TIME: Almanac 1990/1990 Time Magazine Compact Almanac, The (1991)(Time).iso/time/090489/09048900.060

Text File | 1990-09-22 | 10KB | 188 lines

SPACE, Page 54

The Last Picture Show

By Leon Jaroff


    Nearly 2.8 billion miles away, an artifact of man, a mere speck
in the vast expanse of space, closed in on a giant, blue-hued
planet. The spacecraft Voyager 2, alive and well after twelve years
in space, having had dramatic rendezvous with Jupiter in 1979,
Saturn in 1981 and Uranus in 1986, was approaching Neptune, the
solar system's most distant planet. (Pluto, usually the outermost
planet, has a highly eccentric orbit and is currently closer to the
sun than is Neptune.)

    Accelerating to 60,980 m.p.h. relative to Neptune, the one-ton
probe swooped just 3,048 miles above the frozen methane clouds that
shroud the planet's north polar region. Turning sharply, it plunged
toward the mysterious moon Triton, passing within 24,000 miles of
its rocky surface. Then, after its spectacular -- and last --
planetary encounter, it headed for the distant fringes of the solar
system for what its creators hope will be a quarter-century more
of exploration.

    All the while, across the void, Voyager was sending back to
earth images never before seen by man and volumes of information
that will keep scientists busy for years. Signals from its TV
cameras and scientific instruments, carried by radio waves
traveling at the speed of light, arrived at earth four hours and
six minutes later. Decoded at the Jet Propulsion Laboratory in
Pasadena, Calif., the transmissions appeared minutes later in the
form of pictures and data on television screens and computer
consoles.

    At J.P.L. the scientists and technicians who designed Voyager
and have nursed it through its long space odyssey could not contain
their excitement and pride. Champagne flowed, and each new picture
on the TV screens was greeted with cheers and applause. Americans
watching the proceedings live on television shared their sense of
accomplishment. The little unmanned spacecraft had given them an
unexpectedly large bang for the buck.

    On the night of the encounter Triton actually upstaged Neptune,
which had yielded many of its secrets to Voyager in the preceding
weeks. The moon, frosty white with pink and blue blotches, was
covered with cliffs, faults and glacier-like terrain with a
sprinkling of craters. Scientists also spotted a smooth, roughly
circular area several hundred miles across that may be the crater
of an ancient "ice volcano" that once spewed frozen gases.

    Voyager had already revealed more about Neptune than had been
learned in all the years since 1846, when German astronomer Johann
Galle first spotted the planet. While still millions of miles away,
the spacecraft discovered the Great Dark Spot, a violent earth-size
storm in Neptune's midsection, and later it photographed six
previously unknown moonlets. One of the new moons, temporarily
named 1989 N1, turned out to be larger than Neptune's other
long-known moon, Nereid. And Triton, thought to be about 2,500
miles in diameter, was downsized to 1,740 miles. Voyager's sensing
instruments confirmed expectations that Neptune has a magnetic
field and, on the day before the encounter, reported that the
spacecraft had crossed the supersonic shock wave, a front created
where the solar wind meets the magnetic field. By clocking the
rotation of that field, J.P.L. scientists determined that a
Neptunian day is only 16 hours long, rather than the previously
estimated 17 or 18 hours.

    As Voyager sped on toward its rendezvous, its cameras picked
out wispy, white, cirrus-like clouds of frozen methane gas racing
across the equatorial regions. Photographs also revealed shadows
of these clouds cast on a deeper layer of atmosphere some 30 miles
below. Said Bradford Smith, head of the J.P.L. imaging team, "This
is the first time Voyager has ever been able to see cloud shadows
on any of the planets we've looked at."

    Scientists also discerned what seemed to be two partial rings,
or ring arcs, that mysteriously seemed to disappear on subsequent
pictures. But a few days later, Smith borrowed from Hollywood to
announce, "The lost arc our imaging-team raiders have been looking
for is not an arc; it is Neptune's first complete ring." Late last
week Smith announced that five rings in all had been found.

    The high quality of Voyager's pictures and data belied the
difficulties of operating a spacecraft so far from the sun, which
at Neptune's distance looks like an intensely bright point of
light. Because the planet receives only one-thousandth of the
sunlight that falls on earth, Neptunian photography calls for time
exposures of as long as 15 seconds. That required panning the TV
cameras to avoid blurring, which Voyager accomplished by gingerly
swiveling its camera platform and sometimes even yawing itself in
space.

