NASA to remove Discovery's robot arm

A small crack in shuttle Discovery's robot arm was discovered during ultrasound inspections over the weekend, prompting a decision to remove the crane-like boom and ship it back to Canada for repairs.

The inspections were ordered after a bridge bucket carrying technicians from a clean-up site in the shuttle's payload bay inadvertently struck the 50-foot arm.

Marks were noted in two places on multilayer insulation blankets that provide thermal protection to the arm, which is mounted on brackets along the inside of the bay. The blankets were removed and two small indentations were found on the arm's outer bumper, which is a layer of graphite epoxy honeycomb that protects the composite carbon structure beneath.

Engineers used an ultrasound device to examine the arm and found a 1.25-inch crack with a depth of 0.015 inches. The arm will be shipped back to its manufcaturer for repairs.

An identical arm slated to fly on Endeavour would be used to replace the damaged unit if NASA presses ahead with plans to fly Discovery in May. The repaired arm would be returned to Kennedy Space Center and reinstalled in Discovery if the mission is delayed to July.

The robot arm will play a critical role in Discovery's flight.

Astronauts will use it to grapple a sensor-laden boom so the shuttle's heat-shield for the type of damage that doomed Columbia's crew.

Two spacewalkers also will climb aboard the linked arm and boom to determine whether it would be strong enough to be used during heat-shield repair work.