NASA has moved up one day to March 11 the launch of its space shuttle Discovery on its 16-day mission to the orbiting International Space Station (ISS).

The 24-hour change in the launch date reveals NASA's confidence the agency has resolved the valve problems that have led to four delays in Discovery's mission, which was originally set for launch on February 12th. NASA engineers said they replaced Discovery's three control valves, which channel gaseous hydrogen from the shuttle's three main engines, with newer ones, and that they would design a new valve at some point in the future. The delays were implemented as a precaution to test the valves, which had come under close scrutiny after a valve aboard space shuttle Endeavour was found to be damaged during its mission to the space station in November. The damaged valve was found to have a fissure, but it did not compromise Endeavour's safety and worked perfectly during the 8.5 minutes of the shuttle's liftoff to orbit, NASA said.

The new launch date of March 11th will have to be confirmed during a Friday review of launch procedures and preparations, NASA said in a statement. The U.S. space agency is to hold a press briefing on the same day. The Discovery launch was initially scheduled for February 12th. It was delayed until February 19th and then again until around February 27th. A fourth delay was announced February 21st , and five days later the March 12th date was set. Discovery's seven astronauts, including one from Japan, are to deliver the fourth and last pair of power-generating solar panels to the ISS. The double antenna is needed to produce enough electricity to conduct all scientific experiments in the new Japanese and European laboratories that were added to the ISS last year. The solar panels will also increase power generation to accommodate the expansion of the outpost's permanent crew from the current three astronauts to six in May. The ISS is scheduled to be completed in 2010, the same target date for the retirement of the US fleet of three space shuttles. If the tentative launch date holds, there will be no effect on the next two shuttle launches to the Hubble Space Telescope and again to the ISS, NASA said.

(Photo: At NASA's Kennedy Space Center in Florida, technicians prepare to install three gaseous hydrogen flow control valves on space shuttle Discovery. Photo credit: NASA/Chris Rhodes)