Spacewalkers Test Fire Shuttle Repair Goo Gun
Spacewalkers Mike Foreman (left) and Robert Behnken test a shuttle heat shield tile repair tool during the fourth spacewalk of NASA's STS-123 mission on March 20, 2008.
Credit: NASA TV.

This story was updated at 3:47 a.m. EDT.

HOUSTON - Two spacewalking astronauts test fired a high-tech caulk gun filled with goo outside the International Space Station (ISS) Thursday to determine whether it?s a viable repair tool for dinged shuttle heat shields.

Shuttle astronauts Mike Foreman and Robert Behnken squirted a pink, putty-like substance into intentionally damaged shuttle tiles during their six-hour and 24-minute spacewalk to test how the material behaves in the weightless vacuum of space.

?It goes down really well,? Foreman said as he tamped down the thick goop with a sponge-like tool. ?It really is like a loaf of bread with a lot of little bubbles in there.?

NASA engineers developed the shuttle tile repair tool, known as the Tile Repair Ablator Dispenser (T-RAD), in the wake of the 2003 Columbia disaster to fix dings in the thousands of ceramic tiles that line an orbiter?s underbelly.

The shuttle Columbia disintegrated as it reentered the Earth?s atmosphere, killing its seven-astronaut crew, due to a hole in the fragile carbon-composite heat shield panels along its left wing. NASA has developed a black heat-resistant putty for minor panel damage, with the caulk gun and goo, small patches and a gray primer wash reserved for dinged tiles.

?Having this in our bag of tricks is really going to be helpful,? Behnken said before today?s spacewalk.

Thursday?s excursion began at 6:04 p.m. EDT (2204 GMT) and marked the fourth of five spacewalk planned for NASA?s STS-123 shuttle flight aboard Endeavour. The shuttle astronauts have delivered a new station crewmember, a Japanese module and a massive Canadian maintenance robot called Dextre to the ISS during their 16-day mission.

Tile repair test

The T-RAD tool, a slimmed down version of a balky backpack-mounted device, was the only heat shield fix yet to be tested in orbit.

?I?m thrilled with what we saw today,? said ISS flight director Dana Weigel after the spacewalk, adding that T-RAD could also be used to mend torn shuttle insulation blankets. ?It behaved very similar to what we saw on the ground, so that gives me a lot of confidence.?

During the test, Foreman squeezed the trigger of his gun-like T-RAD device, which then mixed two different compounds into an ablative material as it squirted out into the damaged shuttle tile samples. Some of the samples mimicked an actual tile gouge from Endeavour?s last flight in August 2007, as well as ice damage from post-Columbia tests on Earth.

?You are Captain T-RAD today, Mr. Goo,? Endeavour astronaut Rick Linnehan told the goo gun-toting Foreman from inside the shuttle. ?You?re in control.?

NASA engineers hoped to learn how the T-RAD material behaved in space, and whether bubbles would rise to the surface or cause the goo to swell like rising bread.

?We?re really captivated by what you?re doing,? astronaut Steven Robinson told the spacewalkers from Mission Control here at NASA?s Johnson Space Center. ?You?re like brain surgeons up there.?

Foreman and Behnken reported seeing bubbles and some swelling, but were apparently able to pat it down with their tools.

?I expect that this [demonstration] will be successful and may actually teach us something,? said John Shannon, NASA?s space shuttle chief, before the flight. ?Obviously, we have not had any tile damage since returning to flight that has made us seriously consider this repair.?

Thursday?s spacewalk marked the 108th spacewalk outside the ISS and the second career excursion for Behnken and Foreman, both of whom are making their first spaceflight.

Behnken ended the orbital work with 13 hours and 17 minutes of spacewalking time, while Foreman concluded with 13 hours and 32 minutes. Both astronauts will participate in their mission?s fifth spacewalk on Saturday.

The spacewalkers also replaced a faulty circuit breaker outside the station, but could not rewire an electrical line feeding it due to a stuck connector. They also released a series of locks on a station module, hunted for a lost pin in a berthing port - unsuccessfully - and removed a glove-like thermal cover from one of the Dextre robot?s hands.

?It looks pretty good, a monstrosity,? Foreman said of the robot. ?Monstrous.?

NASA is broadcasting Endeavour's STS-123 mission live on NASA TV. Click here for SPACE.com's shuttle mission coverage and NASA TV feed.