NASA and contractor engineers are studying data indicating a possible oxygen leak in the shuttle Discovery's aft engine compartment during its return-to-flight mission last July.

Discovery launches July 26. Credit: NASA

The apparent leak was detected after the mission when so called "catch bottles," used to sample the air in the aft compartment during flight, were removed and checked. Based on a preliminary analysis, a leak rate around 0.08 pounds per second was detected when two of six catch bottles - one on the right side of the compartment, the other on the left - captured 2-second samples a little more than a minute after liftoff.

One catch bottle did not operate properly and the other three, which opened before the two in question, showed normal oxygen concentrations. Engineers initially thought the high readings were a measurement glitch, but they are taking the indications seriously and trying to pin down what, if anything, was amiss.

"There are a couple of ways to look at it," LeRoy Cain, manager of shuttle launch integration at the Kennedy Space Center, said Wednesday in an interview with CBS News. "For sitting on the ground, it was (a) large (leak). For in-flight performance, telemetry from the engines and what not, I don't believe we'd see it. From the standpoint of (over pressurizing) the aft, like during entry, it's not large. It's an order or two magnitude smaller than what would overpress the aft. ... But we've got to go look at it."

Discovery roars off its launch pad for the return to space mission. Credit: NASA

Before launch, the engine compartment is purged with nitrogen and hazardous gas detection systems recorded nothing unusual in Discovery's aft. During ascent, the engine compartment is not purged and vent doors are open to equalize pressure. The aft is pressurized with helium for re-entry.

The catch bottles are the sole means of detecting in-flight leaks in the engine compartment other than changes in the performance of the shuttle's three main engines. During Discovery's launching, all three engines operated normally and there is nothing in the telemetry to suggest a significant oxygen leak.

Possible sources for oxygen leakage include the engines, the plumbing associated with the main propulsion system, the shuttle's life support system and its electricity producing fuel cell system. But those all passed pre-launch leak checks and other than high oxygen concentrations in two of the catch bottles, there is no other evidence of a problem.

"The guys are looking at it very hard," said Cain, who served as the ascent/entry flight director in mission control for Discovery's mission. "It's too soon to know" what, if any, impact the issue might have on Discovery's processing for launch next spring.