The fourth mission of the X-37B robot spaceplane is well underway. We know much of what is happening with the flight. There's a test of a Hall Effect thruster for the US Air Force and a set of materials samples provided by NASA. Beyond this, little else is known.

On previous X-37B missions, there has been plenty of disclosure about the spacecraft itself, but little talk about the payloads concealed under its clamshell doors. This time, it's the reverse. We know a lot about the payloads carried on board, but not much about the X-37B itself!

We do not know which X-37B vehicle is making the flight. This was announced for prior missions, but not this one. We also had absolutely no photography of the X-37B vehicle prior to launch. Again, this is an anomaly. The only photography of this latest mission was taken after the X-37B was encapsulated inside its payload fairing. This meant that you just couldn't see anything at all!

The failure to release images, or even basic information on the vehicle, is highly suspicious. It suggests that there's something different about this X-37B spacecraft.

In a previous article, this analyst suggested that the vehicle could sport infrared cameras on its tailfins, to watch thermal loads during re-entry. This was once done on the Space Shuttle. Such an experiment would be useful, but is it really enough to justify such an opaque cloak of secrecy? After considering the matter, this analyst now thinks that there must be more to the story.

What sort of changes could be so dramatic and so controversial that they would justify a total photography blackout? One option could be a complete redesign of the thermal protection system. The X-37B is testing a new type of reusable heatshield, more robust than the troublesome tiles that covered the Space Shuttle. We have good reason to believe that this shield has performed well on previous missions. X-37B spacecraft have looked very good after their return to Earth. But could there be a desire to replace this with something new and untried on previous flights?

There could be other things on the exterior of the vehicle. Has a navigation and landing camera been added to the vehicle? Does this allow the "robot pilot" software to actually see as well as fly by instruments?

It's also possible that there is some sort of external payload rack mounted on the upper surface of the spacecraft. This would compensate for the small volume of the spacecraft's payload bay.

The payload rack would need to be jettisoned before X-37B could return to Earth. Getting a fragile external payload to launch with the vehicle would be easy, as the whole spacecraft is encapsulated by a payload fairing during launch. But it could not come back. Forget about trying to mechanically transfer it to the payload bay. That's technically impractical, and there's probably no room for it.

What would be placed on such an external rack? There could be secondary experiments that do not need to be returned to Earth for study. Another option is fuel tanks, to extend the mission or allow for additional orbital changes. The rack could even possibly contain another solar array. Hall Effect thrusters run on electricity rather than chemical combustion. Is this thruster powered independently of the spacecraft's main solar array?

The Atlas V rocket that launched X-37B carried some other small spacecraft, including an experimental solar sail for the Planetary Society that recently unfurled. We know of these payloads, but are there others we don't know about? Has a small sub-satellite been piggybacked on the X-37B itself? And if so, who really owns it? This analyst does not believe that any sub-satellites are stored inside the X-37B's payload bay, but they could be attached somewhere else.

If the sub-satellites are released when the spacecraft separates from its launch vehicle, they could be mistaken for debris by foreign space trackers.

We can speculate endlessly, but our speculations should not be too wild. Let's not consider weapons or robot arms on the outside or the inside of the spacecraft.

Let's wait and see if the US Air Force spills any more facts. Somehow, I doubt they will.

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.