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Endeavour crew starts spacewalk as NASA wraps up tests on need for shuttle repairs

    HOUSTON — Two astronauts began a spacewalk Wednesday to prepare a solar array on the international space station for relocation as NASA engineers’ concerns over the shuttle Endeavour’s heat shield eased.
    NASA was finishing up tests to determine whether it would have to devote a fourth spacewalk to repairing a gouge in the belly of the shuttle. On Tuesday, officials said they were cautiously optimistic that wouldn’t be necessary.
    A sliver of the wound penetrates through a pair of inch-deep thermal tiles, exposing a thin felt fabric that is the final barrier before the shuttle’s aluminum frame.
    But thermal analyses have so far shown Endeavour could safely return to Earth as it is, said John Shannon, chairman of the mission management team.
    The 6 1/2-hour spacewalk was astronaut Rick Mastracchio’s third in five days. He and astronaut Clay Anderson, who has been living on the space station since June, were preparing one of the solar arrays for relocation to another spot on the orbiting outpost during a later mission.
    An hour into the jaunt, Tracy Caldwell — who is coordinating the spacewalk from inside the station — asked Mastracchio how he was feeling. A sensor in his spacesuit that monitors carbon dioxide levels failed during his last spacewalk and can’t be repaired in space, so there is no automated means to detect problems.
    ‘‘I feel good,’’ Mastracchio responded. ‘‘No problems.’’
    High carbon dioxide levels would make Mastracchio feel dizzy, have a headache or have trouble breathing.
    The Endeavour crew is halfway through their two-week mission to the international space station. The astronauts have completed most of their main goals, including attaching a new truss segment to the space station and replacing a gyroscope that helps control the station’s orientation.
    Any repairs to Endeavour would be conducted during the shuttle’s fourth spacewalk, scheduled for Friday. If more time is needed to prepare, NASA could bump the spacewalk to Saturday and keep the shuttle at the station longer.
    The gouge on Endeavour was not considered a threat to the crew, but NASA was debating whether to send astronauts out to fix it in order to avoid time-consuming post-flight repairs.
    Even though the repair itself would be relatively simple, the astronauts would be wearing 300-pound spacesuits and carrying 150 pounds of tools that could bang into the shuttle and cause more damage. All spacewalks are hazardous, Shannon noted, and so NASA would not want to add more outside work unless it was absolutely necessary.
    Engineers are uncertain whether it was foam insulation that came off Endeavour’s external fuel tank and struck the shuttle at liftoff, as was the case for Columbia four years ago, or whether the debris was ice or a combination of materials, Shannon said.
    The debris, which weighed less than an ounce, peeled away from a bracket on the tank, fell against a strut lower on the tank, then shot into the shuttle’s belly.
    Engineers speculate more ice could be forming on these brackets, which hold in place the fuel lines that feed the tank, because the super-cold fuel is being loaded an hour earlier than before.
    On Tuesday, teacher-astronaut Barbara Morgan carried out the long-postponed dream of an educator turning the space shuttle into a classroom.
    Morgan, who was Christa McAuliffe’s backup on the Challenger mission, took questions and spoke to hundreds of youngsters packed into the Discovery Center of Idaho in Boise, less than 100 miles from the elementary school where Morgan once taught.
    One child wanted to know about exercising in space. In response, Morgan lifted the two large men floating alongside her, one in each hand, and pretended to be straining. Another youngster wanted to see a demonstration of drinking in space. Morgan and her colleagues obliged by squeezing bubbles from a straw in a drink pouch and swallowing the red blobs, which floated everywhere.
    ‘‘Astronauts and teachers actually do the same thing,’’ she said. ‘‘We explore, we discover and we share. And the great thing about being a teacher is you get to do that with students, and the great thing about being an astronaut is you get to do it in space, and those are absolutely wonderful jobs.’’
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    On the Net:
    NASA: http://spaceflight.nasa.gov
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    AP Aerospace Writer Marcia Dunn contributed to this report from Cape Canaveral, Fla.

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