Just a month after the Boeing X-37B returned to Earth, the US Air Force announced that the secretive space plane's fifth mission will launch in August aboard a SpaceX Falcon 9 for the first time.

"We are very excited for the next fifth X-37B mission," Randy Walden, director of the Air Force's Rapid Capabilities Office, which is responsible for the X-37B's experimental operations, said.

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The previous four X-37B launches were carried out on the back of a Lockheed Martin-Boeing Atlas V rocket. The decision could have been influenced as Falcon 9 is not only partially reusable but also is relatively cheaper at $62 million per launch compared with Atlas V's hefty $109 million, Sputnik News noted.

"We are excited about this new partnership on creating flexible and responsive launch options and are confident in SpaceX's ability to provide safe and assured access to space for the X-37B program," Walden said.

As expected, the main payload of the X-37B remains classified, although a USAF statement revealed that the fifth mission will include the Air Force's experimental Advanced Structurally Embedded Thermal Spreader (ASETS-11), which is used to "test experimental electronics and oscillating heat pipes in the long duration space environment."

The X-37B is also known as the "Orbital Test Vehicle," and it is launched vertically to a rocket but lands like a conventional plane. Although it is smaller than the Space Shuttle, it could carry small cargo loads for its 7 x 4-foot storage bay.

Meanwhile, details of the plane's missions are scarce and have been the subject of several conspiracy theories. Speculations that it is a spy satellite, an on-orbit repair service for satellite, a test sensor for intelligence gathering from space, or a space weapon itself have all surfaced, but none was confirmed by Pentagon. Many people also believe that the controversial EmDrive was secretly tested on X-37B, after the space plane stayed a record 718 days in orbit.