US astronauts were to venture into outer space on Monday for the first time since the shuttle Atlantis linked up with the International Space Station in the final rendezvous of its 25-year career.
Mission Specialists Garrett Reisman and Steve Bowen were 'camping out' in the Quest airlock overnight to purge nitrogen from their circulatory system, the US space agency said.
Their space walk -- the first of three -- was scheduled to begin Monday at 8:15 am (1215 GMT) and last 6.5 hours, according to NASA.
Atlantis and its crew of six astronauts successfully docked with the orbiting space lab on Sunday about 220 miles (350 kilometers) above the South Pacific.
Its arrival was not without drama as a piece of debris threatened to force Mission Control to order the space station to perform an emergency maneuver to avoid a collision.
As things turned out, the unidentified piece of space debris passed by safely some five miles away from the docking procedure and no special operation was required.
After an hour spent checking the soundness of the seal, the shuttle and station crews opened the hatches and held a traditional welcome ceremony before beginning preparations for the first of three planned spacewalks.
NASA said that three station crew members snapped 398 photographs of Atlantis's all-important heat shield on Sunday.
The mission is the 32nd and final scheduled voyage for Atlantis, which first launched in 1985 and has logged some 115 million miles over a career spanning a quarter of a century.
Only two more shuttle launches remain -- one in September for Discovery and the final blast off for Endeavour in November -- before the curtain falls on this era of human spaceflight.
The United States will then rely on Russia to take astronauts to the station aboard three-seater Soyuz spacecraft until a new fleet of commercial space taxis is operational.
During a mission of almost 13 days, most of which will be spent moored to the ISS, Atlantis and the crew will unload more than 12 tonnes of equipment, including power storage batteries, a communications antenna and a radiator.
The biggest single element is the five-ton Rassver research module, or MRM-1, which will provide additional storage space and a new docking port for Russian Soyuz and Progress spacecraft.
The Rassver -- "Dawn" in Russian -- will be permanently attached to the bottom of the space station's Zarya module and carries important hardware on its exterior including a radiator, airlock and a European robotic arm.
The spacewalks were scheduled to install the new batteries and communications antenna on the space station.
President Barack Obama effectively abandoned in February plans laid down by his predecessor George W Bush to send astronauts back to the moon by 2020 and perhaps on to Mars with a new generation of rocket and spacecraft.
Constrained by soaring deficits, Obama submitted a budget to Congress that encouraged NASA to focus instead on developing commercial transport alternatives to ferry astronauts to the ISS after the shuttle program ends.
Nonetheless, Obama set a bold new course in April for the future of US space travel, laying out a vision to send American astronauts into Mars orbit within the next three decades.
He envisaged the design of a new spacecraft by 2025 for human travel deep into space and said he believed missions to asteroids and to orbit Mars by the mid-2030s were achievable.
By the time the final three missions are complete, the space shuttles -- characterized by NASA as the most advanced machines ever built -- will have flown 134 missions into orbit.
The ISS, a joint project involving 16 countries, has cost around 100 billion dollars, mostly funded by the United States.
NASA said Atlantis would be processed on return as normal just in case she was needed for a "rescue mission" in the event of an emergency with the two remaining shuttle flights.
But NASA has not entirely ruled out the possibility of Atlantis taking flight one last time on a comprehensive mission to the ISS next year -- provided Obama gives the go-ahead.
"If it happens to turn into a mission to the space station, we will do what is necessary," said NASA launch director Mike Leinbach.