Countdown clocks began ticking at the Kennedy Space Center Tuesday for the launch of the shuttle Atlantis Friday on a high-priority flight to send a robot mapping probe to Venus.

NASA test director Terry Willingham issued a "call to stations" at 8 a.m. to formally start the second shuttle countdown of 1989, a carefully orchestrated procedure featuring more than 35 hours of built-in "hold" time to give technicians a chance to handle any problems that might crop up.Atlantis's five-member crew - commander David Walker, 44, co-pilot Ronald Grabe, 43, Mary Cleave, 42, Mark Lee, 36, and Norman Thagard, 45 - planned to fly to the Kennedy Space Center from Houston later Tuesday.

The countdown is scheduled to hit zero at 2:24 p.m. Friday when Atlantis is to take off on the 29th shuttle flight, the fourth since the 1986 Challenger disaster and the second of at least five missions planned for this year.

The goal of the flight is the deployment of a costly spacecraft named Magellan, the first U.S. interplanetary probe to be launched in 11 years and the first built for deployment from a space shuttle.

At launch pad 39B Tuesday, technicians worked through the opening stages of the countdown, clearing the way for loading supercold liquid hydrogen and liquid oxygen aboard Atlantis Wednesday to power the ship's electricity-producing fuel cells. The shuttle will be fueled for launch early Friday.

Magellan, made up mostly of spare parts left over from earlier space missions, is equipped with a single science instrument: a complex radar system capable of peering through the clouds that blanket Venus, allowing scientists to create a high-detail map of the planet's surface.

Built by Martin Marietta Astronautics Group of Denver, Magellan is scheduled to be released from Atlantis's payload bay at 8:42 p.m. Friday, six hours and 18 minutes after liftoff from the Kennedy Space Center.

Because of the limited power of Magellan's booster and the positions of Earth and Venus, NASA must get Atlantis off the pad before the end of May or the $530 million mission will face a two-year delay.

The positions of the planets also define the launch time on any given day during the monthlong "launch window," and on Friday, 2:24 p.m. is the earliest the shuttle can go.

But an afternoon launch means it will be nearly sunset in Africa where the ship's crew would have to attempt an emergency landing in the event of a major problem during the climb to orbit.