(Updates with start of spacewalk)

By Irene Klotz

HOUSTON, March 20 (Reuters) - A pair of spacewalking astronauts ventured outside the International Space Station on Thursday to practice a procedure NASA hopes it never needs.

Seared by the memory of shuttle Columbia's demise five years ago due to heat shield failure, the U.S. space agency has been tooling up damage prevention and repair procedures to give future shuttle crews a better chance of surviving a similar accident.

Columbia's seven astronauts were killed as it broke apart during its return to Earth for landing.

In addition to extensive in-flight inspections, NASA has developed techniques for spacewalking astronauts to fix minor heat shield damage. Thursday's outing by Endeavour astronauts Robert Behnken and Michael Foreman is devoted to testing how a putty-like filler for damaged heat tiles behaves in microgravity.

The men floated outside the station's airlock shortly after 6 p.m. EDT (2200 GMT).

Engineers are particularly concerned if the goo will bubble up inside, creating a lip around a repair that could trigger excessive heating as the shuttle plunges through the atmosphere.

Behnken and Foreman will work on sample tiles, some intentionally damaged, some unintentionally broken during previous space flights or during tests that helped investigators trace the cause of Columbia's destruction to a piece of foam debris that fell off during launch and hit the ship's wing.

SURPRISING DAMAGE

After the accident, investigators blasted sample heat shield panels with small chunks of ice and foam and were amazed at how much damage was done.

"That was an incredible, almost horrifying moment," Endeavour pilot Greg Johnson, who helped in the investigation, said in a preflight interview. "Jaws dropped when it made such a huge hole in the panel."

The sample tiles patched during Thursday's spacewalk will be returned to Earth on Endeavour for analysis. NASA wants to have the results before dispatching a shuttle crew to work on the Hubble Space Telescope later this year, as the space station will be too far away to shelter the crew if their ship is too damaged to return home.

"We learn from these accidents and get smarter," Johnson said.

The spacewalk was the fourth of five planned during Endeavour's 12-day construction and maintenance mission at the space station. The crew already has assembled a Canadian-built maintenance robot and installed a storage room for Japan's upcoming Kibo laboratory complex.

Apart from the flight to Hubble, scheduled for late August, NASA has 10 more construction and resupply missions to the space station before the shuttles are retired in two years. (Editing by Jane Sutton and Mohammad Zargham)

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