It's almost time to light a candle for one of the world's most intriguing spacecraft. No, I don't mean ignite the rocket for launch. The second mission of the X-37B robot spaceplane is approaching the one-year mark on its mission. On March 5, it will celebrate this milestone, assuming that the spacecraft is still in orbit.

Nobody knows exactly when X-37B will return to Earth. Right now, there are no indications that it's about to return soon. The US Air Force seemed confident in the performance of their spacecraft when it reached the 270-day mark in orbit. It's probably still working well, and could be expected to do so for months to follow.

Few details have been revealed about the mission, which is a semi-secret test flight. As this author has suggested for years, there are really two missions going on at once. The first mission is a test of the X-37B vehicle and the components used to make it. This is all about testing new technology that will find their way into future re-useable spacecraft.

The second mission concerns some unidentified objects in the spacecraft's small payload bay. This writer believes that these are satellite parts undergoing endurance testing for America's National Reconnaissance Office, the shadowy organization behind America's spy satellites.

Little has been said officially about the X-37B in recent months, but there has been some interesting unofficial discussion. In January, the British Interplanetary Society published an article in the journal "Spaceflight", claiming that X-37B was spying on China's Tiangong 1 space laboratory.

The claim was rapidly dismissed by spaceflight analysts around the world, who pointed to differences in the orbits of the two spacecraft. This author also suggested that the gear on board X-37B doesn't even equip the spacecraft for a spy mission. The claim gained some traction in the mass media before the rebuttals caught up with the original story.

So, how long will it take for X-37B to come home? It seems logical to assume that the US Air Force would want to fly a year-long mission, at least. This makes the flight look like a real marathon. But the robot shuttle probably won't come home on its birthday. There's too much publicity and attention. Mission managers will probably want to wait at least another month before returning the vehicle. They could even wait longer than that.

We don't know the true endurance of this robust little spacecraft. The US Air Force probably doesn't know exactly how long it could function. At some point, a line has to be drawn. We probably won't see a mission that goes more than six months beyond the one-year point. This author guesses that the vehicle won't stay in orbit for longer than three months beyond the birthday mark.

It will be interesting to see what happens in the next few months. We probably won't learn anything new about the spacecraft or the mysterious payload it carries under its clamshell doors. At some point, it will glide downwards from the sky. We will see pictures of the vehicle, no different from the return of the first X-37B mission. Then everything will go quiet again.

A third X-37B mission would certainly be useful. It would use one of the two vehicles that have already been flown, and demonstrate re-usability of the same spacecraft.

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