The two U.S. and Russian crewmen aboard the International Space Station are preparing for a spacewalk later Thursday that will leave the outpost temporarily unoccupied for the first time. The U.S. space agency, NASA, had to overcome initial qualms about the maneuver before accepting it.
The spacewalk takes astronaut Michael Foale and cosmonaut Alexander Kaleri outside the Russian module to attend to Russian, European and Japanese experiments measuring space radiation, micro-meteorite impacts and residue from jet thruster firings. They are also to retrieve reflectors that engineers are studying to help guide the arrival of a European cargo vessel next year.
This is the first spacewalk without a third crewmember inside to oversee station operating systems. The team was reduced to two when the U.S. shuttle fleet was grounded for safety modifications after the Columbia disaster. The shuttle is the only spacecraft big enough to carry sufficient supplies to support three station residents.
When Russia proposed the spacewalk last year, NASA was reluctant. Station operations manager Mike Suffredini says the United States wanted to delay the so-called EVA, or extra-vehicular activity, until after shuttles returned to flight and could support a third station inhabitant.
"From our perspective, at that time, we felt like a few months delay was probably not critical, and we could do the EVA within the time frame we were comfortable with, in terms of doing it with three crew on board," said Mr. Suffredini.
But Moscow was eager for the spacewalk to fulfill its contracts with the European and Japanese space agencies. Besides, it had a lot of experience leaving its former Mir and Salyut stations unoccupied when cosmonauts ventured outside.
The NASA official supervising the current station mission, Peter Hasbrook, says the United States eventually relented. "As we talked with our international partners, especially with the Russian side, as well as the European side and the Japanese side, which are sponsoring some of these objectives, we came to understand from them that they did need these activities performed via EVA, [and] we needed to do that before [the] shuttle was going to return to flight," he says.
Still, it took months of technical talks between U.S. and Russian experts to work out details for the spacewalk. NASA wanted to be sure the space station was safe with no one inside. The chief U.S. flight director for the spacewalk, Sally Davis, says NASA asked the Russian experts many questions, and, over time, both sides developed flight rules to guide operations in these special circumstances.
"We were able to come up with joint responses to some of the scenarios that we might encounter in an EVA," says Ms. Davis. "The idea of the flight rules was to manage the system failures better, more cohesively, between the two control teams, and to reduce the risk to the vehicle while the crew was not inside."
Among NASA's main concerns are a loss of the outpost's stability. The jet thrusters that normally maintain its position must be shut off, so their exhaust will not contaminate the spacewalkers. This leaves gyroscopes to do the job. But one of four gyroscopes is not working. Two are normally sufficient for positioning, but if thrusters must be fired to take over, the crewmen must get out of the way.
The special flight rules allow for termination of the spacewalk in case of a crisis such as a fire, but only after the crewmen finish the task they are working on. If astronaut Foale or cosmonaut Kaleri is injured, medical supplies are positioned at the hatch for quick access. If the ship must be abandoned because of an emergency, some station segments and systems are placed in a secure configuration and the two men are to move into the Soyuz escape craft attached to the outpost.
Station operations manager Suffredini says the robustness of the spacecraft and its backup systems give the spacewalkers ample time to return indoors to deal with a problem. That process takes two hours.
In the end, he says, NASA realized it was better to have the spacewalk now than wait.
"We are mitigating more risks than we are taking by doing this EVA now," says Mr. Suffredini.
