* Shuttle to deliver cargo to the space station
* Four flights remain before the shuttle fleet is retired
Liftoff of the spaceship and seven astronauts is targeted for 6:21 a.m. EDT (1021 GMT) from the Kennedy Space Center in Florida.
Preparing Discovery for Flight
The shuttle will be carrying a cargo pod filled with science racks and equipment for the space station, which is nearing completion after more than a decade of construction 220 miles (350 km) above Earth.
NASA has four flights remaining to finish outfitting the station, a $100 billion project of 16 nations.
The U.S. space agency has contracted with commercial launch service providers Space Exploration Technologies of California and Orbital Sciences Corp (ORB.N) of Virginia to deliver cargo to the station as early as next year.
SpaceX, which was founded by billionaire Internet entrepreneur Elon Musk, plans to debut its Falcon 9 rocket with a demonstration mission next month from a newly refurbished launch pad at Cape Canaveral Air Force Station, just south of the shuttles' launch complex. Orbital's Taurus 2 launcher is expected to make its first flight next year.
Cargo vessels operated by Russia, Japan and Europe also will keep the station stocked with equipment, supplies and gear for six live-aboard crewmembers. All crew transport has been turned over to Russia in preparation for the shuttles' retirement later this year.
NASA had been planning to develop a capsule-style spaceship to fly astronauts to the station, as well as the moon, but U.S. President Barack Obama is calling for the program's cancellation. Instead, the White House wants to see if commercial firms can provide another option besides relying exclusively on the Russians, which charge $51 million per seat for rides on their Soyuz capsules.
Obama is scheduled to host a space summit in Florida on April 15 in an attempt to win support for the new direction he envisions for NASA. An independent panel of advisers, including aerospace executives, astronauts and renowned scientists, last year determined that the moon exploration initiative, known as Constellation, had been underfunded and had no hope of reaching its goal of returning astronauts to the moon by 2020.
The program's cancellation, however, has drawn sharp rebuke from some members of Congress, particularly from districts in Florida, Texas and Alabama that will bear the brunt of an estimated 9,000 engineering and technical job losses due to the shuttle program's end.
Discovery's mission, which is scheduled to last 13 days, is the 131st mission since shuttle flights began in 1981. The program has had two fatal accidents, which took the lives of 14 astronauts, and costs about $200 million a month.
Obama proposes increasing NASA's budget by about $2 billion a year to jump-start commercial space taxi services, as well as invest in new technologies that would eventually lead to a human expedition to Mars. (Editing by Tom Brown and Eric Beech)