As Discovery prepared for lift-off with its crew of seven, including three female astronauts, American Tracy Caldwell Dyson headed for the space station aboard a Soyuz spacecraft with two Russian cosmonauts.

Discovery's arrival at the space station, tentatively scheduled for Wednesday, would mark a first in space, with four women in orbit. It also is also the first shuttle mission with three female crew members.

Joining Dyson from the Discovery will be mission specialists Dottie Metcalf-Lindenburger, 34, a former high school science teacher; Stephanie Wilson, 43, a veteran of two shuttle missions; and Naoko Yamazaki, 39, an astronaut with the Japanese space agency since 1996.

Rounding out the Discovery crew are mission commander Alan Poindexter, 48; co-pilot Jim Dutton, 41; mission specialist and spacewalker Rick Mastracchio, 50; and fellow spacewalker Clay Anderson, 51.

Officials at NASA's Kennedy Space Center gave their thumbs up after their prelaunch briefing for a "go" for Monday's launch just after sunrise at 6:21 am (1021 GMT) from the Florida facility.

The weather was expected to cooperate. Going into the weekend, forecasts put the chances of favorable weather conditions at 80 percent, said meteorologist Cathy Winters.

After this flight, the second shuttle mission this year, only three more are planned before all three remaining US manned orbiters are retired at the end of 2010, ending 30 years of service. The first shuttle flew in April 1981.

During the 13-day mission, Discovery and its crew will deliver nearly eight tonnes of cargo, including spare bunks for the occupants of the space station, a large tank of ammonia coolant and seven racks filled with science experiments.

"We're really excited about this mission... and the science we'll be able to get onboard for the ISS to do what it needs to do and demonstrate its true ability as a national laboratory," said space shuttle launch integration manager Mike Moses.

Among the gear being hauled into space is a freezer to preserve samples of blood, urine, saliva, plants or microbes used in micro-gravity experiments and then analyzed later back on Earth.

Discovery also will be carrying an exercise machine designed to study the effects of micro-gravity on the body's musculoskeletal system. Muscles can atrophy during long sojourns in space so astronauts have to take care to exercise regularly.

The supplies, racks and other gear are packed into a pressurized Italian-built module named Leonardo, carried in the shuttle's bay.

After linking up with the International Space State three days after the launch, Leonardo will be removed from the orbiter and docked to the station for unloading.

Until construction of the orbital outpost is completed, NASA has to ferry spare parts and gear to maintain the space station and service the scientific experiments on board.

Two Discovery astronauts will conduct three space walks lasting six and a half hour each on days five, seven and nine of the mission.

One of their principal, and most complex, tasks will be to replace an empty ammonia tank attached to the rear of the station with one that is full. Ammonia is used in the station's cooling system.

The ISS, a hundred billion dollar project begun in 1998 with the participation of 16 countries, is financed mainly by the United States.

In presenting his 2011 budget proposal in February, President Barack Obama announced that the space station will be maintained until at least 2020.

Once the shuttle program is closed, the United States will depend on Russian Soyuz spacecraft to ferry their astronauts to the ISS until a new US launch vehicle is ready to take over around 2015.

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