TIME: Almanac 1993/TIME Almanac 1993.iso/time/080789/08078900.002

Text File | 1992-09-23 | 3.7 KB | 75 lines

                                                                                SPACE, Page 44

Next and Final Stop: Neptune


Voyager sees some intriguing sights as it nears the eighth
planet

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.