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Hitchhiking Probe Jumps Off at Jupiter

NASA scientists will get their first direct measurements of giant planet's atmosphere

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A YEAR ago, the world watched comet fragments pummel Jupiter. Now, a space-science team is ready to sock the planet with an artificial ''comet'' sent from Earth.

It's a heavily instrumented atmospheric probe that's been hitching a ride on the Galileo Jupiter-explorer spacecraft. As Galileo nears its Dec. 7 rendezvous with the giant planet, the probe is taking off on a complementary mission of its own. It too will reach Jupiter Dec. 7.

Once there, it will plunge into the planet's outer atmosphere in a fiery entry that the National Aeronautics and Space Administration (NASA) has compared to ''flying through a nuclear explosion.'' This will slow the probe's speed from the 115,000 miles per hour of space travel to a mere 100 m.p.h. Then the probe will shed its heat shield, pop open a parachute, and descend gracefully through the mainly hydrogen and helium atmosphere. The data stream it will be sending to Galileo for relay to Earth will give scientists their first direct measurements ever in the atmosphere of one of the giant outer planets. At this writing, Galileo's control team at the Jet Propulsion Laboratory in Pasadena, Calif., was ready to separate the 339-kilogram (747-pound) probe package from the 2-ton main spacecraft. Separation was timed for 1:30 a.m. Eastern time today. This begins the final phase of Galileo's six-year odyssey through the solar system.

Extraterrestrial tango

Ahead lies a 22-month dance around Jupiter and among its four major moons that Galileo project manager William O'Neil has promised will produce ''absolutely stunning'' views of this planetary system. That dance will bring the spacecraft's imaging equipment a hundred to a thousand times closer to the major moons than was possible during the previous Pioneer and Voyager flybys.

Galileo scientists at the California Institute of Technology's Jet Propulsion Laboratory (JPL), which manages the program for NASA, and their university colleagues have waited some 18 years for this adventure to begin. Conceived in the late 1970s, Galileo went through several redesigns - including some prompted by the Challenger accident. The space shuttle Atlantis finally sent the spacecraft on its way in October 1989.


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