TIME: Almanac 1993/TIME Almanac 1993.iso/time/090489/09048900.060

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                                                                                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