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,

Washington, DC.

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

accomplishments include:

* 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,

Washington, DC.

More information about Galileo is available on the Internet


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