A US military space plane has landed in California after spending two years orbiting the Earth from space, but no one knows why it was sent up there.
The aircraft, known as the X-37B, is an orbital test vehicle. The purpose of such vehicles is to "perform risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies,” according to the US Air Force.
It spent 674 days in orbit, but what exaclty it was doing during that time is a mystery. When the vehicle landed, the Air Force simply said that the aircraft had been conducting “on-orbit experiments”.
The classified mission was not the first to be undertaken by the vehicle – it orbited Earth for more than seven months in 2010. A further X-37B mission is due to launch from Cape Canaveral in Florida in 2015.
X-37B was built by Boeing, and at 8 metres in length it is approximately one quarter the size of NASA’s old space shuttles. It has a wingspan of nearly 15 feet and is powered by solar panels when in orbit.
A number of theories have been put forward for the vehicle’s purpose, but at the moment all opinions are speculative. One possibility is that it was sent up to investigate China’s space lab, but this theory has been played down by experts.
Another suggestion put forward by the Secure World Foundation – a non-profit group promoting the peaceful exploration of space – is that the secrecy is linked to the testing of national intelligence-related hardware.
According to BBC Science correspondent Jonathan Amos, no one really knows what the vehicle does. “The only credible explanation I have seen is that it is testing technologies that could find their way on to future satellite missions,” he said.
“If you consider how expensive a satellite mission is - several hundred million dollars - you'd like to be sure that any innovations are going to work straight out of the box.”