Space shuttle Discovery astronauts (L-R) Koichi Wakata of Japan, John Phillips, Richard Arnold, Steve Swanson, Joseph Acaba, Pilot Tony Antonelli and Mission Commander Lee Archambault leave their crew  quarters for launch pad 39A at the Kennedy Space Center in Cape Canaveral, Florida March 15, 2009. (Xinhua Photo)

The flight will also replace a failed unit for a system that converts urine to potable water.

The ISS' Urine Processing Assembly that removes impurities from urine in an early stage of the recycling process is not working. The entire Water Recovery System was delivered and installed during the space shuttle Endeavour's STS-126 mission in November, 2008. Astronauts were able to coax it into use by performing in-flight maintenance, but a distillation unit failed after Endeavour's departure.

Discovery entered a launch countdown last Wednesday, before ground crews at NASA's Kennedy Space Center in Cape Canaveral noticed a hydrogen leak in a liquid hydrogen vent line between the shuttle and the external tank. The leak forced NASA to immediately begin unloading fuel from the tank and cancel the Wednesday launch.

The shuttle was initially scheduled for launch on Feb. 12, but concerns over suspect fuel control valves in the spacecraft's main engines prompted several delays so engineers could replace them.

Like the leaky gas hydrogen line that thwarted Discovery's Wednesday launch, the shuttle's three fuel control valves are also designed to maintain the proper pressure inside the liquid hydrogen fuel reservoir of the orbiter's attached external tank. A similar valve on the shuttle Endeavour cracked during a November 2008 launch and NASA wanted to be sure a similar problem did not pose a risk to Discovery and its crew.

Because of the delays, the mission originally slated to last 14days, with four spacewalks, was shortened by one day with one spacewalk eliminated, to make room for an incoming Russian Soyuz spacecraft set to launch March 26. The Soyuz will carry up a fresh crew for the space station.