U.S. Air Force

This fall, the Air Force will launch another flight of its mysterious X-37B Orbital Test Vehicle. This will be the second launch of the first test vehicle, which had its maiden flight in 2010. A second X-37B vehicle is currently orbiting Earth, having already exceeded its 270-day mission. According to the Air Force, it has no firm return date.

Officially, the X-37B is being launched later this year merely "to gather more test data" Air Force Maj. Tracy Bunko says. Bunko confirms that its payload, like that of the previous X-37B flights, is classified, and that has sparked a barrage of theories about the plane's true purpose as amateur satellite watchers chart its every move.

Although the Air Force won't say what the X-37B's exact purpose is, there is enough known about its orbit (and basic physics) to estimate what this craft is and is not meant to do. Let's take the top three theories one by one.

1. It's a Space Bomber

Forget it, independent experts say. Yes, at one point the Pentagon was funding development of a reusable hypersonic vehicle that was supposed to deliver munitions anywhere in the world within 2 hours. But that concept, called the Common Aero Vehicle, was suborbital. As a spaceplane, the X-37B would suffer from at least one major drawback as a bomber. Changing a spacecraft's orbital plane requires a great amount of thrust—so using something like the X-37B as a bomber would mean changing its orbit to fly over targets, and that would eat up its limited fuel supply, according to University of Maryland professor Mark Lewis, who once served as the Air Force's chief scientist. "If I can't get my alleged bomber to the right location to release its bomb, what good is it?" Lewis says.

2. It's a Spy Plane

This theory could be correct, though it depends what you think the X-37B is spying on.

Earlier this year, the editor of Spaceflight magazine claimed the spaceplane was spying on the Chinese space station, noting that the orbit of the X-37B is close to that of the Tiangong. The theory got a lot of public attention, but Brian Weeden, a consultant with the Secure World Foundation, vigorously disputed it. Weeden, a former Air Force officer who worked at U.S. Strategic Command's Joint Space Operations Center, argues that the China spying theory ignores what is known as right ascension, a critical component of the spacecraft's orbit. The orbit of the X-37B crosses that of the Chinese space station only at two different points—it's not as though the spaceplane could keep a constant eye on Tiangong. "Even though there may be a brief chance at some point for the X-37B to collect intelligence [on the Chinese space station], that certainly is not the main mission, or even something all that feasible," he says.

However, a group of civilian satellite spotters tracking the second X-37B, which is called OTV-2, have noted that the spaceplane's orbit takes it over countries including Iran, Afghanistan, and Pakistan. Weeden agrees that whatever secret payload the X-37B is carrying could indeed be used to capture data from those regions.

3. It's an Experiment

Could the government be telling the truth? The Air Force has always maintained that the X-37B is simply an experimental platform for testing reusable-space-vehicle technologies and for "operating experiments which can be returned to, and examined, on Earth." Weeden says he thinks that really is what the X-37B is doing. "I think the primary mission is the testbed," he says. The X-37B is a good way for the Air Force to fly new technology such as sensors into space, test it, and then examine it back on Earth. Still, reconnaissance and research are not mutually exclusive. X-37B could be doing both.

So, is all the secrecy really necessary? Steven Aftergood, a secrecy expert at the Federation of American Scientists, says it's hard to know at the moment. "But we do know from the history of previous black programs that such secrecy automatically incurs a significant financial cost," he says, pointing to cases like the now-canceled A-12 aircraft.

"[B]lanket secrecy," he says, "invites fevered speculation about the program's true capabilities and intentions, which may or may not serve the national interest."

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