What’s the X-37 Doing Up There?

It’s been five years since the first launch of the Air Force’s X-37B mini-shuttle, and outside observers—meaning those who lack the proper security clearances—still know little more about this mysterious unmanned vehicle than they did in 2010 (see “Space Shuttle Jr.,” Dec. 2009/Jan. 2010). But after three completed flights and a fourth launch last May (the spaceplane was still in orbit as of mid-December), they’re at least able to make educated guesses.

Despite the project’s general secrecy, the Air Force has been perfectly willing to release photos of the vehicle sitting on top of its Atlas V rocket at Cape Canaveral, Florida, and on the runway at Vandenberg Air Force Base in California where it lands, autonomously. The spaceplane’s dimensions are known: It’s small, about 29 feet long, with a cargo bay not much bigger than a pickup truck’s. Project officials have revealed that the X-37B’s maneuvering engine runs on hydrazine and nitrogen tetroxide, and that it uses a different kind of thermal protection than NASA’s space shuttle did. The durations of the first three missions are a matter of public record (224 days, 469 days, and 674 days), and while the orbital parameters aren’t officially disclosed, amateur astronomers have been able to spot the mini-spaceplane through telescopes and figure out that it’s been orbiting at relatively low altitudes.