Liftoff was set for 6:21 a.m. EDT (1021 GMT) from the Kennedy Space Center in Florida.

With an 80 percent chance of good weather for the launch, technicians pumped 500,000 gallons (1.9 million liters) of liquid oxygen and liquid hydrogen into the shuttle's fuel tank for the 8.5-minute climb into orbit.

If the shuttle launches on time, it will link up with the orbiting space station on Wednesday.

The station, a $100 billion project of 16 nations, is nearing completion after more than a decade of construction 220 miles above Earth.

After Discovery's flight, three shuttle missions remain to finish outfitting the orbital outpost.

NASA is preparing to retire Discovery and sister ships Atlantis and Endeavour by the end of the year due to cost and safety concerns. The shuttles have been the cornerstone of the U.S. human space flight program since 1981.

"We've managed to do tremendous things that could never have been done without the shuttle -- building the space station and working with the Hubble (space telescope)," Discovery commander Alan Poindexter said in a prelaunch interview. "It's just been a spectacular experience."

What will follow is uncertain. President Barack Obama's administration is planning to ax a $108-billion program to return astronauts to the moon in the 2020s. Obama plans to visit Florida on April 15 to rally support for a revamped space program built around technology development, environmental monitoring and commercial space initiatives.

Obama's proposals include adding $6 billion to NASA's budget over the next five years to seed development of space taxis to ferry astronauts to and from the space station.

With the shuttle's retirement, Russia's Soyuz capsules will the only vehicles available for crew transport, a service that costs the United States $51 million per seat.

The only other country that has launched people into orbit is China, which is not a member of the space station partnership.

Discovery's four-man, three-woman crew will be delivering about 10 tons of equipment and supplies to the station, including an ammonia cooling system, a washroom, experiment racks and a freezer to hold experiment samples.

In addition to Poindexter, the crew includes pilot James Dutton, spacewalkers Rick Mastracchio and Clay Anderson, flight engineer Dorothy Metcalf-Lindenburger, mission specialist Stephanie Wilson and Japanese astronaut Naoko Yamazaki.