An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

This week, Galileo nears the end of a 14-week-long collaboration with the Cassini spacecraft to study the influence of the solar wind on the Jovian magnetosphere. Galileo is in week 13 of its contribution to this study, which was initiated in late October, 2000. Galileo performs one engineering activity this week. On Wednesday, the spacecraft executes standard maintenance on its onboard tape recorder. Galileo's tape recorder is a reel-to-reel mechanical type and these regular maintenance activities are performed to keep the tape recorder in good operating shape.

Galileo's contribution to the study of the solar wind's influence on the Jovian magnetosphere comes in the form of a low-resolution survey being performed by the Fields and Particles instruments. This suite of six instruments is comprised of the Dust Detector, Energetic Particle Detector, Heavy Ion Counter, Magnetometer, Plasma Detector, and Plasma Wave instrument.

During the past 13 weeks, Galileo has flown from the solar wind, into the Jovian magnetosphere, and now back out into the solar wind. Cassini, on the other hand, was generally expected to remain outside the magnetosphere until after its flyby of Jupiter, when it would fly along the boundary region of the magnetosphere and solar wind as it left the vicinity of Jupiter.

Measurements now indicate that Cassini entered the bow shock region on December 27, three days prior to its closest approach to Jupiter. Cassini has since crossed into and out of the magnetosphere multiple times as the solar wind "blew" stronger and then weaker. The solar wind is a stream of charged particles and gas that emanate from the Sun. Because the solar wind waxes and wanes with changes in the Sun, it exerts varying amounts of pressure on the Jovian magnetosphere. The magnetosphere responds by shrinking when the solar wind pressure is high, and ballooning outward when it is low.

Currently, Galileo is in the solar wind, while Cassini is moving through the magnetotail, or vast "downstream" portion to the magnetosphere that extends as far outward as Saturn. Thus, Galileo will measure variations in the solar wind while Cassini observes the response of the downstream parts of Jupiter's magnetosphere.

This series of joint observations is expected to provide new insights into how the solar wind affects the magnetospheres of Jupiter and other planets, including Earth.