The planned launch of a $378 million Venus probe from the shuttle Atlantis April 28 signals the "rebirth" of America's interplanetary space program, grounded nearly three years by the Challenger disaster, astronauts said.

"I believe that we are looking at a rejuvenation of a planetary space program that's very important and has been in the doldrums for a while due to budgetary concerns and, of course, recently for lack of launch vehicles," Atlantis skipper David Walker said Monday.Atlantis, hauled to the launch pad last week, is scheduled to blast off at 12:24 p.m. MDT April 28 on the year's second shuttle flight, the fourth since the 1986 Challenger disaster.

Atlantis' payload, the Magellan Venus radar mapper, was installed in the shuttle's cargo bay Saturday. The spacecraft is designed to map the surface of Venus with a sophisticated cloud-piercing radar, greatly expanding mankind's knowledge about the second planet from the sun.

Joining Walker, 42, aboard Atlantis for the showcase mission will be co-pilot Ronald Grabe, 41, Mark Lee, 39, Norman Thagard, 43, and Mary Cleave, 41.

Six hours and 18 minutes after liftoff, Lee plans to launch the Magellan. The deployment will mark the first launch of an interplanetary spacecraft from NASA's manned shuttle and the first of three such cargoes scheduled for launch over the next year: Magellan, the Galileo probe to Jupiter and the Ulysses sun-study spacecraft.

Said flight director Milt Heflin: "We're beginning a very scientifically exciting year for the agency. I'm looking forward to the time when we'll send Magellan on its way."

Because of the positions of Venus and Earth, and the power of Magellan's booster rocket, NASA's "launch window" is only 18 minutes long, meaning Atlantis must get off the pad by 12:42 p.m. on launch day or the flight will be delayed 24 hours. The overall launch period for Magellan ends May 28.