Cassini's Flyby of Phoebe Shows a Moon With a Battered Past

View of Phoebe. View of Phoebe.
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June 12, 2004

First images from the Cassini flyby of Phoebe reveal it to be a scarred, cratered outpost with a very old surface and a mysterious past, and a great deal of variation in surface brightness across its surface.

"What spectacular images," said Dr. Carolyn Porco, Cassini Imaging Team leader at the Space Science Institute in Boulder, Colo. "So sharp and clear and showing a great many geological features, large and small. It's obvious a lot of new insights into the origin of this strange body will come as a result of all this."

"What we are seeing is very neat. Phoebe is a heavily cratered body. We might be seeing one of the chunks from the formation of the solar system, 4.5 billion years ago. It's too soon to say," said Dr. Torrence Johnson, Cassini imaging team member at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "It's important to see the big picture from all of the other instruments to get the global view on this tiny moon."

Dr. Gerhard Neukum, an imaging team member from Freie University in Berlin, said, "It is very interesting and quite clear that a lot of craters smaller than a kilometer are visible. This means, besides the big-ones, lots of projectiles smaller than 100 meters (328 feet) have hit Phoebe." Whether these projectiles came from outside or within the Saturn system is debatable.

There is a suspicion that Phoebe, the largest of Saturn's outer moons, might be parent to the other, much smaller retrograde outer moons that orbit Saturn.

Dr. Joseph Burns, an imaging team member and professor at Cornell University, Ithaca, N.Y. said, "Looking at those big 50 kilometers (31 mile) craters, one has to wonder whether their impact ejecta might be the other tiny moons that orbit Saturn on paths much like Phoebe's."

All planned 11 instruments operated as expected and all data was acquired. Scientists plan to use the data to create global maps of the cratered moon, and to determine Phoebe’s composition, mass and density. It will take scientists several days to pour over the data to make more concrete conclusions.

Cassini came within approximately 2,068 kilometers (about 1,285 miles) of the dark moon on Friday, June 11. The spacecraft was pointing its instruments at the moon during the flyby. Several hours later it turned to point its antenna to Earth. The signal was received through the Deep Space Network antennas in Madrid, Spain and Goldstone, in California’s Mojave Desert, at 7:52 a.m. PDT today. Cassini was traveling at a relative speed of 20,900 kilometers per hour (13,000 miles per hour) relative to Saturn. It’s been 23 years since a spacecraft last visited Phoebe. The Voyager 2 flyby in 1981 was at a distance from 2.2 million kilometers, (about 1.4 million miles), 1,000 times farther away.

With the Phoebe accomplished, Cassini is on course for Saturn. A trajectory correction maneuver is scheduled for June 16. Cassini will conduct a critical 96-minute burn before going into orbit around Saturn on June 30 (July 1 Universal Time). During Cassini's planned four-year tour it will conduct 76 orbits around the Saturn system and execute 52 close encounters with seven of Saturn’s 31 known moons.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Office of Space Science, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Carolina Martinez (818) 354-9382
Jet Propulsion Laboratory, Pasadena, Calif.

Heidi Finn (720) 974-5859
Cassini Imaging Central Laboratory for Operations
Space Science Institute, Boulder, Colo.

2004-147



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