The dense clouds that have shielded the surface of Venus from human eyes have been lifted by a small robotic satellite sent to explore a hidden world, and the story of Venus is finally being told.

The mysteries that the Magellan spacecraft have unveiled are of more than pure scientific interest, because they address the fundamental question of why the Earth evolved into a planet that encouraged the blossoming of a diversity of life forms. Venus is Earth's sister planet, similar in size, mass and distance from the sun, yet so inhospitable to life that nothing can live there. Its dense, poisonous atmosphere would smother a human in a split second; its surface is so hot from an intense greenhouse effect that it would melt lead.

Until recently, scientists were frustrated in their efforts to study Venus because of the dense atmosphere that blinded earthbound telescopes. That began to change in 1962 with the flight of NASA's Mariner 2, the first spacecraft to visit the planet. Mariner told them something about Venus, including the surprising fact that it has no magnetic field, but the surface of the mysterious planet remained hidden.

During the following years, while the National Aeronautics and Space Administration concentrated on sending humans to the moon, Soviet scientists developed a series of spacecraft designed to penetrate the heavy atmosphere. That proved a challenge, but in 1970 they succeeded with Venera 7, and what a story it told.

The Soviet craft managed to briefly transmit data about the atmosphere, but no pictures of the surface, and the results were almost unbelievable. They showed that the atmosphere is indeed crushing. A square inch on the surface of Venus is under an atmospheric pressure of two-thirds of a ton--90 times greater than on Earth.

What kind of surface could lie beneath those oppressive clouds? Radiotelescopes on Earth gathered hints, and in the 1970s, scientists began to get their first answers to that question. NASA's Pioneer Venus Orbiters used radar to map 93% of the planet's surface, revealing what many had long suspected. Venus has--or at least had--volcanoes, accounting for an atmosphere rich in carbon dioxide and sulfuric acid. But those pictures were of low resolution, and many a scientist had difficulty trying to decipher the topography.

And then came Magellan.