By Irene Klotz
CAPE CANAVERAL, Florida (Reuters) - Astronauts aboard the space shuttle Discovery prepared on Tuesday to bring NASA's most-traveled spacecraft back to Earth, wrapping up its 39th and final mission.
Touchdown at the Kennedy Space Center in Florida is scheduled for 11:57 a.m. EST (1657 GMT) on Wednesday, with a backup landing opportunity available at 1:34 p.m. (1834 GMT)Meteorologists expect the weather will be suitable for landing.
Discovery blasted off on February 24 to deliver supplies, a storage room and an outdoor platform to hold spare parts for the International Space Station, a $100 billion project of 16 nations that has been under construction 220 miles above Earth since 1998.
The mission completed the U.S. portion of the station, with a final Russian laboratory due to arrive next year.
Two more shuttle flights are planned before NASA ends the 30-year-old shuttle program. On April 19, the shuttle Endeavour is scheduled to launch with the station's highest-profile and most expensive science experiment -- the $2 billion Alpha Magnetic Spectrometer particle detector.
The last mission, a cargo run to the station aboard the shuttle Atlantis, is slated for liftoff on June 28.
"It's bittersweet to see the last flight of an orbiter," said Johnson Space Center director Michael Coats, a former astronaut who served as the pilot on Discovery's first mission in 1984.
"You're sad to see an orbiter on its last mission that's about to go into a museum. But it also makes you feel proud of the team and what they've accomplished in the last 30 years," Coats said. "It's an amazing vehicle and I think we're going to really miss it."
Two other shuttles were destroyed in accidents. Challenger broke apart over the Atlantic Ocean shortly after takeoff on January 28, 1986, killing seven astronauts. Columbia disintegrated as it re-entered Earth's atmosphere over Texas on February 1, 2003, killing seven more astronauts.
The United States is retiring its surviving three space shuttles due to high operating costs and to free up funds for work on a new launch system that can carry people and cargo to destinations beyond the space station's orbit where the shuttles cannot go.
Wrangling over the U.S. budget, however, has blocked NASA from starting any new programs.
"It's always been our plan that at some point we would no longer fly the shuttles. I feel very strongly that the agency is about exploration and about getting beyond low-Earth orbit," said LeRoy Cain, head of NASA's shuttle mission management team. "At some point, we need a system that can do those missions, and the shuttle's frankly not that system."
"The hardest part of this for me is giving up the capability, because if you really look at what this spacecraft does, it can do everything except leave low-Earth orbit," added Discovery commander Steven Lindsey, in an in-flight interview.
"I suspect that sitting on the runway, when it's time for me to get out I don't think I'm going to want to leave my seat," Lindsey said.
The United States will rely on the Russian government to launch astronauts to the space station, though it hopes to eventually buy rides from commercial companies, if any develop the capability.
Cargo runs will be handled by Russia, Europe and Japan, as well as two U.S. firms, Space Exploration Technologies and Orbital Sciences Corp.
(Editing by Jane Sutton and Philip Barbara)