March 22, 2008

Astronauts Embark on Final Spacewalk

Two astronauts from the shuttle Endeavour floated outside the international space station Saturday afternoon on the final spacewalk of a marathon assembly mission.

Mike Foreman and Bob Behnken had a list of tasks: a transfer of the Endeavour's 50-foot-long inspection boom to the station, an examination of a long hobbled mechanism that rotates electricity generating solar panels on the station's right side; and the installation of external science experiments.

Saturday's outing, which got under way just after 3 p.m. CDT, was to last nearly seven hours and was the fifth spacewalk since Endeavour's crew of seven astronauts docked with the space station on March 12. The fliers plan to return to Earth on Wednesday evening, ending a 16-day mission in which they delivered the first piece of a Japanese science laboratory and a Canadian robotic device.

The first task for the two men on Saturday's spacewalk was to assist the astronauts assigned to a late May mission aboard the shuttle Discovery. Discovery will deliver the second piece of the Japanese lab, a boxcar-sized enclosure in which space station tenants can conduct biology and physics experiments.

With the help of two robot arms, Foreman and Behnken planned to transfer Endeavour's heat shield inspection boom from the shuttle to a make-shift storage rack on the outside of the station's central solar power system truss. They will string a 30-foot long extension cord to power the boom as well.

The Japanese lab enclosure is to large there will be no room aboard Discovery to carry one of the cameras and laser tipped booms. Shuttle crews rely on the booms to inspect the thermal armoring of the wings and nose of their ships for damage after they launch.

The inspections were added to missions after the 2003 Columbia tragedy. Columbia's breakup was traced to undetected heat shield damage from a launch-day impact with a chunk of breakaway foam fuel tank insulation.

Discovery's crew will wait until they reach the space station to conduct an inspection with Endeavour's boom.

With the time that remains on today's spacewalk, Foreman and Behnken will split up.

Foreman will float to the station's right side for an inspection of the rotational mechanism that turns outstretched power generating solar panels to track the sun as the orbital base circles the Earth.

NASA was forced to interrupt the "auto tracking" last fall, when flight controllers noticed a worrisome vibration in the gearing of the mechanism.

The station's solar power network generates the highest levels of electricity for life support systems and scientific research with constant tracking. Without a future repair, the hobbled solar panel could force NASA to slow its space station construction plans.

Today's spacewalk is the latest of five outings that have included a troubleshooting inspection of the mechanism.

Space agency engineers suspect an impact with fragments of space debris or a meteoroid or an internal breakdown of some kind is responsible for the damage.

During past inspections, spacewalking astronauts found and collected samples of metal grit found along an internal bearing ring.

Foreman was assigned to lift and check beneath five protective cover plates, the only region of the large gear that has not been inspected. He's to check closely on what appears to be a small pit or a buildup of a dark contaminant that was noticed during a late January inspection.

The space agency plans repairs over multiple spacewalks, perhaps late this year.


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