 24-hour day, and some scientists believe it unlikely that we would happen to witness such a rare event. Another theory, proposed by Rappaport, Goldreich and Tremaine in 1982, is that Pandora's movement includes an element of chaos. The story took a further twist in the mid-1990s, while Saturn's rings were edge-on toward Earth so the timing was good for observing Prometheus and Pandora with NASA's Hubble Space Telescope. Analysis of those and subsequent observations by several researchers in New York and Massachusetts revealed that each moon was about 20 degrees from where normal orbital mechanics would have put it. This was an amazing discovery. For the first time ever, astronomers found that they could not predict the orbits of objects in the sky. Goldreich and Rappaport have demonstrated that chaotic orbits of the F ring shepherds could produce changes in location very similar to those observed with the Hubble Space Telescope. "The chaos is due to the gravitational interactions between the two moons," Rappaport said. "This is the first observation ever of chaotic orbital motions in the solar system." A larger moon of Saturn, Hyperion, is already known to have chaotic rotation around its axis. Goldreich and Rappaport hope they have found the piece of the puzzle that will help resolve the problem of the age of the rings. JPL, NASA's lead center for robotic exploration of the
solar system, is managed for NASA by the California
Institute of Technology, Pasadena.
|
Hubble Calendar
NEW! This remarkable calendar features stunning images of planets, stars, gaseous nebulae, and galaxies captured by NASA's orbiting Hubble Space Telescope . Apollo 12 tribute DVD set ![]() New! Featuring the jovial crew of Pete Conrad, Dick Gordon and Alan Bean, the Apollo 12 mission was struck by lightning shortly after liftoff but proceeded on the second successful exploration voyage to the lunar surface. This three-disc DVD brings the mission to life with extraordinary detail. U.S. - U.K. - E.U. - Worldwide Fallen Heroes special patch This special 12-inch embroidered patch commemorates the U.S. astronauts who made the ultimate sacrifice, honoring the crews of Apollo 1, Challenger and Columbia.U.S. - U.K. - E.U. - Worldwide Women in Space Women of Space: Cool Careers on the Final Frontier is for girls, young women, and anyone else interested in learning about exciting careers in space exploration. Includes CD-ROM.U.S. - U.K. - E.U. - Worldwide Mars rover poster This new poster features some of the best pictures from NASA's amazing Mars Exploration Rovers Spirit and Opportunity.U.S. Apollo 15 DVDs Bring a unique piece of space history to your living room. Two- and six-disc Apollo 15 DVDs will be shipping soon.Hubble Astronomy Now presents Hubble: the space telescope's view of the cosmos. A collection of the best images from the world’s premier space observatory. Gemini 12: The NASA Mission Reports covers the voyage of James Lovell and Buzz Aldrin that capped the Gemini program's efforts to prove the technologies and techniques that would be needed for the Apollo Moon landings. Includes CD-ROM.U.S. - U.K. - E.U. - Worldwide Soviet Space For the first time ever available in the West. Rocket & Space Corporation Energia: a complete pictorial history of the Soviet/Russian Space Program from 1946 to the present day all in full color. Available from our store.U.S. - U.K. - E.U. - Worldwide Viking patch This embroidered mission patch celebrates NASA's Viking Project which reached the Red Planet in 1976.U.S. - U.K. - E.U. - Worldwide Apollo 7 DVD For 11 days the crew of Apollo 7 fought colds while they put the Apollo spacecraft through a workout, establishing confidence in the machine what would lead directly to the bold decision to send Apollo 8 to the moon just 2 months later.U.S. - U.K. - E.U. - Worldwide Gemini 12 Gemini 12: The NASA Mission Reports covers the voyage of James Lovell and Buzz Aldrin that capped the Gemini program's efforts to prove the technologies and techniques that would be needed for the Apollo Moon landings. Includes CD-ROM.U.S. - U.K. - E.U. - Worldwide Get e-mail updates Sign up for our NewsAlert service and have the latest news in astronomy and space e-mailed direct to your desktop (privacy note: your e-mail address will not be used for any other purpose). Expedition 20 The official embroidered patch for the International Space Station Expedition 20 crew is now available from our stores.Current Shuttle Mission Patch The official embroidered patch for shuttle Atlantis' flight to deliver critical spare equipment to the space station.![]() Ares Patch The Ares Project will develop two new rockets to launch astronauts back to the Moon under NASA's Vision for Exploration. The Ares 1 will employ a single space shuttle solid rocket booster to loft the Orion crew capsule. The gigantic Ares 5 will haul the equipment and cargo needed for such lunar voyages. This is the Ares emblem.One Giant Leap
Hosted by Corbin Bernsen, this award winning documentary marks the 50th anniversary of the U.S. space agency and features exclusive interviews with veteran astronauts.Expedition 21 The official embroidered patch for the International Space Station Expedition 21 crew is now available from our stores. |
||||
|
INDEX | PLUS | NEWS ARCHIVE | LAUNCH SCHEDULE ASTRONOMY NOW | STORE ADVERTISE © 2009 Spaceflight Now Inc. |
|||||