CAPE CANAVERAL, FLA., AUG. 3 -- Electrical power surged into the space shuttle Discovery today as NASA began readying it for the first post-Challenger flight next June.

Meeting a date set last January, engineers switched on Discovery's power for the first time in more than a year, checking out the electrical system. As the electricity flowed, a sign flashed "Vehicle Powered" and engineers and technicians working with the vehicle in a hangar clapped and cheered.

Workers then turned on cooling systems and began a systematic check of the shuttle's instrumentation, communications, radar and other systems.

"Significantly, we have reached our first milestone here in returning the shuttle fleet to flight status," said launch director Bob Sieck.

The three remaining shuttles have been grounded since Challenger exploded 73 seconds after launch on Jan. 28, 1986, killing all seven crew members.

The National Aeronautics and Space Administration has set a target date for launch of June 2, 1988, although some NASA officials privately question whether that date is realistic and think it may slip by two or three months. Until the latest date was set in May, NASA had been firmly predicting launch on Feb. 26, 1988.

For several months, Discovery has undergone modifications to several systems. Most are designed to make the shuttle safer in the wake of the Challenger tragedy; others had been planned before the accident.

John Hallmark, Discovery's lead engineer, said 84 of 190 planned modifications have been made, with the rest to be done during the flight processing.

The modifications will improve the spacecraft's structural, communications and instrumentation systems, fuel valves, landing-gear door and the protection that keeps a shuttle from burning up when it reenters Earth's atmosphere.

Meanwhile, in Littleton, Colo., Martin Marietta Corp. rolled out the first of 13 Titan II missiles being converted to space launch vehicles under a $528.9 million Air Force contract.

Air Force Secretary Edward C. Aldridge Jr. said the 103-foot, obsolete intercontinental ballistic missiles would become the workhorses of U.S. space-military programs. Launches are planned through 1995.

The Titans will be used to launch a military weather satellite from California's Vandenberg Air Force Base next spring.