    Transmitting pictures and data back to earth was no less a
task. By the time the 22-watt (equivalent to a small light bulb)
radio signal from Voyager reached the earth, its power was only
about one twenty-billionth of that needed to run an electronic
digital watch. To catch as much of that faint whisper as possible,
NASA had enlarged some of the already huge radio telescopes of its
Deep Space Network in Australia, Spain and California. Even then,
Voyager's voice was practically lost in the electromagnetic noise
of space.

    In order to discern the messages from Neptune above that noise,
J.P.L. controllers instructed Voyager to speak more slowly. The
data transmission rate, which was as high as 115,200 bits a second
at Jupiter, was reduced to a sluggish 21,600 bits as the craft
approached Neptune. As a result, during last week's encounter,
there was time for live broadcasts of only the most important
pictures and data. The rest were stored on onboard tape recorders
to be transmitted later.

    That Voyager is still performing well, let alone functioning
at all, is a tribute to the genius of its designers. By 1989
standards, the spacecraft's technology is hopelessly outdated. Its
command control computers have random access memories of only 8,000
bits of information, compared with today's personal computers,
which have as many as 1.4 million-bit RAMs. The two TV cameras
operate with obsolete vidicon tubes rather than the more reliable
and efficient charge-coupled devices in modern TV cameras. Says
Torrence Johnson, of the imaging team: "You couldn't find anyone
to build one of those vidicon tubes today."

    When the Voyager team first assembled at J.P.L. in 1972, it
had high hopes that NASA in the late 1970s would take advantage of
a once-every-176-years alignment of the outer planets to stage a
planetary grand tour. That plan would have sent two sophisticated
spacecraft (one as a backup) flying by Jupiter, Saturn, Uranus and
Neptune, using the gravity assist of each planet to change their
course and accelerate them onward to the next one. Another pair of
ships would similarly fly past Jupiter, Saturn and Pluto. Without
gravity assists, the J.P.L. team argued, a direct flight to Neptune
was beyond current rocket technology. In addition, the opportunity
for another grand tour would not exist until the middle of the 22nd
century.

    For political and budgetary reasons, however, NASA limited the
program to two no-frills spacecraft that would explore only Jupiter
and Saturn. As a result, Voyagers 1 and 2, both launched in 1977,
were designed by J.P.L. engineers to operate efficiently for only
about five years, more than enough time to reach Saturn.

    "Of course," says chief project scientist Edward Stone, "that
didn't prevent us from launching Voyager 2 on exactly the same
trajectory that the grand tour would have been on, hoping it would
survive for twelve years." Indeed, when Voyager 1 successfully
fulfilled its Saturn mission and headed out of the solar system,
NASA gave the green light for Voyager 2 to proceed to Uranus and
then Neptune. Now, says Stone triumphantly, "in effect, we have
our grand tour."

    And what a tour it has been. Between them, Voyagers 1 and 2
discovered and shot spectacular photographs of new moons orbiting
Jupiter and Saturn, rings around Jupiter, active volcanoes on the
Jovian moon Io, and spokes and braids in Saturn's complex and
brilliant rings. At Uranus, Voyager 2 found two new rings, ten new
moons and a set of dust bands. It also discovered a magnetic field
with an axis tilted a remarkable 59 degrees from the planet's axis
of rotation.

    Now the spacecraft has seen its last planet and J.P.L.
staffers, many of whom have spent their entire careers on the
Voyager missions, are calling the Neptune encounter "the last
picture show." "Between Uranus and Neptune, I went to graduate
school," says Candy Hansen, who has designed many of Voyager's
picture-taking programs. "I bought my house between Jupiter and
Saturn. That's how I measure my life."

    While Hansen and most of the other members of the Voyager team
will soon turn their attention to other space projects, some will
stay on, as Voyager 2's mission is not yet completed. Like Voyager
1, the spacecraft is heading out of the solar system. Its cameras
will soon have little to record, but its other instruments, powered
by heat from radioactive plutonium in its thermal generators, will
continue such tasks as ultraviolet astronomy of the stars and
seeking out the edge of the solar system, the boundary marking the
farthest reaches of the solar wind and the beginning of
interstellar space.

    Finally, in 25 years or so, declining power levels and
increasing distance will end any communication between Voyager and
earth, and the little spacecraft will speed silently toward the
stars. With it will go a recording on a twelve-inch copper disk
installed aboard Voyager on the remote chance that an alien
traveler will someday come upon it. The record, called Sounds of
Earth, carries greetings in 60 languages and a message from then
President Jimmy Carter that ends, "This record represents our hope
and our determination, and our goodwill in a vast and lonely
universe."


-- Edwin M. Reingold/Pasadena