Backgrounder: Introduction to U.S. shuttle Discovery's mission

WASHINGTON, April 5 (Xinhua) -- U.S. space shuttle Discovery lifts off on Monday morning from the Kennedy Space Center in Florida, on a 13-day construction mission to the orbiting International Space Station. The following is a brief introduction to Discovery's STS-131 mission.

Discovery's flight will deliver eight tons of supplies and equipment to the station, including spare bunks for the occupants of the space station,

a large tank of ammonia coolant and seven racks filled with science experiments. Discovery is also 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 Multi-Purpose Logistics Module, carried in the shuttle's cargo bay.

Leonardo serves as basically a moving van for the space station, allowing the shuttle to, first of all, deliver shipments of equipment and supplies larger than any other vehicle could accommodate, and, second, to return science experiments, unneeded hardware and trash to the ground -- all other cargo transfer vehicles burn up in the Earth's atmosphere.

The mission has three planned spacewalks, with work to include replacing an ammonia tank assembly, retrieving a Japanese experiment from the station's exterior, and switching out a rate gyro assembly on the S0 segment of the station's truss structure.

STS-131 is the 33rd shuttle mission to the station. Three flights to the station remain after STS-131 before the shuttles retire in 2010.