Flying Rings Around Saturn

By Frederic Golden

After a spectacular mission, a mysterious little glitch

It was a bit late--some 3.1 seconds behind schedule--and 41 miles off target.

Still, after four years of traveling across 1.24 billion miles of space, there was no faulting Voyager 2's marksmanship. Indeed, one golf-minded scientist likened it to sinking a 500-mile putt. Superlatives were certainly in order last week as the semiautonomous robot completed the second lap of its epic flight: a rendezvous with the giant ringed planet Saturn, the spectacular finale to two ambitious decades of planetary exploration by unmanned U.S. spacecraft.

There was, however, a mysterious glitch. While Voyager 2 was hidden behind Saturn it developed the space-age equivalent of a crick in the neck, reducing the mobility of its cameras. But the robot had already accomplished most of its goals, and the trouble will almost certainly not prevent use of the cameras on the rest of Voyager 2's flight--past Uranus in 1986 and Neptune in 1989.

Racing toward Saturn at 54,000 m.p.h.--20 times as fast as a speeding bullet--the 1,800-lb. spacecraft came within a cosmic hair of the planet's stormy cloud tops, clearing them by 63,000 miles. Then it plunged downward behind the huge gaseous sphere and passed through a large gap near the edge of the thin disc of icy debris that forms Saturn's multi-hued rings. Finally, like a pebble in a great celestial slingshot, it was sent hurtling off toward Uranus on a new course created by the powerful pull of Saturn's gravity. NASA's new boss, James Beggs, hailed the flight as "one of the really great scientific achievements of our age." But he refused to commit the Reagan Administration to any new space endeavors, not even a mission to intercept Halley's comet on its reappearance in 1986 for the first time in 76 years.

Until the sudden failure, Voyager 2's performance was almost flawless. Picking up where its twin, Voyager 1, left off last November, it provided new insights into Saturn's turbulent weather. Banded with powerful jet streams, like those of neighboring Jupiter (which Voyager 2 surveyed in 1979), the planet has even greater winds--up to 1,100 m.p.h. just north of the equator. In Saturn's higher latitudes, Voyager 2's cameras spotted a storm system larger than all of Europe and Asia, as well as numerous smaller storms, some whirling clockwise, others counterclockwise.

Zeroing in on Saturn's moons, Voyager 2 discovered surprising contrasts between those icy little worlds. Hyperion, for example, is a small, distant moon shaped like a battered Idaho potato (or, as NASA scientists variously had it, a "hamburger patty" or a mouse-gnawed "hockey puck"). Tethys, closer in, is scarred by a huge crater more than a third as wide as the satellite's own 670-mile diameter, and by a canyon that extends at least two-thirds of the way around it. Apparently the moon was struck by a huge object and badly cracked, but the pieces froze together again like chunks of reformed ice cubes. And why is the strange moon lapetus ten times darker on one side than the other? Scientists found indirect signs that the inky black stuff may be carbon.

The most intriguing moon is Enceladus, which seems to be caught in a kind of gravitational tug-of-war. Pulled by nearby moons, its interior heats up, causing surface fissures and creating glacier-like ice flows that obliterate craters made earlier in its turbulent history.

For earthbound viewers, and even for scientists at the Jet Propulsion Laboratory outside Pasadena, which masterminds Voyager's progress, the show-stopper was, once again, Saturn's rings. Better illuminated than they were during Voyager 1's flyby, they showed up in greater detail and in far greater number: literally by the thousands. Before Voyager 2's visit, scientists had a ready hypothesis to explain why the icy materials of the rings--fragments ranging in size from dust specks to boulders--follow only certain orbits and not others, leaving gaps of hundreds of miles. The gaps, the theory went, were opened by small moons, less than 20 miles across, that plowed through the debris, clearing out groovelike paths. Alas, after a close look, Voyager 2 found no trace of these moon sweepers.

One alternate theory holds that the rings and the gaps between them are in effect peaks and troughs in Saturn's own gravitational field, like the concentric ripples created by a rock dropped into the surface of a still pond. Another line of speculation attributes the formation of at least some of the rings to a periodic gravitational attraction, or "resonance," between the ring particles and Saturn's passing moons.

Voyager came closest to Saturn just before the 1 1/2-hour period when the spacecraft was behind the planet, totally cut off from the earth. But the semi-intelligent machine continued operations under the command of its preprogrammed computers, taking pictures, performing experiments and storing the information on tape. Not until Voyager 2 emerged from behind Saturn and again began radioing back data did scientists learn that something had gone wrong. As Voyager 2 crossed the rings, the playback showed, the cameras began missing their targets. Somehow the spacecraft's movable "scan platform," which acts as an aiming mechanism for the narrow-and wide-angle cameras as well as several other optical instruments, had slowed, then stuck. The platform could swing up and down but not sideways, leaving Voyager 2's cameras looking out into the dark void of space.

Had a stray ring particle smashed the small gearbox that controls the platform's horizontal movements? Or was there a failure in the electronics or perhaps in the gears themselves? No one could say, even after hours of patient long-distance troubleshooting (it takes nearly an hour and a half to send a radio command to the far-off spacecraft). By the next day JPL controllers had found they could at least get the frozen platform to swing through a few degrees of arc, though not smoothly or precisely enough to aim the instruments properly. By moving it through ever larger arcs, they hoped that they might eventually work it free. Such tactics helped overcome a similar problem on Voyager 1, caused by a stray bit of plastic that got into its gears.

At week's end, it appeared that they may well have succeeded. With a swivel that jubilant controllers described as "right on the money," the platform brought fading Saturn back onto their TV monitors again. This week they hope to aim the cameras at Phoebe, the planet's outermost moon. Even if the problem recurs, though, it should not spoil the photographic reconnaissance of Uranus or Neptune. The controllers can simply "pan" the cameras by rolling the entire spacecraft with blasts from its small thrusters.

--By Frederic Golden. Reported by Jim Schefter/Pasadena

With reporting by Jim Schefter/Pasadena

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