A year has passed since the US Air Force sent up the mysterious X-37B.
But the question of why remains unanswered.
Read also: Elon Musk wants to send a craft to Mars in two years.
On all occasions, it has worked as intended, then it can be used again.
Jonathan McDowell is an astronomer at the Harvard-Smithsonian Centre for Astrophysics and says the long journeys are perfect to carry around sensitive loads to be tested in space.
Security experts have argued that there is spy equipment being tested, as The Guardian reported, the military continues probably his work in silence.