Science: Hurtling Through the Void

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Pioneer 10 becomes the first object to leave the solar system

Eleven years ago a small, unmanned spacecraft looking like the product of a child's Erector Set took off from Florida on a historic mission. Equipped with primitive electronic eyes and other instruments, Pioneer 10 flew past the giant planet Jupiter, providing the first startling close-up view of that distant world. Now the surprisingly durable robot, whose working parts were designed by its builder, TRW, to last only two or three years, has scored another remarkable achievement. Propelled by a gravitational boost from Jupiter, it has become the first man-made object to leave the solar system and enter the realm of the stars.

The epochal departure, determined by faint signals from Pioneer 10 picked up by NASA'S tracking antennas, occurred early this week when Pioneer crossed the orbit of Neptune, currently the outermost planet.* At that moment, it was 2,813,685,909 miles from the sun and moving at a brisk 30,558 m.p.h.

The event, said Astronomer Carl Sagan, "is filled with symbolism. Lots of things have entered the solar system during its 5 billion-year history—comets, asteroids, every sort of cosmic debris. This is the first time, however, that something associated with life and intelligence has left it."

Anticipating Pioneer's wanderings into the cosmos, where it might be intercepted by intelligent beings, Sagan and a Cornell colleague, Frank Drake, along with Sagan's former wife Linda, an artist, created a gold-anodized aluminum plaque that was affixed to the spacecraft's side. The plaque shows a nude male and female, a representation of the solar system, plus other scientific clues that might help inhabitants of other worlds trace Pioneer's origins.

The chance of extraterrestrial discovery is, to be sure, infinitesimally small. As it hurtles through the interstellar void, Pioneer will not approach another star for 10,000 earth years. Even then, it will hardly be a close encounter: there will be a gulf of 3.8 light-years (some 23 trillion miles) between Pioneer and Barnard's star, a small, cool, red celestial object that does not seem to have life-supporting planets. Still, as scientists at NASA's Ames Research Center note, Pioneer should survive indefinitely in the vacuum of interstellar space. The machine may even outlast the solar system itself, which is expected to expire in another 5 billion years when the sun swells into a red giant and engulfs the inner planets.

One surprise of Pioneer's voyage so far is the discovery that the boundary of the sun's atmosphere, or heliosphere, the bubble of particles blown into space by the solar wind, is much farther out than supposed. Researchers had expected the region to end near Jupiter's orbit. But even now the spacecraft's instruments show no decline in the heliosphere's strength, only what seems to be a swelling and contraction connected with cycles of solar activity.

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