The largest number of astronauts ever to leave Earth at once and the first commercial cargo ever to be carried away from Earth by astronauts were poised today to put America into the space trucking business.

If the weather in Florida and California cooperates, astronauts Vance Brand, Robert Overmyer, William Lenoir and Joseph Allen will take off at 7:19 a.m. Thursday to spend the next five days in space.

When they touch down in California at 9:27 a.m. (EST) Monday, they will have left behind a pair of $50 million communications satellites to serve the United States and Canada for the next 10 years.

Lenoir and Allen will also put on space suits and make the first walk in space since two Skylab astronauts ventured outside their spacecraft eight years ago.

"This flight has been a long time in the planning," Space Shuttle Project Manager Glynn S. Lunney said today at the Kennedy Space Center, where the space shuttle Columbia left Earth four times before on test flights but carried no satellite cargo. "We are really looking forward to deploying the satellites chartered for this flight."

The satellites in Columbia's cargo bay were booked aboard the shuttle almost five years ago at a cost to their owners of about $9 million each. Identical in size and cylindrical in shape, the satellites are owned by Satellite Business Systems of McLean, Va., and Telesat of Canada and each is built to handle as many as 10,000 transcontinental telephone calls at once.

This is the first time astronauts have served as "truck" drivers and cargo handlers in space. Each of the satellites weighs five tons and astronauts Lenoir and Allen have trained for the task of dropping off the massive satellites for more than two years.

If the flight goes as planned, the satellite owned by Satellite Business Systems (SBS) will be powered up by computer command from the shuttle cabin Thursday afternoon and spun like a top on a cradle that sits in Columbia's cargo bay just behind the crew's cockpit.

At 3:20 on Thursday afternoon, the spinning satellite will be sprung out of the cargo bay into space by Lenoir and Allen as Brand and Overmyer pilot Columbia on its sixth revolution, over the Atlantic Ocean.

The satellite is spun to prevent it from tumbling and as it spins away from Columbia, Brand and Overmyer will back the shuttle to a spot about 12 miles away to protect it from the satellite's rocket exhaust. Forty-five minutes later, the satellite will start its engine and rocket away from the low orbit occupied by the shuttle.

By the time the satellite's rocket engine comes on, the four astronauts will be positioned so that they cannot see the satellite. They will be flying with the underside of the shuttle facing the satellite and their windows facing straight up away from the satellite. This will be done so that when the satellite's engine fires, the rocket exhaust strikes the black-colored shuttle belly instead of its windows.

"As much as they want to see what's happening, the rocket exhaust could contaminate our windows even at that distance," Brand said in explaining the maneuver. "There's nothing in space to stop that rocket exhaust from moving hundreds of miles before it dissipates itself."

The same procedures will be followed at the same time on Friday afternoon to deploy the Canadian satellite owned by Telesat. The only difference is that Lenoir and Allen will drop Canada's satellite -- named ANIK or "little brother" in Eskimo -- out over the Pacific Ocean instead of the Atlantic.

Both satellites are destined to hover above the equator, where they will stay for at least the next 10 years by matching the speed of the Earth's rotation 22,400 miles above the globe. The American satellite will be positioned on a straight line to the equator from Texas, the Canadian satellite on a straight line to the equator from the Canadian Rockies, where both will sit in permanent view of all of North America.

The two satellites must be dropped off at precise times in daylight so that sensors can line the satellites up at precise angles to the sun. This must be done so that the satellites' engines can rocket them to fixed positions 22,400 miles above the Earth where they will match the Earth's rotation.

The daylight restrictions also dictate Columbia's launch time. The 100-ton spaceliner was due to leave launch pad 39A at 7:19 a.m. and could leave no later than 7:59 a.m. if anything were to delay its launch.

Another reason for the 40-minute launch window is that alternate landing sites must be in daylight if the flight is aborted for any reason. The alternate landing sites for Columbia are in Spain, Africa and California. Columbia could land in Spain or in Africa if its engines fail to take it into orbit. It could land in California near the end of one revolution if it only has enough engine power to reach an orbit of about 100 miles instead of the 160-mile-high orbit it plans to reach.

One thing still in doubt tonight was the weather in California, where more than an inch of rain had soaked the desert runways at the shuttle landing site at Edwards Air Force Base. While there is a concrete runway at Edwards for the shuttle to use at landing, a chance existed tonight that more rain might force a change in landing plans or even a delay in launch to wait for the runway at Edwards to dry out.

"We don't think there will be that much rain in the next few days," said Air Force Gen. James A. Abrahamson, associate administrator of the National Aeronautics and Space Administration. "It's just our luck that we have the only desert in the world that floods every time we want to use it."