The varied effects of Ionian volcanism can be seen in this false color infrared composite image of Io's trailing hemisphere. Low resolution color data from Galileo's first orbit (June, 1996) have been combined with a higher resolution clear filter picture taken on the third orbit (November, 1996) of the spacecraft around Jupiter. Photo: NASA/JPL

NASA's Galileo spacecraft is trying to go "three for three" as it attempts its third and closest flyby of Jupiter's fiery moon Io, the most volcanic body in the solar system.

The spacecraft will dip to only 200 kilometers (124 miles) above Io's surface -- roughly the distance between Los Angeles and San Diego -- at 1432 GMT (9:32 a.m. EST) on Tuesday, February 22. Galileo gathered a wealth of pictures and other scientific information during its flybys of Io in October and November of 1999.

"Io's volcanoes are so active that the moon's surface is always changing, and with each flyby we get new and different observations," said Dr. Torrence Johnson, Galileo project scientist at NASA's Jet Propulsion Laboratory. "This time we expect to be able to observe the effects of the eruptions we saw in the October and November Io flybys."

The Io flybys represent a classic case of "no pain, no gain," since Io orbits close to Jupiter in a region bombarded by radiation from the huge planet's radiation belts. That radiation can disrupt spacecraft systems or even knock out instruments, but mission planners believe potential gains in scientific knowledge outweigh the risks of the Io flybys. Nonetheless, the encounters were planned near the end of Galileo's extended missions, when the spacecraft has already returned volumes of pictures and information from Jupiter and its moons.

"Although we gathered some great images and data during the previous Io flybys, the radiation did cause some problems, and we won't be surprised if that happens again this time," said Galileo Project Manager Jim Erickson of JPL. "Galileo has already survived more than twice the radiation it was designed to withstand, so we're keeping our fingers crossed that it will complete this encounter with flying colors."

Galileo engineers often say that the spacecraft has "lived well past its warranty." Galileo entered orbit around Jupiter in December 1995. It was originally assigned to spend two years studying Jupiter, its moons and its magnetic environment. When that original mission ended in December 1997, it was followed by a two-year extended mission, which ended in January 2000. This Io flyby is part of an additional extension, called the Galileo Millennium Mission.