Four men and a woman are to leave Earth aboard the space shuttle Discovery Wednesday morning on one of the most ambitious missions astronauts have ever flown: to recover two errant satellites and return them to Earth for repairs and re-use at the behest of insurance underwriters.

In each intricate rendezvous, two astronauts will venture outside Discovery and one will latch onto the spinning satellite with a special device, then maneuver it toward the shuttle so that its robot arm can grab and stow the satellite.

A consortium of insurance underwriters had to pay $180 million when the satellites -- one owned by Indonesia and the other by Western Union Co. -- went into useless low orbits in February. But the underwriters hope to recover most of the loss -- and the $5.5 million they are paying the National Aeronautics and Space Administration for the recovery -- by repairing and reselling the satellites.

"We are intending to sell them to the highest bidder," said Stephen Mewrrett, an underwriter for Lloyd's of London.

Meanwhile, the program to launch a shuttle each month was jolted when officials announced that insulation problems have forced grounding of Challenger, which was scheduled to ferry a top-secret military satellite into orbit Dec. 8.

NASA said Challenger is not expected to be ready until "three to six weeks" after the original December date.

Communications-satellite customers around the world are watching the salvage operation, which was undreamed of until the pair of $35 million drums were lost in space eight months ago.

The recovery is the top priority of the space shuttle's 14th flight. It is due to lift off at 8:22 a.m. EST Wednesday and set two other communications satellites into orbit, one for Canada and one for the U.S. Defense Department.

"It's just the kind of challenge that a lot of people in this business thrive on," shuttle Program Director Glynn S. Lunney said at the Johnson Space Center in Houston, where the salvage scheme was hammered out from scratch over the last eight months.

When the 1,200-pound satellites, built by Hughes Aircraft Corp., were deployed from the space shuttle's cargo bay in February, their onboard engines flamed out within seconds. Both satellites ended up no more than 600 miles above Earth instead of the 22,400-mile orbit for which they were targeted.

NASA and Hughes Aircraft engineers immediately set to work on a salvage scheme.

"It was almost God-given that the satellites ended up in an orbit that at least permitted consideration of their retrieval," said Lunney.

The ultimate scheme to salvage two satellites that were never built to be salvaged grew out of a combination of hard work and engineering genius. Said Lunney: "We had to invent things as we went along."

First, Hughes Aircraft had to find a way to disarm the satellites' solid rocket motors, which still carried 1,200 pounds of fuel that could endanger the astronauts. It did this last March by firing the engines to move the two satellites up and down like yo-yos. The maneuvers also placed the satellites in safe orbits to await their retrieval.

In August, a second set of maneuvers placed the two satellites precisely on opposite sides of the Earth from each other. This permitted ground stations in six countries to send commands to one satellite and then the other, ordering them down from their 600-mile-high orbit to one within reach of the shuttle. Those maneuvers were finished a week ago, and the satellites are now in orbits that take them as close as 230 miles to Earth.

The salvage operation will begin late in the week when Discovery Commander Frederick R. Hauck and Pilot David M. Walker perform the first of a series of 44 intricate maneuvers to catch up with the Palapa satellite built for Indonesia.

"I think once we get up there and we're station-keeping on this satellite with it right over the bay at 35 feet, I'll be breathing a little sigh of relief and saying, hey, the hard part is over," Hauck said.

The hard part will have, in fact, just begun. While Hauck, Walker and Anna L. Fisher stay inside Discovery's cockpit, astronauts Joseph P. Allen and Dale A. Gardner, wearing space suits, will move into the cargo bay and begin the Rube Goldberg part of the mission. Wearing a jet-powered backpack and attached to a six-foot "stinger" device that resembles an umbrella held backwards, Allen will fly out to the Palapa satellite. Its electrical circuits will have been turned off by ground command, leaving it a dark, passive drum spinning at one revolution a minute.

Allen will insert the probe into the bell-shaped engine nozzle at the bottom of the satellite and pull a lanyard to force eight large toggle bolts onto the sides of the engine to fasten Allen to the satellite.

Allen will then use his backpack jets to stop the satellite's rotation and move it toward the shuttle. Fisher, the fourth U.S. woman in space and first mother, will use the shuttle's 50-foot mechanical arm to grapple the satellite and maneuver it into the cargo bay.

Gardner, anchored to the bay in two specially-built foot restraints, will be the "muscle man" in the operation. He and Allen will remove the stinger from the engine nozzle and, assisted by Fisher and the robot arm, will wrestle the satellite into the bay and batten it down with large electrically driven bolts.

The scheme is to take six hours, almost the limit of a spacewalker's endurance and oxygen supply.

Reversing their roles, Gardner and Allen will repeat the same procedures next Tuesday when they attempt to retrieve the Westar VI satellite built for Western Union.