ON DECEMBER 3, 1973, Pioneer 10 passed within 81,000 miles of the cloud tops of the largest planet, Jupiter, taking the first close-up images of it and charting its intense radiation belts. Pioneer 11 passed by on December 2, 1974, and used Jupiter¿s gravity to accelerate to Saturn. Image: NASA/ARC
A Deep Space Network antenna in Madrid picks up a feeble and unexpected radio signal--a cry from a lost spacecraft. It is April 2001. Pioneer 10, missing, feared dead for eight months, was phoning home to let scientists know that it was still alive and kicking. Intrepid Pioneer 10 went on to celebrate a milestone 30th birthday in 2002, and today 10 belongs to an elite group of spacecraft that just refuse to die.
More than a quarter of a century after their launches, Pioneer 10, Voyager 1, Voyager 2 and the IMP-8 spacecraft are still going strong. During their primary missions in the Seventies and Eighties, they gave us a unique insight into the atmospheres of Jupiter, Saturn, Uranus and Neptune, and the solar-wind conditions around Earth. And today, thanks to some nifty long-distance repair work by dedicated space scientists on the ground, the probes continue to radio-signal postcards home to tell us about the path to interstellar space.
Pioneer 10 was launched on March 2, 1972, on an expedition to Jupiter. To mark its 30th birthday, scientists sent a special transmission to the retired spacecraft. After 22 hours, having crossed 7.4 billion miles, Pioneer 10¿s reply--equal in power to a nightlight--was heard loud and clear. But without careful remote control maneuvering, Pioneer¿s latest messages home could well have been lost in space.
Maneuvers in the Dark
After two and a half decades in outer space, Pioneer 10¿s antenna had drifted off-point. In 1997, NASA researchers realized they needed to re-point the antenna to Earth to keep in contact with the craft. But the targeting process carried the risk of muting Pioneer 10 permanently. To garner enough power for the maneuver, Pioneer 10¿s transmitter had to be shut down. "We were concerned that turning the transmitter¿s traveling wave tube off in the deep cold of space, and then back on again, would cause a thermal shock that might shatter the helix [a component that¿s critical for transmission] in the tube," explains Larry Lasher, Pioneer project manager at NASA Ames Research Center. But "the hearty spacecraft successfully executed the [maneuvering] procedure in the blind for 90 minutes," he says.
Not bad for a craft originally slated only for a 21-month mission to Jupiter. To reach the giant planet, Pioneer 10 first had to cross the (then) seemingly impenetrable asteroid belt. By the 1960s, when Pioneer's mission was being planned, the orbits of more than 3,000 large asteroids had been determined and thus could be avoided, but tiny particles in the belt were still thought to pose a big threat. Scientists had no way of estimating the number of grain-size particles that might severely damage a craft after multiple collisions. "Only by going there could the danger be properly assessed, " says Ed B. Massey, manager of the Voyager and Ulysses projects, describing the debt later missions owe to Pioneer. "When Pioneer successfully traversed the asteroid belt, it had demonstrated that a concentration of small debris sufficient to harm the spacecraft did not exist."
After three decades that saw Pioneer 10 blaze a trail for all future spacecraft through the asteroid belt, take the first close-up images of Jupiter and its satellites and then head out of the solar system, the spacecraft was officially decommissioned in 1997. But, as Lasher notes, "in retirement Pioneer 10 still served the public as a valuable space resource at no extra cost to the taxpayers." It has been used to train spacecraft controllers in tracking station protocol and continues to be tracked as a part of study in communication technology.
Edge of Darkness
Pioneer 10 also continues to investigate a puzzle posed by its own early findings: Where exactly is the edge of the sun¿s influence? One of the greatest surprises raised by the original Pioneer mission was, according to Lasher, the persistence of the solar wind, the stream of charged particles that radiates from the sun. Prior to Pioneer¿, the effect of the solar wind was thought to extend out to Jupiter; but even now, with Pioneer at a distance of 7.5 billion miles, it has yet to escape the long arm of the sun¿s reach.