At the end of November, the mysterious X-37B robot spaceplane will reach a critical point in its mission. This time will mark the end of the official 270-day endurance limit of the semi-secret military shuttle. The X-37B could be getting ready to come home. But will it?

Let's rewind and review what this is all about. The X-37B is a small, experimental winged spacecraft operated by the US Air Force. It's roughly the size of a car, and has wings and a small cargo bay with clamshell doors that open up in orbit.

There's no cockpit and no crew. The cargo bay contains a boom that unfurls a solar panel while the vehicle is in orbit.

The first flight of an X-37B vehicle was launched on April 22, 2010, and flew a lengthy classified mission throughout the year. On December 3, 2010, the spacecraft returned to a runway landing, following a mission that lasted roughly 224 days.

We know for sure that the spacecraft itself was testing components and procedures that could be useful in future reusable spacecraft.

The "X" designates that this is an experimental vehicle. Prior to its adoption by the US Air Force, the X-37B was actually a civilian project connected to NASA, and these goals for the spacecraft were clearly stated in open channels.

The US Air Force has also reiterated these points in official statements. This part of the mission is beyond debate.

We also suspect that something else was carried in the cargo bay, but we don't know the details. The secrecy surrounding the X-37B's mission generated much curiosity and fear.

Some pundits suspected there was a spy camera on board, to investigate objects on Earth or in space. Some suggested the small shuttle was a robot satellite mechanic, ready to refuel or repair other spacecraft.

The wildest suggestion of all was that weapons were aboard, ready to destroy other spacecraft or targets on the ground. The spy camera theory is plausible, but not exactly proven. The other two theories are dubious.

This author considered the overall picture of military spaceflight, and reached a more mundane conclusion. It was my opinion that the cargo bay contained mechanical parts and other components to be used on military satellites. The parts would be exposed to space and operated for a long period.

Afterwards, they would be returned to Earth for analysis. This isn't very sexy, but it's absolutely vital. The US military and the shadowy intelligence community have lost too many satellites in recent years.

Problems with their components must surely be a cause of these losses. Fixing these bugs will ensure that these large, complex and highly expensive spacecraft will be more reliable in the future.

The first flight of the X-37B seems to have been highly successful, judging from the limited evidence we have. Photographs and video of the spacecraft were released soon after it landed, showing it intact.

There was a gap of only a few months between the landing of the first X-37B and the launch of the second vehicle. Evidently, there was no need for a major overhaul or redesign.

So we return to the second flight of the X-37B. Everything we know suggests that it's identical to the first vehicle. There has been even less discussion of this second mission than the first.

We don't know if is carrying or doing anything different to the first mission. But it has clearly done something new. It's already been in space for much longer than its predecessor.

Will it come down before its 270 days are up? I suspect not. I believe that the second X-37B will try to push the endurance of this spacecraft for a bit longer.

We don't really know how long this vehicle can survive in space. The official endurance figure could be a conservative estimate, or possibly tinged with a little strategic deception.

The next few days will be educational. Nobody has specified a re-entry date in open forums. Let's see what happens.

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