Discovery astronauts prepare to return to Earth Monday after a successful supply mission to the International Space Station, leaving NASA's shuttle program with just three more flights.
The crew aims for touchdown at NASA's Kennedy Space Center in Florida at 1248 GMT with no threat from the huge ash cloud from an erupting Icelandic volcano that has shut down air traffic over Europe, the US space agency said.
"Iceland's erupting volcano won't affect Discovery's landing Mon (Monday). Reentry course isn't near ash cloud," read a message from the Kennedy Space Center on micro-blogging service Twitter.
Over a 14-day mission, Discovery's seven-member crew 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.
Once the shuttles are retired, with Discovery's final flight on September 16, the United States will rely on Russia to take astronauts to the station until a new fleet of commercial space taxis is operational.
President Barack Obama reiterated this controversial policy during a visit to the shuttle's Florida landing site last week.
Monday's forecast at Cape Canaveral includes overcast skies and a chance of showers, but shuttle commander Alan Poindexter and his crew have a backup landing opportunity in Florida later in the morning at 1423 GMT.
The astronauts took power conservation measures Sunday in case the meteorological conditions delayed their return by a day.
"The weather situation is always fluid, and we will keep watching it," Mission Control in Houston told Discovery.
The shuttle is provisioned to remain in orbit until Wednesday if necessary, said Bryan Lunney, NASA's supervising flight director.
Monday's descent will follow a rare northwest to southeast course over the United States, leaving a glowing contrail visible to observers across several states should skies remain clear.
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.
The linkup 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 observations 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 Discovery delivered will hold blood and other specimens collected for experiments.
During his April 15 visit to Kennedy, Obama made no mention of a shuttle program extension, disappointing some in Congress and those employed by the multi-billion-dollar space flight program.
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," insisted Poindexter to 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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