The Pioneer 10 spacecraft

Soon after it was launched on March 2, 1972, the 570-pound planetary probe became the fastest object made by humans ever to leave Earth. Going 32,400 miles per hour, it reached the moon in just 11 hours. In 12 weeks, it crossed Mars's orbit. Still, it didn't arrive at its primary target, Jupiter, until Dec. 3, 1973 -21 months later.

Pioneer 10 sent back the first close-up views of the giant planet, and later contributed to many important discoveries about cosmic rays and the solar wind. It also became the first object of human design to travel beyond the solar system.

As it went farther out in space, Pioneer 10 went farther out of the public's mind, too, though it kept sending data. NASA didn't officially retire it until March 31, 1997, well beyond its expected operational lifetime.

Later, by special arrangement, it was reinstated as part of NASA's Lunar Prospector controller-training program.

Pioneer 10's activity is much reduced today. At a distance of 6.8 billion miles from Earth, its radio signals are very feeble.

But it still has a mission. It carries a plaque bearing the images of a man and a woman. The metal postcard is intended for unknown recipients in space. The next object the craft is likely to pass is a star in the constellation Taurus - in about 2 million years.

But Pioneer 10 may not be there first. Voyager 1, launched in 1977, has already overtaken it. It's likely many more spacecraft will, too. Who knows? In 2 million years, descendants from Earth may be at the star to greet Pioneer 10.

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