With more than 500 days in orbit, the third mission of the mysterious X-37B spaceplane is not only longer than any previous flight, but also much stranger.

The X-37B is a robotic spacecraft launched by the US Air Force on a secret mission that has generally eluded boffins and analysts. It's not unusual for nations to launch secret spacecraft, but it's often the case that their true missions can be deduced. The size of the rocket, the orbit and other factors can give analysts clues that a certain satellite is an optical spy satellite, a radar observation satellite or a classified communications satellite.

Knowing that the secrecy is sometimes just a formality, some missions have patches that hint at their true missions, like owl's eyes and large mesh arrays with the motto "We own the night". Suspicions that this was a radar observation satellite that could see targets in the dark were confirmed.

No such clues have been dropped for X-37B. Part of its mission is known, thanks to US Air Force statements and the fact that X-37B was once a civilian project operated by NASA. This is an experimental spaceplane designed to test new technologies, ranging from "robot pilot" avionics to an advanced heatshield system. Photography of the exterior of this spaceplane has been extensive, both before and after landing.

There's more mystery surrounding what lies inside. X-37B features a small shuttle-like payload bay in its middle, with clamshell doors that open in space. We know that there's a solar panel inside that unfurls from a mast once the X-37B is in orbit. But there's room for other items too. Exactly what else is inside, and what it's doing, has confounded the media and the space community.

This analyst has suggested that X-37B is conducting mundane but critical tests of electromechanical components intended for future US national security satellites. The parts are operated in open space for long periods then returned to Earth for study. Other analysts have suggested that the vehicle is conducting a semi-operational mission that involves the use of sensors for Earth observation.

The fact that the X-37B isn't the most ideal platform for a spy satellite could be an advantage: There's a cover story for the real mission. Most theories have their pros and cons. Amazingly, nothing has been conclusively proved, disproved, or favoured above all other theories. The cloak of secrecy is compounded by a lack of clear deduction. This is indeed mysterious, even by the normal standards of classified "black" space missions.

The highly extended mission hints that there's more than mundane testing on this flight. Perhaps components that were undergoing testing on previous flights are now on a true operational mission. Or perhaps we are witnessing a long endurance run to prove that certain critical satellite parts can work reliably for a long-duration spy satellite. Both theories are plausible.

Adding to the confusion, nobody knows when the X-37B will come back. Will it attempt to stay in orbit for 18 months? Will it go even longer? It's possible that even its mission controllers don't know for sure at this stage.

Dr Morris Jones is an Australian space analyst who has written for spacedaily.com since 1999. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email. Dr Jones will answer media inquiries.