Billed as a small shuttle, the unmanned X-37B heralds the next generation of space exploration. It will be the first craft to carry out an autonomous re-entry in the history of the US programme.
But its mission - and its cost - remain shrouded in secrecy. The Air Force said the launch was a success but would give no further details.
However, experts have said the spacecraft was intended to speed up development of combat-support systems and weapons systems.
There have already been accusations that the programme could lead to the 'weaponisation' of space.