The U.S Air Force's mystery-shrouded X-37B space plane has already been in orbit for the past 700 days.

The reusable aircraft was launched on top of a SpaceX Falcon 9 rocket from NASA’s Kennedy Space Center (KSC) in Florida on Sept. 7, 2017 for its fifth and latest mission known as Orbital Test Vehicle 5 (OTV-5).

The military plane, which looks like a smaller version of the U.S space agency’s space shuttle, has since been circling our planet.

The objective of the mission, however, remains a mystery. The Air Force only talks about its X-37B program in general terms and provides small details about the missions.

The X-37B Orbital Test Vehicle mission 3 (OTV-3), the Air Force's unmanned, reusable space plane. Vandenberg Air Force Base/Boeing

Air Force officials say that the space plane is testing technologies for future reusable spacecraft and transport experiments up to space and back. They also revealed that the unmanned vehicle’s payloads include the Advanced Structurally Embedded Thermal Spreader (ASETS-II), which was designed to measure how oscillating heat pipes and electronics perform in space.

“The fifth OTV mission continues to advance the X-37B’s performance and flexibility as a space technology demonstrator and host platform for experimental payloads,” the Air Force has said of the OTV-5 mission.

“This mission carries small satellite ride shares and will demonstrate greater opportunities for rapid space access and on-orbit testing of emerging space technologies.

Particular details about the mission and most of the plane’s payloads remain classified so the public could only speculate on what the space plane is doing up in orbit.

Last month, former Secretary of the Air Force Heather Wilson revealed to an audience at the Apsen Security Forum that the X-37B plane can pull off maneuvers that can drive potential adversaries “nuts.”

Experts think she was referring to the plane’s ability to change orbit, which makes its movement unpredictable.

“When it's close to the Earth, it's close enough to the atmosphere to turn where it is,” Wilson said, according to

"Which means our adversaries don't know—and that happens on the far side of the Earth from our adversaries—where it's going to come up next. And we know that that drives them nuts. And I'm really glad about that."

The X-37B program has two known reusable vehicles that were both built by Boeing. Each of these planes measures 29 feet long and 9.6 feet tall, and has a wingspan of nearly 15 feet. Just like the space shuttle, the solar-powered X-37B launches vertically and lands on a runway like a plane.