January 29, 2008

Astronauts to Repair Solar Wing Motor in Wednesday Spacewalk

Astronauts aboard the International Space Station (ISS) are preparing
for a Wednesday spacewalk to restore some of the power-generating ability of
the orbiting laboratory's expansive solar wings.

ISS Expedition 16 commander
Peggy Whitson
and flight engineer Dan Tani will replace a broken joint
motor at the base of the station's two starboard solar wings during a planned
six-hour spacewalk.

The solar wing has been unable
to track the sun
continuously since early December, when the joint motor
suffered a series of electrical shorts. Without the repair, the space station will
have enough power to make it through at least the next shuttle mission - targeted
for a Feb. 7 launch - but not much further said Kirk Shireman, NASA's ISS deputy
program manager.

If the Wednesday spacewalk is successful, the ISS will have
power to last through the planned arrival of a massive Japanese laboratory in
April and into the summer, he added.

The broken motor controls a beta gimbal joint that pivots
one of the station's two starboard solar wings to face the sun. After the motor
experienced electrical failures in December, Whitson and Tani made a spacewalk
to inspect the joint and found no outward signs of damage. NASA hopes replacing
the whole motor, a garbage-can sized device that weighs about 250 pounds (113
kilograms), with a backup will fix the problem.

For safety reasons, the astronauts can only work during
eclipses, when the sun is blocked by the earth. If the sun was shining on the
solar panels while Whitson and Tani were working on the joint, they would be at
risk of shocks due to the high power levels surging through the arrays.

"We have to be very conscientious when we're opening
connections that will expose us to that power," said Tani, adding that he
and Whitson must also take special care since they will be working with latches
that physically connect the solar wing to the ISS. "That is probably the
biggest danger of this [spacewalk]."

Once they begin working, the astronauts will have 33 minutes
of complete shade at a time. If they can't replace the motor during one
eclipse, they'll have to wait till the next one. The space station orbits the
Earth about once every 90 minutes to make a single circuit above the planet's
day and night sides.

"They will be in a little bit of a timeline crunch when
they're performing activities during the eclipse," said astronaut Thomas
Marshburn, who has rehearsed the repair in a giant NASA swimming pool used for
spacewalk training. "We found that it is very doable to get all this

The space station is currently afflicted by two unrelated power
system glitches with its starboard solar arrays. In addition to the broken joint
motor, known as a Bearing Motor Roll Ring Module, the orbital lab is also
suffering from damage to a massive gear designed to rotate the station's
starboard solar arrays like a paddlewheel to track the sun.

Astronauts discovered
metal grit
from the gear's attached metal ring during past spacewalks.
Whitson and Tani will take another look at the 10-foot (3-meter) wide gear if
they have extra time during Wednesday's excursion, mission managers said.

Space station managers are still discussing when to repair
the larger gear, called a Solar Alpha Rotary Joint, later this year, though any
effort will likely require a series of spacewalks.

Wednesday's excusion will mark the sixth career spacewalk for
both Whitson and Tani, and the fifth for the station's Expedition 16 crew.

"Any opportunity to go outside is a very special
event," Whitson said. "The huge structure that we've built up there
is just amazing to see. In its own way it's just as beautiful as looking at

NASA will broadcast the Expedition 16 crew's fifth
spacewalk live on NASA TV beginning at 4:00 a.m. EST (0900 GMT) on Jan. 30. Click here for SPACE.com's live
coverage and mission updates.