Like handymen caulking a bathtub, two spacewalking astronauts squirted pink putty into deliberately damaged tile samples to test a new technique for repairing the space shuttle's fragile heat shield.
NASA declared the experiment a huge success. "We're just thrilled with the way it turned out," Zebulon Scoville, Endeavour's lead spacewalk officer, said early Friday after the 6 1/2-hour spacewalk ended.
The space agency has been researching repair techniques since the shuttle Columbia disintegrated in 2003, killing its seven astronauts. The putty method is the last to be tested in orbit.
Using a high-tech kind of caulk gun, spacewalker Michael Foreman squeezed the goo into holes in the tile samples. He then used a small foam brush to tap down and smooth the putty, which comes out like toothpaste and feels like hard plastic once it solidifies.
"We are absolutely captivated by what you guys are doing here," Mission Control radioed Foreman and his spacewalking partner, Robert Behnken. "You're like brain surgeons up there."
Goo performed well
NASA was curious to see how the goo would behave in space. Engineers weren't sure whether bubbles would form and rise to the top as they do on Earth or whether any bubbles would remain inside the material and cause it to swell.
This so-called rising bread-loaf effect could jeopardize a repair and endanger a crew during re-entry.
The goo performed just as NASA thought it would, if not a little better, space station flight director Dana Weigel said. There was a small amount of expansion, but not as much as expected.
Endeavour will ferry the tile samples back to Earth for analysis.
None of NASA's new repair methods would have been capable of saving Columbia after breakaway foam gashed a hole the size of a dinner plate in its left wing.
The goo tested Thursday night is intended for the approximately 24,000 silica tiles on the shuttle's belly and elsewhere, not the 22 reinforced carbon panels that line each wing and take the brunt of re-entry heat.
Goo to the rescue
Flight director Mike Moses said the test would give NASA "that extra confidence boost" if astronauts ever have to use the repair method for real.
All but one of the 11 remaining shuttle flights will be headed to the space station, where a shuttle crew could take refuge if its ship were irreparably damaged.
For the Hubble Space Telescope mission at the end of August — where no such refuge exists — a second shuttle will be ready to launch to the rescue if necessary.
It was the fourth spacewalk since Endeavour arrived at the international space station just over a week ago to deliver a robot and the first section of a Japanese lab. Foreman and Behnken are scheduled to participate in the mission's final spacewalk on Saturday.
But first, the crew planned to spend Friday afternoon checking the ship's nose and wings for any micrometeorite damage that may have occurred in orbit.
The examination normally is done on the way back to Earth. But the crew has to leave the 100-foot, laser-tipped inspection boom at the space station to make room in Discovery's payload bay for the Japanese lab that is to be delivered in late May.