NASA has postponed the launch of the space shuttle Discovery for a fourth time, but without setting a new target date to send the orbiter to the International Space Station (ISS).

The decision was announced after a 13-hour meeting of a team in charge of preparation for the launch, which said it wanted to conduct further tests on three flow control valves.

The launch was initially scheduled for February 12. Then it was delayed until February 19 and then again until around February 27.

“At this point I want to make sure we get a measured approach to fly and we are ready to move in the right configuration,” Bill Gersteinmaier, an assistant administrator at the National Aeronautics and Space Administration, told reporters.

“We will take a look at it and we will know for sure on Wednesday,” he added.

The delays were announced as a precaution to test three flow control valves that 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.

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.