What could become the most hair-raising rescue attempt in space history is to begin Saturday morning as five astronauts fly the space shuttle Discovery away from Earth on a planned eight-day mission, during which they will try to revive a Navy communications satellite that has been lifeless in orbit since April.

Next week's attempt to salvage the fuel-laden Leasat satellite promises to be the highlight of Discovery's mission, the 20th of the shuttle program. The launch is scheduled for 8:38 a.m. EDT.

"Even the weather forecast is all good news right now," Air Force Lt. Scott Funk, the shuttle weather officer, said at Kennedy Space Center here. "There might be some isolated rain showers offshore at launch time, but that's all."

The flight commander is Joe H. Engle, who in 1981 flew the second shuttle mission. Engle, an Air Force colonel, will be 53 on Monday. Other crew members are Air Force Lt. Col. Richard O. Covey, 39; Dr. James D. van Hoften, 41, an engineer; Dr. William F. Fisher, 39, a physician, and John M. Lounge, 39, an engineer.

Discovery is to launch communications satellites for the American Satellite Co., the Australian government and the Hughes Communications Corp., which will lease its satellite to the Navy.

The ASC satellite is to be deployed from the shuttle's cargo bay Saturday, Australia's satellite on Sunday and the Hughes-Navy satellite on Monday.

The National Aeronautics and Space Administration is being paid nearly $40 million for the delivery service. In addition, Hughes is paying $8.5 million for the rescue attempt on its Leasat, almost a twin of the Hughes satellite about to be launched.

The job of reviving the 15,000-pound Leasat may be the hardest task an astronaut crew has attempted. The satellite holds more than 5,000 pounds of liquid rocket fuel in two tanks that have been through thousands of freeze-thaw cycles, which may have clogged the fuel lines with ice or even ruptured the pipes.

"These liquid fuels are the key to this whole salvage attempt," said Marvin Mixon, a Hughes vice president. "If we can warm up whatever liquids are frozen, we think we'll be okay."

The satellite also carries 7,500 pounds of solid rocket fuel in its Minuteman motor, identical to the engine in the Minuteman missile. "There is a remote chance this motor could ignite at any time," Mixon said, "but the fact that it hasn't done anything since April 13 is a strong suggestion it is dormant like the rest of the satellite."

Hughes and NASA believe the satellite is dormant because the timing device to set everything in motion never came on. But since the clock could turn on accidentally, it will have to be bypassed by Discovery's crew if the salvage attempt is to succeed.

"We have to safe the timer and safe the Minuteman motor," Mixon said. "These issues have been examined in enormous detail and we believe we can carry all of it out in a safe way."

The task of reviving the satellite, which Mixon rated "50-50," will be attempted by van Hoften and Fisher, who will don spacesuits after the shuttle has rendezvoused with the satellite and walk in space during the salvage attempt.

Van Hoften, who will be attached to Discovery's 50-foot-long mechanical arm, will attempt to immobilize the 14 1/2-foot-wide satellite, which is spinning slowly, with his hands. Van Hoften has trained in underwater tanks dozens of times for the task.

Fisher will be standing by inside the cargo bay, about 35 feet below the satellite. Once van Hoften has immobilized the satellite, he will turn it around so that Fisher can disarm a lever on it that could trigger the timing device. When that is done, Fisher will remove a panel on the side of the satellite and plug in a pair of electrical cables that will link the satellite with the ground.