The astronauts of the space shuttle Discovery were within 1,000 miles tonight of an abandoned Navy communications satellite that they will try to salvage this weekend in what promises to be one of the program's most daring and ambitious missions.

Closing the gap each time they circled Earth, the five astronauts expect to be flying Discovery in formation early Saturday with the drum-shaped Leasat 3 satellite dead in orbit since April 13.

Tonight, Discovery was making up the 1,000-mile gap at a rate of more than 140 miles every 90 minutes.

"The crew has had an outstanding day," Flight Director William Reeves said at Johnson Space Center in Houston. "We've had two good course corrections today, and everything we've seen points to a good rendezvous on Saturday morning."

Crew members Joe H. Engle, Richard O. Covey, John M. Lounge, James D. van Hoften and William F. Fisher face a major project when Discovery moves within 35 feet of the satellite.

Leasat is a lifeless drum 20 feet long, 14 1/2 feet wide and weighing 15,000 pounds, more than half of which is rocket fuel.

Almost half of the fuel is liquid hydrazine, which has been through so many freeze-thaw cycles in its four months of orbit that ice may have formed in the fuel lines and ruptured some pipes.

The satellite's first-stage engine is identical to the motor that drives the Minuteman missile and could conceivably ignite at any time as the astronauts maneuver to make repairs.

"We have never given this salvage attempt better than a 50-50 chance of success, and we still don't," said Marvin Mixon, vice president of Hughes Aircraft Corp., which built the satellite and is paying NASA $8.5 million for the salvage attempt, which involves two space walks.

The first is to begin at 8:08 a.m. EDT when van Hoften and Fisher don spacesuits and step outside Discovery into its cargo bay.

Inside the cockpit, Lounge will be at the controls of the shuttle's mechanical arm, which he will use to hold van Hoften in place next to the satellite and to position the satellite so van Hoften and Fisher can work on it.

Van Hoften's first task will be to stop Leasat's motion. Engineers believe it is spinning very slowly, no more than one revolution a minute.

Once van Hoften has stabilized Leasat, he will fasten a capture bar to its side so he can grasp it with gloved hands and move it around, enabling Fisher to begin repairs.

Fisher first must disarm a timing lever that protrudes from the bottom of the satellite and must be disabled so it does not set in motion an internal clock that would ignite the rocket engine.

Then he must begin trying to install two electronic boxes and a wire harness to bypass a malfunctioning electrical timing circuit in the satellite. The maneuver is much like jump-starting a dead automobile battery.

So difficult, tedious and time-consuming are the countless tasks faced by the two men that flight directors have given them two days to complete the work.

Saturday's space walk is due to last almost seven hours. A second space walk of at least four hours is scheduled Sunday.

"We still think we need two space walks to do everything," Reeves said. "The chances of finishing with one are entirely dependent on how lucky they are."

Reeves said the shuttle's robot arm, whose damaged elbow motor is normally operated with computer assistance, can be worked only by using its backup drive motors to move the shoulder, elbow and wrist joints independently.

This means that the shoulder must be moved before the elbow and the elbow before the wrist so Lounge can maneuver the end of the arm to perform tasks.

"Some of these tasks depend on how much light Lounge has and how well he can see what he's doing from the cockpit window," Reeves said. "There might be times when everybody has to just sit there and wait for the sun to come up . . . ."

The crew also had Hurricane Elena on its mind today.

"I've got a lot of folks down there who I know are hoping it misses them," pilot Richard O. Covey said from orbit 220 miles up. He is from Fort Walton Beach in the Florida Panhandle where his mother, Patricia, lives.

Meanwhile, the nation's fourth shuttle, Atlantis, was moved to the launch pad at Kennedy Space Center here before dawn today to begin preparations for test firing Sept. 12, officials said. Atlantis is expected to be launched for the first time Oct. 3 on a secret military mission