March 21, 2008

Astronauts Test Heat Shield Repair Goo

Two spacewalking astronauts, working outside the international space station late Thursday, wrapped up tests with a gooey repair material for damaged shuttle heat shielding that NASA hopes it never has to use.

The testing was a milestone of sorts in NASA's efforts to rebound from the fatal 2003 breakup of the shuttle Columbia, a tragedy blamed on undetected breach of the thermal armoring.

The pink caulk-like material offered few surprises as astronauts Mike Foreman and Bob Behnkensquirted it into intentionally damaged samples of the soft tiles that cover the shuttle's underside.

The goo bubbled as it was dispensed from the gun-like applicator Foreman wielded. But using a foam brush, he tamped and sculpted the ropey lines of caulk into a smooth texture and rounded shape.

"It's really like a loaf of bread, with all these little bubbles in there," Foreman informed Mission Control. There, experts watched live video of the test transmitted by cameras in the helmets of the two spacewalkers.

"You are our tile-and-grout specialist," said astronaut Rick Linnehan, who choreographed the spacewalk from inside the space station and kept the video flowing.

"I hope we don't need one," said Foreman.

Columbia's loss was traced to a hole in the brittle heat shielding of the left wing that was blasted apart by a launch-day impact with a chunk of breakaway fuel tank insulating foam.

In the tragedy's aftermath, the space agency added inspections of the heat shielding to every mission. NASA agreed to retire the shuttle in 2010 and send all but one of its remaining missions to the space station. The outpost would offer a refuge where the astronauts of a cripped spaceship could wait for rescue.

The space agency also equipped shuttle crews with tools and materials to attempt repairs from impacts with launch debris or meteoroids. Patches can fill holes in the brittle wing thermal shielding.Cover plates and a dark primer can be applied atop or dabbed onto gouged tiles to reject heat.

The pink goo was the last of the repair techniques to be tested in space.

"I consider it to be kind of the last thing we're going to do on the return to flight (heat shield) repair tasks that we took on," said John Shannon, NASA's shuttle program manager. "We have high confidence in it, but this will just be the final activity that we do to verify that it's indeed a good repair capability."

The spacewalkers worked with nearly a dozen samples, some with damage from past shuttle missions or from ground tests.

The test samples will return to Earth aboard the shuttle Endeavour, which has been docked with the station since March 12. The samples will be examined in a NASA lab and undergo tests that expose them to the heat buildups that accompany the shuttle's high speed descent to Earth.

The space agency hoped for a goo test before it launches the one shuttle mission that will not head to the station. That flight, a 12-day overhaul of the Hubble Space Telescope, is scheduled for a late August liftoff.

A favorable test outcome is not considered a prerequiste for the Hubble mission. NASA will have a rescue shuttle ready to liftoff as the astronauts assigned to the Hubble overhaul begin their mission.

Twice before, Thursday's test had been postponed. In October, a shuttle crew ran out of time for the exercise when the astronauts encountered problems with the transfer of a space station solar power module. In mid-2006, the test was scrubbed from a shuttle flight to give engineers more time to make an unwieldy applicator smaller and fend off some of the unwanted air bubbles that form in the goo.

As they started their spacewalk, Foreman and Behnken replaced a faulty external space station circuit breaker.

Though they managed a quick exchange, Foreman ran into difficulty tying the new circuit breaker into the station's solar power grid.

As the spacewalk drew to a close, Behnken returned to the stubborn circuitry. He, too, was unsuccessful at making a power cable connection that would have established the link.

Endeavour's seven astronauts, including Foreman and Behnken, are in the midst of a 16-day space station assembly mission, the longest by a shuttle crew to date.

Thursday's spacewalk was the fourth of the five they plan.

The fifth outing is is scheduled for Saturday night. Endeavour departs the station on Monday for a return to Earth on Wednesday evening.


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