The US space agency called off the launch minutes before the shuttle was due to blast off toward the orbiting International Space Station and said it would make another attempt Wednesday at 1:10 am (0510 GMT).

"We have a scrub for the day," said launch director Pete Nickolenko. "Unfortunately, the local weather has not cooperated."

Cloud formations, rain and storms were within a 20-mile (36-kilometer) radius of launch pad 39A at the Kennedy Space Center in Cape Canaveral, Florida.

The weather unexpectedly worsened hours before the liftoff. It cleared minutes before takeoff -- but too late to give the launch a green light.

The astronauts were onboard, along with a new bedroom, a treadmill, a freezer, food and other supplies Discovery is set to deliver during its 13-day mission.

It will also be dropping off the newest ISS resident -- US astronaut Nicole Stott.

The delay was a reminder of the turbulence that surrounded the previous shuttle mission, in July, which was been postponed five times by weather woes and technical glitches.

Forecasters saw a 70 percent chance of favorable weather on Wednesday, dropping to 60 percent on Thursday.

Stott will be taking over from engineer and fellow American Tim Kopra, who has been aboard the ISS since July and is returning to Earth with Discovery.

The shuttle crew, led by Commander Rick Sturckow, will be delivering 6.8 tonnes of cargo transported in a pressurized module called Leonardo that was built by the Italian space agency.

Two astronauts from the team are scheduled to conduct three spacewalks of six and a half hours each during the mission, which is the fourth of five planned for the shuttle this year. The last is scheduled for November.

A key task during the spacewalks was to replace an old liquid ammonia tank, which will be substituted with a new, 1,760-pound (800-kilogram) replacement brought aboard Discovery. The substance is used as a coolant.

The astronauts will also 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.

Hernandez will provide bilingual updates from space, via the micro-blogging website Twitter, during the mission -- the 128th for the space shuttle program, and the 30th mission to the ISS.

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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