"Galileo has already survived more than twice the radiation it was designed to withstand."

If intense radiation doesnt cause any glitches, as has been the case with previous encounters, Galileo will image Io and gather other scientific data during the flyby.

"Galileo has already survived more than twice the radiation it was designed to withstand, so were keeping our fingers crossed that it will complete this encounter with flying colors," said Jim Erickson, the Galileo project manager at NASAs Jet Propulsion Laboratory.

Galileo flew by Io in October and November of 1999. During Tuesdays pass by the tortured moon, one of Jupiters 16 natural satellites, scientists hope to detect changes since the fall.

"Ios volcanoes are so active that the moons surface is always changing, and with each flyby we get new and different observations," said Torrence Johnson, the Galileo project scientist.

Because of the gravitational tug of nearby Jupiter, the whole of Io experiences incredible amounts of tidal distortion. The flex think of a piece of iron wire bent back and forth repeatedly heats the moon, making it the most volcanically active body in the solar system.

Tuesdays flyby of Io, which is about the size of our own moon, is part of Galileos second mission extension since arriving at Jupiter in 1995. Called the Galileo Millennium Mission, the $1.4 billion spacecraft will spend the year continuing to probe Jupiters moons, including flybys of Ganymede on May 30 and December 28.

Plans also call for Galileo and the Saturn-bound Cassini probe to make joint observations of Jupiter when the latter spacecraft flies by for a gravity assist in December.

After that, Galileo will have exhausted all but about 20 pounds (9 kilograms) of its propellant, severely curtailing its ability to carry on much longer.