An artist's concept of Galileo. Photo: NASA/JPL

NASA's Galileo spacecraft completed its fifth year of orbiting Jupiter yesterday, continuing to send home new information after enduring more than twice the time in orbit and three times the radiation dosage that it was originally planned to withstand.

It is heading back toward Jupiter after the most elongated of its 28 loops around the planet since entering orbit on Dec. 7, 1995. As it moves closer to Jupiter, Galileo is making a 14-week continuous study of the planet's magnetosphere, a vast bubble of magnetic force that surrounds the planet and contains its dangerous radiation belts.

The study began in October while Galileo was still outside the magnetosphere. The information being collected as Galileo re- enters it will be paired with measurements taken as the craft exited the magnetosphere last spring. The study is also part of collaborative research with NASA's Cassini spacecraft, which is flying past Jupiter this month for a gravity assist to reach Saturn. From a position outside of Jupiter's magnetosphere, Cassini is monitoring the solar wind of particles streaming away from the Sun.

"The data we're collecting right now are part of a collaboration with Cassini to understand how the magnetosphere of Jupiter responds to changes in the solar wind," said Dr. Duane Bindschadler, Galileo manager of science operations at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Initial science results from Galileo's May 20 flyby of Jupiter's largest moon, Ganymede, will be presented at the fall meeting of the American Geophysical Union, beginning Dec. 15, in San Francisco. On Dec. 28, Galileo will fly by Ganymede once again, this time while the moon is in eclipse behind Jupiter. Galileo's flight team is preparing for more studies of Jupiter's volcanic moon Io in 2001 and other possible encounters in a further extension of the mission.

Several instruments and subsystems have suffered some damage during the mission, but Galileo is still able to collect valuable scientific information. "Galileo is showing some signs of battle fatigue, but it is still a capable spacecraft," said Jim Erickson, project manager. Most of the degradation resulted from cumulative radiation damage, particularly during Io flybys in 1999 and 2000, he said.

JPL, a division of the California Institute of Technology in Pasadena, manages Galileo for NASA's Office of Space Science, Washington, D.C.