Press Release

Spacecraft Double-Team the King of Planets

By SpaceRef Editor
October 24, 2000
Filed under

Don Savage

Headquarters, Washington, DC

(Phone: 202/358-1547)

Guy Webster

Jet Propulsion Laboratory, Pasadena, CA

(Phone: 818/354-6278)

RELEASE: 00-168

Two NASA spacecraft are teaming up to scrutinize Jupiter
during the next few months to gain a better understanding of the
planet’s stormy atmosphere, diverse moons, faint rings and vast
bubble of electrically charged gas.

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, CA.

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 six million miles (10 million kilometers)
away, well outside the orbits of Jupiter’s four large moons — Io,
Europa, Callisto and Ganymede — but within the orbits of nine
small ones.

The spacecraft began transmitting Jupiter pictures and data this

“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

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

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

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


More information on the joint spacecraft study of Jupiter is
available at:

SpaceRef staff editor.