Washington: Indian American astronaut Sunita Williams prepared for a six month long stay at the International Space Station as the countdown began shortly before midnight Monday for Space Shuttle Discovery's Dec 7 launch.

 
Williams, who will replace German Thomas Reiter as a flight engineer on the space station, and six other crew arrived Sunday at National Aeronautics and Space Administration's (NASA) Kennedy Space Center in Florida for final preparations.

Reiter, a European Space Agency astronaut, who flew aboard Discovery July 4 to take up residence at the Space Station, will return to Earth with the rest of the STS-116 Shuttle crew after its 12-day mission.

For Ohio born Williams, 41, the second person of Indian origin taking NASA's fourth space flight since the Columbia disaster killed Indian born Kalpana Chawla and six other astronauts in 2003, would be a dream come true.

"So many people in the Astronaut Office are from different countries and cultures, and every time someone goes up that can identify with a group of people on the ground, you get that group of people wondering, 'wow, maybe that could be me one day.' There are a million possibilities out there," she said in a pre-flight interview.

 "I am half Indian and I've got a, I'm sure, a group of Indian people who are looking forward to seeing this second person of Indian origin, flying up in space. So it's nice to know that everybody brings along with them a group of people from all over the world that get interested in space," said Williams.

With the clock starting at the T-43 hour mark, there are 27 hours, 36 minutes of hold time built into the countdown, leading to a preferred lift-off time at 9:35 p.m. EST (8:05 IST Friday).

 At Launch Pad 39B, Space Shuttle Discovery is safely enveloped by the pad's rotating service structure, which protects the shuttle assembly from the elements while providing access for technicians. The structure will be rolled back to the 'park' position early Thursday morning, revealing the shuttle poised for launch.

 NASA considers Discovery's Thursday mission as the most complex of the final 14 assembly flights to finish the International Space Station.

"What makes this one singularly unique is the fact that we're going to rewire the space station," Mark Polansky, Discovery's commander, said.

 Since it went into orbit in 1998, the space station has been running on a temporary electrical system. Lead Space Station Flight Director John Curry compared it to the way you might build a house on the ground  until your electricity is hooked up, you probably plug your saws into a generator.

That's basically what the astronauts building and living on the station have been doing for the past eight years. But with the installation of two new electricity-generating solar array panels in September, all the pieces are now in place to switch to the permanent system.

 The plan is to send astronauts out on two spacewalks, each devoted to rewiring half of the station. Though it sounds complicated, that part shouldn't be too difficult. Spacewalks are inherently dangerous and should only be done if there is no alternative, Polansky said, but as spacewalks go, these are pretty straightforward.

 
The astronauts will head outside, wait for the team on the ground to send commands to switch off the power, and then unplug the power cables and plug them in new places.

 
Theoretically, everything should go fine. But there are a few things that could cause some big hitches. Several of them have to do with the unpredictability of equipment that's been in space for years.

 
For instance, before any of the rewiring can be done, half of the solar array that's been providing the temporary electricity must be folded up to make room for the new solar arrays to rotate. That's never been tried before, and it may not be as easy as it sounds.

 
"It's been sitting out there taking thermal cycles (moving from minus 200 degrees Fahrenheit to plus 200 degrees Fahrenheit every 45 minutes) since November 2000," Curry said.
 
"It's like a map  if you keep a map out in your car for six years and then you decide to fold it up again, you may get some waves in it or it may not fold back the same way at all," he said.

Many of the main components of the electrical system have been flying that long and could cause similar large headaches. It's impossible to know for sure if the equipment will work until the power has been turned off, rewired and turned back on.
 

And if it doesn't work, the astronauts can't leave it like that - the essential systems on the station would be running on whichever half of the station has power, but without both halves they won't have any backup.