GALILEO GETS ONE LAST FREQUENT-FLYER UPGRADE
Donald Savage, Guy Webster
The resilient Galileo spacecraft doesn’t know when it
call it quits. So, NASA has outlined the details of one last
mission extension, which includes five more flybys of the
Jovian moons before a final plunge into the crushing pressure
of the giant planet’s atmosphere.
Galileo has been orbiting Jupiter for more than five years and
survived radiation exposure more than three times what it was
built to withstand. Galileo’s mission has previously been
extended twice and during that time it has returned an
enormous wealth of scientific information, including evidence
of a sub-surface ocean on Jupiter’s moon Europa.
“We’re proud that this workhorse of a spacecraft has kept
performing well enough that we can ask it to keep serving
science a little longer,” commented Dr. Jay Bergstralh, Acting
Director of Solar System Exploration at NASA Headquarters,
On May 25, Galileo should pass about 123 kilometers (76 miles)
above the moon Callisto, the second largest of Jupiter’s 28
known moons. The effects of Callisto’s gravity will set up the
space probe for a swing over both polar regions of the
intensely volcanic moon Io in August and October.
Galileo will take pictures, measure magnetic forces, and study
dust and smaller particles. Science goals include studying the
extent of volcanism on Io, both in new and previously active
sites; determining whether Io generates its own weak magnetic
field; and gaining a better understanding of a doughnut-shaped
ring, the Io Torus, that encircles Jupiter and contains
electrically charged gases.
In 2002, having completed its imaging mission, Galileo will
continue studies of Jupiter’s massive magnetic field with
seven instruments. In January, the orbiter will fly near the
equator of Io.
In November, it will swing closer to Jupiter than ever before,
dipping within about 500 kilometers (about 300 miles) of the
moon Amalthea, which is less than one-tenth the size of Io and
less than half as far from Jupiter. Scientists will use
Galileo measurements to determine the mass and density of
Amalthea. They will also study dust particles as Galileo flies
through Jupiter’s gossamer rings and seek new details of the
magnetic forces and the densities of charged particles close
to the planet.
Galileo’s final orbit will take an elongated loop away from
Jupiter. Then in August 2003, the spacecraft will head back
for a direct impact and burn up as it plows into Jupiter’s
60,000 kilometer-thick atmosphere. This final act was approved
by the National Research Council of the National Academy of
Sciences last December.
“Galileo has already succeeded beyond expectations, and we
have the opportunity to learn still more in coming months, but
it is sad to see the end of the road up ahead,” said Eilene
Theilig, Galileo project manager at NASA’s Jet Propulsion
Laboratory, Pasadena, CA. “Exposure from Jupiter’s intense
radiation belts has impaired some of Galileo’s instruments,
but it is still producing valuable scientific results.”
The science program for the Galileo mission extension was
recommended to NASA by a blue-ribbon panel of planetary
scientists, who met last July, and will cost $9 million. “This
mission extension accomplishes the highest priorities of the
review panel in a cost effective way,” said Paul Hertz,
Galileo Program Executive at NASA Headquarters.
Galileo was launched Oct. 18, 1989, aboard NASA’s Space
Shuttle Atlantis. On Dec. 7, 1995, a probe released earlier
from Galileo made measurements while dropping through
Jupiter’s upper atmosphere. Galileo’s top scientific
* Produced strong evidence that Europa has a melted saltwater
ocean under the ice layer on its surface. The spacecraft has
also found indications that Ganymede and Callisto have layers
of liquid saltwater, too.
* Detailed the varied and extensive volcanic processes on Io,
catching plumes erupting, fire fountains in process and lava
flows expanding, among other observations.
* Delivered a probe that made the first measurements of
Jupiter’s atmosphere from within the atmosphere.
* Made the first close approach to an asteroid and made the
first discovery of a satellite orbiting an asteroid.
* Discovered the first internal magnetic field of a moon.
Ganymede’s intrinsic magnetic field actually creates a “mini-
magnetosphere” embedded within Jupiter’s vast magnetosphere.
* Provided the only direct observation of Comet Shoemaker-
Levy’s impact into Jupiter.
JPL, a division of the California Institute of Technology in
Pasadena, manages Galileo for NASA’s Office of Space Science,
More information about Galileo is available on the Internet
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