Technicians moved into Discovery's cargo bay to begin plugging a tiny gas leak that threatens to delay the first post-Challenger shuttle launch.

With the shuttle poised on the launch pad, the technicians Tuesday were to drill into the cargo bay wall and an engine compartment to repair the tiny leak discovered a month ago.The leak in a steering engine compartment must be stopped before Discovery can be launched in late September or early October.

The plan was to use a routing tool to cut two holes in the rear bulkhead of the 60-foot-long cargo bay and two more in the adjoining aluminum hull of the steering engine compartment.

The dual openings would allow a technician to reach about 12 inches into the compartment to clamp a clamshell-shaped device over the leak and fill it under pressure with a sealant. The device measures 4 inches by 1 inch and should fit easily over a fitting that is slowly leaking nitrogen tetroxide gas from a vent line.

Because Discovery is poised vertically on the launch pad, the technicians would have to do much of the work on their hands and knees or lying on their stomachs on the cargo bay's rear bulkhead.

On Tuesday, workers opened the bay's doors and erected a tentlike structure over the work area to prevent debris from getting into the rest of the shuttle or into a nearby enclosure where Discovery's payload is waiting to be loaded. The payload, a communications satellite, would be placed aboard once the repairs are complete.

The line with the leak services a steering engine system that is separate from the three main engines test-fired on the launch pad last week.

The repair and subsequent testing are expected to take about a week, said Lisa Malone, a National Aeronautics and Space Administration spokeswoman.

Although the leak was detected a month ago, NASA decided to put off repairing it until after the main engine test.

The launch team is continuing to troubleshoot the only fault resulting from the test firing: a small amount of hydrogen gas detected after the test by a sensor in an area where lines join the shuttle with its external fuel tank. For the firing, the tank was loaded with liquid hydrogen and oxygen to fuel the main engines.