By Jove, It's Hydrogen

After a perilous 21-month journey across more than half a billion miles of space, a tiny, unmanned spacecraft named Pioneer 10 passed within 81,000 miles of the giant planet Jupiter last December. That first close-up peek at the sun's largest satellite lasted barely a day.

But before the little ship was boosted along by Jovian gravity on a flight path that will eventually carry it out of the solar system, it gathered more information about Jupiter than had all astronomers since Galileo first pointed his crude telescope at the planet more than three centuries ago. Now, after a lengthy study of Pioneer's wealth of data--including 80 photographs--scientists have put together a totally new image of the king of planets.

As it sped toward its historic rendezvous, the 570-lb. spacecraft was battered by a blizzard of charged particles so intense that scientists at NASA'S Ames Research Center near San Francisco feared that all of its eleven instruments would be destroyed. Recalls Physicist J.

Walker Fillius, whose detector picked up electrons at a rate of 13 million per sec.--more than enough to penetrate several inches of steel: "We were biting our nails to see if the spacecraft would survive--and it did, just by a hair."

Whirling Giant. Those frightening moments came as the ship plunged through Jupiter's unexpectedly strong magnetic field and highly lethal radiation belt. Spinning far more rapidly than the earth (one revolution every 9 hr. 55 min.), the planet whips its lines of magnetic force deep into space --sometimes as far as 6.5 million miles.

As a result, they form a huge disk-shaped magnetic field; the electrons and protons that swirl within it create powerful electrical currents that may discharge lightning bolts all the way to Io, the innermost of Jupiter's nine moons.

Packing more mass than all of the other planets combined, the whirling giant bulges at the middle like the earth, only far more so. Pioneer found that Jupiter's equatorial diameter (88,298 miles) is nearly 6,000 miles greater than the spread between its poles. The data returned by the spacecraft also support the long-held theory that Jupiter is unique among planets: a great ball of whirling gases and liquids with no solid surface. Its outermost 600 miles consist of an atmosphere of hydrogen and helium gases laced with clouds composed of crystals of ammonia, ammonia hydrosulfide and water ice. The rest of the planet is mostly a seething cauldron of liquid hydrogen, except perhaps for a small, rocky, possibly iron-bearing core. Scientists suspect that Jupiter's extreme interior heat--about 54,000DEG F. at the core--may be left over from the planet's creation nearly 5 billion years ago.

Carried outward from the planet's interior by eddies and currents like those in a boiling kettle, Jupiter's heat helps shape its most prominent features. Pioneer's photographs showed that the great gray-white stripes circling the planet seem to be hot, rising clouds and gases that have been drawn into bands by Jupiter's rapid rotation. The darker orange-brown belts that run parallel to the light bands are probably troughs of cooler, descending gases. Despite the planet's tranquil appearance from afar, it hardly seems hospitable to life. Its atmosphere is apparently ravaged, not only by great bolts of lightning but also by winds with velocities of more than 300 m.p.h. In fact, Jupiter's great red spot, long a puzzle to astronomers, could be the vortex of a violent Jovian storm.

This file is automatically generated by a robot program, so viewer discretion is required.