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The United States Air Force's X-37B space plane will launch on its fourth mystery mission next month.

The unmanned X-37B, which looks like a miniature version of NASA's now-retired space shuttle orbiter, is scheduled to blast off atop a United Launch Alliance Atlas V rocket from Florida's Cape Canaveral Air Force Station on May 20.

"We are excited about our fourth X-37B mission," Randy Walden, director of the Air Force Rapid Capabilities Office, said in a statement. "With the demonstrated success of the first three missions, we’re able to shift our focus from initial checkouts of the vehicle to testing of experimental payloads." [See photos of the X-37B's third mission]

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The X-37B Orbital Test Vehicle mission 3, the Air Force's unmanned, reusable space plane, landed at Vandenberg Air Force Base at 9:24 a.m. Oct. 17, 2014. The OTV-3 conducted on-orbit experiments for 674 days during its mission. Boeing

The X-37B's payloads and specific activities are classified, so it's unclear exactly what the spacecraft does while zipping around the Earth. But Air Force officials have revealed a few clues about the upcoming mission.

"The Air Force Research Laboratory (AFRL), Space and Missile Systems Center (SMC) and the Air Force Rapid Capabilities Office (AFRCO) are investigating an experimental propulsion system on the X-37B on Mission 4," Capt. Chris Hoyler, an Air Force spokesman, told Space.com via email.

"AFRCO will also host a number of advance materials onboard the X-37B for the National Aeronautics and Space Administration (NASA) to study the durability of various materials in the space environment," Hoyler added.

The Air Force owns two X-37B space planes, both of which were built by Boeing's Phantom Works division. The solar-powered spacecraft are about 29 feet long by 9.5 feet tall (8.8 by 2.9 meters), with a wingspan of 15 feet (4.6 m) and a payload bay the size of a pickup-truck bed. The X-37B launches vertically atop a rocket and lands horizontally on a runway, like the space shuttle did.

One of the two X-37B vehicles flew the program's first and third missions, which were known as OTV-1 and OTV-3, respectively. ("OTV" is short for "Orbital Test Vehicle.") The other spacecraft flew OTV-2. Air Force officials have not revealed which space plane will be going to orbit on the upcoming mission.

— Mike Wall, Space.com