The Little Spacecraft That Could

By Leon Jaroff.

Largely overshadowed by the tragic loss of Challenger, the feats of the indomitable Voyager 2 last week provided the only bright notes during the U.S. space program's darkest hours. As the 1,800-lb. spacecraft sped away from its close encounter with Uranus, it continued its flawless performance, transmitting data and pictures that are gradually stripping away some of the mysteries of the planet. At NASA's Jet Propulsion Laboratory in Pasadena, Calif., nearly 2 billion miles away, William McLaughlin, the Voyager flight- engineering manager, could speak only in superlatives as he reviewed the data. Said he: "I think it is the most successful space mission of all time."

Indeed, having brilliantly explored Jupiter in 1979 and Saturn in 1981, Voyager had already compiled an enviable record. Now the spacecraft was on the verge of duplicating its earlier, spectacular accomplishments. At week's end it had already discovered ten tiny Uranian moons and sent back incredibly detailed photographs of the five larger, previously known satellites. It had photographed the nine known rings and found at least two more. The versatile spacecraft also managed to pry a bewildering volume of information from Uranus itself, despite the fact that the giant planet is shrouded by a thick and opaque blue-green atmosphere.

Scientists at J.P.L. seemed most fascinated by Voyager's close-up views of the five major Uranian moons. By far the most exotic was Miranda, about 300 miles across and the closest of the large moons to the planet. Miranda, Geologist Laurence Soderblom explained, "is a bizarre hybrid," combining at least ten different types of terrain, some similar to the "valleys and layered deposits of Mars . . . the grooved terrain of Ganymede (a moon of Jupiter) and the depression faults of Mercury." The crusts of Miranda and three of the four other major moons, Soderblom said, "have been tectonically shuffled in a cataclysmic fashion," probably by the powerful tug of gravity from Uranus, which has a diameter four times as large as the earth's.

Both Titania and Oberon, each some 1,000 miles in diameter, have huge, distinctive features. Voyager spotted a three-mile-high mountain on Oberon and a valley running all the way across the visible surface of Titania. On the moon Ariel, 730 miles across, three linear patterns seemed to resemble the tracks left by terrestrial glaciers. Only Umbriel, 740 miles in diameter and covered with overlapping meteorite craters but with few other features, seems to have been largely unaffected by Uranian gravity--for reasons scientists cannot explain.

As Voyager swung behind Uranus, it bounced radio waves off the rings and discovered that they are quite different from those of Saturn, which contain an abundance of fine particles. The Uranian rings are made largely of dark "boulders," most of them more than a yard wide, that circle the planet once every eight hours. Many scientists believe they may be the remnants of a large moon that shattered in an ancient cataclysm.

In fact, some astronomers have long suspected that it was a catastrophic event, perhaps a collision with an earth-size object, that toppled Uranus on its side (see chart); it spins with its rotational axis practically perpendicular to those of most of the other planets. The spacecraft raised even more questions about Uranus when it discovered that the planet has a magnetic field about as strong as earth's but topsy-turvy by terrestrial standards, with the north magnetic pole displaced by 55 degrees from the south geographic pole. The odd arrangement led scientists to speculate that Voyager had caught the magnetic field in the process of reversing its polarity, a phenomenon that has occurred often on earth, most recently about 700,000 years ago.

The magnetic field also helped scientists calculate the length of a Uranian day. By detecting the changing radio emissions caused by the interaction of the field with the solar wind as the planet turns on its axis, the spacecraft established that Uranus rotates once approximately every 17 hours. The technique, explained Physicist James Warwick, can be likened to standing on a lawn and "feeling the water drops every time a sprinkler goes around." By tracking clouds in the atmosphere, Voyager discovered high-altitude winds moving around the planet at 220 m.p.h., more than twice as fast as they travel above the earth.

Other conclusions from Voyager's findings, according to Project Scientist Edward Stone: Uranus has a core consisting of rock and liquid, is covered by a deep ocean of water laced with dissolved ammonia, and is wrapped in a 5,000- mile-thick atmosphere consisting largely of hydrogen, with 10% to 16% helium and a scattering of methane and other gases.

At week's end, as it looked back and saw Uranus receding in the distance, Voyager seemed to be in perfect health. Scientists at J.P.L. are confident that it will stay that way not only through its encounter with Neptune in August 1989 but perhaps until 2010, when it will be far out of the solar system. Says Richard Laeser, the Voyager project manager: "I have no desire to do much else except to ride this thing all the way out into interstellar space."

With reporting by Jon D. Hull/ Pasadena