CAPE CANAVERAL, NOV. 9 -- NASA's space shuttles are strong enough to safely carry heavier payloads than previously allowed, enabling the agency to launch 50 tons of additional cargo into orbit through 1993, officials said today.

The increase will help the National Aeronautics and Space Administration and the Defense Department relieve a large backlog of satellites grounded by last year's Challenger disaster and a string of unmanned rocket failures.

It also will play a key role in space station operations in that more experiments and cargo can be returned from space on a single flight.

"The total space shuttle performance capability requires a balance between lift capacity to orbit and the allowable return weight during reentry and landing," Rear Adm. Richard H. Truly, chief of the shuttle program, said in a statement.

"This new capability will improve this balance and add considerable flexibility and efficiency to our space transportation system."

The increase in the shuttle's allowable "down weight" -- the weight of both the shuttle and any on-board cargo -- was made possible by an extensive structural analysis of the forces experienced by the craft during landing maneuvers.

Previously, the overall weight limit was 211,000 pounds. It now is 230,000 pounds.

"Our initial analysis indicates that this change will allow the space shuttle to carry a cumulative weight in excess of 100,000 pounds of additional payloads into orbit through 1993," Truly said.

In the wake of the Challenger disaster, NASA engineers have been reexamining a variety of issues to improve flight safety, including the structural "loads" experienced during liftoff and landing. The first post-Challenger flight is scheduled for next June.

If a shuttle is too heavy, the forces experienced during landing maneuvers could cause structural damage or even catastrophic failure.

In a related effort, NASA is considering restricting all future Spacelab flights to the shuttle Columbia, which is heavier than its counterparts, Discovery and Atlantis.

Doing so, NASA said, would allow the agency to make "optimum use" of Discovery and Atlantis, presumably to carry additional cargo into orbit.