The Air Force sent the unmanned, Atlas V mini-shuttle into orbit on Wednesday – making it the fourth X-37B flight for a top-secret military research programme.
The last X-37B – also known as an orbital test vehicle – mission returned after 674 days in 2014 while the first three flights spanned 1,367 days. The Air Force won’t reveal how long this mission will last or where it will end.
The programme started with NASA before it was passed to the military seven years later, sparking many theories about its purpose.
One theory is that it is a drone spy ship used to monitor Chinese satellites, while Iran believes it is a space warplane carrying weapons – a claim dismissed by the Air Force.
Another theory is that it is working as a regular surveillance satellite over areas like the Middle East or sweeping up disused American orbiters to bring them home to be fixed.
The Boeing-built spacecraft is 29ft-long and has a wing span of 15ft, roughly one-quarter the size of NASA’s now-retired space shuttles.
The vehicles are intended to serve as orbital test beds for future sensors and technologies.
For the first time, the military unveiled a few experiments that are on board.
The equipment includes an experimental plasma-based propulsion system, known as a Hall thruster – which ejects fuel carried by a satellite more efficiently than widely-used chemical thrusters.
NASA also is flying samples of almost 100 polymers, composites and other advanced materials to test how they fare in the harsh environment of space.
Most intriguingly it will carry a solar-sail demo sponsored by the Planetary Society. The society’s chief executive officer, Bill Nye, said a large, light, shiny Mylar sail measuring 32 square metres was tucked into a small box for lift-off.
The intent is for sunlight to propel it, once it is released and deployed in orbit.
Such technology could be useful for future missions to monitor solar weather and keep a lookout for possible asteroids headed our way.
“We’re very hopeful that the thing will deploy properly, sunlight will hit it and get a push,” Mr Nye said after lift-off.
A similar experiment by the group was lost in a Russian launch accident 10 years ago. This time, it turned to United Launch Alliance, a venture between Boeing and Lockheed Martin Corp.