Years behind schedule, NASA is finally ready to fire a robot probe to Venus from the shuttle Atlantis, kicking off a new "golden age" of space exploration and ending an 11-year hiatus in U.S. interplanetary launches.

Atlantis, carrying five astronauts and a probe named after 16th-century explorer Ferdinand Magellan, is scheduled to take off on the year's second shuttle flight, the fourth since the 1986 Challenger disaster, at 2:24 p.m. EDT Friday, April 28.Six hours and 18 minutes after liftoff, powerful springs will gently push the Magellan probe away from its cradle in Atlantis' cargo bay, and an hour later, a solid-fuel rocket is scheduled to fire, boosting Magellan out of Earth orbit to begin a 15-month voyage to Venus.

Magellan was designed to spend at least 243 days - one venusian "day" - mapping the surface of Venus in unprecedented detail, producing photographlike radar images that may help determine what caused Venus, similar to Earth in many ways, to end up the 900-degree inferno it is today, the apparent victim of a runaway "greenhouse effect."

The mission, including the Magellan spacecraft and the cost of management and data analysis, has a price tag of about $530 million.

If successful, Magellan will revolutionize knowledge about Earth's sister planet and re-establish America's leadership in interplanetary exploration after three years of post-Challenger criticism and comparison with the steady, if unspectacular, Soviet space program.

It is perhaps ironic, then, that Magellan's target is Venus, the site of past Soviet space triumphs, because the mission comes at a time when the Russian space program is in disarray, reeling from internal criticism and the failures of two costly Mars probes.

For the crew of Atlantis, the launch of Magellan will mark the high point of a four-day, 56-minute mission.

Because of the probe's weight - 40,208 pounds including the weight of its booster - Atlantis was unable to carry many "secondary" payloads into orbit.

For that reason, Commander David Walker, 44, co-pilot Ronald Grabe, 43, Mary Cleave, 42, Mark Lee, 36, and Norman Thagard, 45, plan to devote the rest of the 29th shuttle flight to extensive Earth photography, engineering and medical tests and a few relatively minor experiments.

Landing is scheduled for 12:20 PDT May 2 at Edwards Air Force Base, Calif.

The flight of Magellan will mark the first launch of an American planetary spacecraft since Pioneer Venus took off aboard an unmanned rocket in 1978, an 11-year hiatus caused by shuttle development costs and delays, tight budgets and finally, the Jan. 28, 1986, Challenger disaster.

"Magellan will be the first planetary program to launch on the space shuttle," Grabe said in an interview. "We're excited about that. It creates a nice partnership between the two elements of the space program: the manned program of the shuttle and the unmanned planetary experimentation and exploration side."

In August, for example, Voyager 2, launched in 1977, will race past distant Neptune, 2.7 billion miles away, providing scientists with the first closeup views of the distant planet, its two known moons and what may be a rudimentary ring system.