# Galileo Crosses Boundary Into Jupiter's Environment

NASA's Galileo spacecraft radioed confirmation late this week that it has entered Jupiter's environment, crossing over the boundary from interplanetary space into the giant magnetic cocoon around Jupiter called the magnetosphere.

"With the spacecraft now in the magnetosphere, we begin our first direct measurements of the Jupiter system," said Galileo Project Manager William J. O'Neil at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.

Data from Galileo's magnetometer confirmed that the spacecraft passed the milestone on November 26 at a distance of about six million miles (nine million kilometers) from Jupiter's cloud tops, scientists announced today.

After a six-year voyage through the Solar System, Galileo is less than a week away from taking up permanent residence around Jupiter. On Thursday, December 7, Galileo's previously deployed atmospheric probe will plunge into Jupiter's cloud tops at 5:56 p.m. EST and descend into the giant planet on a parachute.

Overhead, the Galileo spacecraft itself will collect and record data radioed from the probe during the 40- to 75-minute probe mission. At 8:19 p.m. EST, an hour after the probe mission is completed, Galileo will begin to fire its onboard rocket to slow down and allow itself to be captured into orbit around Jupiter to begin a two-year mission of closeup studies of Jupiter's large moons, the planet itself, and continuous measurements of the magnetosphere.

Jupiter's magnetosphere is like a giant bubble around the planet. A shock wave -- called "bowshock" after the wave that builds before the bow of a ship -- exists where the magnetosphere faces the stream of charged particles flowing outward from the Sun, called the solar wind. As the solar wind flows around Jupiter, the magnetosphere tapers off like a wind sock, with the whole invisible structure moving in response to buffeting by the solar wind.

Galileo scientists said they first saw signs of the bowshock on November 16, but the bowshock apparently moved back and forth in response to alternate gusts and waning of the solar wind. "As the solar wind velocity increased, the shock moved inside the position of the spacecraft leaving Galileo again in the solar wind," said Dr. Margaret Galland Kivelson of the University of California at Los Angeles, the principal investigator on Galileo's magnetometer experiment.

This crossing and recrossing of the shockwave happened several times, she said, between the first shock encounter on November 16, when the spacecraft was about nine million miles (15 million kilometers) from Jupiter, and November 26 when Galileo finally crossed the main bowshock at 1 p.m. EST at about six million miles out from Jupiter's cloud tops.

The magnetometer science team also found the first direct evidence that the jovian magnetosphere was either unaffected or had recovered in the aftermath of last year's impact of Comet Shoemaker-Levy with Jupiter. Some scientists had theorized that the magnetosphere might have been modified signficantly by the violent impact, but that appears not to be the case, according to data from Galileo.

Meanwhile, Galileo engineers report that work has been completed on the spacecraft's tape recorder to assure its readiness for recording data during Thursday's atmospheric probe descent. Final fine-tuning of the spacecraft's flight path is scheduled for this Saturday, December 2.

Two Internet home pages exist to provide information on the atmospheric probe, Galileo orbiter spacecraft, mission operations and science returns. The Galileo Project home page may be accessed at http://www.jpl.nasa.gov/galileo/. A home page sponsored by the atmospheric probe team at NASA Ames Research Center, Mountain View, CA, may be accessed at http://ccf.arc.nasa.gov/galileo_probe/.

    Douglas Isbell
    Headquarters, Washington, DC        December 1, 1995
    (Phone:  202/358-1753)

    Franklin O'Donnell
    Jet Propulsion Laboratory, Pasadena, CA
    (Phone:  818/354-5011)

    RELEASE:  95-215

Space History Galileo

##### Author: Calvin J. Hamilton