By Irene Klotz

HOUSTON, July 8 (Reuters) The first spacewalk slated for the shuttle Discovery crew today may be one small step for NASA, but a giant leap for the two men trying to discover if a spindly 100-foot boom can put distant parts of the spaceship within reach for emergency repairs.

NASA is developing techniques, materials and procedures for shuttle crews to repair their ships' heat shields in orbit should they become too damaged to risk the trip home. The work stems directly from the 2003 Columbia accident, which killed seven astronauts when their damaged ship attempted to return through the atmosphere for landing.

In addition to fixing the shuttle's fuel tank, which shed a chunk of the type of debris that downed Columbia, NASA developed a sophisticated imaging system so shuttle crews can inspect their ships for launch damage after reaching orbit. The sensors are on the end of a 50-foot boom, which is then attached to the shuttle's robotic arm of equal length for scanning.

Today's spacewalk, which is the first of three planned during Discovery's newly extended 13-day mission, is focused on tests to determine if the 100-foot-long span is stable enough to serve as a work platform. The spacewalk is scheduled to start about 1843 hrs IST CDT (1313 GMT) and last 6 1/2 hours.

Spacewalker Piers Sellers will take the plunge first. He planned to strap his feet into a foot grip at the end of the boom, then bend and twist and bounce to simulate an actual heat shield repair job. His partner, Michael Fossom, will go next.

Throughout the exercise, NASA expects a running commentary from the astronauts about the boom's stability.

The primary purpose of Discovery's mission is to test changes made to the shuttle's external fuel tank following the Columbia accident and the first post-Columbia mission last year.

The shuttle also delivered new equipment and supplies to the International Space Station and ferried a new resident crew member to join the two already aboard. The shuttle arrived at the station on Thursday following launch two days earlier on only the second flight since the accident.

NASA was still analysing images and other data today to determine if the new fuel tank design was successful. The shuttle program cannot withstand any more serious problems or it likely would be terminated, the agency administrator has said.

NASA hopes to resume space station construction, which has been on hold since the Columbia disaster, with two more flights this year and about four a year thereafter. The station must be finished before the shuttle fleet is retired in 2010, as there are no other vehicles that can carry the laboratories, structural trusses and solar power arrays into space and assemble them in orbit.

REUTERS KD HT1205