Two astronauts went on history's first unrehearsed spacewalk today and rigged the shuttle Discovery's mechanical arm for a bold attempt to revive a Navy communications satellite stranded in orbit since Saturday.

Discovery is scheduled to begin nudging up to the satellite about 7:30 a.m. and to perform the critical life-restoring maneuver at about 9 a.m.

Without planning or practice, astronauts Jeffrey A. Hoffman and S. David Griggs spent more than three hours today in Discovery's open cargo bay putting three extension "hands" on the 50-foot-long robot arm. They looked as if they had rehearsed the spacewalk for months.

"Is that beautiful or is that not beautiful?" Griggs exclaimed as he floated back from the mechanical arm to admire his handiwork.

Said astronaut Margaret Rhea Seddon from Discovery's cockpit: "Face the cameras and take a bow."

"We did this contingency EVA extra vehicular activity as well as any we've ever planned," flight director John Cox said at Houston's Johnson Space Center. "We've come a long way in 20 years."

Still ahead of the crew lay the difficult tasks of rendezvousing with the drifting, 7 1/2-ton satellite, pulling up alongside it and using the arm to turn the satellite's power on and start its clock. If the daring attempt is successful, the satellite will be boosted into a permanent geostationary orbit 22,300 miles above Earth.

In any case, the crew is scheduled to return to Earth Friday -- though a Thursday landing is possible.

The first maneuver in preparation for the rendezvous was made this afternoon with the shuttle and the satellite about 45 miles apart. Commander Karol J. Bobko and pilot Donald E. Williams flew Discovery onto a path below the satellite so the shuttle would slowly catch up with the satellite.

The crew was sent to bed just after 5 p.m. and was to be awakened at midnight to begin the final steps in the rendezvous that is to bring Discovery about 50 feet below the satellite at 7:30 a.m. EST on Wednesday. By that time, the crew should have looked over the satellite from a safe distance with powerful binoculars to make sure its 10-foot-long antennas are not deployed and it is not spinning faster than two revolutions per minute.

"This means it's safe to move in the last 10 miles or so to the satellite," flight director Cox said. "This means the timing mechanism aboard the satellite has not been turned on, which means nothing on the satellite is armed."

The last steps of the rendezvous are expected to be the toughest. Bobko and Williams are to fly Discovery with its nose pointed away from Earth to bring its open cargo bay no closer than 30 feet from the satellite. At the same time, Seddon is to maneuver the robot arm so its three extension hands almost make contact with the solar cells covering the satellite's drum-shaped body.

Seddon spent more than an hour today talking with astronaut Sally Ride in Houston about Wednesday's task. Three other female astronauts -- Judith Resnik, Anna Fisher and Mary Cleave -- took turns with Ride working the arm at the Johnson Space Center to minimize the chances that Seddon will encounter a surprise when she tries the real thing.

Female astronauts are trained to use the shuttle's robot arm for the same reason they're employed on electronic assembly lines. The average woman's fingers are smaller, more sensitive and more dexterous than the average man's.

Ride warned Seddon not to catch the arm's three extension hands on four screws protruding from the satellite:

"Stand the arm back about two feet, then lay the swatters two extension hands look like fly swatters against the satellite right after a vertical yellow stripe goes by. That's where the screws are. The swatter will either catch the arming lever or it won't."

Seddon has four minutes on each of two consecutive orbits to catch the satellite's arming lever. If she misses with one extension hand or tears it in trying, she can try another. If all her attempts fail, the astronauts will have to leave the $40 million satellite adrift in orbit.

If Seddon flips the arming lever with the makeshift extension, the crew has about 40 minutes to fly to safety. The arming lever is supposed to trip a timing mechanism that deploys the satellite's antennas, fires tiny jets to start it spinning faster and then fires the satellite's Minuteman missile engine to carry it to a permanent position in space.

"We have confidence this job can be done and done safely," Cox said. "The satellite is loaded with fuel, but we don't think the timer has ever come on and so we think those engines are safe."