Venus may be "a terrible place to live," but scientists believe the planet's torrid temperatures, incredible pressures and sulfuric acid rain may be a great place to learn about the solar system's evolution.

A robot craft called Magellan, the first American space probe in 11 years, is on its way to survey Venus for clues to the forces that molded all the inner planets orbiting the sun.The $550 million craft will use a powerful radar imaging system to map Venus' surface, where scientists believe the geologic history of the planet is imprinted. By studying that history, scientists hope to learn what's contained in "the missing chapters" of solar system evolution.

Lennard Fisk, NASA's chief scientist, said Venus is known as the Earth's sister planet because it is about the same size as Earth and close to the same distance from the sun, in astronomical terms. Venus, the second planet from the sun, is 67 million miles away, while Earth, the third planet, is 93 million miles out.

Despite the similarities, Venus "has evolved into a substantially different object," Fisk said.

"It has an atmosphere that is 90 times that of the Earth. It has a runaway greenhouse effect. It has a surface temperature of 900 degrees. And it rains sulfuric acid," Fisk said. "It would be a terrible place to live."

The Earth and Venus, along with Mercury and Mars, are known as the "inner planets" because of their grouping nearer to the sun than the "outer" planets, such as Jupiter, Saturn and Uranus.

The family of inner planets are all thought to have accreted, or formed, 4.5 billion years ago. In the beginning, it's believed, Venus and Earth were near-twins in size and character.

But while the Earth went on to become a lovely, gentle world thriving with life, Venus became a hellhole - so hot that lead melts on its surface.

"You've got to ask yourself why?" Fisk said. "You've got to ask yourself if our own environment is that fragile? With only small changes, could Earth evolve into a different place?"

Magellan may help answer those questions.

The spacecraft is scheduled to slip into Venus' orbit in August 1990 and spend the following 243 days mapping the planet's surface with the most sophisticated radar ever sent to deep space.

Radar signals are able to penetrate the dense cloud cover that conceals Venus from telescopes on Earth. Radar views from Magellan will be enhanced by NASA computers to give clear pictures of powerful geologic forces at work on Venus.

All the data could help experts ponder a fundamental question: Is the Earth, like Venus, fated to one day become a barren, lifeless world backing in superheat and bathed by acids?

Scientists expect to find thousands of volcanoes rising from the baked Venusian plains. Venus also has vast mountain ranges with peaks towering higher than the Earth's Mount Everest.

Most scientists believe the runaway greenhouse effect on Venus, caused by its thick atmosphere of carbon dioxide, long ago caused all water on the planet to boil away.

The experts also will look for indications of continental drift, the force that causes rock plates to inch about the globe over millions of years.

If there is evidence of volcanism and crustal drift on Venus, it will prove that the planet retains enough internal heat to melt rock. The internal heat machine is what creates the lava for volcanoes and drives the movement of continents.

Meteors crashing into the shallow surface crust of the planet are expected to have left craters in every sector of Venus. Fisk said these scars will be of particular interest to experts. The age of a solid planet or moon, he said, can be calculated by the number and distribution of craters.

"Knowing whether Earth is a fluke or just one of the natural consequences of the evolution of a generic solar system" will help answer such questions, said Laurence Soderblom of the U.S. Geological Survey in Flagstaff, Ariz.