A Crescendo of Discovery

By Jamie Murphy.

In the feeble light of the distant sun, Voyager 2 hurtled toward Uranus last week, rapidly accelerating under the gravitational pull of the huge gaseous body. Aboard the spacecraft, two television cameras and an array of instruments focused on the ever enlarging sphere and its rings and moons, snapping pictures and taking readings that were beamed to earth, almost 2 billion miles away. At week's end, as Voyager whipped past the mysterious blue-green planet, soaring as close as 50,679 miles to its cloudtops at 42,143 m.p.h., streams of new data from the craft poured into the control room at the Jet Propulsion Laboratory in Pasadena, Calif. "We're quite excited," said J.P.L. Project Scientist Edward Stone. "It's the crescendo of discovery."

Stone was not exaggerating. Within a few days, Voyager had taught scientists more about Uranus than they had learned in the entire 205 years since it was discovered by Amateur Astronomer William Herschel. The spacecraft detected a tenth ring and ten tiny, previously undiscovered moons and discerned craters and other surface features on the five large moons that until now had been seen only as featureless spots of light through telescopes. It observed a reddish-brown haze at the planet's north pole and tracked cloud formations as they passed over the middle latitudes. The movement of the clouds seemed to confirm earlier estimates that a Uranian day is about 16 hours long. The spacecraft also determined that Uranus has a magnetic field, and measured it.

"What we have seen thus far has been spectacular," said Ellis Miner, Voyager's deputy project scientist. "What has remained unseen to this point is going to turn out even better." For as it swung past Uranus, Voyager took thousands of pictures and gathered reams of scientific data, accumulating information faster than its systems could process and transmit it toward the earth. The unsent information, stored on magnetic tape, was to be gradually beamed to J.P.L. over the next several days. In these transmissions, scientists expected to find, among other things, images of more tiny moons.

Even as new Voyager transmissions were being received at J.P.L., scientists were feverishly interpreting the early results. Computer-enhanced photographs of the moons Titania and Oberon, each about 1,000 miles in diameter, showed that Oberon had a three-mile-high peak and that the surfaces of both moons were pocked with white spots from which streaks of bright material radiated. To Bradford Smith, head of the Voyager imaging team, the spots were evidence that meteorite impacts had pulverized the gray surface, exposed an underlying layer of ice and spewed it out in all directions. The haze covering the Uranian north polar area,* he suggested, may be smog--not unlike the Los Angeles variety--resulting from a photochemical reaction caused by sunlight acting upon gases in the planet's atmosphere.

Physicist Norman Ness reported that Voyager's magnetometers had "detected firm evidence" of a Uranian magnetic field about a third the strength of earth's. The existence of the field suggests that the giant planet, which is 64 times as large as the earth in volume but has a mass only 14 1/2 times as great, has a liquid core. Some scientists speculate, however, that the magnetic field may be generated by an electrically charged ocean covering the planet. Some of the larger moons apparently have, or at one time had, crustal movements that created the fault zones and valleys evident in the Voyager photographs. Geologist Laurence Soderblom, for one, was surprised at what he called "the degree of geological activity on the Uranian satellites." Along some of the faults on Titania, he said, "some sort of material is leaking out of fractures and perhaps freezing on the surface."

The fact that any pictures and information reached J.P.L. from Uranus is remarkable in itself. Signals from Voyager's transmitter, traveling at the speed of light, required 2 hours and 45 minutes to traverse the 1.84 billion miles between the spacecraft and the earth, and were incredibly faint when they arrived. To enhance the transmissions, nasa picked them up with antennas at all three of its Deep Space Network complexes in California, Australia and Spain, and combined them electronically. Still, the combined signals were so weak that NASA engineers had to slow down the transmission rate so that information could be distinguished from normal radio background noise. As a result, it took Voyager at least four minutes to transmit a single picture. Then too, the images picked up by the spacecraft's cameras were extraordinarily dim; the sunlight reaching Uranus is only about 1/400th as intense as it is on earth. But computer

enhancement of the pictures at J.P.L. brought out detail that would otherwise have been invisible in the screening of the transmitted images.

Perhaps most remarkable of all was the continuing performance of Voyager 2. Launched in August 1977, it was originally designed to operate for five years and assigned to encounter only Jupiter (in 1979) and Saturn (in 1981), missions that it carried out with great success, sending back data and thousands of spectacular photographs of both planets. But even before Voyager sped by Saturn, NASA decided to take further advantage of a onceevery-175- years alignment of the outer planets by using Saturn's gravitational pull to hurl Voyager on to Uranus, then to use Uranian gravity to speed the craft to an enounter with Neptune. Now, after 8 1/2 years aloft, Voyager has far outlived its design specifications, nursed by its controllers through only two minor crises, one caused by a faulty radio receiver, the other by a balky scanning platform. Said Mission Controller Bruce Brymer: "The craft is beautiful. No matter what we throw at it, it keeps on going. You've got to love it."

As it passed Uranus last week, Voyager looked back at the planet, now silhouetted against the distant sun, seeking to learn more about the rings by observing sunlight passing through them. One early finding: the rings contained far less dust than those circling Saturn. Then, its direction changed by the tug of Uranian gravity, the hardy little spacecraft began a 3 1/2-year trip to Neptune, which it is scheduled to encounter in August 1989.

NASA controllers had reason to be confident that Voyager would arrive at Neptune right on time. When it swooped by Uranus last week, it was only a minute ahead of the schedule that NASA engineers had set for it five years earlier.

FOOTNOTE: *Uranus lies on its side, its axis of rotation more or less perpendicular to those of the other eight planets. Its north pole now points at the sun and thus receives more solar radiation than other regions of the planet.

With reporting by Jon D. Hull/Los Angeles