Shuttle Discovery astronauts delayed their return to Earth until Tuesday after twice being waved off attempts to land in Florida because of rain and fog, NASA said.
"There was a lot of cause for optimism, but at the end of the day, it was too low of a (cloud) ceiling," Mission Control told shuttle commander Alan Poindexter.
"We know you worked it real hard," said Poindexter as the delay extended the shuttle's mission to 15 days. "We appreciate everything."
The shuttle will now aim to land at 7:33 am (1133 GMT) on Tuesday at the Kennedy Space Center in Florida, but with rain and low clouds again in the forecast, NASA has also laid plans for a possible landing at Edwards Air Force Base in California at 1300 GMT.
Bryan Lunney, NASA's supervising flight director, said the shuttle and its seven member crew have enough provisions to remain in orbit until Wednesday if necessary.
Although its return to Earth was postponed by rain, the US space agency said Discovery faces no threat from a huge ash cloud spewed by an erupting Icelandic volcano, which has shut down air traffic over Europe.
Discovery lifted off on April 5 and docked with the space station two days later, overcoming a communications antenna failure that crippled their rendezvous radar.
During its two weeks in space, Discovery delivered nearly eight tons of scientific equipment and 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 from the two craft for 10 days. Four were women, the highest 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.
Another new experiment will measure changes in muscle and joint health of astronauts during their long exposures to weightlessness, and a new freezer that Discovery delivered will hold blood and other specimens for experiments.
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 of 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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