Hitchhiking Probe Jumps Off at Jupiter

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

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.

This was hardly the mission plan the original program designers had in mind. The spacecraft no longer had the booster rocket power to head directly for Jupiter. It had to steal the needed momentum from Earth and Venus. Instead of heading outward to Jupiter, it headed inward to Venus. It swung around Venus and then came back to swing around Earth. This put Galileo into a long loop that brought it back past Earth a second time. That swingby at last set it on course for Jupiter in December 1992.

Space navigators call a course-adjusting planet swingby a gravity assist. It changes both the direction and speed of the spacecraft as the planet's gravity whips the craft around somewhat like a stone in a sling. Galileo will also use this technique as it wanders among the Jovian moons during the 10 scheduled orbits of the giant planet. The spacecraft will pass close by one or another of these moons on precisely the right course for that moon's gravity to hand it on to the next dance partner.

In the course of its wanderings so far, Galileo has sent back data on Venus, transmitted new views of the moon, and provided planetary scans of Earth. It also took the first close-up images of two asteroids - Gaspra (October 1991) and Ida (August 1993).

Pesky antenna

The craft gave the scientific team a major handicap when the main antenna refused to deploy. This has slowed the data transmission rate significantly. The team has learned how to overcome this handicap to a large extent, however, by compressing data into more compact form. Mr. O'Neil says, ''We're confident we're going to get at least 70 percent of the mission [objectives].'' He adds that, when the books are finally closed, the data transmission team may well ''do better than that.''

Galileo's next big test will be to fire its main retro-rocket engine for a course correction in about two-weeks time. Memory of how the Mars Observer craft was lost when it fired up its engine in August 1993 has made the Galileo team cautious. But O'Neil explains that the team has examined every conceivable contingency.

Reflecting on the many obstacles the program has already overcome, O'Neil adds, ''It's great to be where we are today.''