AFP - A ruptured hydrogen valve found while fueling the shuttle Discovery led NASA to postpone a launch planned for Wednesday -- the second to be scrapped in 24 hours.
   
An agency spokesman said Tuesday that fixing the broken valve, discovered while filling the shuttle's external fuel tank, would push back the launch at least 48 hours.
   
"At this point we don't know when the next attempt will be," said NASA spokesman Allard Beutel.
   
Officials said a press conference to discuss the US space agency's revised launch plans was likely later Tuesday.
   
NASA had begun refilling Discovery's fuel tank in preparation for a second liftoff attempt, when the decision to call of the launch was made. The space agency scrapped a first launch attempt late Monday due to thunderstorms.
   
The delays were a reminder of turbulence that surrounded the previous shuttle mission aboard the space shuttle Endeavour, which was postponed five times by weather woes and technical glitches.
   
The Discovery crew is scheduled to conduct three spacewalks of six and a half hours each during the mission.
   
A key task during the spacewalks will be to replace an old liquid ammonia coolant tank, which will be substituted with a new, 1,760-pound (800-kilogram) replacement brought aboard Discovery.
   
The seven shuttle astronauts also will be retrieving experiment equipment from outside the ISS and returning it to Earth for processing.
   
The new freezer will store samples of blood, urine and other materials that will eventually be taken back for study on the effects of zero-gravity.
   
A treadmill named after popular US comedy talkshow host Stephen Colbert will be the second aboard the ISS. Exercise is key for astronauts spending long periods of time in space, where zero-gravity can result in muscle atrophy.
   
The shuttle flight is the first with two Hispanic astronauts: veteran mission specialist John "Danny" Olivas, 44, of El Paso, Texas, and rookie Jose Hernandez, 47, of Stockton, California. Veteran European astronaut Christer Fuglesang, 52, of Sweden, is also among the crew.
   
Once the Discovery mission is complete, just six more shuttle flights remain before NASA's three shuttles are retired in September 2010.
   
The ISS is a project jointly run by 16 countries at a cost of 100 billion dollars -- largely financed by the United States.