Spacecraft Double-Team The King Of Planets

The joint studies of the solar system's largest planet by the Galileo and Cassini spacecraft will also resemble the passing of a baton from the durable veteran to the promising rookie, say mission controllers at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Galileo has been running laps around Jupiter since December 1995, continuing to produce scientific discoveries after surviving more than double the orbital time and triple the radiation exposure originally intended. It will pass close to Jupiter's largest moon, Ganymede, on Dec. 29.

Cassini left Earth on Oct. 15, 1997, bound for Saturn with a dozen scientific instruments to carry into orbit there and a European-made probe, Huygens, to drop onto Saturn's biggest moon in 2004. Cassini will make its closest approach to Jupiter on Dec. 30. It will still be nearly 10 million kilometers (six million miles) away, well outside the orbits of Jupiter's four large moons -- Io, Europa, Callisto and Ganymede -- but within the orbits of nine small ones.

Cassini began transmitting Jupiter pictures and data this month.

"We have a chance to make observations with a well-instrumented spacecraft that has more capabilities than any spacecraft that has previously visited Jupiter," said Robert Mitchell, JPL's Cassini program manager. "Fortunately, Galileo is still operating there, so we can get a synergistic effect in studies of Jupiter by having spacecraft at two different locations in the vicinity of Jupiter at the same time. That's not something we could have counted on in 1995."

One joint study will examine how the "solar wind" of charged particles speeding away from the Sun buffets Jupiter's magnetosphere, the bubble of charged gas rotating around Jupiter under the control of the planet's magnetic field. In November, Cassini will be in the solar wind upstream of where the wind hits the magnetosphere, while Galileo will be inside the magnetosphere. Cassini will monitor fluctuations in the solar wind while Galileo watches the response of Jupiter's magnetosphere to those fluctuations.

During the past five years, Galileo has measured frequent changes in the density of particles in the magnetosphere, but researchers have not had the opportunity to connect the effects to specific changes in the solar wind, said Dr. Torrence Johnson, Galileo project scientist at JPL.

JPL physicist Dr. Scott Bolton, on science teams for both Cassini and Galileo, said, "Having two spacecraft there at once is possibly the only chance in our lifetime to simultaneously connect changes in the solar wind to conditions inside Jupiter's giant magnetosphere."

Getting a better grasp on how Jupiter's magnetosphere acts and reacts will advance understanding of the smaller magnetosphere surrounding Earth and larger ones affecting areas of the galaxy where stars are being born, Bolton said. Disturbances in Earth's magnetosphere can disrupt electrical and communications systems.

Another study taking advantage of dual vantagepoints will focus on a stream of dust, finer than particles in cigarette smoke, originating from volcanoes on Jupiter's moon Io. Patterns in the stream as it passes first one satellite, then another, could give information about the dust's movement. Researchers also hope to identify its composition, which would be a sampling of material from Io.

Both spacecraft will study eclipses of Jupiter's large moons. While the moons are in the shadow of Jupiter, glows can be seen that are overwhelmed by reflected sunlight at other times. Excitation of the moons' thin atmospheres by energetic particles in Jupiter's magnetosphere causes the glows. Researchers hope to learn more about gases on the moons by studying these glows.

Cassini will study Jupiter's atmosphere from October through March as the craft approaches from the sunny side, then recedes from the dark side of the planet. "If we're lucky, we may even see a storm arise, and see how it starts and how it evolves," said Dr. Dennis Matson, Cassini project scientist at JPL. The Jupiter studies will also provide a dress rehearsal, checking out equipment and procedures for Cassini's main mission at Saturn, Matson said.

JPL manages the Cassini and Galileo missions for NASA's Office of Space Science, Washington DC. JPL is a division of the California Institute of Technology, Pasadena. Cassini is a cooperative endeavor of NASA, the European Space Agency and the Italian Space Agency.

More information on the joint spacecraft study of Jupiter is available at http://www.jpl.nasa.gov/jupiterflyby . An expanded press kit on the study is available at http://www.jpl.nasa.gov/presskits/jupiterflyby .