With all the hoopla and attention paid to the space shuttle Atlantis as it successfully soared into Earth orbit this week, the real importance came six hours later when the crew launched an unmanned robot spacecraft toward the planet Venus. It was the first time in 11 years that the United States has engaged in interplanetary exploration.

Once the jewel of the American space program with dramatic picture-taking flights past Jupiter, Saturn and Uranus, robot landings on Mars and earlier probes of Venus, interplanetary exploration dwindled to a halt in 1978, mostly because of budget problems.However, the flight of the Venus robot - named Magellan after the 16th-century Portuguese explorer - opens the door to an ambitious new exploration of the solar system and beyond, a program stretching over the next 17 years.

The nation's space agency has been accused of a lack of imaginative planning, but the projects that lie ahead are exciting ones. In October, another shuttle crew will launch the Galileo robot probe to Jupiter. Early next year, the Hubble Space Telescope will be put into orbit, giving an unprecedented clear look at the universe, unhampered by the Earth's atmosphere. Another robot probe will be put in orbit about the sun in 1990 and an unmanned spacecraft will be dispatched to Mars in 1992.

The Magellan flight to Venus is fascinating because Venus, a close neighbor to Earth and about the same distance from the sun, astronomically speaking, is so different from Earth and mysterious because it is perpetually shrouded by dense clouds.

What is known about Venus is enough to make clear that while it would be interesting to fly past, one wouldn't want to live there. The atmosphere is dense, the surface temperature is about 900 degrees, and when it rains, it rains sulfuric acid.

Magellan, after a looping 15-month voyage covering 806 million miles, will drop into orbit around Venus and radar map some 90 percent of the planet's surface. The high-resolution radar will pick up objects as small as a football field, 10 times the clarity of any previous radar pictures of Venus.

It promises to be a fascinating project that will contribute vast amounts of knowledge to the understanding of the solar system.