A second tiny crack has been found in a bearing assembly in a high-pressure main engine liquid oxygen pump used by the shuttle Atlantis during its climb to space Dec. 2, NASA officials said Friday.

Engineers do not yet know what caused the cracks or what impact, if any, they might have on upcoming launches.Data from Atlantis's launch showed higher-than-normal vibration levels in the powerhouse "LOX" pump right at, or shortly after, ignition. During a post-flight inspection last month at the Kennedy Space Center, a tiny, .005-inch-wide crack in a bearing race was discovered in main engine No. 3, serial No. 2029. The second crack was found Thursday.

The race is the circular housing that contains ball bearings that are critical to pump operation.

Atlantis landed Dec. 6, 1988, at Edwards Air Force Base, Calif., to close out the 27th shuttle mission, the second since the 1986 Challenger disaster. Discovery is scheduled to blast off on the next shuttle flight around Feb. 23.

A shuttle's No. 3 main engine occupies the lower right position in the triangular engine layout when looking toward the front of a shuttle form the back. Atlantis' engine was removed over the holidays and the pump was shipped back to engine-builder Rocketdyne in Canoga Park, Calif., last week for a detailed inspection.

Jerry Berg, a spokesman for NASA's Marshall Space Flight Center in Huntsville, Ala., said late Friday engineers disassembled the pump and reached the bearing assembly for an "eyeball inspection" Thursday.