A countdown rehearsal was under way Friday for the launch of Atlantis and a crew of five on the first space shuttle planetary mission.

The astronauts were to board the shuttle cabin Friday to practice the final two hours of launch day procedures.Also being monitored was the Magellan spacecraft in the cargo bay. After reaching orbit, the astronauts are to release this $378 million payload on a mission to map the cloud-hidden surface of Venus.

After a 466-day outward trip, Magellan is to sweep into orbit around Earth's nearest planetary neighbor and aim a high-powered imaging radar at the surface. During 243 days, the radar is to send back the sharpest images yet of much of this mysterious planet.