Power mission for space shuttle

The flight has been delayed to allow engineers to investigate a value issue

Nasa's Discovery shuttle is about to launch on a mission to complete the electricity generation system on the International Space Station (ISS).

The orbiter will deliver the fourth and final set of solar array wings to the platform and the last segment of its truss, or backbone.

The 73m-long arrays will provide the power necessary to fully exploit the science labs now attached to the ISS.

Their electricity will also support the expansion of the crew to six people.

Currently, the normal live-aboard complement is three individuals; and the station partners want to double this in May.

Discovery is due to lift off from the Kennedy Space Center in Florida at 2120 local time (0120 GMT, Thursday 12 March).

The 14-day mission will feature four spacewalks to help install the S6 truss segment to the starboard, or right, side of the station and the deployment of its solar arrays.

Discovery's astronauts will also replace a failed unit for a system that converts urine to drinking water.

Another key task is ISS crew rotation. Discovery will drop off the Japanese space agency's (Jaxa) first resident crew member.

Dr Koichi Wakata is an experienced astronaut, having flown on two previous shuttle missions. He is expected to stay on the ISS until June.

"This week will be an historic week for Japan's space programme," said Kuniaki Shiraki, executive director of Jaxa's human space programme.

"We have been waiting a long time."

Dr Wakata takes the place of Nasa astronaut Sandra Magnus, who has served as a flight engineer on the platform since November.

Last missions

Discovery's flight is the 125th to be made by a shuttle; the 28th to the ISS; and the 36th flight for Discovery itself.

Once the final truss segment is installed, the station's backbone - which supports not just the arrays, but radiators and other equipment - will be 102m long.

Koichi Wakata: The first Japanese long-duration ISS resident

When viewed from the Earth's surface shortly after sunset, the ISS appears as a very bright star moving swiftly across the sky. The addition of a larger reflective area will make the platform an even more brilliant spectacle.

Altogether, the station's arrays can generate as much as 120 kilowatts of usable electricity, says Nasa.

The addition of the final set of solar arrays will nearly double the amount of power available for scientific experiments aboard the station - from 15kW to 30kW.

Eight more flights are required to finish construction of the station and to fill it with supplies prior to the retirement of the shuttle fleet in 2010.

Nasa also plans one additional shuttle mission to repair and upgrade the Hubble Space Telescope.

The agency is still awaiting the appointment of an administrator following the departure of the Mike Griffin. President Obama has yet to fill the post.

Discovery's mission has been delayed by several weeks to give engineers time to investigate the cause of a fractured fuel valve on the last shuttle mission.