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A spacecraft that could one day take astronauts to Mars will undergo an unmanned test flight today.
The Orion craft, launched into orbit on a Delta IV heavy-lift rocket from Florida, will circle the Earth twice before re-entering the atmosphere at 20,000 mph.
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.
Programme manager Mark Geyer said: "We're going to test the riskiest parts of the mission. Ascent, entry and thinks like fairing separations, Launch Abort System jettison, the parachute, plus the navigation and guidance - all those things are going to be tested.
"Plus, we'll fly into deep space and test the radiation effects on those systems."
Orion will launch from Cape Canaveral Air Force Station on the end of a Delta IV Heavy, the largest US rocket currently in service, and will splash down in the Pacific off the coast of Baja California.
Unlike the Apollo module, which carried three astronauts, Orion is designed to carry a crew of four.
Measuring 16.5 feet across, Orion is larger than Apollo, which had a diameter of 12.8 feet. Its three RS-68 engines will produce about two million pounds of thrust at lift-off.