Two U.S. astronauts took a spacewalk Tuesday to perform exterior maintenance on the International Space Station. It was the last opportunity to fix hardware while the outpost has enough spare hands before the crew size soon decreases from three to two.
The grounding of the U.S. shuttle fleet after the Columbia disaster means a smaller station crew for the foreseeable future. The big shuttles can carry enough supplies to support a large station crew, but the U.S. space agency NASA says the smaller Russian rockets now filling the supply job can support only two crewmembers adequately.
To take advantage of the trio of astronauts and cosmonauts now on the outpost, NASA decided to advance the maintenance schedule before the two-man replacement crew arrives at the end of this month.
NASA's space station manager, Bill Gerstenmaier, says the smaller crew could do the work, but it is better to have a third crewman inside helping the others into their bulky spacesuits and assisting in other ways. "So we went ahead and looked at tasks that we thought we could go ahead and do now while we had three crew on board that would put us in a better posture to go into this period with just two crew and limited consumables," he says.
As a result, station commander Ken Bowersox and science officer Don Pettit floated outside the space station to make several components more durable. Among their tasks, they added a backup power source for gyroscopes that stabilize the outpost's position and reconfigured power connections to make them more stable.
In a few weeks, the two crewmen and Russian flight engineer Nikolai Budharin are to transfer command to their replacements, astronaut Ed Lu and cosmonaut Yuri Malenchenko. Because U.S. shuttles are grounded after the Columbia accident, the crew exchange will occur with the help of Russian Soyuz rockets about one month later than originally planned.
In Houston, Texas Tuesday, experts investigating the Columbia disaster found fault with the analysis by which NASA concluded that the shuttle's wing was not damaged by hard foam insulation falling from its external fuel tank during launch.
The experts are checking the possibility that the foam strike, indeed, caused a wing opening that superheated atmospheric gases penetrated during re-entry. NASA engineers had determined while the flight was in progress that falling foam could not have hurt the wing.
But the chairman of the board of experts, retired Admiral Harold Gehman says the analytical model they used was flawed. "The model has a lot of limitations. It's a rudimentary kind of model. It's essentially a spreadsheet. It's not a computational model," he says. "It's really just a bunch of data based on previous experience and some testing. But it is not a predictive model."
The investigators say the conclusion reached from that analysis influenced NASA's decision not to request long range military photography to determine if there was wing damage that could threaten the shuttle.
NASA has argued that even if wing damage had been found during Columbia's mission, it could not have been repaired in orbit.