(AP)
Updated: 2007-08-12 09:56

CAPE CANAVERAL, Fla. - A routine shuttle mission, highlighted by a teacher's first spaceflight and space station construction, is now overshadowed by a potentially serious gash in Endeavour's thermal shield.

A detailed laser inspection on Sunday of the difficult-to-reach area on Endeavour's belly may send astronauts out to repair the 3-inch wound later in the week. A severe penetration could let in searing gases when the shuttle returns to Earth in a possible replay of the Columbia accident.

In this photo made from the NASA-TV, Canadian Space Agency astronaut Dave Williams works outside of the Space Shuttle Endeavour during a space walk while orbiting Earth, Saturday, Aug. 11, 2007.[AP]

As a pair of spacewalking astronauts installed a new beam to the international space station on Saturday, engineers back on Earth scrutinized images of the gouge, believed to be the result of a strike by ice at launch.

The ice, which would have come from the external fuel tank, is denser than the tank's insulating foam and even a small piece could cause major damage to the shuttle's thermal covering.

In a scene eerily reminiscent of the foam smack to Columbia four years ago, radar showed a whitish spray or streak emanating from Endeavour's right side 58 seconds after Wednesday evening's liftoff. The chairman of NASA's mission management team was quick to point out that the spray looked much smaller than the one during Columbia's launch.

Nonetheless, the similarities immediately grabbed the attention of mission managers. The 1986 Challenger launch explosion already was at the forefront of everyone's minds, given the presence of teacher-astronaut Barbara Morgan — Christa McAuliffe's backup — aboard Endeavour.

Mission managers promptly ordered a focused inspection of the gouged area Sunday, using Endeavour's 100-foot, laser-tipped robot arm and inspection boom. They also had engineers and other experts poring over the radar launch imagery as well as photographs of the damage that were taken by the space station crew right before the shuttle's docking Friday.

The inspection boom and augmented photography became mandatory after the Columbia disaster. Every shuttle crew also has been supplied with a repair kit to handle precisely this type of damage.

Sunday's inspection will ascertain the depth of the gouge, a vivid white against the surrounding black thermal tiles. If NASA decides a repair is needed, two astronauts will venture out to either cover the gouge with black paint, screw on a plate or squirt goo into the cavity.

The damaged area, just a few feet from the right main gear landing door, is subjected to as much as 2,300 degrees Fahrenheit during re-entry. The wings and nose cap get considerably hotter, and any damage there would be even more worrisome.

Columbia's left wing ended up with a hole estimated between 6 inches and 10 inches after being slammed by a 1 1/2-pound wedge of foam shortly after liftoff.

Mission managers failed to listen to engineers' concerns during Columbia's flight, and no effort was made to check the wing in orbit. Now, everyone's concern counts, including the astronauts on board. Mission Control notified them once the gouge was discovered Friday and also sent up pictures.

Aboard the joined shuttle-station complex, work continued as usual Saturday, with astronauts successfully carrying out the first spacewalk of Endeavour's mission.

Spacewalkers Rick Mastracchio and Dave Williams installed a 2-ton square-shaped beam to the backbone of the station, now stretching 246 feet end to end. The beam, or truss, was delivered by Endeavour along with other station equipment that will be hooked up during at least two more spacewalks in coming days.

Astronaut Charles Hobaugh lowered the beam into place, using the space station's robot arm, as Mastracchio and Williams floated nearby, offering guidance. Once the beam was attached to the station, the spacewalkers bolted it down and hooked up grounding straps, then performed some extra outdoor chores.

Midway through the six-hour spacewalk, NASA's main command-and-control computer aboard the space station mysteriously shut down. The backup automatically kicked in, and the third computer went from standby to backup. Mission Control said the problem did not affect the spacewalk or the health of the station.

Even with all the improvements to the fuel tank, NASA has readily acknowledged that it is impossible to launch a shuttle with absolutely no threat of debris. Engineers have focused their efforts on preventing large pieces of foam from coming off the tank, an effort that has mostly paid off.

As for ice, technicians have always inspected the fuel tank right before liftoff — the tank is filled with super-cold fuel — but some spots are difficult to see. The chairman of the mission management team, John Shannon, said NASA will review the ice inspection that was carried out before Endeavour took off, to see if anything was missed.

Shannon cautioned Friday that space shuttles have safely returned to Earth with thermal tile damage in the past. Almost every mission if not all in the 26 years of shuttle flight, in fact, has ended with gouges of at least an inch in the thermal tiles that cover the belly. In one flight, nearly 300 dings that big were recorded.

"We have a rich flight history of tile damage, some of which is more significant looking than what we have right here," Shannon told reporters. "In the past, we didn't even know we had damage and we flew back home. So what I would tell you is we're going to do all the work required to understand it ... I would not even venture to guess what the probability is that we would have to go repair this."

NASA hopes to keep Endeavour at the space station for at least seven days and quite possibly a record 10 days. The shuttle is equipped with a new system for drawing power from the station, and mission managers are expected to approve the extra docked days on Sunday.

In an emergency, Shannon said, Endeavour could remain at the space station for at least two months and a rescue shuttle could be launched as early as October.