Weeks into its fourth flight, the X-37B robot spaceplane is finally yielding some more data on its generally secret mission. The entire X-37B program has amazed and confounded space boffins with its mix of cutting-edge technology and deep secrecy.

Roughly the size of a car, the spacecraft looks like a shrunken, stubby version of the NASA Space Shuttle. Since its launch, there have been no more official statements from the US Air Force (owners of the vehicle) on the course of this mission. There have also been no leaks through unofficial channels, an amazing achievement in an age when secrets seem to spill so frequently.

That doesn't stop analysts from at least partially lifting the veil of secrecy. We can make educated guesses to fill in the blanks. Satellite trackers even watch X-37B in orbit and report on its movements. This analyst has been posting his own theories on the real payloads and missions for years.

Now we know just a little more through official channels. We were told before launch that the spacecraft was testing a Hall Thruster, which uses electricity instead of chemical combustion for propulsion.

The thruster is believed to be stored inside the X-37B's payload bay. We have now been told that the thruster has "completed initial on-orbit validation testing" in a media release from Aerojet Rocketdyne, makers of the thruster. This may seem like a small point, but it's the only solid news we have received on this mission since its launch.

Why talk about this now? The thruster is not a classified payload, so any news of it is probably not sensitive. But there could be other agendas. This is probably a marketing pitch, designed to help sell the thruster to other clients beside the US Air Force. Perhaps the successful operation of the thruster needs to be advertised right now to sway some clients.

So far, so good. But testing of the thruster is far from complete. This media release was made less than a month and a half after X-37B was launched. That's not exactly a marathon operational run. We can expect that the thruster will continue to be tested for months, and possibly more than a year. This would more accurately reproduce the conditions the thruster would experience on an operational mission.

The mission is still keeping many things veiled. Let's recap what we already know. This flight is highly different from the previous three missions. Back then, we could see the little spaceplane itself, but we did not know what was inside its payload bay.

This time, we have been told about experiments inside the payload bay but we have not seen any photography of the X-37B itself. That's strange. There's no point in covering up something that's been made so public so many times before. This suggests that there's something different on the outside of the vehicle, and the US Air Force doesn't want us to know about it.

What do we know for sure? Inside the payload bay, apart from the aforementioned Hall Thruster, there is a collection of small materials samples on board, supplied by NASA. But there are still some mysteries for these openly acknowledged payloads. A photograph of the Hall Thruster has been released by its manufacturer, showing it independently of the X-37B, but we have no photography of the NASA materials experiment.

NASA has released photography of sample experiments flown previously on the International Space Station, but did not say if the X-37B experiment used the same housings. This analyst suspects that it's different. It would be impractical to carry and deploy the Station-used sample panels on X-37B. The payload bay is small, and it also contains other items. The samples also need to face the space environment directly.

Thus, we can deduce how the NASA samples are being carried. They are attached to the underside of the clamshell payload bay doors on the X-37B. They are probably stored in several individual panels that are shaped like strips instead of squares. These strips are arranged in parallel to the long central axis of the vehicle, with each strip conforming to the curves of the payload bay doors. It's a similar arrangement to the planks in an old barrel forming a circle.

If this is true, it would be a fairly mundane change. So why all the secrecy? Giving away details on anything inside the payload bay could give clues to everything else inside.

The payload bay is small, and the placement of one item affects the dimensions and volume that's free for others. In addition to the two official experiments, there's also a solar panel that unfurls from a deployable mast after the payload bay doors open. We could work out how much space is taken up, and how much could be left for other payloads. Are there other payloads inside the bay that haven't been officially announced? It's possible.

All things considered, the contents of the payload bay are probably less mysterious than the outside of the vehicle itself. What's so different about this spacecraft that we are not even allowed to see it? For the record, the US Air Force has also refused to disclose which X-37B vehicle is making the flight. Two X-37B spacecraft have flown in the past.

We don't know if one of these is flying again. We don't know if it has been heavily modified. We don't know if additional experiments have been added to the outside of the spacecraft. Are the changes so radical that this isn't really a conventional X-37B spacecraft anymore? In this case, the semi-secret X-37B program could now serve as a cover for an even more secret program that isn't even known!

We can speculate ad nauseam, and some speculation can become too wild. It can sometimes be difficult to know what's plausible, let alone what is actually real about the program. There's a lot of mystery to be solved about X-37B. There could be even more mystery about something else that's concealed behind the X-37B program itself.

Dr Morris Jones is an Australian space analyst who has written for spacedaily.com since 1999. Email morrisjonesNOSPAMhotmail.com. Replace NOSPAM with @ to send email. Dr Jones will answer media inquiries.