Soon, America's mysterious robot spaceplane will lift off for the fourth time. Since its maiden launch in 2010, the X-37B has mystified analysts and prompted some fairly wild theories about its secret missions. X-37B is a small shuttlecraft with no cockpit and no crew.

It has a small payload bay with clamshell doors like NASA's Space Shuttle. The project began life as an experimental project run by NASA, before custody was transferred to the US Air Force.

There's no secret about the fact that the X-37B is testing new technologies used to build the vehicle itself, but mystery has always surrounded what was inside its cargo bay. The Air Force never talked about it in the past.

This time, they sang! The Air Force has confirmed that the upcoming mission will carry a Hall Effect thruster package, which will be tested for its performance in space and studied closely after its return to Earth. Hall Effect thrusters use electricity as their source of energy rather than chemical combustion, making them highly efficient for small velocity changes on long missions.

At the time of writing, no images of the payload have been released, and we can probably expect that this will not change in the future. We have been given pictures of previous X-37B missions, and that's all. Never mind. It's a big step for the US Air Force to simply describe the payload.

Why is this being disclosed now, when nothing was said about the previous three launches? Has there been a change in media policy by the USAF? This analyst does not believe so. We are being told about the Hall Effect thruster because this information is not particularly sensitive.

According to the USAF, the thruster is a modified version of one used on previous USAF satellites. It will thus fly again on future USAF satellites. The Air Force can say whatever it chooses to say about its own payload.

Lips were sealed for the previous X-37B missions, and there is a simple and plausible explanation for this. The Hall Effect thruster was not carried on these flights. The other payloads were more secretive, and were probably not connected to any US Air Force program.

This analyst has previously suggested that the first three missions were also performing tests on satellite parts, but they were designated for America's fleet of secret spy satellites operated by other government agencies. These agencies didn't talk about their experiments, and the Air Force was instructed to remain silent as well.

So, what was really lurking under the payload bay doors? We can only speculate. This analyst has previously suggested that the payloads were probably less sexy than some of the more exotic theories circulated on the Internet. No weapons. No devices for snooping on other satellites.

No robot arms or servicing systems for other satellites. The real payloads were probably mundane components such as batteries, electronics, actuators and material patches. Such testing may seem uninspiring, but it's absolutely vital to ensure the health of some very expensive covert satellites.

Back to the upcoming mission. The Hall Effect thruster will not be alone in the payload bay. In fact, it won't even dominate it. This analyst suggests that the largest object in the payload bay will be the xenon propellant tank for the thruster.

We can also add an electronics box and possibly an independent antenna for power, control and communications of the thruster. There will probably also be a collection of small engineering cameras to check the state of the vehicle when it is in orbit.

The payload bay also contains a solar panel that unfurls on a mast after the payload bay doors open in orbit. There is also believed to be a small antenna on a mast. With all this gear, there is probably no room for anything else.

Like a logic puzzle, the disclosure of the payload for the fourth mission gives clues to not only the next flight, but also the missions that came before it.

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.