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Big Moon Near Jupiter Will be in the Dark When Spacecraft Visits

When NASA's Galileo spacecraft zips past Jupiter's moon Ganymede on Dec. 28, Ganymede will have slid into the shadow of Jupiter, giving scientists an excellent chance to examine faint but informative glows that would be overwhelmed by sunlight at other times.

"By timing the encounter to happen while Ganymede is in eclipse, we're putting Galileo in the right place at the right time to see auroras," said Dr. Eilene Theilig, deputy project manager for Galileo at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

In its 29th orbit of Jupiter since arrival five years ago, the durable spacecraft is on course to pass about 2,300 kilometers (about 1,430 miles) above the surface of the darkened moon 25 minutes after midnight, PST (3:25 a.m., EST). Galileo last visited Ganymede in May, when it passed within about 800 kilometers (about 500 miles) of the surface, collecting information that scientists announced this month they see as evidence for a liquid ocean hidden under Ganymede's surface. The Dec. 28 flyby will be a special opportunity to study what's above the surface.

With direct sunlight blocked by Jupiter, scientists expect to see shimmering auroras on Ganymede, comparable to Earth's Northern Lights. "The auroral glows we plan to observe occur because Ganymede has a very tenuous atmosphere of gases," said Dr. Torrence Johnson, Galileo project scientist at JPL. "When these gases are hit by electrons from Jupiter's radiation belts, they glow. It's similar to what goes on in a fluorescent light bulb when you turn on the electricity."

Studying Ganymede's aurora could provide information about the chemical makeup of gases in Ganymede's atmosphere and also about Ganymede's magnetic field. Ganymede is not only the largest moon in our solar system, it is the only one known to have its own internally generated magnetic field. Paths of electrons approaching Ganymede from Jupiter's radiation belts are determined by lines of magnetic force, so the location of the glows triggered by those electrons reveals something about the shape of the magnetic field around the moon, Johnson said.

Galileo's trajectory for the Ganymede flyby will give the orbiter an exposure to Jupiter's intense radiation belts, said Jim Erickson, project manager for Galileo at JPL. With extensions to its original two-year mission, Galileo has survived three times the cumulative radiation dose it was designed to tolerate. Some of its 12 scientific instruments have been impaired by the radiation to varying degrees, but the spacecraft is still returning valuable scientific information. The effects of additional exposure next week cannot be predicted with certainty, Erickson said.

More information about the Galileo mission is available at http://galileo.jpl.nasa.gov .

Galileo is collaborating with NASA's Cassini spacecraft on several studies of Jupiter and its surroundings this fall and winter, while Cassini passes Jupiter for a gravity boost toward its 2004 appointment with Saturn.

JPL, a division of the California Institute of Technology in Pasadena, manages Galileo and Cassini for NASA's Office of Space Science, Washington, D.C. Cassini is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.

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