The space shuttle Endeavor's mission has gone well so far, but NASA engineers are concerned about a divot three inches square in the silica tiles providing the spacecraft's main defense against the heat of re-entry. Likely caused by ice (although possibly by foam insulation) on liftoff, the damage will be carefully examined and measured by a camera and a laser tool on the Shuttle's robot arm extension. (The robot arm operator, by the way, is Barbara Morgan, originally the backup "Teacher in Space," who went through the rigors of becoming a fully qualified astronaut while she waited for her chance to fly.) If the gouge is determined to pose a threat, the crew will be directed to perform an EVA and use one of three types of on-board repair kits to fix it before reentry.
COMMENT: The tile system has never caused an accident, but it is unacceptably fragile and enormously labor-intensive to maintain. It's an outgrowth of the desire to keep development costs for the Shuttle down (which was successful) by trading off maintainability and reusability during the spacecraft's operational life. Of several options for the heat shield, the tiles were the cheapest to develop but the most expensive to maintain. All indications are the same short-sighted approach is afflicting the Crew Exploration Vehicle program.
UPDATE 8/12/2007: NASA has confirmed the damage is from a piece of foam, estimated as grapefruit-sized, that came off a strut on the External Tank (ET). Further study is still underway to determine whether the gash is deep enough to require repair. See: