Contact: Jane Platt

June 30, 1999

Galileo Spacecraft Midway Through Successful Flyby

NASA's Galileo spacecraft is now midway through a successful flyby of Jupiter's pockmarked moon Callisto, with flight engineers reporting that all systems are operating normally. The spacecraft has performed flawlessly so far, swooping over Callisto this morning at 12:47 a.m. Pacific Daylight Time at an altitude of 1,048 kilometers (651 miles). Galileo's camera and other instruments have taken pictures and gathered all the science information planned up to this point. That data is stored on the onboard tape recorder and will be played back over the coming weeks.

Galileo's gyroscopes have performed well during this encounter. It appears a software patch and other modifications have corrected problems with the gyros that occurred during some previous encounters. After the close approach to Callisto early this morning, the gyros were turned off. They'll remain in the off position until after Galileo passes its closest point to Jupiter, which exposes the craft to very high radiation and can damage the electronics of the gyroscopes. That closest point, called perijove, will take place tomorrow, July 1 at 10:26 p.m. PDT.

During this period, Galileo is being operated by its star scanner, which is less vulnerable to radiation than the gyros. The gyros will be switched on again in preparation for a calibration test on Saturday, July 3.

This is the second of four consecutive Callisto flybys, with the next scheduled for August 12. Valuable science information is being gathered during the Callisto flybys, but the primary purpose is to lower Galileo's orbit to prepare it for two close flybys of Jupiter's fiery moon Io this coming autumn. Next Thursday, July 8, engineers will perform a flight path correction to make sure Galileo is positioned properly as it approaches Io's orbit.

During this series of Callisto flybys, Galileo's instruments will monitor Io to gather as much information as possible. The science community is eagerly anticipating the Io encounters, which will provide the closest-ever look at the most volcanic body in the solar system. Because of the intense radiation in the area of Io's orbit, there is some risk to the health of the spacecraft.

The spacecraft has been orbiting Jupiter and its moons for three-and-a-half years. The spacecraft is more than halfway through a two-year extended Galileo Europa Mission, a follow-on to the primary mission that ended in December 1997. JPL manages the mission for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology, Pasadena, CA.