A secret robotic space plane developed by the US Air Force has just recorded its 500th day in orbit around Earth.
And nobody knows what it's for.
The X37-B mini space shuttle is a relatively small reusable space plane, designed to orbit the Earth and feed back to military commanders on the ground.
The mysterious craft is unmanned, obviously, but its actual functions are still heavily classified.
The most recent craft launched on 11 December 2012 and has not yet returned to Earth, smashing its previous records for long-term flight above the atmosphere.
"Clearly, and understandably, the Air Force initially set the expected parameters [of the X-37B] conservatively, not really knowing what it could do until it was tested — which is what they are doing now," said Joan Johnson-Freese, professor of national security affairs at the Naval War College, to Space.com.
X37B
The craft looks just like the retired Space Shuttle, but is just 29 feet long and 10 feet tall. It launches vertically on a rocket and returns via a robotic horizontal landing.
The US Air Force has so far sent three missions into orbit with the solar-powered plane, and there is no telling how long this one will last. The best guess from most experts is that the plane is being used to test new reconnaissance equipment.
Flying at a low altitude of 218 miles and a range of around 43.5 degrees north and south latitudes, it is thought the plane is mostly able to provide surveillance of the Middle East, rather than Europe, Russia or China.