CAPE CANAVERAL -- NASA is investigating the possibility that highly flammable gaseous oxygen might have leaked into shuttle Discovery's rear engine compartment during its first post-Columbia launch, officials said Thursday.

Scouring over data from the test flight to the International Space Station, NASA engineers uncovered evidence indicating there might have been high concentrations of the hazardous gas in the compartment about two minutes after liftoff.

Engineers initially thought the data was erroneous and discounted the possibility of a leak. But they still have not been able to determine whether a leak in fact occurred, so a formal investigation is being carried out.

The issue must be resolved before NASA's next shuttle mission. A leak could lead to a fire or even an explosion in flight.

"We're going to err on the side of caution," said Kyle Herring, a spokesman for NASA's Johnson Space Center in Houston. "We're going to run this thing to ground and make sure we understand it."

Discovery blasted off July 26 and there were no indications of a leak in performance data beamed back from the shuttle during its 9-minute climb into orbit.

Fueled by 500,000 gallons of liquid hydrogen and liquid oxygen in the shuttle's external tank, Discovery's three main engines performed as expected and the ship reached its intended orbit with no apparent problems.

A serious leak likely would have left the shuttle short of its target, and an extra firing of its twin orbital maneuvering engines might have been required to make up for any shortfall.

NASA also monitors the engine compartment during pre-launch fuel-loading operations and the final launch countdown to make certain there is no build-up of hazardous gasses.

Strict NASA rules call for a launch attempt to be scrubbed if gaseous oxygen concentrations in the compartment reach 500 parts per million, Kennedy Space Center spokeswoman Jessica Rye said. But there was no evidence of a build-up before Discovery and its seven astronauts were launched.

The data in question was retrieved from six so-called "catch bottles" that gather samples of air within the engine compartment during flight. Data from the devices cannot be retrieved until a shuttle returns to Earth.

Data from three of the devices showed that gaseous oxygen levels during flight were normal. Data from a fourth was corrupt.

Two of the catch bottles generated data that indicate higher-than-allowable levels of gaseous oxygen were present in the compartment about two minutes into flight.

The investigation into the apparent leak is not expected to have any immediate impact on preparations for NASA's next shuttle mission, which the agency hopes to launch in May.

Routine prelaunch work will continue at KSC and other NASA installations. Engineers investigating the matter will report back to shuttle managers in January.

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