No one seems to know much about the Air Force's X-37B secret space plane except that it appears to be working exactly as designed. The unmanned Boeing-built craft, which resembles a miniature space shuttle, returned to Earth on Friday after nearly two years — 674 days, to be exact — in space. It's the X-37B program's third mission to space and by far the longest.

The plane landed at 9:24 a.m. local time on Oct. 17 at Vandenberg Air Force Base in California, the Air Force's 30th Space Wing announced.

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"The 30th Space Wing and our mission partners, Air Force Rapid Capabilities Office, Boeing, and our base support contractors, have put countless hours of hard work into preparing for this landing and today we were able to see the culmination of that dedication," Colonel Keith Balts, 30th Space Wing commander, said in a release. "I'm extremely proud of our team for coming together to execute this third safe and successful landing. Everyone from our on console space operators to our airfield managers and civil engineers take pride in this unique mission and exemplify excellence during its execution."

But just what did the X-37B do up there? Officially, the Air Force isn't telling.

To be fair, experimental military-funded space projects aren't exactly the kind of thing you expect the brass to talk about in public. Inquiries by NBC News have been met officially with a polite but firm "no comment." (However, NBC News reported back in 2001 that the X-37 concept was being promoted by the Pentagon as a next-generation space bomber.)

What is known is that the X-37B has no human pilot, or at least not one in its windowless cockpit. It's operated remotely and lands on its own. The details of its launches aren't secret, but neither are they particularly interesting: It rode an Atlas 5 booster into space on Dec. 12, 2012, and assumed orbit about 180 miles above the Earth. That last part was figured out by a network of curious astronomers, not released publicly by the Air Force.

The plane's size means there isn't room on board for much except avionics equipment, fuel for the thrusters, and a mysterious cavity about the size of a truck bed that could contain all manner of sensors, experiments, hardware — perhaps some bacterial colonies, or a bomb. No one can be sure what's inside.

Until the Air Force decides it's time to spill the beans, the X-37B will keep its secrets, even if they happen to be ordinary testing of still-classified radio hardware or radiation-resistant materials.

For now, the Air Force's two X-37B space planes are locked away at Vandenberg Air Force Base in California. Eventually they'll be heading for new hangars at Kennedy Space Center in Florida — getting ready, perhaps, to break the record for days in orbit once again.