The space plane will be used to test ion propulsion, among many other classified missions.
The military's highly secretive, miniature space plane is set to return to space Wednesday to continue doing its secret experiments. But for the first time in its history, we actually know a little bit about some of the missions the X-37B will carry out.
The X-37B looks just like a miniature NASA shuttle, except it's unmanned, highly autonomous, and it's operated by the Air Force. The space plane has flown three separate missions; this one will be its fourth tour in space.
Most recently, the X-37B spent 22 months orbiting Earth, where it was testing "advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, re-entry, and landing," according to the Air Force.
Specifics on any of those missions have been classified, and Air Force Space Command has repeatedly turned down any opportunity to talk more about the mission.
The good news is that, finally, the Air Force has dropped some hints about what it'll be testing this time. The Air Force said that it will carry Hall thrusters, a type of ion-based propulsion that uses noble gases to efficiently move a spacecraft around in orbit.
"While producing comparatively low thrust relative to conventional rocket engines, Hall thrusters provide significantly greater specific impulse, or fuel economy," the Air Force wrote in a statement. "This results in increased payload carrying capacity and a greater number of on-orbit maneuvers for a spacecraft using Hall thrusters rather than traditional rocket engines."
NASA is also running an experiment on the X-37B—it is testing how roughly 100 different materials respond to being in space for long durations.
Finally, the military is launching LightSail, a solar sail built by the Planetary Society, on the same rocket as the X-37B. The solar sail itself won't be deployed in this mission, but the satellite is going to simulate its full mission, scheduled for 2016.
Though we have relatively little information on those two experiments, it's much, much more than we've been privy to in the past (there are surely more, classified experiments on board).
More than two years ago, I spoke to the then-head of Air Force Space Command about the X-37B. He told me that the plane is "spectacular" but would not give me any details about the plane and wouldn't even discuss its budget—he called the secrecy a "good, strategic national security decision."
That secrecy has led conspiracy theorists to suggest that the space plane has been used to spy on the Chinese and North Koreans; others have said it could possibly be a space weapon. Neither of these theories are seen as very credible.
That the Air Force is willing to discuss anything about the plane suggests that it wants to dispel those rumors, according to Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics who works for the Chandra X-Ray Center. McDowell also runs Jonathan's Space Report, an email newsletter that has detailed satellite launches since 1989.
"A lot of the confusion is that this is the first space program by the Air Force Rapid Capabilities Office, which is mainly an airplane organization, rather than coming out of the Air Force Space and Missile Center like most USAF space programs," McDowell told me. "The specific experiments and sensors carried are secret, but I don't think the overall program is really that mysterious."
McDowell said he thinks that the Air Force is using the X-37B as something of a classified, small space station to test experiments that it can't or doesn't want to test on the International Space Station.
"I remain of the opinion that they are using the X-37B like NASA used the Shuttle for some of its Spacelab missions," he said. "Stuff a bunch of experiments in the cargo bay and then bring them home for refurbishment."
Those experiments will go back to space on Wednesday. And we're just as curious as ever.