Galileo has dark encounter with Jovian moon Ganymede
Posted: December 28, 2000
When Galileo makes its nearest pass at 0825 GMT (3:25 a.m. EST), Ganymede will have slid into the shadow of Jupiter, giving scientists an excellent chance to examine the 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 in December 1995, the durable spacecraft is on course to pass about 2,300 kilometers (about 1,430 miles) above the surface of the darkened moon.
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. Thursday's flyby is a special opportunity to study what's above the surface.
NASA says studying Ganymede's aurora could provide information about the chemical makeup of gases in the moon's atmosphere and also about its 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.
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.
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.
NASA animation shows the Galileo space probe making a close flyby of Jupiter's moon Ganymede. The moon is in eclipse by the giant gas planet during Galileo's encounter.
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With the sun behind us, this animation clips shows Ganymede slipping into the eclipse, which makes its auroral glow visible as Galileo passes closest to the moon.
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