The examination took about three hours as the imaging devices atop a 30-meter-long (100 foot) robotic arm coupled with the Orbiter Boom Sensory System (OBSS) scanned five areas on the shuttle underside that may have been damaged during Wednesday's launch from Cape Canaveral, Florida, the National Aeronautics and Space Administration said.

The gouge, 30.5 x 25.5 millimeters (1.2 x 1.0 inches) -- smaller than initially reported -- and 28.5 millimeters (1.12 inches) deep, was made near a landing gear hatch by a piece of foam, possibly covered with ice, that broke off the shuttle's external fuel tank shortly after blast-off.

Mission Management Team chairman John Shannon said an exact mold of the gash will be reproduced in thermal tiles and tested in a laboratory that simulates the extreme heat and friction the shuttle encounters on re-entry to Earth.

NASA engineers will be able to "do a thermo analysis model... to understand what the actual heating impact of re-entry will be for a damage of this type," he told a press conference.

The tests, to be carried out "in the next 24 to 48 hours," should provide engineers enough data to determine whether repairs are needed to the damaged heat shield before the shuttle undocks from the International Space Station on August 20, he said. ABC News reported Sunday that the ding in Endeavour's underside is deep enough to go through the thermal tiles to the aluminum skin of the orbiter itself. The available repair options include heat reflecting paint, a paste to fill in the hole and a metal plate to completely cover the damaged thermal tile.

"We are really prepared for exactly this case since Columbia," Shannon said, referring to the February 2003 disaster of the shuttle Columbia that killed seven astronauts.

The foam came off the shuttle's fuel tank, which holds super-cold liquid hydrogen fuel for the takeoff and is jettisoned before orbit is reached. An insulation layer on the tanks is supposed to prevent icing.

NASA keeps a watchful eye on the shuttle's thermal shield since the Columbia disaster.

Columbia's protective heat shield was pierced by a piece of insulating foam that peeled off its external fuel tank during liftoff. The breach resulted in the shuttle disintegrating as it re-entered Earth's atmosphere, killing all seven astronauts on board.

Also, space officials decided Sunday to extend the mission by three days, which means that Endeavour, launched Wednesday from Cape Canaveral, will now return to Earth on August 22, a NASA spokesman said.

The extra time will allow a fourth, additional spacewalk to the mission to continue construction work on the International Space Station. On Saturday the ISS was expanded with a new truss segment attached by two astronauts.

Endeavour's tour was initially planned for 11 days, with three space walks. NASA however said from the start it would extend the mission after testing a new system that transfers electricity from the ISS to the shuttle.

The system, which prolongs the life of the shuttle's batteries and allows it to remain aloft longer, was tested successfully on Sunday.

Also on Sunday, preparations began for the mission's second spacewalk, due to start at 1531 GMT Monday, during which astronauts Dave Williams and Rick Mastracchio will replace a faulty gyroscope aboard the ISS.

The space station has four gyroscopes that are used to control its attitude. On Saturday, Mastracchio, of the United States and Canadian Dave Williams spent six hours and 17 minutes installing and activating a new, 1.58-ton segment for the ISS the Endeavour brought up.

Endeavour docked on Friday with the ISS bringing seven astronauts, including 55-year-old Barbara Morgan, the first school teacher in space.

earlier related report
Astronauts on the International Space Station on Sunday are to examine and measure a troublesome gash in the shuttle Endeavour's heat shield, by means of a camera and a laser atop a robotic arm.

The 56-square-centimeter (nine-square-inch) gouge near a landing gear hatch was apparently made by a piece of ice that broke off the shuttle's external fuel tank 58 seconds after Wednesday's launch from Cape Canaveral, Florida.

The laser device will provide the exact measurement and depth of the gash so NASA engineers can decide whether repairs are need, for which the Endeavour mission would be prolonged to allow for an additional spacewalk.

Endeavour's mission was initially planned for 11 days, with three space walks.

NASA may extend the mission by three days, and add a space walk, after they test a new system that transfers electricity from the ISS to the orbiter, prolonging the life of the shuttle's batteries.

Separately, if NASA decides that the damaged shuttle tiles need fixing, an additional space walk is possible.

NASA on Saturday studied pictures of the damage taken on Friday, while two astronauts completed the first spacewalk of the shuttle's 11-day mission.

Mission specialists Rick Mastracchio of the United States and Canadian Dave Williams spent six hours, 17 minutes installing and activating a new, 1.58-ton segment for the International Space Station that the Endeavour had delivered.

The two astronauts attached the Starboard 5 truss segment to the Starboard 4 segment, with the help of shuttle pilot Charles Hobaugh, who from inside the ISS operated the station's robot arm holding the 3.37 x 4.24-meter (11 x 14-foot) truss.

They also retracted a radiator on the Port 6 truss, which will be moved and attached to the Starboard 5 truss during a future mission, replaced a defective gyroscope on the ISS and installed an external stowage platform.

Back on Earth, meanwhile, National Aeronautics and Space Administration experts analyzed 296 pictures of Endeavour's heat shield taken during a routine inspection Friday just before the orbiter docked with the ISS.

In addition to the gash, small white marks were also visible on other thermal tiles surrounding the damaged area, mission manager John Shannon told reporters.

He said NASA was trying to estimate the extent of the apparent damage, which will not be exactly determined until a closer inspection is carried out with the ISS robotic arm bearing a high-resolution camera and laser device.

The ice that caused the damage presumably was formed by humid air coming in contact with the cold surface of the shuttle's fuel tank, which holds super-cold liquid hydrogen fuel for the takeoff and is jettisoned before orbit is reached. An insulation layer on the tanks is supposed to prevent icing.

The US space agency has carefully inspected the orbiter's protective thermal tiles during each of the missions that have followed the shuttle Columbia disaster of February 2003.

Columbia's crucial protective heat shield was pierced by a piece of insulating foam that peeled off its external fuel tank during liftoff. The breach resulted in the shuttle disintegrating into a ball of fire as it re-entered Earth's atmosphere, killing all seven astronauts on board.

Endeavour docked on Friday with the ISS bringing seven astronauts, including 55-year-old Barbara Morgan, the first school teacher in space.

Morgan's space mission came 21 years after the shuttle Challenger launch explosion in 1986 killed another woman intended to become the first teacher-astronaut, Christa McAuliffe.

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