The apparent failure of a clock on a communications satellite deployed today from the space shuttle left the satellite like "a time bomb" trailing Discovery and its crew of seven in orbit.

As Discovery's crew and flight directors at the Johnson Space Center in Houston pondered ways to repair and recover the satellite -- including a possible spacewalk -- the shuttle was kept 45 miles away and turned bottomside toward the fuel-laden satellite to protect it from a possible blast.

The second of two satellites deployed in orbit by Discovery, the Navy's Leasat, was pushed out of the shuttle's cargo bay this morning on time and apparently without difficulty by commands sent to Leasat by astronauts Margaret Rhea Seddon and Jeffrey A. Hoffman. However, the satellite's engine, which was to lift it into a permanent geostationary orbit 22,300 miles above Earth, failed to fire.

The failure apparently was caused by a faulty timing mechanism on the satellite. It was to have directed the satellite through a sequence of steps that would have sent it into permanent orbit.

"Our best guess is that the post-deploy sequencer timer never was activated," flight director John Cox said at the Johnson Space Center.

"The reason we think that is that the antenna should have popped out about a minute and 20 seconds after deployment, and the satellite should have been put into a 15-rpm revolutions per minute spin to stabilize it. Neither of those things happened."

Engineers at Hughes Aircraft Corp. in California tried to simulate the conditions in which the 14-foot-diameter, 7 1/2-ton satellite failed.

"If we can figure out what happened and duplicate it here on Earth, one of the astronauts might be able to put on a space suit and go outside attached to the end of the shuttle's robot arm and fix it," Hughes Vice President Marvin Mixon said. "If he can get close enough to the satellite, there's a chance he may be able to move the lever to start the timing mechanism and get back out of there and inside in enough time to be far away from the satellite when its engine fires."

The Leasat clock is supposed to be triggered as the satellite is deployed.

"We're taking pictures, guys," Seddon said from Discovery's cockpit after astronauts Karol J. Bobko and Donald E. Williams maneuvered Discovery to a position about 45 miles ahead of the satellite so exhaust fumes from the engine firing would not damage the shuttle or fog its windows. "Unfortunately, we're looking at the dark."

The satellite's main engine is the same as one used in the Minuteman III intercontinental ballistic missile and contains 7,370 pounds of solid rocket fuel. The engine is ignited by electrical connectors directed by the satellite's clock.

The satellite's second-stage engine uses liquid fuel to take it to its final orbit. This engine burns 4,092 pounds of nitrogen tetroxide and dimethyl hydrazine, extremely combustible chemicals that ignite on contact with each other. The satellite also carries fuel tanks holding 352 pounds of hydrazine for use in controlling its position in orbit.

"That thing is literally a time bomb right now," one source at the Johnson Space Center said, "especially since we don't know if the clock started or not."

Discovery and its crew of Bobko, Williams, Seddon, Hoffman, S. David Griggs, Charles D. Walker and Sen. Jake Garn (R-Utah) were orbiting 45 miles in front of the satellite this evening. Bobko had positioned Discovery so its underbelly faced the satellite in case the timing mechanism started and fired an engine. There also was a chance the satellite could blow up and send fragments scattering through space.

One plan discussed by engineers at the Johnson Space Center and at the California headquarters of Hughes Aircraft, which built the Leasat satellite, was to have Discovery rendezvous with the satellite and inspect it to determine whether anything is visibly wrong with it.

"Of course, we don't know how safe or unsafe the satellite is right now," a Johnson Space Center source said. "Any attempt to rendezvous with that satellite might be too dangerous."

If Discovery's crew could get close enough, it might be able to use the space shuttle's 50-foot robot arm to nudge the satellite and get its timing mechanism to start.

A lever on the satellite is supposed to be engaged to start the timing mechanism when the craft is deployed. Close inspection might reveal that this lever was never operated. The possibility of using the shuttle's robot arm to trip the lever was being discussed, in addition to the possibility of a spacewalk.

The lever cannot be operated by remote control from the ground.

Engineers also were considering leaving the satellite where it is, having the shuttle fly as far away from it as possible and then just keeping watch in hope that it falls out of orbit and burns in the upper reaches of Earth's atmosphere.

Ironically, the man who might have helped Discovery's crew make quick decisions about the satellite was bumped from this flight more than a month ago and reassigned to a mission in August. He is Gregg Jarvis, an engineer with Hughes Aircraft Corp., who led the team that designed Leasat. Jarvis was dropped from this flight when the National Aeronautics and Space Administration reassigned French scientist Patrick Baudry to a flight in June to make room on this flight for Garn.

The satellite reportedly is insured for $80 million to $85 million by the company that insured the two Hughes satellites that failed to reach proper orbits last year after their engines misfired. Both were later retrieved.