By Jupiter! -- Galileo Probe To Begin First Exploration Inside Planet's Atmosphere

Space. Six years after it was launched, Galileo is finally reaching its destination - Jupiter. Even with its checkered history, scientists expect the space probe to help them unravel some mysteries about this strange planet. -----------------------------------------------------------------

The Galileo spacecraft mission to Jupiter has been dogged with bad luck and patched with long-distance scientific ingenuity.

The payoff for persistence is to come Thursday, when Galileo climaxes its six-year, 2.3 billion-mile journey. That's when a probe it released in July enters Jupiter's atmosphere at 50 times the speed of a bullet. At the same time, the mother craft begins a two-year elliptical orbit of the gas planet.

While a stuck antenna means Galileo will return only 1,500 photographs to Earth instead of the 50,000 originally intended, planetary scientist Torrance Johnson of NASA's Jet Propulsion Laboratory estimates the $1.46 billion mission will complete at least 50 percent of its original objectives.

Included will be the first exploration inside Jupiter's atmosphere, close-up examination of four of its moons and detailed examination of its powerful magnetic field, its radiation, and dust.

Because smaller surviving antennas take so long to transmit, initial data from the atmospheric probe may not be available until Dec. 19. Pictures from the main craft may not appear for months.

For scientists who have been working on planning and building Galileo for 20 years, however, a bit more time hardly matters. As University of Idaho electrical engineer David Atkinson, who started working on Galileo's probe in 1981, notes, "Of all the principal investigators on the probe science team, I'm the youngest." He is 40.

First scheduled for launch in 1982, Galileo was not sent on its way until 1989 after problems with the space shuttle and booster rockets. Still lacking a powerful booster, the spacecraft had to be sent on a long, looping six-year-long course instead of flying directly to Jupiter. The roundabout course used the gravity of the inner planets to fling the spacecraft that far.

Astronomers have persisted because the gas planet is a wonderfully weird place to visit. As big as 1,400 Earths, it is only one-quarter as dense because it is made primarily of the hydrogen (89 percent) and helium (10 percent), the gases used to lift airships and blimps.

Galileo's nose-cone-shaped probe will hit the atmosphere at 106,000 mph and then in two minutes be braked by friction and its 8-foot parachute to just 100 mph. The friction will generate 28,000-degree Fahrenheit heat and the probe will be wrenched with forces 230 times that of Earth's gravity.

If it survives this welcome, the 30-inch-high probe will transmit atmospheric data for up to 75 minutes and descend 100 miles into Jupiter's turbulent skies, to where temperatures reach 400 degrees F.

Western Washington University's H. William Wilson, director of scientific technical services, helped test and calibrate the probe instrument to measure the planet's heat.

Then it will succumb to heat and pressure, having penetrated only the skin of the onion. At that point it will be approximately 44,000 miles to Jupiter's center.

Scientists believe the planet has no surface we can easily visualize, but rather its atmosphere gradually condenses with depth into a hot liquid, a churning sea of hydrogen penetrated by a constant rain of heavier helium. Huge lightning bolts flash in its upper atmosphere, which may also contain water rain clouds.

The hydrogen ocean eventually becomes dense enough to act like a stewing metal, providing the electrical conductivity to generate the most powerful magnetic field of the planets. There may also be a hard core the size of several Earths at Jupiter's center.

Jupiter is in essence a star that never became big enough to ignite, but it still radiates more heat from its interior than it receives from the sun, possibly from the energy of its own gravity.

Even though it doesn't receive enough solar energy to generate even Earth-size storms, Jupiter's upper atmosphere is banded by howling winds of up to 250 mph. Its Great Red Spot, centuries old, is a hurricane as big as three Earths.

"All these gas planets have enormous winds and no one knows why," said the University of Idaho's Atkinson. He is leading a team that will deduce the speed of winds below Jupiter's cloud tops by calculating how far the parachuting probe is diverted as it drops into the thickening atmospheric soup.

Containing 70 percent of the total mass of all the planets in the solar system, Jupiter is like a miniature solar system itself, surrounded by 16 moons. The planet's atmosphere is also believed to be a time capsule, the best surviving example of the nebula of gas and dust from which the solar system originally formed.

In July 1994, Jupiter was also the site of the biggest comet collision ever recorded, when Comet Shoemaker-Levy slammed into its atmosphere. Galileo took pictures of the resulting atmospheric explosions.

