 On
April 28th, a Deep Space Network antenna in Madrid detected a
curious radio transmission from the constellation Taurus. The
feeble signal registered little more than a billionth of a trillionth
of a watt -- nonetheless, it had a powerful effect on scientists.
The signal
was intelligent and it came from an interstellar spacecraft about
twice as far from the Sun as Pluto. It was Pioneer 10!
Ground controllers
had been listening for the distant space probe since last August
with no success, raising fears that its radio transmitter had
finally run out of power after 29 years in space. But, says delighted
Pioneer 10 project manager Larry Lasher at NASA/Ames, "Pioneer
10 lives on!"
Pioneer 10
was launched on March 2, 1972, from Cape Kennedy aboard an Atlas
Centaur rocket. At the time it was the fastest spacecraft ever
to leave Earth. It was the first spacecraft to travel through
the asteroid belt, the first to visit Jupiter, and the first to
use a planet's gravity to change course and reach solar-system
escape velocity. Now, as it races for interstellar space, Pioneer
10 faces its toughest challenge yet: the inexorable march of time.
"We're way beyond
our warranty," says Lasher. "Pioneer 10 was only intended to last
21 months, but it's been going for nearly 30 years." The craft is
powered by electricity derived from the warmth of decaying plutonium
238. Although the half-life of the isotope is 92 years, the thermocouples
that convert heat energy to electricity are degrading faster. Mission
controllers suspect there won't be enough electricity to power the
radio transmitter much longer.
Scientists
are monitoring Pioneer's faint signals as a test of communications
technology for future interstellar missions. "The Advanced Concepts
Program is funding this effort," he continued. "They're learning
how to extract coherent data from a noisy signal using chaos theory."
Far-flung
Pioneer 10 is the perfect guinea pig for such an experiment. Its
feeble signals travel nearly 11 hours to reach Earth from a distance
of 7.3 billion miles -- that's 78 times farther from the Sun than
our planet.
On December
8, 1992, when Pioneer 10 was "only" 5.2 billion miles away, the
craft experienced an unexpected course change. Astronomers think
it was diverted slightly by the gravitational pull of a Kuiper
Belt Object. Circling the Sun at vast distances beyond the outermost
planets, Kuiper Belt Objects are asteroid-sized bodies similar
in composition to icy Pluto.
Above:
Several NASA spacecraft are searching for the boundary between
interstellar space and the heliosphere. Voyager 1, at 81 AU, is
currently the most distant. As this diagram shows, only Pioneer
10 is moving in the opposite direction to the Sun's motion through
the galaxy.
The unusual
encounter happened as Pioneer 10 was exiting the realm of the
planets and exploring for the first time the outer limits of the
heliosphere -- a giant bubble carved out of the gaseous interstellar
medium by the solar wind. No one knows the exact dimensions of
the heliosphere, but Pioneer 10 is probably approaching its edge.
"We could cross into interstellar space within a few years," says
Lasher. "We'll know we're there when the flux of cosmic rays recorded
by the onboard Geiger Tube Telescope suddenly increases." (Prof.
James van Allen, principal investigator for the Geiger Tube Telescope,
still analyses Pioneer 10's cosmic ray data even though van Allen
himself has retired from the University of Iowa.)
Hopefully
Pioneer's radio will still be transmitting when the historic crossing
occurs. It could happen, says Lasher. After all, Pioneer 10 has
beaten the odds before.
For instance,
Pioneer 10 was the first spacecraft to cross the asteroid belt
between Mars and Jupiter. Before that crossing, no one knew how
many rocks and grains of sand speeding through space at thousands
of miles per hour would hit and possibly disable the spacecraft.
Pioneer 10 made the journey nearly unscathed, thus opening the
way for other probes to explore beyond Mars.
 Later, the craft
endured an intense shower of radiation inside Jupiter's magnetosphere
-- and kept right on going!
Right:
Herb Lassen, one of Pioneer 10's original design engineers at
TRW, discusses what the long-lived spacecraft might encounter
as it travels among the stars.
After Pioneer's
power runs out, the 570 lb spacecraft will have a new job: ambassador
to the stars. The probe will have its first stellar encounter
in about 300,000 years when it passes within three light years
of the red dwarf Ross 248 in the constellation Taurus. Ross 248
is an 11th magnitude star 10.3 light years from Earth. In the
next million years, Pioneer 10 will pass ten stars at distances
ranging from three to nine light years, and will probably still
be traveling through the Milky Way galaxy when the Sun becomes
a red giant and destroys our planet five billion years hence.
The probe
wasn't so long-lasting in its first and only movie role, Star
Trek V: the Final Frontier. A trigger-happy Klingon named
Captain Klaa blasted Pioneer 10 to smithereens for target practice.
Of course that was just a model of the spacecraft.
 TRW, the company that designed and built Pioneer
10 under contract with NASA Ames, provided Paramount Pictures
drawings for the Star Trek model. Says Paul Morgan, a video
producer for TRW's IMAGE Center, "The computer model was a good
one, but a cameraman complained that it was hard to light."
The Klingon
hit it anyway.
Pioneer 10
bears a message for any life forms that it might encounter on
its trek across the galaxy. A gold-anodized aluminum plaque (pictured
above) was designed by Frank Drake and the late Carl Sagan and
bolted to the spacecraft before it blasted off in 1972. The plaque's
engraving depicts a man and a woman, a map of Earth's solar system,
and other symbols that may help intelligent beings interpret the
message and understand something about the spacecraft's creators.
As an emissary to the galaxy, Pioneer 10's greatest and most bizarre
adventures may still lie ahead.
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