During previous flybys of Jupiter's moons, a recurring electrical glitch caused the spacecraft computer to reset and enter "safing" mode, shutting down all non-essential functions until ground controllers could restore normal operations.

The Galileo team pooled engineering and problem-solving talents to develop special software, known as a "bus reset patch." The name refers to Galileo's "data bus," which transfers information to various parts of the spacecraft. The software was designed to boost Galileo's I.Q. by teaching it to recognize symptoms of the glitch and correct the problem itself, without entering safing mode.

The spacecraft was put to the test twice on Monday, May 3, when the glitch popped up as Galileo was approaching Callisto. Galileo quickly diagnosed the problem, determined there was no threat to spacecraft health, and decided for itself not to enter safing mode. This allowed all spacecraft and scientific functions to continue uninterrupted, with Galileo snapping pictures and gathering observations from an altitude as close as 1,322 kilometers (821 miles) above Callisto.

"Galileo proved it's an 'A' student," said Project Manager Jim Erickson. "This is an example of our efforts at JPL to make the spacecraft more independent and better able to evaluate and deal with problems without intervention from ground controllers."

"We're so thrilled that our efforts paid off and Galileo performed as we had hoped," said Nagin Cox, who helps oversee the team of current and former Galileo personnel that developed the new software patch.

In an unrelated occurrence during this morning's Callisto flyby, the pointing control for the scan platform, which aims Galileo toward observation targets, switched on its own from a very accurate gyro-controlled system to a less accurate backup mode that uses the star scanner without gyros.

Nonetheless, the spacecraft continued to record its observations, but observations taken by an instrument called the near infrared mapping spectrometer may be less sharp than planned. Preliminary analysis shows this anomaly occurred just after Galileo's closest approach to Callisto, or 7:56 a.m. Pacific Daylight Time, the time the signal was received on Earth. The Galileo team is investigating this anomaly, and preliminary analysis shows it may be related to previous gyro anomalies.

Galileo has been orbiting Jupiter and its moons since December 1995. Its original, two-year mission ended in December 1997, and the spacecraft is currently more than halfway through a two-year extended tour, called Galileo Europa Mission.