updated 7/20/2007 3:42:33 PM ET 2007-07-20T19:42:33

NASA engineers are examining seals known as O-rings that go between space shuttle booster rockets because recent batches have shown a higher-than-usual amount of unmixed rubber.

The seals are of special concern to the U.S. space agency since their failure was blamed for the 1986 Challenger disaster that killed six astronauts and teacher-in-space Christa McAulliffe.

Challenger blew apart 73 seconds after liftoff when seals that had stiffened from cold weather allowed hot gases to escape the booster and ignite fuel.

Rubber specks that are too large or too close together can make the seals stiffer.

McAulliffe’s backup for the Challenger flight, Barbara Morgan, is set to launch on her first spaceflight with six other astronauts aboard the shuttle Endeavour on Aug. 7 for a mission to the international space station.

NASA engineers are checking to make sure the O-rings in Endeavour’s boosters aren’t part of the batches under examination, but they don’t expect them to be, NASA spokesman Rob Navias said Friday. The investigation was first reported by USA Today.

“It’s not serious,” Navias said. “We fully expect it will have no impact on launch.”

“It’s a paper chase that we typically do when another batch of hardware shows an issue that we want to take a look at a little more carefully ... We don’t even know if the O-rings in question are bad,” he said.

NASA managers expect an update on the O-rings when they meet next week at Kennedy Space Center to give final approval for the August launch.

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