NASA's Mission Control is now looking to a possible landing at Edwards Air Force Base, California, at 9:01 am (1301 GMT), where favorable weather was forecast, after passing up the 7:34 am (1134 GMT) Florida touchdown.

There was also another opportunity to land at Kennedy at 9:08 am EDT (1308 GMT), if conditions improved.

"The fog is hanging in," Mission Control told Discovery commander Alan Poindexter before the seven astronauts passed up the first of five landing opportunities. "The trends are unfavorable."

Rain and fog in Florida on Monday meant passing up two landing opportunities, stretching Discovery's re-supply mission to the International Space Station to a 15th day.

Earlier Monday, after two aborted attempts, Mission Control told shuttle commander Alan Poindexter that there was "a lot of cause for optimism, but at the end of the day, it was too low of a (cloud) ceiling."

Bryan Lunney, NASA's supervising flight director, meanwhile said the shuttle and its seven-member crew have enough provisions to remain in orbit until Wednesday if necessary.

NASA also said that Discovery faces no threat from a huge ash cloud spewed by an erupting Icelandic volcano, which has shut down air traffic over Europe, because its return into Earth's atmosphere does not take it over the affected area.

During two weeks in space after its April 5 launch, Discovery delivered nearly eight tonnes of scientific equipment to the International Space Station along with other supplies intended to fortify the orbiting science laboratory for operations beyond NASA's final shuttle launch.

The link-up united 13 US, Russian and Japanese astronauts for 10 days. Four were women, the highest ever number of females in space at any one time.

Over the course of three spacewalks, astronauts replaced a bulky external coolant tank. The ammonia reservoir circulates a coolant through outstretched radiators to disperse the heat generated by the station's internal electronics, including the life-support systems.

The science hardware delivered by Discovery included an Earth observation rack to hold cameras, and spectral scanners for studies of the atmosphere, land forms, coastal areas as well as weather-induced crop damage.

Discovery also delivered a new freezer that will hold blood and other specimens for experiments, including one that will measure changes in muscle and joint health of astronauts during their long exposures to weightlessness.

The mission is one of the last by the space shuttle program and comes just days after US President Barack Obama laid out a new future for the space program that made no mention of extending the multi-billion dollar shuttle program.

Once the shuttles are retired, the United States will rely on Russia to take astronauts to the station until a new fleet of commercial space taxis is operational.

At NASA, the looming reality that the United States will soon be unable to launch its own astronauts for the first time in three decades has begun to sink in.

"We're very excited about the future direction of human exploration in space," Poindexter told reporters Sunday, saying the crew in space had been able to follow Obama's remarks last week.

"I'm sure that it's running through people's minds, but we are professionals and we are working really hard on the missions in front of us," Richard Jones, lead NASA flight director for the Discovery mission, said earlier in the day.

"As we get closer, that will be forefront on people's minds."

Discovery's pilot Jim Dutton, who was making his first and possibly last space flight, echoed the sentiments.

"I think everyone feels a little bittersweet," Dutton said. "We love the shuttle, but we have to press on into the future."

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