Two spacewalking astronauts, armed with a caulk gun and high-tech kind of Silly Putty, floated out the hatch Thursday to test 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.

"You ready for a good stroll today?" astronaut Richard Linnehan asked from inside. The reply: "We're ready."

Astronauts Michael Foreman and Robert Behnken had to replace a bad circuit breaker at the space station before tackling the long-awaited repair test, which was ordered up after the 2003 Columbia tragedy.

NASA was anxious to see how well Foreman could squeeze the salmon-colored goo from the caulk gun-like dispenser and fill holes in deliberately damaged tile samples that were carried up aboard Endeavour.

Even more of a mystery 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 will be nice to have this under our belt, to know ahead of time," said flight director Mike Moses.

"This will give us that extra confidence boost" if astronauts ever have to use the repair method for real, he added. The goo-filled tile samples will be returned to Earth aboard Endeavour for analysis.

Moses described the goo as sticky orange toothpaste. While the recipe is secret _ for proprietary reasons - the material is said to have a silicone base with an alcohol polymer to make it flow. 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 being 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.

One more spacewalk is planned for Endeavour's 12-day space station visit.

On Saturday night, Foreman and Behnken will go back out to move their ship's inspection boom over to the space station for use by the next shuttle crew. The Japanese lab, Kibo, is so big that there won't be enough room for a boom in Discovery's payload bay. That mission is scheduled for late May.

___

On the Net:

NASA: http://spaceflight.nasa.gov