NASA engineers on Monday pored over new imagery of the space shuttle Endeavour’s underbelly to decide if its damaged heat shield needed repair, as astronauts prepared for the mission’s second spacewalk.

The three-dimensional images of a gouge in the shield were taken on Sunday by a camera, and measured by a laser, both of which were trained on the shuttle's protective surface.

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 on  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.

Repair options
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.

Also, space officials decided to extend the mission by three days, which means that Endeavour, launched on 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.