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Last Updated: Friday, 8 December 2006, 13:47 GMT
Shuttle night lift-off postponed
Space shuttle Discovery
The shuttle needs to be able to launch at night to meet schedules
Nasa has called off the launch of its third space shuttle mission in six months because of poor weather.

The crew were seated inside the space shuttle Discovery at the scheduled launch time of 2135 (0235 GMT), but low clouds prevented lift-off.

The flight was set to be the first night launch of the orbiter since the Columbia accident in 2003.

Nasa is racing to finish work on the International Space Station (ISS) before the fleet is retired in 2010.

A British-born astronaut, Nicholas Patrick, is among Discovery's crew.

A new launch time has been set for Saturday at 2047 local time (0147 GMT Sunday).

No opening

A cold front was forecast to bring low clouds into central Florida on Thursday evening.

We gave it the best shot
Mike Leinbach
Nasa launch director
A lift-off appeared possible until the last moments of the launch window, despite weather forecasts putting the chances at just 40%.

But controllers called off the flight when weather systems revealed no break in the clouds over the Kennedy Space Center.

"We gave it the best shot and didn't get clear and convincing evidence that the cloud ceiling had cleared for us," launch director Mike Leinbach told the shuttle's seven astronauts.

Commander Mark Polansky urged the crew "not to be too disappointed".

Five of the astronauts on the mission are going on their first shuttle flight.

The US space agency decided to skip a Friday launch attempt because the weather forecast is even worse than for Thursday.

'Complex mission'

Nasa has described the construction mission as one of the most complex to date.

On the previous shuttle flight in September, astronauts delivered new solar arrays to provide power for additional modules scheduled to be installed next year.

Space shuttle Discovery  (Nasa)
The astronauts were strapped in their seats and ready to go
The tricky task of wiring the arrays into the ISS power grid falls to the crew of shuttle Discovery. This will need to be done without interrupting the station's life-support and other critical systems.

Half the outpost will be powered down while astronauts make the new electrical connections during two separate spacewalks. It will be a tense time, with little back-up power if other problems arise.

"Many of us consider this the most challenging flight that the International Space Station will have done since we began the effort of assembling it," said space station manager Mike Suffredini.

In addition, this mission will attach a small truss, or backbone, connector to the existing structure, and retract a solar array into a safe position to allow future work to be completed unhindered.

Nicholas Patrick  Image: AP
Nicholas Patrick, now a US citizen, is a robotics expert
At least 14 more missions are needed to finish the $100bn (50bn) station.

Discovery's flight is scheduled to last 12 days, with a landing targeted for 19 December at the Kennedy Space Center in Florida.

Discovery's crew consists of Commander Mark Polansky, pilot William Oefelein and mission specialists Robert Curbeam, Joan Higginbotham, Nicholas Patrick, Sunita Williams and the European Space Agency astronaut Christer Fuglesang.

Sunita Williams will be making a one-way trip. She is to remain aboard the space station, replacing Germany's Thomas Reiter, who will return with the rest of the Discovery crew.

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