NASA began its three-day countdown on Tuesday for the launch of the space shuttle Endeavour on a mission to hang "wings" on the International Space Station.
The wings are actually panels of photoelectric cells that convert sunlight to direct-current electricity. They will vault the space station past its predecessor, the Russian Mir, where the quality of scientific research was limited by low power provided by smaller arrays.
At 240 feet (73 m) in length, the $600 million wings will be twice as long as Endeavour once unfurled.
"That's a little larger than the wingspan of a 747 aircraft, and they'll provide enough electricity, if they were down here on Earth, to power 30 homes," said Bob Cabana, an astronaut who doubles as the space station's launch integration manager.
"The crew on this particular mission will attach the heaviest, largest and most complex piece of station hardware launched on the shuttle so far," said Ron Dittemore, the shuttle program manager.
Endeavour and its five-astronaut crew is scheduled to lift off on Thursday about 10:06 p.m. EST. (0306 Friday GMT). U.S. Air Force meteorologists have predicted a 90 percent probability of fair skies and acceptable weather.
If launched on time, Endeavour will be the third shuttle mission in three months.
The Endeavour crew arrived at the Kennedy Space Center launch complex in T-38 training jets on Monday night from the astronaut training facility in Houston.
Shuttle commander Brent Jett will be joined on the mission by pilot Michael Bloomfield, Canadian Space Agency astronaut Marc Garneau and spacewalkers Joseph Tanner and Carlos Noriega. Tanner and Noriega will conduct three spacewalks, securing the solar panels and connecting power couplings so the electricity can flow to the station.
The wings will ride into space rolled up in tubes and deploy once Garneau attaches them to the station.
The current space station crew, made up of Russians Yuri Gidzenko and Sergei Krikalyov and their American commander, Bill Shepherd, can live and work in only two of three existing modules because there is not enough power to heat a third, the U.S. Unity module.
Once the solar panels are in place, they will be able to keep Unity open and also work in a fourth module, a U.S. laboratory named Destiny, when it arrives in January.
Endeavour's crew will be the sixth shuttle crew to visit the space station, but the first to have a crew aboard the station awaiting them. Shepherd, Gidzenko and Krikalyov arrived earlier this month and will be replaced by another crew after a four-month stay.
The two crews will not spend much time together during this mission. The hatches between the shuttle and station will not be opened until all three spacewalks are complete. For a variety of reasons, air pressure is kept lower aboard the shuttle than on the station when astronauts are moving back and forth through airlocks.
In the end, the two crews are scheduled to spend only about a day together, most of that spent stowing space-station trash on the shuttle for a return flight to Earth.
The $60 billion space station is a joint project of the United States, Russia, Europe, Japan and Canada. Completion is targeted for 2006. By that time it will have as much living space as three suburban homes and will be one of the brightest objects in the night sky.