Was NASA exercising the proper degree of caution when it allowed a tiny crack to postpone the flight of space shuttle Discovery? Or has the space agency become too timid?

Sen. Jake Garn, R-Utah, who flew aboard Discovery in 1985, says NASA lacks the courage to "take the smallest chance" in the aftermath of the Challenger accident that claimed seven lives.The space agency denies it.

"If there was a national defense or some emergency reason why we needed to go fly, I don't think any of us would have any hesitation to go fly this bird," said William Lenoir, head of NASA's space flight program. "We're being conservative."

The problem that confronted space agency executives wasn't as clear-cut as the one last year when two space shuttles developed fuel leaks. The issue then was whether flawed parts needed to be replaced for safety.

There are two 4-foot-long doors on the underside of the shuttle that are open during liftoff to accommodate huge pipes from the vehicle's fuel tank. The doors should shut tight when the empty tank drops away.

"If that door is opened anything greater than a half inch, you have big troubles," said Dan Germany, manager of orbiter systems at the Johnson Space Center in Houston.

Any crack in the thermal shield would provide a path for the intense heat of re-entry into the atmosphere and threaten the survival of the crew.

"If you get on orbit and you don't have the doors closed, it's not a good day," Germany said.

Motors on the left and right sides drive the mechanisms that close the doors. Cracks an eighth of an inch deep were found in metal "lugs" that surround a half-inch pin on each side.

Engineers thought they knew why the left lug was cracked - pad crews had tried to close it while it was latched open - but there was no explanation for the right lug failure.

"I don't think we are being too cautious," said Robert Crippen, a former astronaut who heads the shuttle program. "A lot of people believed the shuttle was safe to fly. But some elements of the (engineering) community were uncomfortable, primarily because we could not define the event which would have caused the right hand door lug to crack."

Garn, a space booster who wangled a ride on Discovery six years ago, said he'd fly the shuttle "tomorrow morning, even without fixing the cracks in the door."

The Rogers Commission, which investigated the 1986 Challenger disaster, blamed NASA for "a silent safety program" and a slavish devotion to schedule.

The shuttle was destroyed when its right-hand booster rocket leaked hot gas at a joint - a phenomenon that had been seen before and ignored.