Six billion miles and counting....
- Six billion miles and counting....
Last month NASA received a weak signal from Pioneer 10, twice as far from
the Sun as Pluto and speeding toward the constellation Taurus.
March 2, 2000 -- With its red eye glittering in the southwestern sky just
after sunset, Taurus the Bull is one of the most arresting winter
constellations. Most stargazers know it well, but what many don't know is
that the familiar constellation is also a very far out tourist destination.
In about 30,000 years Taurus will receive a remarkable visitor from Earth --
a well-traveled spacecraft named Pioneer 10.
Pioneer 10 was launched on March 2, 1972 from Cape Kennedy aboard an Atlas
Centaur rocket for a two-year mission to Jupiter. Twenty-eight years later,
the probe is about twice as far from the Sun as Pluto. It's bound for
interstellar space at 13 km/s (28,000 miles per hour) heading in the general
direction of the first magnitude star Aldebaran.
After many years in space, including dangerous passages through the asteroid
belt and Jupiter's magnetosphere, Pioneer 10 might finally be nearing the
end of its active scientific life. 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 think that there will
not be enough electricity to power Pioneer's transmitter for much longer.
Right: Where is Pioneer 10 heading? You can see for yourself. Just step
outdoors around 8 p.m. local time and look to the southwest. This diagram
shows the constellations Orion and Taurus and the bright planets Jupiter and
Saturn. Pioneer 10 is coasting toward the red star Aldebaran, which lies 71
light years away and shines 155 times more brightly than our own sun.
The spacecraft's 8-watt signal, equal to the power of a night light, now
reaches NASA's Deep Space Network antennas with the strength of .3
billionths of a trillionth of a watt! Scientists are tracking the feeble
transmissions as part of an advanced concept study of chaos theory and to
learn more about conditions in the solar system beyond Pluto.
In February, ground controllers sent commands to Pioneer instructing the
craft to maneuver to improve the reception of its signal on Earth. Pioneer
is so low on power that its transmitter had to be turned off to allow it to
execute the turn. After 90 minutes of blind flight the transmitter was
reactivated. After more than 10 hours -- the time it takes for light to
travel 11 billion km (6.8 billion miles) from Pioneer 10 to Earth -- anxious
ground controllers received a signal that the maneuver had been a success.
The distinction of being the first human artifact to venture beyond Pluto's
orbit is just one in a long list of firsts for Pioneer 10. 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
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Many scientists rank the first crossing of the asteroid belt between Mars
and Jupiter as Pioneer 10's most important achievement. Before the crossing,
no one knew how many rocks, as well as grains of sand, speeding through
space at thousands of miles per hour would impact and possibly disable the
spacecraft. Pioneer 10 made the crossing nearly unscathed, thus opening the
way for other spacecraft to explore beyond Mars.
On December 8, 1992, when Pioneer was 8.4 billion km (5.2 billion miles)
away, the probe experienced an unexpected course change. Astronomers think
that the craft was diverted slightly by the gravitational pull of a Kuiper
Belt Object (KBO). KBOs are frigid asteroid-sized bodies, similar in
composition to Pluto, that circle the sun at vast distances beyond the
outermost planets. If confirmed, the 1992 event would mark just the second
time in history that a Solar System object has been discovered by its
gravitational effect alone. The first was the planet Neptune which was
discovered in 1846. Its position was predicted because of its gravitational
tug on the planet Uranus.
Above: Several NASA spacecraft are searching for the boundary between
interstellar space and the heliosphere (a giant bubble blown by the solar
wind). As a larger version of this diagram shows, only Pioneer 10 is moving
in the opposite direction to the Sun's motion through the galaxy.
Pioneer 10 is now exploring the outer limits of the heliosphere, a bubble
carved out of the gaseous interstellar medium by the solar wind. It was once
thought that this cavity didn't extend much farther from the Sun than
Jupiter. Thanks to the Pioneer and Voyager space probes it's clear that the
heliosphere is much bigger -- at least twice as large as the orbit of Pluto.
The exact boundaries of the heliosphere are still unknown. Scientists want
to monitor Pioneer 10 for as long as possible in hopes of recording the
historic crossing into interstellar space.
After Pioneer's power runs out, the 570 lb spacecraft will have a new job
as ambassador to the stars. The probe will have its first stellar encounter
in about 30,000 years when it passes within three light years of the red
dwarf star Ross 248 in the constellation Taurus. 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
Pioneer 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 Dr. Frank Drake and the late Dr. 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 which 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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