The fourth flight of the X-37B robot spaceplane will soon begin. This mission is groundbreaking! It's hardly the first launch, but it's the first time that we have been openly told about the payloads carried inside the spaceplane. This caught the spaceflight community by surprise, given the tight secrecy that has surrounded the previous three missions.

X-37B is roughly the size of a car, and carries a small payload bay with clamshell doors that open in orbit. It deploys a small solar panel from this bay after the doors open. There is no cockpit. The spaceplane is launched atop an Atlas V rocket, covered by a large payload fairing. It stays in orbit for months or even more than a year before gliding to a runway landing.

Let's recap what we know about this flight. We were first told by the US Air Force that the X-37B is carrying a Hall Effect thruster in its payload bay, along with the associated parts to run the thruster. Hall Effect thrusters are different from normal chemical fuel rockets. They use a single propellant (such as xenon gas) and ionize it with the use of electricity. The electrically charged propellant is thus expelled from the engine at a high velocity, making this thruster very fuel-efficient. Hall thrusters are great for making minor orbital corrections over long periods of time, but they are useless for launching objects from the ground.

The thruster is a modified version of a Hall Effect thruster used on some USAF satellites. The new, upgraded version will find its way on board future satellites if it proves its worth on this flight.

Soon afterwards, NASA explained that the mission will also carry a materials test experiment provided by the space agency itself! This will expose "more than 100 different materials" to space. Like the Hall thruster, these material samples are intended for use in future spacecraft.

So, we have a lot of firm knowledge of these payloads. What about the previous X-37B missions? We know that the missions were largely about testing the X-37B itself. It's an experimental vehicle built with a lot of new technologies. The overall design of the spacecraft is no secret. But we were never told anything about the contents of the payload bay for the previous launches. This was suspicious, and generated some wild speculation. This analyst has long suspected that the secret cargo was provided by another clandestine US government agency, and was probably testing parts for covert US satellites.

Although we know a lot about this mission, some questions still remain unanswered. We don't know how long this upcoming mission will remain in orbit. The previous flight went for more than a year and ten months. That marathon mission was designed to test the performance of the X-37B itself. While X-37B is still somewhat experimental, it has now largely proved its capabilities, so the performance of the payloads is probably the deciding factor. The Air Force has said nothing about the mission length, but NASA has. NASA's media release on its own experiment states that the samples will be exposed to space "for more than 200 days." But how much longer than this will it fly?

Previous NASA materials tests on the International Space Station have run for a year or more. This analyst would thus suggest that we can expect at least this long for the upcoming test. We know the spacecraft is capable of this, and more. But this does not necessarily mean any endurance records will be broken. A flight of roughly eighteen months could be possible. But if the Air Force wants, we could see the little spaceplane go the distance and celebrate two birthdays for its mission in orbit.

And what of the Hall Effect thruster? Like the accelerated testing of mechanical components on Earth, we can expect that the Hall thruster will be fired a lot more on this flight than it would be on an operational mission. Several years of on-orbit performance will be compressed into several months.

We are also unsure of which vehicle will fly on this mission. Two X-37B spacecraft have flown in space. One vehicle flew the first and third missions, giving the program its first taste of reusability. Another vehicle was used on the second mission. Will this vehicle be used again for the fourth mission? It seems plausible but it is simply not known at the time of writing. The USAF may prefer to send the first vehicle on a third trip into space, thus testing the reusability factor even further. Why prove once again that the spacecraft can fly twice when you can see how it performs on three missions?

The whole X-37B program is proving to be far more active and successful than some observers suspected at its start. Clearly, the little spacecraft is useful for many purposes and multiple customers. We can expect more missions in the future. We can expect different payloads, and probably some different agencies taking part in these missions. The cloak of secrecy will probably return for some missions, and be removed for others. We have seen this before with the Space Shuttle, which was periodically used to launch classified payloads.

Enjoy the flight. Space enthusiasts around the world are all watching with you.

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.