After a marathon 15-month mission in orbit, the second X-37B spaceplane has finally landed. Launched by the US Air Force on a semi-secret mission, the mission has apparently flown a successful mission, judging from the clear, on-the-record statements issued by the USAF.

The USAF has confirmed that the mission was successful, but as usual, they will not say exactly what the mission was. Half the mission is known for sure, and has been confirmed by the Air Force. X-37B has been testing a group of advanced technologies that will find their way into future spacecraft.

These technologies include new heatshield materials, automatic pilot systems and others that are probably not so obvious. Then there's the second half of the mission, still shrouded in secrecy.

X-37B has a shuttle-like payload bay in its centre, with clamshell doors. It's only a small space, but the cargo carried inside is a secret. This has led to wild speculation by some pundits, ranging from spy missions to other spacecraft, to a space weapon. The real secret mission is probably more mundane.

This author will again reiterate his theory that the payload bay contains more experimental spacecraft components, but these ones were supplied courtesy of the secretive National Reconnaissance Office. The parts inside the bay are destined for use on future US spy satellites.

There's been some speculation that X-37B was spying on ground targets with cameras inside the payload bay. This is certainly possible, but it's important to distinguish a camera test from an operational spying mission. We don't even know for sure if spy cameras were inside the payload bay. This author believes that the parts under the hood were less exciting, and probably include mechanical hinges, batteries, insulation samples and electronic boards.

Such things are not very sexy, but they're vital. Several NRO spy satellites have suffered untimely demises when parts inside them suddenly failed. Developing and testing quality satellite parts is probably the most critical challenge currently facing the NRO.

The X-37B offers a unique opportunity to fly these parts in space for lengthy periods, expose them to the rigours of orbit, and return them to Earth for analysis. Plus, the spacecraft is operated by the military, which can provide an appropriate level of secrecy for the mission.

We will probably receive no further official comment on this second mission. Little was said at the end of the first X-37B flight in 2010. The cloak of secrecy will be maintained by both the US Air Force and the NRO.

A third flight of X-37B is planned for the near future, and it will use the same X-37B vehicle that flew the first mission. This will test the re-usable nature of the spacecraft, including the heatshield.

It will also extend the on-orbit testing of the spacecraft's components for an even greater period. The combined on-orbit time of this first spacecraft on its two flights will probably exceed the X-37B's second mission, and could reach two years!

At a time when the USA seems to be unsure about its future in space, recent weeks have offered some strong morale boosts, and also some insight into the future of space transportation in general. Prior to the return of X-37B, we witnessed the splashdown of the first Dragon cargo carrier mission

to the International Space Station. The contrasts between these two spacecraft are striking. One is operated by private enterprise. The other is strictly government. One is a conventional capsule, the other a winged spaceplane. One is civilian, the other military. One is highly public, the other highly secret.

Bring on the third mission of X-37B. It's time for more progress.

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.