Atlantis leaves station as NASA eyes shuttles' end

CAPE CANAVERAL, Florida (Reuters) - Space shuttle Atlantis left the International Space Station on Wednesday after a seven-day stay to deliver gear to keep the outpost operating after the shuttle program is retired next year.

Space station flight engineer Nicole Stott joined the six Atlantis astronauts for the return trip home. She is expected to be the last station crewmember to catch a ride on the shuttle. Atlantis is due back at the Kennedy Space Center in Florida on Friday.

"Thank you for a great mission," Jeff Williams, commander of the station, radioed to Stott as the shuttle prepared to leave at 4:53 a.m. EST (0953 GMT). "Bon voyage."

The United States is retiring its three-ship shuttle fleet in 2010 after five more missions to complete construction and outfitting of the $100 billion space station. The station, a project of 16 nations, then will be serviced by Russian, European and Japanese cargo ships, though none can transport the heavy pumps, tanks and gyroscopes carried by the shuttle.

Astronauts will fly exclusively on Russian Soyuz capsules at a cost of about $50 million a seat.

The United States has been developing a replacement for the space shuttles, but it is not expected to unveil it until 2015 at the earliest. An advisory panel tapped by the Obama administration to assess NASA's human space programs determined the new spaceship won't be ready until 2017.

In addition to flying to the station, which currently is funded only through 2015, the new ships are being designed to fly astronauts to the moon and other destinations in the solar system. The program, however, is under review.

Stott's departure comes six days before three more station crewmembers are due to return to Earth, leaving the complex with a two-man crew for the first time since 2006. Replacements are due to arrive December 23. The station won't reach a full, six-member crew again until March.

During Atlantis' mission, astronauts conducted three spacewalks to install antennas, science experiments, cargo mounts and other tasks. They are returning with the station's broken urine-recycling system, which purifies wastewater into potable water.

Also returning to Earth are six mice that have been aboard the station since late August for science experiments. Three of the mice died while in space, though scientists are not sure why.

NASA's next trip to the outpost is expected in February when shuttle Endeavour delivers the station's final connecting hub and a six-sided cupola that will give the outpost a 360-degree viewing port.

(Editing by Paul Simao)