When the USAF launched its secretive X-37B spaceship the conspiracy theories went off the charts.
With its futuristic design and almost aggressive wing shape it wouldn't have been too alien to suggest that it was the US military's first space fighter.
Well after two years in space, and a landing that was just as mysterious, the USAF is finally ready to open up about its secret mission.
The X-37B is testing a revolutionary new form of engine called a Hall thruster. Hall thrusters vary from conventional rockets by using electricity to ionise and propel xenon gas. The results are a 'whisper quiet' thruster that -- although low in power -- uses a fraction of the fuel and weight that current methods do.
The trade-off is time. While current propulsion methods can move satellites extremely quickly, the fuel is heavy and expensive.
This Hall thruster is far better suited to orbiting objects however that require only minor adjustments and are expected to be in service for periods of five years or more.
According to Spaceflightnow, the USAF is going to use the Hall Thruster for its Advanced Extremely High Frequency satellites. Capable of surviving a nuclear blast, the $1bn AEHF is the US Military's backup plan should ground communications fail.
The X-37B is going to be heading back into space in May to further test the propulsion system.