The US Air Force has officially confirmed that the second flight of their X-37B robot spaceplane is on overtime. This small experimental spacecraft, which resembles a car-sized space shuttle, has been in orbit since early March. At the end of November, the spacecraft passed its semi-official endurance limit of 270 days in space.

The USAF had been very quiet about this mission for months, but broke the silence as the deadline approached. We have been told that the X-37B is in good health, and is now flying an extended mission. Exactly when the vehicle will return to Earth remains unclear.

Why stretch the mission beyond its previously advertised endurance level? It's probably because the true endurance of the spacecraft is known to be much longer. Engineers are conservative, and tend to favor safe numbers in their predictions. The publicized endurance figure is probably fudged with wide safety margins. There's also the benefit of the previous mission in 2010, which lasted around 224 days.

Data from the spacecraft returned during this first flight, and obtained after its return, probably showed that it was performing better than expected. Earlier predictions of how some of its systems would degrade have probably been found to be inaccurate. The result is a new, better-informed estimate of the endurance for X-37B. How large is the new figure? Beyond closed doors, nobody seems to know.

The second mission could also have been extended through the careful management of the spacecraft in orbit. Making fewer on-orbit manoeuvers and taking care with thermal exposure limits could extend on-board supplies, and reduce wear and tear. So X-37B could remain in orbit for a lot longer. Controllers could even be trying for an entire year in space!

What's happening on board the spacecraft? While it remains on orbit, it continues to demonstrate the reliability of the parts and systems used to build it. This is the primary goal of the whole X-37B program. There's also the question of what lies inside the small cargo bay of this shuttle-like vehicle. This author has previously written about his theories on the matter.

My conclusion is that the cargo bay is also carrying things to be tested, which will probably be small mechanical parts designed for use on military satellites. Some of these parts will be identical to ones carried aboard the first X-37B flight.

In fact, some of them could be the very same parts that actually flew on this mission. Flying them again after a long stay in space would allow them to be tested to the limits of normal endurance, and also allow them to be inspected before flying, halfway through their on-orbit lifetimes, and then again at the end of the second mission. Three points on a graph allows for a better model than a simple "before and after" study.

There's another possible reason for extending the mission. Recently, engineers have begun to discuss a potential Son of X-37B, provisionally dubbed X-37C. This would not be a small experimental vehicle, but a large operational spacecraft. It could be a potential ferry vehicle for astronauts and cargo to the International Space Station and other destinations. So the need to test this spacecraft is now more important than ever.

A third flight in the X-37B is a possibility. To make it useful, it would need to use a vehicle that had previously flown. The first two missions have each used a separate spacecraft, although the designs are identical. Flying a used vehicle would demonstrate reusability. It isn't clear when this mission will fly, but it will obviously not start until well after the second flight has returned to Earth.

The extended flight of X-37B is a good omen. It suggests that the mission is going normally, and that the spacecraft is more capable than engineers had previously thought. This, in turn, bodes well for any future spacecraft that will build on its experience.

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