STILL TICKING

It was sent aloft in 1972, when Richard Nixon was in the White House, Bobby Fischer was the world’s chess champion, and the IBM 360 mainframe still dominated the computer industry. It was designed to have a useful life of three years, at most. Yet today, after a quarter-century, when Fischer has disappeared from the chess scene and the IBM 360 is merely a nostalgia item on display at Boston’s Computer Museum, the doughty little spacecraft Pioneer 10 is still plugging along.

As of this week, Pioneer is 6 billion miles from Earth, making it the most distant man-made object in the universe. Its signals–barely discernible against the cacophony of background space noise–continue to register on the giant radio telescopes of NASA’s Deep Space Network, reassuring scientists that Pioneer is still alive and calling home. Perhaps most remarkable, those signals emanate from an 8-watt transmitter, which radiates about as much power as a bedroom night light, and take more than nine hours to reach Earth.

Cruising through the cosmos at 27,700 m.p.h., the Energizer bunny of NASA spacecraft is measuring cosmic rays and solar emissions, probing for the outer boundary of the solar system and even abetting the efforts of scientists pursuing SETI, the Search for Extraterrestrial Intelligence. When Pioneer was launched in March 1972, its primary assignment, ordained by NASA, was to reach the environment of Jupiter. At the time, says physicist James Van Allen, the discoverer of Earth’s radiation belt and a principal contributor to Pioneer’s achievements, “this objective was regarded as a bold one.” While unmanned U.S. and Soviet spacecraft had visited the inner planets, none had flown beyond Mars and braved the asteroid belt to reach Jupiter and the other outer planets. Indeed, many scientists feared that, in addition to the asteroids, the belt was laden with unseen particles that would collide with and destroy an intruding craft. But Pioneer 10 sailed through with nary a scratch.

As it swung by Jupiter in November 1973, Pioneer took the first close-up photographs of the giant planet and its moons, detected and mapped its immense magnetic field and intense radiation belts, analyzed its turbulent atmosphere and discovered that it was encircled by a faint Saturn-like ring.

Then, assisted by a powerful boost from Jovian gravity, the spacecraft hurtled toward deep space. Not far beyond Jupiter, scientists had expected Pioneer to find the boundary of the heliosphere, beyond which the solar wind (charged particles emitted from the sun) can no longer be detected. Yet as distant as Pioneer is today, it is still being wafted by solar breezes, and scientists now believe the elusive boundary could lie as far as 10 billion miles from the sun, and perhaps farther.

Pioneer will eventually cross that boundary but will be unable to convey the news to Earth. Unlike spacecraft operating closer to the sun, it cannot rely on solar panels to generate power. Instead it is equipped with a thermocouple-like generator heated by radiation from a clump of plutonium 238. Unfortunately, the radiation is also slowly degrading the generator, which is producing only two-thirds of its original output.

Last week in the Pioneer control room at NASA’s Ames Research Center, in Mountain View, California, project manager Fred Wirth watched Pioneer’s data, in the form of multicolored blocks of numbers flashing across a computer screen. To make the best use of the spacecraft’s dwindling power, he has shut down all but three of its 11 scientific instruments, and by early next year only one–Van Allen’s cosmic ray detector–will be able to function. Citing operational costs and the diminishing scientific return, NASA has ordered Wirth to halt all communications with Pioneer next year on March 31.

That is melancholy news not only for Wirth and other members of the Pioneer team but also for astronomer Frank Drake and his crew at the nearby Mountain View headquarters of seti. There, scientists hoping to pick up radio signals from a distant civilization have been using Pioneer’s signals to test the efficacy of their detection systems. Says Drake: “It’s been our prime diagnostic tool.”

Yet even a silent Pioneer 10 may someday effect a kind of communication with extraterrestrials. Attached to one of the spacecraft’s antenna support struts is a plaque, designed by Drake and astronomer Carl Sagan, that is inscribed with symbols, binary numbers and drawings conveying what they hope is a universally understandable message. It locates the solar system, shows that Pioneer was launched from Earth and portrays a terrestrial man and woman.

If the plaque is one day recovered by advanced extraterrestrials, however, it is unlikely that any of us will be around to receive their reply. At Pioneer’s current cruising speed, even if the spacecraft were headed directly toward the closest star, Proxima Centauri, and any possible nearby residents, it would not arrive for some 100,000 years.