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.

The US Space Shuttle Discovery lifts off, on March 15, at Kennedy Space Center in Florida. Discovery's flight is delivering the space station's fourth and final set of solar array wings, completing the station's truss, or backbone. [Xinhua]

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 14 days, 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.

The mission's first spacewalk is expected to take place Thursday to install the new solar wings. The canceled spacewalk chores will be tackled by the space station crew after Discovery leaves.

Discovery's delay has also hindered plans to launch a new US military communications satellite from the Cape Canaveral Air Force Station near Discovery's seaside pad at the Kennedy Space Center. The Wideband Global SATCOM-2 satellite was due to launch Saturday atop an Atlas 5 rocket, but will stand down until next week.