CAPE CANAVERAL, Fla. (AP) _ Venus is the Earth's ugly sister, a place of killing temperatures, acid- laced clouds and high pressures. But scientists think Venus may know planetary family secrets about the solar system childhood.

The Magellan robot craft, America's first space probe in 11 years, is on its way to Venus to learn some of those secrets.

The $550 million craft will use a powerful radar imaging system to map the Venus surface where scientists believe is imprinted the geologic history of the planet. And by studying that history, scientists hope to learn about what they call ''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.

''And yet,'' said Fisk, ''it has evolved into such a substantially different object. 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.

''It would be a terrible place to live,'' he added.

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?''

Science hopes Magellan will answer some of those questions.

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

Signals from the radar 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 views of evidence of powerful geologic forces at work on Venus.

Scientists expect to able to detect thousands of volcanoes erupting from baked Venusian plains, along with vast mountain ranges with peaks that soar far higher than the Earth's Mt. Everest.

The experts also will look for indications of continental drift, the tectonic force that causes rock plates to move about the globe at the rate of inches per epoch.

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.

Craters, carved out by boulders from space smashing into the shallow surface crust of the planet, should scar virtually ever sector of Venus. These will be of particular interest to experts, said Fisk, because the age of a body can be calculated by the number and distribution of craters.

By the time Magellan completes its mission, in 1991, Fisk said science may know more about the geologic youth and middle age of Venus than they do of the Earth. These are the ''missing chapters'' obscured on Earth by erosion and oceans.

''Since the Earth is 70 percent covered by water and we have surveyed the oceans bottom in only a limited way, there is much we don't know about this planet,'' said Fisk. ''By looking at virtually all of Venus, we will really have a better geologic history of Venus than we do of the Earth.''