WASHINGTON (AFP) - After four launch delays, NASA says it now believes the space shuttle Discovery could be sent on a mission to the orbiting International Space Station (ISS) by mid-March. While the launch is "tentatively targeted" for March 12, an exact date "will be determined as work progresses with the shuttle's three gaseous hydrogen flow control valves," the space agency said in a statement late Wednesday. The delays were announced as a precaution to test the control valves, which channel gaseous hydrogen from the shuttle's three main engines. The shuttle's valves have come under close scrutiny since a valve aboard space shuttle Endeavour was found to be damaged after its 16-day mission to the orbiting ISS in November. 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 February 21, with no date set for a launch. 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 people 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.