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

NASA's Galileo spacecraft has left the powerful influence of Jupiter's magnetosphere, marking the first time since early 1996 that Galileo has been outside Jupiter's magnetic area. The spacecraft has now entered the solar wind, which is a stream of particles emitted continually from the Sun that flows at roughly 400 kilometers per second (about 1 million miles per hour.)

This transition from the magnetosphere to the solar wind could be thought of as marking the beginning of the joint data gathering by the Galileo and Cassini spacecraft. Galileo will return close to Jupiter in October of this year, and Cassini is preparing to swing by Jupiter in December 2000 to slingshot toward Saturn. While both spacecraft are in Jupiter's neighborhood, their measurements will be compared to gain new understanding about how the solar wind changes as it flows outward near Jupiter's orbit. Later this year, the simultaneous, joint observations of the two spacecraft will allow investigators to discover more about how the solar wind influences Jupiter's magnetic field and the charged particles trapped within it.

"One of the elements of study will be to try to determine the influence of the solar wind on Jupiter's magnetosphere," said Galileo Project Scientist Dr. Torrence Johnson. "We know from previous missions that the magnetosphere is affected by the solar wind -- expanding and contracting depending on solar wind conditions -- but this will be new territory, an opportunity to find out exactly what the solar wind is doing to the magnetosphere."

"We have now passed through the boundary of Jupiter's magnetosphere and look forward to studying its properties," said Dr. Margaret Kivelson, principal investigator for Galileo's magnetometer instrument at the Institute of Geophysics and Planetary Physics, University of California, Los Angeles. "We know that the boundary has two components -- the shock, which is like the sound barrier, and the magnetopause, where the direct influence of Jupiter's magnetic field ends. In the last week, Galileo passed though this boundary."

An artist's concept of NASA's Cassini probe flying past Jupiter. Photo: NASA/JPL

In preparation for its observations of Jupiter, Cassini performed a flight path adjustment last week, on June 14. It is now on the path to fly by the huge planet and will be closest to Galileo at the end of December. This maneuver will also allow it to pass by Saturn's outermost moon, Phoebe, at a distance of 2,000 kilometers (about 1,250 miles). Cassini will leave the solar wind and enter Jupiter's magnetosphere at the end of the year. Until then, Cassini will be analyzing cosmic dust and continuing to make fields and particles measurements.

Galileo has been orbiting Jupiter and its moons since December 7, 1995, and successfully completed its two-year primary mission on December 16, 1997. That was followed by a two-year extended mission which concluded in December 1999, and Galileo is now continuing its studies under yet another extension, called the Galileo Millennium Mission. 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. Cassini is a joint mission of NASA, the European Space Agency and Italian Space Agency, and is managed by JPL for NASA's Office of Space Science, Washington, D.C.