Mystery X-37B robot spaceplane returns to orbit on Tuesday
Second flight for small, secretive 'space warplane'
The United States Air Force will mount the third mission into space by its small space-shuttle lookalikes, the X-37Bs (once memorably dubbed "secret space warplanes" by the Iranian government) on Tuesday if all goes to plan.
The third Orbital Test Vehicle mission, aka OTV-3, will see the same X-37B which flew the inaugural OTV mission take off from Cape Canaveral, Florida, just after 1pm EST, contractor United Launch Alliance  announced.
The X-37Bs are prototype space planes that, in the vague words of the American military, supports "space experimentation". It is understood the goals of the X-37B project include aiding the development of reusable spacecraft and to test new technologies for long-duration space flight.
What distinguishes a space plane from other spacecraft is its ability to return to Earth and land on a runway. The wings, wheels and propulsion kit needed to successfully touch down in such a way make the planes significantly heavier, more expensive and harder to control than other types of craft, which either don't return in useable condition or drop in by parachute.
A rendering of the X-37B plane
The Air Force keeps the details of the space planes strictly classified, meaning that there has been no public justification of these expensive experiments, and the life and times of the X-37B planes remain something of a mystery.
The second super-secret space shuttle OTV2 returned to Earth  in June after 469 days in space.
It was rumoured to be hovering over China quietly slurping in wireless data as well as other wild speculation about its possible purpose. It's likely we won't hear many more hard facts about the third plane either.
In general we here on the Reg space desk consider that the X-37B is really more or less what the US Air Force say it is - a platform for trying out new secret technologies in space quickly without the expense and delay of building a dedicated satellite, and without the downsides of capsule parachute landings which naturally would tend to involve some risk of one's top-secret spysat tech being recovered by someone else.
However the craft's shuttle-like wings do suggest that it could potentially be used for more interesting missions, which we have analysed previously here . ®