For more than three years, the International Space Station has floated half-built above the Earth. Maintained by a skeleton crew, the station -- an assemblage of modules and girders -- has not come close to its stated goal of becoming a world-class research outpost.
But now construction, which has hung in limbo since NASA's space shuttle fleet was grounded after the 2003 Columbia disaster, is scheduled to resume. The shuttle Atlantis is scheduled to lift off on Sunday, carrying a bus-size segment of the station's backbone that includes a new set of solar-power arrays.
Since the project began in late 1998 with the joining of two US and Russian modules, the US and 15 other nations have slowly put together a structure that weighs more than 180,000kg, with a habitable volume of almost 425m3. When completed, it is to weigh almost 450,000kg and have a cabin volume of more than 930m3, larger than a typical five-bedroom house.
Getting to that goal will require some of the most difficult shuttle missions ever mounted by NASA, starting with the Atlantis' launching from the Kennedy Space Center at Cape Canaveral in Florida. The shuttle and its crew of six will haul a 13,600kg, 14m truss segment to the station, delicately remove it from the cargo bay and install it during three spacewalks by two teams of astronauts.
"The flights ahead will be the most complex and challenging we've ever carried out for construction of the International Space Station in orbit," said Michael Suffredini, the station program manager at NASA.
NASA has allotted about 15 flights to complete the project before the shuttles are retired in 2010.
The next four missions will carry other massive truss segments to extend the station's central girder to more than 107m. The girder will eventually support four huge sets of solar-power arrays, batteries and heat-dispensing radiators.
The additional truss segments, which will increase the mass of the station by 36 tonnes, will also include 3m wide rotary joints shaped like wagon wheels that will allow the solar arrays to track the sun for optimum power as the station moves in orbit. The Atlantis is delivering the second array, joining one put on the station in 2000.
"The assembly of the station on these flights has no parallel in space history," Suffredini said. "We have planned, studied and trained for these missions for years. We know they will be hard, and we may encounter the unexpected. But we are eager to get started."
Work on the station stopped because of the Columbia accident, which forced NASA to redesign many shuttle systems and its own safety procedures.
The Atlantis' mission, which will last about 11 days, will be an exhausting one for its six-member crew.
"We've never had to fit so many activities back to back on consecutive days," said Captain Brent Jett Jr. of the Navy, the mission commander and a veteran of three shuttle flights.
Once in orbit, the crew will spend a full day inspecting the shuttle's heat shield with a Canadian-built robot arm and an attached 15m boom tipped with a camera and sensors.
As the shuttle approaches the space station for docking the next day, Jett will stop it about 180m away and execute a pitch maneuver that will rotate the orbiter nose-up, allowing the station crew to take detailed photographs of the shuttle's underbelly. Then he and the pilot, Christopher Ferguson, another Navy captain who is on his first mission, will dock with the station.