The X-37B, which has been likened to a scaled-down space shuttle, blasted off from Cape Canaveral Air Force Station at 0052 BST (1952 EDT).
The military vehicle is unpiloted and will carry out the first autonomous re-entry and landing in the history of the US space programme.
The spacecraft can return experiments to Earth for inspection and analysis
It looks like some smaller cousin of the shuttle. While no one really is talking about what it can do, the idea that it essentially can become a maneuverable satellite, moving to higher r lower orbit as needed and perhaps returning when it needs new equipment, is a fascinating idea.
It will be interesting to see what this unmanned ship is capable of.