The U.S. space shuttle Atlantis is set to take off with a crew of seven and a supply of hardware to expand the international space station. Spacewalking astronauts are to install the first section of a long girder that will hold critical life support and power-generating equipment.
Instead of hard hats and blue jeans, the astronauts will be wearing hard helmets and spacesuits on this assembly mission. Otherwise, they will look very much like high-rise construction workers when they and the space station crane attach the centerpiece of cross-beam truss across the outpost's spine.
The mission's chief spacewalk trainer, Dina Barclay, said the 13-meter-long unit, called S-Zero, is the first of 10 that will eventually stretch more than 100 meters and support power and cooling systems for future research laboratories.
Mr. Barclay said, "The truss plays a vital role in the space station's future because future truss segments will attach to either side of it and all the avionics and ammonia cooling that go out to and come in from the outer truss segments go through S-Zero for power distribution and cooling for the modules in which our astronauts live and work."
Eventually, when all the truss segments are linked together, they will hold nearly half a hectare of solar panels and radiators to dissipate heat.
This first truss segment has 475,000 parts, making it the station's second most complicated piece of hardware after the U.S. laboratory, to which it will connect. Among those parts are those that make up first space railroad a flatcar and section of track spanning the truss's length.
Station flight director Bob Castle said the railcar one day will roll along the completed truss carrying the outpost's robot arm. Mr. Castle said, "It allows us to take the robotic arm, which now is limited to one location, it has to stay on the lab right now - it will allow the robotic arm to travel the length of the truss so we can actually grapple things and then roll along the length of the truss and install things at the end of it."
The station's robot arm is critical to this mission because unlike the smaller shuttle version, it is long enough to lift the 13-ton truss from Atlantis and reach the truss's final location on the U.S. lab.
However, one of the arm's seven joints freezes when commanded through one of the two redundant computer circuits that operate the arm. It works properly with the second command circuit, but in case that fails, too, the Canadian engineers who designed the crane have relayed a revised software program to the station that will allow it to maneuver with only six working joints.
In that case, spacewalkers would manipulate the seventh joint manually, according to space station operations official Michael Suffredini. "They have a way to attach a tool to that joint and rotate that joint about a little over 180 degrees," he said. "Then the crew would continue to fly the arm as they are used to all the way to the lab."
A subsequent shuttle team will replace the faulty joint during a June visit to the outpost.
The work of this mission is to be accomplished by four shuttle astronauts in alternating pairs over four consecutive spacewalks. One of the pairs includes Jerry Ross, the U.S. space agency's most experienced spacewalker.
Atlantis commander Michael Bloomfield said this is Mr. Ross's seventh space flight, a world record. Command Bloomfield said, "He became an astronaut in 1980, before the shuttle even flew. After the flight, he will have completed nine spacewalks, more than any other American astronaut. Almost four percent of his time in space will actually have been outside."
Commander Bloomfield said Jerry Ross, at age 54, is one of two spacewalking grandfathers on this shuttle mission. Their colleagues affectionately call them the gray team.