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

A detailed laser inspection on Sunday of the difficult-to-reach area on Endeavour’s belly could send astronauts out to repair the 3-inch wound later in the week, although NASA said that prospect appeared less likely than it did a day earlier. A penetration, if severe enough, could let in searing gases when the shuttle returns to Earth in a possible replay of the Columbia accident.

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, the result of a strike by fuel-tank foam insulation at launch.

NASA said late Saturday that the piece of foam came off a bracket on the tank, then bounced off a strut farther down on the tank and shot into Endeavour.

“It was a bad bounce off that external tank strut,” said John Shannon, chairman of the mission management team. “We had that piece of foam come off and it shot straight up.”

The encouraging news was that the debris was not ice, which would have been denser and potentially more dangerous. It’s possible, however, that some ice was attached to the piece of foam, Shannon said. There was no information on the size of the debris, although Shannon said what came off the tank may have been about the size of a grapefruit.

Foam has come loose from these brackets on previous flights, Shannon said. The brackets hold the long fuel feed line to the tank, and the struts connect the tank to the shuttle for launch. Ice tends to form near these brackets and cause the foam to pop off at liftoff.

“It’s a little bit of a concern to us because this seems to be something that has happened frequently,” Shannon said.

The gouge is an estimated 3« inches long and just over 2 inches wide. The thermal tiles in that location are just over an inch deep. Sunday’s inspection will ascertain the depth of the gouge, a vivid white against the surrounding black thermal tiles.

Directly beneath the damage is part of the aluminum framework of the starboard wing, which would provide additional protection during the intense heat of re-entry, Shannon said. He called that a lucky break.

It’s possible the astronauts will have to perform repairs, Shannon said, but it’s less likely than managers thought Friday, when they feared a piece of ice had struck Endeavour. In fact, if Endeavour had to make an emergency landing right now, NASA would take the chance based on all the risks, he added.

Video cameras retrieved from Endeavour’s booster rockets, which were towed back from the Atlantic, confirmed Saturday that the debris was foam.

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. Shannon 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.

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«-pound wedge of foam about the size of a small suitcase shortly after liftoff.

Aboard the joined shuttle-station complex, astronauts successfully carried 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.

Shannon cautioned 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.

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.

Associated Press writers Rasha Madkour and Liz Austin Peterson in Houston contributed to this report.