That bit of serendipity was typical of the mixture of good and ill fortune that has accompanied Galileo. As the Grateful Dead sang, what a long, strange trip it's been.

Technologically, Galileo is old stuff by now, its on-board computer no more powerful than the first IBM PCs and its tape recorder using old reel-to-reel technology. It was supposed to be launched by the space shuttle in January 1982.

Shuttle and booster problems pushed that back to 1986, and then the space shuttle Challenger blew up. NASA subsequently concluded that the liquid-hydrogen rocket the shuttle was to have carried into space to power Galileo was too dangerous, and the replacement solid-fuel rocket was too small to push it directly to Jupiter.

Meanwhile, Galileo - trucked from California to Cape Canaveral, Fla., before the Challenger explosion - was trucked back to California while space missions were put on hold and then trucked again to Florida for its 1989 launch from the space shuttle Atlantis. Scientists believe all those road miles jostled Galileo's graphite lubricant. On April 11, 1991, when they tried to deploy the spacecraft's main antenna, it stuck.

The deployment motor was turned on and off more than 13,000 times and the spacecraft was sent tumbling in hopes it would jar loose, but nothing worked. Instead of being able to transmit about a picture a minute at 134,400 bits per second, Galileo is limited to a smaller antenna transmitting at best 1,000 bits per second, even after ingenious software enhancements beamed from Earth to the spacecraft.

More bad news showed up on Oct. 11 of this year, when the tape recorder malfunctioned. Scientists partly got around that problem by avoiding a suspected weak spot in the tape, but as a result Galileo will not take pictures of the moons Io and Europa, instead reserving the tape recorder to store data beamed from the probe dropping into Jupiter's atmosphere.

The Cassini mission to Saturn and its moon Titan, scheduled to launch in 1997, is being designed to avoid these problems. It has an improved antenna design, a solid-state tape recorder, a far-superior computer and will be launched from a simpler Titan 4 instead of the space shuttle.

Even with the problems, Galileo's leisurely six-year transit to Jupiter has had some benefits.

On Oct. 29, 1991, it became the first spacecraft to make a close pass of an asteroid, in this case the 12-mile-long Gaspra.

Galileo flew by the Moon in 1992, better mapping its North Pole, confirming the existence of an ancient impact basin on the Moon's far side and supplying evidence that our satellite had been more volcanically active than previously thought.

On Aug. 23, 1993, Galileo flew by the potato-shaped, 34-mile-long asteroid Ida and discovered it had a tiny, one-mile-diameter moon (named Dactyl) orbiting about 60 miles from the asteroid's center.

In 1994 Galileo was in a position to photograph the comet Shoemaker-Levy - something it would not have done if it had been on its earlier schedule - and in 1995 it detected an interplanetary dust storm, possibly particles thrown out by a volcano on Jupiter's moon Io.

For its two-year orbit around Jupiter, the spacecraft is to come as close as 125 miles to the moons Europa, Callisto and Ganymede. It will also photograph the main planet. The pictures will have 20 to 1,000 times the resolution of those taken by Voyager in 1977 because Galileo will go up to 300 times closer than the earlier craft.

Lost with the antenna are some planned atmospheric and magnetic field studies. Still, what is expected to be learned is not bad for a 5,000-pound machine about as big as a car, powered internally by a tiny nuclear reactor generating less electricity than needed to run a toaster.

Earth receivers will detect radio signals from the spacecraft that are about a billion times fainter than the sound of a transistor radio in New York as heard from Seattle.

Galileo Galilei, whose telescope spotted the first four Jupiter moons in 1610 after building a crude telescope, would no doubt be impressed. ----------------------------------------------------------------- Galileo's orbital tour

Galileo will make 11 different elliptical orbits around Jupiter during the next two years. It will encounter these moons on the following dates:

1995

Dec 7: Jupiter arrival, Io flyby. ------------------------------------ 1996

July 4: Ganymede.

Sept. 6: GAnymede.

Nov. 4: Callisto.

Dec. 19: Europa. ------------------------------------ 1997

Feb. 20: Europa.

April 5: Ganymede.

May 7: Ganymede.

June 25: Callisto.

Sept. 17: Callisto.

Nov. 26: Europa.

Dec. 7: mission ends. --------------------------------------