The crew of Discovery made a valiant attempt today to revive a Navy satellite adrift in orbit but failed to bring it to life despite repeated nudges by their only available salvage instrument, a makeshift extension of the shuttle's 50-foot mechanical arm.

While flight directors held out some hope for a future effort to retrieve the satellite -- insured for $85 million -- none was optimistic about bringing back a craft loaded with six tons of rocket fuel.

It was the danger posed by the fuel that led flight directors to order the Discovery crew to give up and back away from the satellite.

"The window is closed," astronaut David Hilmers told the crew from Mission Control in Houston. "Perform the separation maneuver."

For a few minutes the astronauts seemed reluctant to quit, but they finally yielded and Discovery's daring and unplanned rescue mission was over.

"You guys did everything you could," Hilmers said. "It was a great job, a great try."

Almost as a reward for the exhausting three days that Discovery's crew put in trying to pull off the unplanned rescue mission, flight directors tonight gave them the option of landing back at Florida's Kennedy Space Center on either Thursday or Friday morning.

"We consider the mission over," Hilmers said to the crew. "It wouldn't bother us if you prefer to land on Friday."

Replied Commander Karol J. Bobko: "We'd just as soon stay up an extra day."

"We're giving you the choice."

"We'll stay up."

Until the last few minutes, this morning's rescue mission had proceeded flawlessly. In a matchless display of flying skill, Bobko and Pilot Donald E. Williams brought the 100-ton spaceliner so close to the spinning 7 1/2-ton satellite that television viewers could see Earth's clouds reflected in the satellite's shiny metal skin.

Again and again, astronaut Margaret Rhea Seddon moved a set of makeshift extensions fastened to the robot arm onto an arming lever suspected to be the culprit in the satellite's failure to move out of the orbit it has been drifting in since Saturday.

Nothing worked. Twice, Seddon appeared to snare the arming lever hard enough to trigger a response, but nothing happened. Once, the arm put a strong enough hold on the lever to slow the satellite's spin, but still nothing happened.

Flight directors and mission managers at Houston's Johnson Space Center had decided in advance to limit Discovery to a six-minute attempt on a single orbital path if the robot arm made any solid contact with the satellite. Their ruling was made with crew safety in mind since the stranded satellite's fuel tanks could ignite on any disturbance.

"We think we made hard contact on at least two occasions," Bobko said.

"We broke the rungs on the swatter the arm extension looks like a fly swatter , and we hit it hard with the base of the cone," he added.

"We concur," Hilmers said. "It was a great effort."

Bobko made one last plea, asking Hilmers if ground control wanted him to stay close enough to the satellite to see if its clock might start, its antennas unfurl, its spin speed up and its engine fire.

"Negative," Hilmers replied. "That isn't necessary. It doesn't look like it's going to do anything."

With that, flight directors called off the salvage attempt and ordered the crew to back away to a safe distance from the satellite.

Seeming disappointed and a little disheartened, the flight directors on the ground tried to put the best face on today's events.

"This team's been under a lot of tension the last few days," flight director Randy Stone said, "but I just want to say I'm extremely proud to be part of a team that took on such an enormous task."

No one was optimistic about recovering the satellite on a future mission. "The question is can you make those two motors on the satellite safe," flight director Jay Greene said. "I think there are possibilities, but I think it's premature to discuss these possibilities."

Steven Dorfman, president of Hughes Communications Corp., which owns the Navy-leased satellite, went a little further than Greene.

Hughes almost lost two other communications satellites last year when engines built by McDonnell Douglas failed to fire, but in November a shuttle crew retrieved them in orbit and brought them home for repair.

"There are more problems associated with this recovery than the last one," Dorfman said. "These problems the loaded fuel tanks may make it impractical to recover this satellite."

If recovery turns out to be impractical and unsafe, the Navy satellite will remain adrift in an egg-shaped orbit for the next few years before the high point of its orbit -- now 270 miles -- begins to decay. The satellite finally will fall out of orbit and burn up in Earth's atmosphere in what could be a spectacular show of "fireworks."

"There's no reason to expect there will be anything hazardous about the reentry of this satellite," Greene said. "There's no danger to anybody on Earth from this satellite."

Dorfman said the stranded satellite is insured for $85 million with Merrett Syndicates Ltd. of London and International Technology Underwriters of Washington, the underwriters that insured the Westar VI and Palapa B satellites that Hughes almost lost in orbit last year.

"The insurance money is part of a policy that covers four of these satellites," Dorfman said.

"We have two in orbit now and must put two more in orbit to fulfill our contract with the Navy. The $85 million covers the successful launch of a fifth satellite if one should fail," he added.