Mysterious robotic space plane returns to U.S. after 22-month mission

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By Rachel Crane

(CNN) -- There is a mysterious robotic space plane that has been orbiting the earth for the past 22 months. It's the Air Force's X-37B, also known as the Orbital Test Vehicle, and it's set to return to Earth this week to Vandenberg Air Force Base in California.

The Air Force has 2 X-37B space planes in it's fleet, and has been sending them into space on secretive missions since 2010. The latest mission is the third of it's kind. And this plane remained in space much longer than it's predecessors.

So what information are they gathering and why the secrecy? There are many theories out there: spy satellites capturing images of political hotspots, space weaponry being tested, the rumors go on and on. The Air Force maintains that this is solely an experimental test vehicle. The primary objectives of the missions are officially, "to test spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth." Sounds pretty vague huh?

Here's what we do know:

The planes resemble NASA's space shuttles, but are about 1/4 the length and have payloads the size of a pickup truck bed. They use an atlas 5 rocket to blast off and then land on a runway. They can also stay in space for such long periods of time because they are unmanned, and use a solar array for power. Boeing built the planes but because it's a classified project, we don't know the price tag.

We know when they take off, we know when they land…these missions are not a secret. But what goes on in between is shrouded in mystery.