Astronauts on board the space shuttle Discovery will attach an extension to the orbiting International Space Station during a spacewalk today, in what is said to be one of the most difficult tasks ever carried out in space.

Discovery, which docked last night local time, will remain at the ISS for eight days, during which its crew will perform three spacewalks.

In the first, Christer Fuglesang, a Stockholm physicist and the first Swede in space, along with Robert Curbeam, the mission specialist, will attach a two-tonne, $11 million cube-shaped addition to the space station.

During the subsequent spacewalks astronauts will rewire the US-made portion of the space station, which will involve cutting off the power and activating extra solar panels to double the current electrical output.

The National Aeronautics and Space Administration says it will be the trickiest manoeuvre ever carried out in space.

Meantime, Nasa was deciding whether there was any need for further inspection of the shuttle’s left wing after a sensor detected a "very low impact" less than three hours after docking.

Two astronauts had used a robotic arm to examine the wing leading edge. But John Shannon, chairman of the mission management team, said the impact was considered to be of concern but the agency wanted to play if safe.

"It looks like something happened," said Mr Shannon, adding that the wing looked fine during a focused inspection performed Sunday.

Discovery launched late Saturday from Cape Canaveral in the first night launch in four years. The crew comprises of two women and five men all of whom, with the exception of Mr Fuglesang, are American.

Six of Discovery’s seven astronauts will spend a week at the space station and the seventh, Sunita Williams, will live there for six months, replacing German astronaut Thomas Reiter of the European Space Agency.

Before docking, Discovery performed a back flip under the orbiting stations so that the crew could film there in order to detect any potential damage to the heat shield.

This has become a routine examination since the Columbia tragedy in 2003 when the shield was pierced by foam insulation which peeled off its fuel tank during lift off causing the shuttle to disintegrate during its return to earth.

Discovery, which will land on December 21, is one of 14 shuttle flights Nasa has planned over the next four years.

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