Shuttle Astronauts Prepare to Leave Space Station

* Shuttle to leave station at 11:55 p.m. EDT Sunday

* Landing scheduled for 2:35 a.m. EDT Wednesday

* NASA's final shuttle mission to fly in July

By Irene Klotz

CAPE CANAVERAL, Fla. (Reuters) - The shuttle Endeavour astronauts packed gear and prepared to leave the International Space Station on Sunday, wrapping up a 12-day visit to complete construction of the U.S. side of the orbital outpost.

The crew cleaned up from their four spacewalks and reinstalled the station's upgraded air purification system, the last maintenance task on a long to-do list that included servicing the station's cooling and power systems and extending the reach of its robotic crane.

Hatches between the shuttle and station were to be closed at 6:56 a.m. EDT, with Endeavour's departure slated for 11:55 p.m.

Landing is scheduled for early Wednesday at the Kennedy Space Center in Florida, where workers were preparing NASA's last space shuttle to be rolled out to the launch pad. Atlantis and a crew of four are scheduled to close out the 30-year-old shuttle program with a final cargo run to the station in July.

"We're really pleased to be able to help round out the program with the successful work that we've had up here," Endeavour lead spacewalker Drew Feustel said during an inflight interview.

"There's one more flight after us and they'll finish out the space shuttle program. We really believe that we're ending on a very strong and positive note and looking forward to future opportunities," he said.

The United States is retiring its three space shuttles due to high operating costs and to free up funds to develop new spaceships that can carry astronauts beyond the station's 220-mile high orbit where the shuttles cannot go.

"Once the space shuttle retires we're going to lose a lot of capability of moving large payloads out in space, but then that opens the doors for new things that are going to come across the horizon," said Endeavour's pilot, Greg Johnson.

The primary goal of Endeavour's flight, the 134th for NASA's space shuttle program, was to deliver and install the $2 billion Alpha Magnetic Spectrometer particle detector, designed to study dark matter, antimatter and other high-energy phenomena that cannot be detected by telescopes.

That job was accomplished shortly after Endeavour's May 18 arrival at the $100 billion space station, a project of 16 nations that has been under construction since 1998.

Atlantis will be delivering a year's worth of supplies to the station. NASA is turning over U.S. cargo runs to two companies, Space Exploration Technologies and Orbital Sciences Corp., and hopes commercial flights will be available to ferry station crew members as well within four or five years.

In the interim, Russia will provide space taxi services, at a cost of more than $50 million a seat. Russia, Europe and Japan also fly cargo ships.