A US space capsule that could help get humans to Mars is about to make its maiden flight.
Orion will be launched on a Delta rocket out of Cape Canaveral in Florida on a short journey above the Earth to test key technologies.
The conical vessel is reminiscent of the Apollo command ships that took men to the Moon in the 1960s and 1970s, but bigger and with cutting-edge systems.
Given that this is a first outing, there will be no people aboard.
Nonetheless, the US space agency describes the demonstration as a major event.
Nasa has a window in which to launch Orion of about two-and-a-half hours, which begins at 07:05 local time (12:05 GMT).
The launch preparations had to be stopped shortly before the opening of the window because a boat strayed into the eastern part of the launch range.
But Nasa says that there are no technical issues with the rocket and the weather is clear for lift-off.
“This is huge; Thursday is a giant day for us,” said Nasa administrator Charlie Bolden.
Orion is being developed alongside a powerful new rocket that will have its own debut in 2017 or 2018.
Together, they will form the core capabilities needed to send humans beyond the International Space Station to destinations such as the Red Planet.