NASA now is looking at a Friday liftoff said mission management team chairman Mike Moses, who said the launch attempt would be August 28 at 12:22 am (0422 GMT).

Before then, engineers will evaluate the valve that developed problems during Tuesday's fueling before Discovery's tank is loaded with propellant ahead of Friday's launch.

NASA had begun filling the tank in preparation for the early Wednesday liftoff, when the decision to postpone was made -- the second scuttled launch in a 24-hour period.

The US space agency late Monday scrapped a first launch attempt due to thunderstorms.

Space officials said the problem valve cycles liquid oxygen from the external fuel tank to the shuttle's main propulsion system, and technicians will attempt to fix it while Discovery remains at the launch pad over the next 48 hours.

The delays were a reminder of turbulence that surrounded the previous mission, when space shuttle Endeavour's launch 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 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.

The seven shuttle astronauts also will be retrieving experiment equipment from outside the ISS and returning it to Earth for processing.

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 to be 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.

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