The Air Force's secretive X-37B spaceplane just set a new orbit record. Why? That's top secret.

Next week, the Air Force's unmanned X-37B spaceplane will complete two years in orbit, assuming it doesn't land before then. On Monday, it set a new mission record of 718 days in orbit, surpassing the in-space voyages of its four predecessors. It's likely return date — like almost everything else about the X-37B — is shrouded in mystery. Air Force spokesman Major William Russell tells CNN that there is no scheduled end to the mission, and the spaceplane will land back on Earth only after its objectives have been completed. What objectives? That's classified.

Because the X-37B missions are secret, the Air Force "tends to speak of the vehicle and its activities in general terms," Space.com notes.

We know, for example, that the X-37B is about 29 feet long, launches vertically on a rocket, and lands like an airplane or the now-retired space shuttles. It is also, the Air Force says, part of "an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform," and its "primary objectives" are developing "reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth." The experiments include "advanced guidance, navigation, and control," "thermal protection systems," "high temperature structures and seals," "conformal reusable insulation," and "autonomous orbital flight, reentry, and landing."

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You can get glimpses of the spaceplane and hear more of the Boeing and Air Force marketing jargon surrounding it in this video:

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"The X-37B program tends to attract public interest because of that secrecy," CNN reports, and "speculators have guessed the planes could be involved in spying activities or testing out a space weapon." What is it really doing? "We have no idea," says Matt Novak at Gizmodo. "But given how bad things are going on Earth ... here’s hoping that the Air Force is developing some kind of human escape plan."