Galileo makes its flyby of Ganymede while in eclipse by Jupiter as soon in animation. Photo: NASA TV/Spaceflight Now

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.

An artist's concept of NASA's Galileo spacecraft in orbit around Jupiter. Photo: NASA/JPL

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.