The US Air Force’s X-37B robotic space plane landed on 7 May after 718 days in orbit. This was the unmanned spacecraft’s fourth trip into space and, as before, the precise details of the sojourn are classified.
The only payloads that the military revealed upon the vehicle’s launch on 20 May 2015, were a new form of “ion engine”, which is an extremely efficient thruster that could extend the life of communications or spy satellites; and a pallet of around 100 samples that were to be exposed to space to see how they degenerate.
The X-37B is a quarter the size of Nasa’s now retired space shuttle. It has a payload bay with a capacity of an American pick-up truck and once in orbit deploys a solar panel to provide power.
Although the military do not provide details of the orbit to the public, amateur satellite spotters found it soon after launch and published details on the internet. It was circling Earth every 91 minutes at an altitude of 315km, and with an orbit inclined at 38° from Earth’s equator.
Upon returning to Earth, the X-37B touched down at 11:47 GMT using an autopilot programme to guide it to the Kennedy Space Centre. It used the runway on which the space shuttle landed 78 times from 1984 to 2011.
The US Air Force owns two identical X-37B spaceplanes. They were constructed by Boeing and there has been talk of larger versions, including one that could transport astronauts to the international space station.
A fifth flight for the X-37B is already planned for launch later this year.
