Washington - NASA is eyeing early Friday as the next launch window for the delayed Discovery shuttle after lightning and a valve problem scuttled two earlier attempts this week.

NASA officials said the next launch attempt could occur as early as 0422 GMT (12:22 am in Florida) Friday, but they are awaiting the final results of testing and data review by NASA experts.

NASA said on its website that it was pleased with partial results from the testing of the faulty valve in Discovery's main propulsion system, but said added pressure tests were being conducted and showing leakage of small amounts of liquid hydrogen.

Fuel valve problems have dogged recent shuttle launches as the ageing craft and launch apparatus enters the final phase of the decades-old shuttle programme.

By late next year, NASA hopes to have completed the build-out of the orbiting International Space Station so it can retire the shuttles. In their place, a new spacecraft is being built that will be available in 2015 at the earliest.

Delayed takeoffs are common with space shuttle missions. The last shuttle to blast off, Endeavour, only did so in July after five scrubbed missions due to technical problems and bad weather.

The current planned 13-day mission is designed to transport new equipment and experiments to the International Space Station (ISS). Astronaut Nicole Stott is expected to relieve Timothy Kopra aboard the ISS, and three spacewalks are planned for the mission.

Discovery's crew for this mission includes six Americans and Swede Christer Fuglesang, flying for the European Space Agency. (dpa)