NASA’s agency officials said Friday that the launch of the shuttle Discovery's mission to the International Space Station will be delayed for three more days, to February 22, to allow more time to test potentially troublesome fuel valves.

Discovery was slated to launch no earlier than February 19 pending the completion of tests to ensure the shuttle's three fuel flow control valves are safe to fly. I other words NASA says it needs the extra time to finish testing hydrogen gas fuel valves. A small part of one valve broke during the launch of the last shuttle flight in November.

Discovery and its seven-man crew are set to deliver the last set of solar wings to the international space station. NASA spokesman Kyle Herring said engineers are still conducting tests, which are taking longer than expected. The worry is that if the valve tip comes off, it could damage part of the shuttle.

During NASA's last space shuttle launch in November, one of the valves aboard the Endeavour orbiter was damaged, sending a small chip about the size of a thumbnail tip into the plumbing lines leading back to the external tank. Endeavour successfully reached orbit and maintained fuel tank pressure despite the glitch. Engineers found a crack in the suspect valve after Endeavour returned to Earth.

"We want to make sure we've got this right," said NASA's space operations chief Bill Gerstenmaier late Tuesday. "So we think standing down a little bit of time, and letting the folks do a little more work is a good thing."

NASA plans eight more flights to the space station to complete construction, which has been under way for more than a decade. One final mission to service the Hubble Space Telescope also is planned before the shuttles are retired in 2010.