Engineers are trying to fix a tiny gas leak in the space shuttle, but there's a 50-50 chance Discovery will have to be rolled back from the launch pad to a hangar for repairs, says Kennedy Space Center's director.

Taking Discovery off the pad would delay the first launch since the Challenger disaster, now scheduled for early September, by up to two months, Forrest S. McCartney told reporters Tuesday."We wish we had found it (the leak) earlier," McCartney said. "If we had found it when it was accessible, there would have been no problem. But it's in an inaccessible place, and we don't know what we're going to do about it.

"We've got people looking at the prudent and safe thing to do. Whatever course we take there will be no compromise in flight safety.

"The odds are 50-50 on a rollback," McCartney said.

He said no decision was likely until after Discovery's three main engines have been test-fired on the pad in a critical test set for July 28. The main engines are separate from the steering engine system, where the leak is.

The tiny nitrogen tetroxide gas leak was detected Friday in an engine compartment and was traced to a fitting on a line leading to an oxidizer tank. Nitrogen tetroxide normally combines with fuel to power the steering engines.

Shuttle managers were combing through documents to determine whether workers might have missed a similar leak as early as last January.

Launch Director Bob Sieck said paperwork showed that a pressure check was made on the oxidizer tank manifold in an engine compartment before Christmas, and that a second reading taken two weeks later showed a pressure drop.