SCIENCE

A historic voyage to a distant planet

JOHN BARBER February 3 1986
SCIENCE

A historic voyage to a distant planet

JOHN BARBER February 3 1986

A historic voyage to a distant planet

SCIENCE

In the annals of exploration, few developments match the event that took place on Jan. 24 at precisely 12:59 p.m. EST. At that time, the U.S. spacecraft Voyager 2 reached its best point for viewing the dark-ringed planet Uranus, two billion miles out in space. It was an impromptu excursion for Voyager, which was never intended to visit any other planet after a spectacular rendezvous with Saturn in 1981. Then, scientists at the National Aeronautics and Space Administration’s Jet Propulsion Laboratory in Pasadena, Calif., used Saturn’s gravity to turn the spaceship toward the seventh planet orbiting the sun—one which was then seen as little more than a cloudy, bluish-green disc even from the most powerful Earth-based observatories. The scientists did their work well, predicting Voyager’s arrival 50,000 miles above Uranus’s dense, gaseous surface within one minute and 125 miles.

As it neared the planet, Voyager began transmitting photographs and data on radio waves that travelled at the speed of light—186,000 miles per second—but still took 2V2 hours to reach Earth. And during the six-hour period of Voyager’s closest encounter

with Uranus, those faint transmissions dramatically reduced the mystery cloaking one of the most remote members of the farflung solar system.

The discoveries began early in the week as Voyager detected the presence of nine previously unknown Uranian moons, bringing the total of the planet’s known moons to 14. And that finding also confirmed a theory of Scott Tremaine of the University of Toronto’s astronomy department. Tremaine and two colleagues had theorized that the nine thin rings around the planet would break up without the influence of at least 10 “shepherd” moons—so called because their gravity herds the jet-black debris orbiting Uranus into rings.

At week’s end, the reasons why the rings were so much darker than any others in the solar system remained unclear. But NASA scientists had been able to detect radio waves emanating from the planet. Previously, the apparent absence of radio waves led scientists to speculate that Uranus had no magnetic field. But the waves not only confirmed the presence of a field, they will also provide researchers with valuable clues about the planet’s composition.

The stream of fresh information arriving from a distant planet has excited scientists and laymen alike, but the exploits of the spaceship transmitting those discoveries are equally dramatic. Since its launch in 1977 the nuclearpowered Voyager has proven capable of adapting to a wide array of unexpected problems. Most recently, scientists reprogrammed its computers to compensate for the difficulties of Uranian photography. The reason: dim sunlight at Uranus required exposure times of as much as 16 seconds, which would cause ordinary pictures to blur. Engineers solved that problem by programming the entire spacecraft to rotate as it sped by Uranus, in effect “panning” the planet to keep its image steady in the two camera lenses. Many of the machine’s Earth-bound admirers say that that flexibility has amazed and delighted them. And it may continue to have that effect as it speeds on to the next encounter in its extraordinary and historic voyage—an Aug. 25, 1989, rendezvous with the planet Neptune, 2.7 billion miles from its home base.

— JOHN BARBER in Toronto with KEN MCBRIDE in Pasadena

KEN MCBRIDE