Next And Final Stop: Neptune

By MICHAEL D. LEMONICK

After a marathon journey of twelve years and more than 4 billion miles, the remarkable Voyager 2 space probe is finally approaching its last port of call. Having made historic flybys of Jupiter in 1979, Saturn in 1981 and Uranus in 1986, it is poised for an Aug. 24 rendezvous with Neptune, the most distant of the giant planets. (It will not encounter Pluto, whose bizarre orbit now places it closer to the sun than Neptune is.) Voyager's aging cameras and electronic sensors are somewhat impaired, and the probe is so distant that its signals take four hours to travel to earth. Still, scientists expect mounds of fresh data and some 8,000 photographic images, entirely new information about a little known object that is almost four times the size of earth but appears in earthly telescopes only as a fuzzy blue-green ball.

Though Voyager is still about 22 million miles from Neptune, it has already made several discoveries. It has found a new moon to add to the known duo, % Triton and Nereid. Labeled 1989-N1, the object is between 125 and 400 miles across and has a surprisingly ordinary orbit. Like most moons, 1989-N1 orbits nearly over its planet's equator and in the same direction as the planet's rotation, implying that it formed with or soon after Neptune.

By contrast, Triton, which is about the size of earth's moon, orbits in the opposite direction. That has led astronomers to guess that Triton might be a large asteroid that was captured by Neptune's gravity. Such an intrusion should have disrupted the paths of any existing moons. This would explain tiny Nereid's highly elongated and tilted orbit. But 1989-N1 is just "sitting there," says Voyager project scientist Torrence Johnson, of the Jet Propulsion Laboratory. Johnson expects that the probe will discover more moons, shedding light on Triton's origins. "All of the outer planets have lots of junk around them," he notes. Jupiter, Saturn and Uranus have at least 15 moons apiece. "It would be amazing if we got to Neptune and didn't find a bunch of these things."

Like Jupiter and Saturn, but unlike its near-twin next-door neighbor, Uranus, Neptune appears to have distinct weather patterns. The probe's cameras have glimpsed a streak of white that may be an atmospheric jet stream, longitudinal bands that could mark prevailing winds, and a dark blotch, perhaps similar to Jupiter's ancient high-pressure system known as the Great Red Spot. Neptune, Jupiter and Saturn all generate more heat than they receive from the sun, while Uranus does not; the excess heat may be the source of the turbulence.

Another focal point of scientific interest is Neptune's rings. Indirect evidence suggests that they exist, but as arcs rather than true rings. Voyager's photographs may help explain how they formed. The space probe will also examine reddish Triton, whose methane atmosphere is believed to overlie a surface puddled with liquid nitrogen.

While astronomers are eager to solve existing riddles about Neptune, the most exciting prospect is that Voyager 2 will find something unanticipated. That happened at Jupiter, where its sister probe, Voyager 1, found volcanoes on the moon Io. It happened at Saturn, where both spacecraft found many more rings than anyone had predicted. And it happened at Uranus, where Voyager 2 found that the planet's magnetic field was tilted an unprecedented 60 degrees from the axis of rotation. Given that track record, the unexpected is a virtual certainty.

With reporting by Edwin M. Reingold/Los Angeles