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 southeastern 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 southeast. 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.

Spacecraft Emeritus

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 solar-system-escape velocity.

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 hence.

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.

Related Links:

Pioneer Home Page - background, images, and mission status reports from NASA/Ames.

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