Two US astronauts were to step out on the last spacewalk of the shuttle era Tuesday at the International Space Station where Atlantis is docked on the final mission of the 30-year US program.

Americans Ron Garan and Mike Fossum were chosen for the six and a half hour spacewalk, aimed at retrieving a failed ammonia pump from the orbiting lab and moving it to the shuttle's payload bay for return to Earth.

The duo, who were already aboard the ISS when Atlantis arrived on Sunday as part of the six-member international Expedition 28 crew, will also attach a Robotic Refueling Module experiment to the lab.

The joint US-Canadian project aims to test technologies for repairing and refueling satellites in space.

Atlantis's crew of four American astronauts will help support the spacewalk, set to begin at 8:44 am (1244 GMT).

"We have come a long way since we first started doing spacewalks," said deputy shuttle program manager LeRoy Cain, reflecting on the 159 floating trips by astronauts and cosmonauts to build the ISS over the past decade.

The last-ever spacewalk by a US shuttle crew was completed on May 27 during Endeavour's final visit to the orbiting outpost.

American astronauts Mike Fincke and Greg Chamitoff's seven and a half hour trip helped move the global space tally past 1,000 hours of spacewalking to construct, maintain and repair the ISS.

Tuesday's spacewalk will be the 160th at the research lab. The shuttle and ISS crews began waking up at 2:59 am (0659 GMT) to start preparing, NASA said.

Garan and Fossum have already stepped out together on three spacewalks in June 2008 as part of the STS-124 mission that delivered the Japanese Kibo lab to the ISS.

On Monday, NASA's damage assessment team concluded that the shuttle heat shield sustained no major harm during liftoff and would not need a more focused inspection, which Cain described as "really good news."

NASA also decided to extend Atlantis's mission by a day, so the astronauts will now spend 13 days on their final journey to space before the US shuttle program closes down forever.

The main objective of Atlantis's trip is to take advantage of the massive cargo space available one last time in order to restock the International Space Station with a year's worth of food and supplies.

The Atlantis crew began work on Monday with their six colleagues at the ISS to transfer nearly five tons of goods to the orbiting outpost.

The Raffaello multipurpose logistics module was lifted out of the shuttle's cargo bay and, with the help of a Canadian robotic arm, was placed onto the space station's Harmony node at 6:46 am (1046 GMT).

Other supply ships from Europe, Japan and Russia will be able to stock the ISS when the shuttle program retires after Atlantis's mission, but the space available on the shuttle is unparalleled.

The container is "packed with 9,403 pounds (4,265 kilograms) of spare parts, spare equipment, and other supplies -- including 2,677 pounds (1,215 kilograms) of food -- that will sustain space station operations for a year," NASA said.

Over the coming days, the combined crew will be transferring items from the Raffaello to the station and moving more than 5,600 pounds (2,540 kilograms) of old station gear back into the module for the return to Earth.

"It is pretty much all hands on deck," said flight director Jerry Jason. "It is going to be a very busy time period."

Atlantis's flight marks the end of an era for NASA, leaving Americans without their own vehicle for sending astronauts into space until private industry comes up with a new capsule, likely by 2015 at the earliest.

With the shuttle gone, only Russia's three-seat Soyuz capsules will be capable of carrying astronauts to the ISS at a cost of more than $50 million per seat.

With the extra day in space, Atlantis is now scheduled to land back on Earth July 21 at 5:56 am (0956 GMT), mission control in Houston said.

The four remaining US shuttles -- Discovery, Endeavour, Atlantis and the prototype Enterprise -- will be sent to museums across the country after the shuttle program ends.

Two shuttles were destroyed in flight: Challenger blew up in a post-takeoff explosion in 1986 and Columbia disintegrated during its return to Earth in 2003. The disasters killed 14 crew members.