Alone in the vastness of the outer solar system, an intrepid spacecraft is quietly making history. At 8:00 a.m. Eastern daylight time today, Pioneer 10 sped past the orbit of Neptune and became the first device of human design to go beyond all the known planets.
It is yet another first after an 11-year, 3.6 billion-mile voyage noted for spectacular achievements.
And as Pioneer hurtles on its own momentum 30,545 miles deeper into the unknown each hour, scientists are excited about discoveries that may lie ahead.
Pioneer may confirm the presence of yet-to-be-observed gravity waves, echoes of cataclysmic cosmic events. It might help pinpoint the source of unexplained forces distorting the orbits of Uranus and Neptune, which some say suggest the existence of a dim companion star of the Sun or even the elusive, hypothethical 10th planet. And if technicians can keep track of the ever-fading signal from Pioneer's 8-watt transmitter, it may well tell them where the realm of the solar system yields to the forces of interstellar space.
''We are constantly entering unexplored territory,'' says Aaron Barnes, an astrophysicist at the Ames Research Center in Mountain View, Calif., where instructions are dispatched daily to the craft.
''Pioneer marks the beginning of an historical epoch,'' adds Pioneer project manager Richard O. Fimmel, also of the National Aeronautics and Space Administration's (NASA) Ames Research Center. ''One might look at it as man's first step into interplanetary travel.''
Since its launch March 3, 1972, from Cape Canaveral, Fla., Pioneer's experience has been that of a trailblazer. Before Pioneer, no craft had ever traveled past Mars. It was the first to negotiate the intimidating asteroid belt , an area between the orbits of Mars and Jupiter cluttered with rocky debris. In December 1973, it swept to within 81,000 miles of Jupiter's turbulent cloud tops. Pioneer relayed the first close-up pictures of that planet's great red spot, documented cyclonic forces of the dense hydrogen-and-helium Jovian atmosphere, and found Jupiter to be a rapidly spinning liquid planet. It then became the first spacecraft to use the il21l,0,22l,6pgravity and orbital motion of a planet as a slingshot to power it toward the outer planets, thus serving as a model for future missions through at least the end of the century.
More than seven years after its scheduled 21-month mission expired, Pioneer is still working. Having survived Jupiter's intense radiation belts and bombardment by minute micrometeoroids, the hardy, 500-pound satellite performed almost flawlessly as it crossed the orbits of Saturn in 1976, Uranus in 1979, and Pluto April 25.
Normally Pluto is the outermost planet. But because its orbit is so elliptical, in contrast to Neptune's roughly circular path, Pluto will be the closer of the two for the next 17 years. (See illustration.)
Yet even as it leaves the planetary realm, Pioneer will not escape the Sun's influence for some time. ''We're out of the castle,'' says NASA's Fimmel, ''but not yet across the moat.'' Much of Pioneer's daily 16-hour broadcast is devoted to data detailing the size and characteristics of that ''moat,'' or, more precisely, the heliosphere.
Generated by an electrically charged gas made of protons and neutrons streaming a million miles an hour from the Sun's corona, the heliosphere acts as a magnetic bubble shielding the solar system from cosmic rays - interstellar nuclear ash caused perhaps by the deaths of massive stars.
As the solar system moves through this interstellar gas, the bubble takes on a streamlined tear shape. The blunt end corresponds to the leading edge of the solar system. Pioneer is traveling down the ''tail'' of the heliosphere, or the trailing edge - by some estimates 5 to 10 billion miles long. Pioneer 10's target is the heliopause, or ''skin,'' which acts as a barrier between the sun's atmosphere and interstellar space.
Sometime in the early- to mid-1990s, Pioneer is expected to pierce the heliopause and enter interstellar space. About that time, the radioisotopic generators that power its instruments and transmitter are expected to give out.
Even then, aimlessly spinning through the galaxy for billions of years, Pioneer 10 will have a purpose. For the benefit of any intelligent life that might happen upon Pioneer, its designers affixed a plaque showing the location of the Earth and solar system, a man and a woman, and some points of basic science.