Craft Flies 16 Miles From Moon Of Saturn

The Cassini probe, which took this image, flew close enough to nab particles.
The Cassini probe, which took this image, flew close enough to nab particles. (Nasa Via Associated Press)
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By Marc Kaufman
Washington Post Staff Writer
Friday, October 10, 2008

The international Cassini space probe flew within 16 miles of the surface of Saturn's moon Enceladus yesterday -- a breathtakingly close flyby designed to gather dust and water particles that will help scientists better understand the recently discovered geysers that spew constantly from the moon's south pole.

"Cassini flew closest to the equator of Enceladus to collect those particles and then went into the plume coming out of the south pole at a much greater height," said project scientist Robert Pappalardo. The main goal of the mission, he said, is to determine if the dust and ice particles drifting above the moon's equator are the same or different from those that spit out of the geysers.

"This is how we hope to learn more about the history and evolution of Enceladus, and about whether there's liquid water involved in the generation of the plume," he said.

On an earlier Cassini flyby, scientists discovered that the plume contained ice particles, gases and some carbon-based organic (though not ever living) material.

The flyby was the closest ever of any moon of Saturn and led to great excitement among NASA scientists, now that the tiny moon is known to contain large amounts of water. Although conditions are frigid on Enceladus, scientists believe that liquid water, or even an ocean, might exist below the icy surface, possibly creating conditions that could support life.

Pappalardo said the team would not have dared to fly the spacecraft so close to the moon earlier in its four-year exploration of Saturn and its surroundings, but the scientific and engineering team is now very familiar with the orbit of Enceladus and with its gravitational force. Because the moon is so small -- its diameter is roughly the width of the state of Arizona -- it could not pull Cassini into a collision with the surface, as a larger moon or planet would do.

"Because we know the dynamics of Cassini and Enceladus so well, we can fly very close to the equator and pick up those dust particles that probably wouldn't be found higher up," he said. Results from the mission are expected by mid-November.

Cassini will return to Enceladus on Halloween night for more picture-taking from about 120 miles above the surface. The current mission will take some photos, but the pass is not designed to produce the kind of dramatic images taken when the geysers were discovered in 2005. The next flyby will be designed to use the spacecraft's cameras and other optical remote sensing instruments and will focus on taking images of the fractures that slash across the moon's south polar region like stripes on a tiger.

Pappalardo said scientists are only beginning to understand the dynamics of the geysers. Some researchers theorize that they consist of liquid or frozen water escaping from the warmer center of the moon, while others believe that they are the result of heat generated by large fragments of the surface rubbing back and forth along several large, visible fractures.

The plume on Enceladus is often described as the only one in the solar system, but Pappalardo said others may exist on Neptune's moon Triton and on Jupiter's moon Europa. He said those plumes would be much smaller in relation to their moons, which are much larger than Enceladus.

The Cassini mission is a joint NASA, European Space Agency and Italian Space Agency project. The spacecraft was launched in 1997 and reached Saturn four years ago.

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