NASA said on Wednesday it had 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 shows NASA's confidence it has resolved the valve problems that led to four delays in Discovery's mission that was originally set for launch on February 12.
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 11 will have to be confirmed during a Friday review of launch procedures and preparations, NASA said in a statement. The US space agency is to hold a press briefing on the same day.
The Discovery launch was initially scheduled for February 12. It was delayed until February 19 and then again until around February 27. A fourth delay was announced on February 21, and five days later the March 12 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, the National Aeronautics and Space Administration said.