The robot known as Dextre is shown in the grasp of the International Space Station's robotic Canadarm2 in this image photographed by a crewmember at the station. Also pictured are solar array panels against the backdrop of a blue and white Earth. Having worked on Dextre in three spacewalks, astronauts on Thursday ventured outside a fourth time to test an emergency repair caulk that could be used for patching damaged thermal tiles.

Cape Canaveral, Fla. ? Two spacewalking astronauts pulled out a caulk gun and high-tech kind of Silly Putty on Thursday night and tested a method for patching shuttle thermal tiles.

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.

The long-awaited repair test was ordered up following the 2003 Columbia tragedy.

Astronaut Michael Foreman – “Mr. Goo” – squeezed the salmon-colored material from the caulk gun-like dispenser and filled holes in deliberately damaged tile samples that were carried up aboard Endeavour. He used a small foam brush to tap down and smooth the material, but small bubbles kept popping up.

His spacewalking partner, Robert Behnken, floated nearby, wiping away excess goo from the nozzle of the caulk gun.

Even more of a mystery than how the caulk gun would work was how the goo would behave. Engineers were curious about 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.

“It really is like a loaf of bread with all the little bubbles in there,” Foreman observed. He reported seeing bubbles popping at the surface of the goo and some expansion of the material.

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. The goo-filled tile samples will be returned to Earth aboard Endeavour for analysis.

Moses described the goo as sticky orange toothpaste. The spacewalkers will dispense about a couple milk jugs’ worth of the stuff, Moses said.

Since the loss of Columbia, astronauts in orbit have tested other methods for repairing a shuttle’s thermal shielding, but never this one. It was supposed to get a tryout on a shuttle flight last fall, but a torn solar wing at the space station took priority.

The hole in Columbia’s wing was so big – about the size of a dinner plate – that none of NASA’s new repair methods would have kept the shuttle safe during re-entry, even if they had been available back then. 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.

Of the 11 remaining shuttle flights, all but one will be headed to the space station, which could serve as a refuge if there was irreparable damage to the shuttle and it was too risky to bring it back to Earth. For the Hubble Space Telescope mission at the end of August – where no such refuge exists – a second shuttle will be at the launch pad ready to fly to the rescue if necessary.