The shuttle uncoupled from the orbiting space laboratory at 11:22 am (1522 GMT) after its six-member crew unloaded the last equipment, which included a crucial communications antenna, power storage batteries and a radiator.

The biggest single element was the five-ton Rassvet research module, or MRM-1, which will provide additional storage space and a new docking port for Russian Soyuz and Progress spacecraft.

The Rassvet -- "Dawn" in Russian -- was permanently attached to the bottom of the space station's Zarya module. It carries important hardware on its exterior including a radiator, airlock and a European robotic arm.

Astronauts completed the 12-day mission's final spacewalk on Friday, plugging a new ammonia jumper cable into the station, transferring a grapple fixture from the shuttle to the station and reconfiguring some tools.

The trip back to Earth caps the 25-year career of one of NASA's iconic spacecrafts. The shuttle, which has logged some 115 million miles (185 million kilometers), is due back in Florida Wednesday at 8:44 am (1244 GMT).

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 have to rely on Russia to take astronauts to the station aboard three-seater Soyuz spacecraft until a new fleet of commercial "space taxis" is operational.

The impending end of the mission is a bittersweet moment for NASA, which is preparing to retire the shuttle fleet.

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.

There is a chance, however, that the current mission may not be the final chapter for Atlantis.

NASA administrator Charles Bolden is considering flying Atlantis to the station once more time, with additional supplies, before mothballing the spacecraft for good.

The ISS, a joint project involving 16 countries, has cost around 100 billion dollars, mostly funded by the United States.

Atlantis astronaut complete final spacewalk
Two US astronauts completed their final spacewalk Friday at the International Space Station, replacing batteries before heading home.

Garrett Reisman and Michael Good reentered the orbiting station's decompression chamber at 1:13 pm (1713 GMT) after the Atlantis crew's third spacewalk in five days. Their sortie lasted six hours and 46 minutes.

The pair plugged a new ammonia jumper cable into the orbiting space station's huge framework and installed two new batteries that store energy collected by the station's large solar panels.

Each battery weighs 375 pounds (170 kilograms) and measures about the size of a three-foot (one-meter) box. Good and another fellow astronaut swapped out another four batteries during another spacewalk on Wednesday.

Batteries usually last five to six years but the ones that were replaced had functioned for nine years. The old batteries will be brought back to Earth aboard Atlantis.

During their spacewalk, the two astronauts also transferred a grapple fixture from the shuttle to the station and reconfigured some tools.

During the 12-day mission, Atlantis and its six-member crew unloaded over 12 tons of equipment, including the communications antenna, power storage batteries 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 -- was permanently attached to the bottom of the space station's Zarya module. It carries important hardware on its exterior including a radiator, airlock and a European robotic arm.

Atlantis will undock from the ISS for the very last time on Sunday, capping the shuttle's 25-year career during which it has logged some 115 million miles (185 million kilometers). It is due back in Florida on Wednesday at 8:44 am (1244 GMT).

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.

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