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A spacecraft that could one day take astronauts to Mars will undergo a historic unmanned test flight today.

The Orion craft below), launched into orbit on a Delta IV heavy-lift rocket from Florida, will circle the Earth twice before re-entering the atmosphere at 20,000mph.

It is the first mission since the Apollo moon landings to take a spacecraft built for manned flight into deep space beyond the limit of orbiting satellites.

During the 4.5-hour flight Orion will travel 3,609 miles from Earth, 15 times the distance to the International Space Station.

A key part of the mission will be to see how Orion's heat shield withstands the 2,200C heat of re-entry.

Nasa hopes to use Orion to put astronauts back on the moon by 2020 and take them to Mars in the 2030s. A midway mission to an asteroid is also on the cards.

Mark Geyer, the project manager, said: "We're going to test the riskiest parts of the mission. Ascent, entry and things like fairing separations, Launch Abort System jettison, the parachute, plus navigation and guidance - these things will be tested. We'll fly into deep space and test the radiation effects on those systems."

Orion will splash down in the Pacific off Baja California.