The small silver and gold spacecraft called Pioneer 10, which left Earth 11 years ago, will depart the solar system tomorrow, becoming the first manmade spacecraft that has journeyed beyond the planets on its way to a rendezvous with the stars.

At 8 a.m. EDT Monday, Pioneer 10 will move at 30,558 mph beyond the orbit of the planet Neptune, 2.8 billion miles from Earth, and pass into the void that is no longer interplanetary space.

Although Pluto normally is regarded as the outermost of the nine planets that circle the sun, its eccentric orbit carried Pluto inside Neptune's more circular orbit three years ago. It will stay there for the next 17 years, and Pioneer 10 will be beyond Pluto's orbit forever.

Moving through airless and frictionless space, where the chance of collision with another body is so remote that it is beyond imagination, Pioneer 10 could continue its journey for infinity. Scientists have estimated Pioneer's shortest possible lifetime at 2 billion years and then only if it encounters a shower of cosmic dust or cosmic rays that so corrode its 570-pound aluminum frame that it begins to break up.

Space scientists have charted Pioneer's course for the next 800,000 years and calculate that its first encounter with a star will occur in 10,507 years, when it passes the small, red Barnard's Star at a distance of 3.8 light years.

Among the largest stars it will encounter is Altair, which is 10 times brighter than the sun. Pioneer will pass Altair in about 800,000 years. The spacecraft's nearest encounter will be with a "flare" star named Ross 248, which it will pass in 32,000 years at a distance of only 3.2 light years.

By the time Pioneer 10 makes its first stellar encounter, it will have long since lost radio communication with Earth. Its 8-watt radio signal is so weak now that, by the time the transmission reaches Earth, it measures one-trillionth of a watt. Most scientists think that they will no longer be able to hear Pioneer's signal in about eight years, when the spacecraft is about 3.7 billion miles from Earth.

Scientists have been surprised at hearing Pioneer's signal for so long. Not only has its radio transmitter never failed, but its four radioisotope generators that produce power from atomic heat are still generating enough electricity to run most of its instruments and power its radio.

Only two instruments aboard the spacecraft have failed: a magnetometer burned out, and a star sensor was damaged when it crossed through Jupiter's giant belts of radiation Dec. 4, 1973.

In Pioneer's more than 11 years in space, Earth has witnessed Watergate, a president's resignation, the end of the Vietnam war, an Israeli-Egyptian peace settlement, an Arab oil embargo that triggered a rise in energy prices and worldwide inflation and recession, the revolution in Iran that culminated in the taking of 52 U.S. hostages, and the invasions of Afghanistan by the Soviet Union and of Lebanon by Israel.

"The key to the concept of this spacecraft was simplicity," said Dr. Herbert Lassen, the scientist at TRW Inc., in Redondo Beach, Calif., who conceived the idea of a spacecraft that would use Jupiter's gravity as a slingshot to propel itself out of the solar system.

"We built this spacecraft with the idea that it would be built with the fewest parts possible to fail," he said.

Never was a spacecraft more aptly named than Pioneer 10. It was the first spacecraft to survive the asteroid belt between Mars and Jupiter and first to survive Jupiter's searing radiation belts that are millions of times stronger than the Earth's Van Allen Belt. It is also the first spacecraft to carry a greeting from Earth to any alien civilization that may find Pioneer wandering among the stars.

Pioneer's greeting is engraved on a gold plaque fixed to the antenna support struts. On the plaque are the images of a naked man and woman, the man with his right hand raised in friendship. To their left is a radial pattern of lines depicting the 14 pulsating stars (pulsars) in the Milky Way galaxy. A 15th line reaches far to the right and symbolizes the center of the galaxy where Earth is located.

"The pulsars are the only stars in the galaxy that are running down at known rates, like clocks," explained Dr. Carl Sagan of Cornell University, one of three persons who conceived and designed the plaque. "They are ideal symbols to specify where the spacecraft came from and when."