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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

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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