Artist's concept of Galileo passing near Jupiter's small inner moon Amalthea.
Artist's concept of Galileo passing near Jupiter's small inner moon Amalthea. Image credit: Michael Carroll

While approaching Jupiter's moon Io on Thursday, during the seventh year of its mission around Jupiter, NASA's Galileo spacecraft placed itself into standby mode, awaiting further commands from Earth.

"We're not totally surprised, because Galileo has already outlived expectations and we knew that it might encounter additional difficulties from the high-radiation environment on this flyby," said Dr. Eilene Theilig, Galileo project manager at JPL. "Galileo has already lasted more than four years past its original mission and has survived three-and-a-half times the radiation it was designed to withstand, so it's not unexpected that this flyby would be interrupted by a problem."

Images and other data were not collected during the closest phase of the encounter. The Galileo flight team at NASA's Jet Propulsion Laboratory, Pasadena, Calif., is sending commands aimed at switching the spacecraft out of standby or "safing" mode for the later portion of the planned encounter period, which lasts into Sunday.

Galileo hit its intended flyby point, achieving one of the encounter's primary goals of using Io's gravity to put the spacecraft on course for a September 2003 impact into Jupiter. This flyby is the closest and last for Galileo at any of Jupiter's four major moons. The spacecraft sped within 102 kilometers (63 miles) of Io's volcanic surface.

At about 13:41 Universal Time (5:41 a.m. Pacific time) today, the spacecraft detected a computer reset, which caused Galileo to enter a so-called "safe" mode. In this mode, onboard fault protection software instructs the spacecraft cameras and science instruments to stop taking data and places them in a safe state awaiting further instructions from the ground. The situation is similar to some that occurred in previous orbits and appears to result from the radiation environment near Jupiter.

Engineers remain hopeful that they'll be able to restore normal spacecraft functioning by transmitting new commands to Galileo to restore data collection, Theilig said.

The path of today's encounter was chosen to use Io's gravity to put Galileo on course to send it plunging into the crushing pressure of Jupiter's atmosphere in September 2003. Galileo is running low on the propellant needed to steer the spacecraft and keep its antenna pointed toward Earth. The intentional collision course with Jupiter was chosen as a way to end the mission before losing control of the spacecraft.

Additional information about the Galileo mission is available at .

Galileo was launched from the Space Shuttle Atlantis on Oct. 18, 1989. After a long journey to Jupiter, Galileo began orbiting the huge planet on Dec. 7, 1995, and successfully completed its two-year primary mission in 1997. That has been followed by three mission extensions. JPL, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.

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