In 1972, a tiny spacecraft named Pioneer 10 soared into space to explore the outer planets. Its designers, unsure of the perils of the asteroid belt and interplanetary space, hoped the mission would last two or three years.

Sixteen years later, Pioneer 10 is alive and well. Now almost 4.2 billion miles away, 45 times the distance from the Earth to the sun, it has traveled farther than any other human-made object.As the durable wayfarer nears the edge of the sun's huge bubble of activity, it is forcing scientists to change their theories about the dynamics of our solar system.

"Pioneer 10 is one of the superb achievements of the human race," said James Van Allen, a physicist at the University of Iowa. "It has yielded more scientific results than the entire U.S. manned space program to date, at a minute fraction of the cost."

Last week, scientists celebrated the fifth anniversary of Pioneer's passage beyond the orbit of the most distant planet. Despite its enormous distance, much lies ahead for the 565-pound spacecraft.

Its most important task is to search for the edge of the heliosphere, a vast balloon of space that the sun dominates with a million-mile-an-hour blast of charged particles known as the solar wind. Laced with a magnetic field, this wind shields the solar system against most interstellar gases and cosmic radiation.

Scientists think Pioneer 10, cruising at 28,400 miles an hour, could fly out of the heliosphere within the next few years. It would then provide the first direct samples of the virtually empty expanses between the stars.

Pioneer is also looking for a possible 10th planet or other dark object at the fringes of the solar system, and for elusive and powerful gravity waves predicted by Albert Einstein's theory of relativity. Both searches are almost impossible to perform from Earth.

"The Pioneer data are unique," said Deputy Flight Director Robert Jackson of the Ames Research Center in Mountain View, Calif. "When it dies, it will be a very long time before we can sample this part of space again."

Pioneer's radioactive power generators are slowly losing energy. By 1995, they may not produce enough power to send signals back to Earth. Pioneer's signal is already so weak that if it were collected over the entire surface of Earth for 11 billion years, it would turn on a 71/2-watt night light for just one one-thousandth of a second. Only an extremely sensitive receiver can pick up the signal.

Pioneer has lasted far longer than its planners dared dream in 1972. "All of our original objectives look incredibly timid today," said Van Allen. "During our early work, we thought we'd be jolly lucky to get to Jupiter."

To reach Jupiter, Pioneer had to cross the asteroid belt beyond Mars. Some scientists had visions of tumbling boulders pulverizing the craft in the infancy of its mission.

"One of our primary objectives was to fly Pioneer through the asteroid belt and see if it got killed," said Jackson. "But it didn't, and that paved the way for the future exploration of the planets."

Pioneer 10 was the first spacecraft to fly past Jupiter and Saturn, revealing the intense magnetic fields and churning atmospheres of those planets. It is now well beyond the orbits of Neptune and Pluto, where there isn't much to look at. But there is plenty for Pioneer to do.

Scientists used to think that the sun's influence would not extend beyond Jupiter. Pioneer, now almost nine times past that point, reports that the solar wind is still going strong.

But within the last six months, Pioneer's experiments have shown that the edge of the heliosphere may not be far off. The most telltale signs are cosmic rays - high-energy atomic nuclei spewed out of exploding stars.

Like fish struggling up a turbulent stream, the cosmic rays must fight past the tangled magnetic fields of the heliosphere to reach Earth. The closer Pioneer gets to the edge of the heliosphere, the more cosmic rays it should detect, said physicist John Simpson of the University of Chicago.

In recent months, the number of cosmic rays hitting Pioneer has shot up dramatically.

Scientists will know that Pioneer has left the heliosphere when the cosmic ray impacts level off. Then, said Simpson, it will sample true outer space, undisturbed by the sun, for the first time.

"Cosmic rays are the Rosetta Stones of astronomy," Simpson said. "They show how elements were cooked inside of stars. It's like doing nuclear physics in space."

Most scientists think the distance to the heliosphere's boundary - which Pioneer will reach in a few years - is 50 or 60 times greater than the distance from the sun to the Earth. "I'll be astonished if we don't encounter the boundary by 1994," said Iowa's Van Allen.

Pioneer scientists also are looking for signs that the heliosphere expands and contracts as the sun's activity changes. The sun has an 11-year cycle of high and low activity. The resulting changes in the solar wind may make the heliosphere "take deep breaths in and out," said physicist Darrell Judge of the University of Southern California.

The heliosphere may be shaped like a teardrop instead of a sphere, scientists note, because the solar system moves through the galaxy at a high speed. Pioneer is heading toward the tail of the teardrop, so it may take longer to reach the edge than calculations show.

In another project, astronomers carefully check Pioneer's trajectory for evidence of another object in the solar system. But the spacecraft is moving along just as expected, said John Anderson of the Jet Propulsion Laboratory in Pasadena, Calif.

If there is a 10th planet, or if the sun has a dark stellar companion that burned out, it must now be too far away for its gravity to affect Pioneer, Anderson said.

Pioneer's search for gravity waves has proven equally fruitless, said Deputy Flight Director Jackson.