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

NASA's long-lived Galileo spacecraft achieved partial success in a dash through Jupiter's inner radiation belts and past the small moon Amalthea on Tuesday, its final flyby before a deliberate impact into Jupiter next September.

This was Galileo's last flyby, after 37 other close encounters with various planets, asteroids and Jupiter's four large moons since launch 13 years ago.

As the orbiter headed closer to Jupiter than it had ever ventured before, it gathered measurements of the energy fields and charged particles in the inner region of Jupiter's magnetic environment. It also examined dust grains that form a "gossamer" ring around the planet.

However, Galileo placed itself in a standby precautionary mode after its closest approach to Amalthea. It flew past Amalthea at a targeted altitude of 160 kilometers (99 miles) at 06:19 Tuesday, Universal Time (10:19 p.m. Nov. 4, Pacific time), then went into "safe" mode about 30 minutes later. In that mode, onboard fault-protection software suspends many operations until receiving further instructions from the ground.

"We knew this would be a challenging encounter for Galileo, so we are not surprised to have some things go awry," said Dr. Eilene Theilig, Galileo project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "Not only was the spacecraft going through a region with more intense radiation than it had ever experienced before, but it was also doing this after it had already survived four times the cumulative radiation dosage it was designed to take and had already operated nearly five years past its original mission."

The flight team at JPL is working on recovery operations, diagnosing what happened and preparing new commands to Galileo that will restore the orbiter to normal and enable the playback of scientific data stored on the spacecraft's tape recorder. Two tape tracks of science data were recorded during the encounter period, out of four tracks planned. Also, the intended type of two-way radio link with Earth for the period closest to the flyby was not achieved.

Now receding again from Jupiter, the spacecraft left the region of radiation danger about 11 hours after passing Amalthea.

Amalthea orbits about halfway between Jupiter and Io, the innermost of the planet's four large moons. Amalthea is an elongated body, about 270 kilometers (168 miles) long.

Galileo has nearly depleted its supply of the propellant needed for pointing its antenna toward Earth and controlling its flight path. While still controllable, it has been put on a course for impact into Jupiter. That's so there will be no risk of it drifting to an unwanted impact with the moon Europa, where Galileo discovered evidence of a subsurface ocean that is of interest as a possible habitat for extraterrestrial life.

Sixty-four minutes after speeding over Amalthea's cratered surface, Galileo passed within about 71,400 kilometers (44,366 miles) of Jupiter's cloud tops. That marked the beginning of Galileo's final orbit, which will end with a plunge into the crushing pressure of Jupiter's atmosphere on Sept. 21, 2003.

Going into a standby mode does not jeopardize Galileo's disposal at Jupiter. No further commanding of the spacecraft is necessary to assure that it is on an impact trajectory, Theilig said.

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.