On March 5, the X-37B robot spaceplane celebrated a birthday in orbit. This mysterious, semi-classified spacecraft is still up there, and it's not clear when it will return to Earth.

It can't stay up there forever. At some point, mission controllers will order the small vehicle to descend to a runway. But it's not clear when that will happen, or exactly why the US Air Force will decide to end the mission at a specific time.

There would certainly be calculations of the endurance of the vehicle. Some of this will be based on data obtained from the first flight of the X-37B in 2010, which lasted around 224 days.

It would be easy to extrapolate this data, coupled with the known properties of certain parts, to work out the state of the vehicle after a certain period.

There are also in-situ measurements. Using telemetry, controllers know how much fuel the spacecraft has, the performance of its solar panel, the state of its batteries, its internal temperature, and a few other vital signs.

This author also suspects that there are small engineering cameras on board the X-37B that photograph its solar panel and other components that are exposed to space. It would be easy to see if things looked right.

If things are working well, it would be useful to continue the mission for a considerable period of time. Launches of this spacecraft are rare and expensive. It makes sense to squeeze as much experience as possible out of the mission.

Furthermore, the longer it flies, the longer the parts the X-37B is testing can be certified. That is the ultimate purpose of the mission.

So we can probably expect X-37B to remain in orbit for a while longer. It could stay in space for a few months more. It's possible that a return date has not even been set, or even a rough range of re-entry dates.

Managers could elect to remain in orbit for a couple of months, then perform another review of the state of the vehicle. If things are still working well, the mission could be extended gradually in a series of regular reviews.

It is entirely possible that X-37B will still be in space 18 months after its launch. After that, managers will probably start to consider re-entry scenarios. Even if they wish to add a few more months to the mission, it will probably not stay in space for two years.

Dr Morris Jones is an Australian space analyst and writer. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email.