By Irene Klotz
CAPE CANAVERAL, Florida (Reuters) - Astronauts on board the space shuttle Discovery closed the ship's payload bay doors on Wednesday in preparation for a landing in Florida that will complete the 27-year flying career of the world's most-traveled spaceship.
Touchdown at the Kennedy Space Center is targeted for 11:57 a.m. (1657 GMT). The shuttle is returning from an extended, 13-day construction mission at the International Space Station.
Discovery, on its 39th and final voyage, delivered a combination storage room-research lab to the station, as well as an external platform to house large spare parts. It also carried tons of supplies and science gear, including a prototype humanoid robot.
Discovery's last flight completes the U.S. side of the station, a $100 billion project of 16 nations that has been under construction 220 miles above Earth since 1998.
Russia plans to launch a last laboratory module to the station next year.
The United States is retiring its three-ship shuttle fleet, beginning with Discovery, its oldest surviving shuttle, due to high operating costs and to free up funds to begin work on new spaceships that can travel to the moon, asteroids and other destinations beyond the station's orbit.
Wrangling over the U.S. budget, however, has blocked NASA from starting any new programs.
Discovery's sister ships Endeavour and Atlantis are scheduled to make their final flights to the space station in April and June to deliver the $2 billion Alpha Magnetic Spectrometer particle detector and a final load of cargo.
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 will now rely on the Russian government to launch astronauts to the space station, although 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 Pascal Fletcher and Jackie Frank)