A surprisingly hardy U.S. space probe marked 10 years in orbit around Venus Sunday, revealing the cloud-shrouded secrets of Earth's closest but hostile planetary neighbor.

Scientists, ecstatic at the unexpected longevity of the Pioneer Venus Orbiter, hope to receive another four years of steady, valuable data from Earth's mysterious twin 35 million miles away.The instrument-laden 800-pound NASA satellite is expected to finally plunge to a fiery destruction on scalding Venus sometime in 1992.

The near flawless mission has already been "spectacular," marveled Harold Masursky, a U.S. Geological Survey scientist in Flagstaff, Ariz., and a leader of the Pioneer Venus radar imaging team.

The Pioneer orbiter has helped enable NASA-Ames Research Center scientists to penetrate the choking dense layer of carbon dioxide and sulphuric acid vapor over the totally dry planet, whose hellish surface of 800 degrees Fahrenheit is hot enough to melt zinc.

The probe went into orbit on Dec. 4, 1978, after a seven-month journey from Earth. Its current path ranges from just above the atmosphere to about 41,000 miles from Venus.

"Ames has done itself proud for running a mission for many years beyong its nominal lifetime," Ma-sursky said. The orbiter has survived years beyond what NASA envisioned.

Scientists have been intrigued by Venus because its cauldronlike atmosphere is so different from Earth's. Finding out why could shed light on how unique Earth might be. The Soviets have also sent probes to study the planet.

Venus, the brightest light in Earth's predawn sky other than the sun and moon, is the second closest planet to the sun. Earth, its closest neighbor, is only slightly bigger in size and mass.

"It's a planet that started life as a virtual twin of Earth and went wrong," said mission official Thomas Donahue of the University of Michigan.

Donahue will chair a scientific symposium at NASA Ames Research Center, 40 miles south of San Francisco, on Monday to hail the achievements of the Pioneer Venus Orbiter Mission.

The Pioneer Venus Orbiter, or Pioneer Venus 1, was the lead spacecraft in a multi-probe U.S. mission to study the secrets of Venus.

It was sent into orbit around the steamy planet to take ultraviolet infrared pictures, radar maps and daily measurements for transmission back to Earth.

Closely following were four other NASA robot probes that were sent crashing to the surface on Dec. 9, 1978, from a Pioneer Venus 2 spacecraft after radio-ing back detailed information about the planet's puzzling atmosphere. They were not designed to survive the impact.

The first scientific reports from the barren landscape of Venus came in 1971 from a Soviet Venera spacecraft. NASA conducted Mariner spacecraft flybys of Venus in 1962, 1967 and 1974.

NASA plans to launch a new probe, Magellan, toward Venus from a space shuttle next April to make radar maps of the planet's surface far more detailed than the ones made by the cruder instruments aboard Pioneer-Venus and other now-defunct Soviet probes.