Developed in the 1970s, following NASA's glory days of moon missions, the shuttle was the first US manned launch vehicle designed to be reused, and the only one still in service.

When it heads to the museum after three decades of service, it should be replaced by the Crew Exploration Vehicule, a craft yet to be designed that will take astronauts back to the moon in 2020 in a prelude to missions to Mars and beyond.

The shuttle, officially called the Space Transportation System, first roared into orbit on April 12, 1981.

Since then, Columbia, Challenger, Discovery, Atlantis and Endeavour have flown 113 space missions with 600 men and women.

The five orbital craft have carried more than 1,360 tonnes of freight into space, including the Hubble Space Telescope and elements for the construction and servicing of the ISS.

But there have been tragedies. Challenger exploded on takeoff January 28, 1986, and Columbia broke apart as it re-entered Earth's atmosphere on February 1, 2003. A total of 14 astronauts were killed.

Discovery's launch will be the first since the fleet was grounded following the Columbia disaster.

The shuttle, which has more than two million components, is a complex and fragile piece of engineering, and one which critics say is far too costly.

Its centerpiece is the plane-like orbiter that can hold eight astronauts.

It takes off vertically, attached to a large external fuel tank, to which a pair of reusable solid-fuel booster rockets are affixed.

The shuttle's three main engines ignite six seconds before liftoff, and together with the solid rocket boosters, they provide the thrust to lift the orbiter off the ground for the initial ascent.

About two minutes after liftoff, when the shuttle reaches an altitude of about 45 kilometers (28 miles), the two rocket boosters burn out and jettison into the ocean, where NASA vessels recover them.

The shuttle takes eight minutes and 40 seconds to reach orbit, at which stage the main engines shut off and the external tank separates, mostly disintegrating in the atmosphere.

About 35 minutes later, the two orbital maneuvering systems' engines are used for about three minutes to place the shuttle into final orbit.

Upon completion of the mission and re-entry into Earth's atmosphere, the orbiter glides to a landing, as it has no means of propulsion at that stage.

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