The reusable robotic vehicle, which looks like a miniature version of NASA's space shuttle orbiters, launched atop a SpaceX Falcon 9 rocket from NASA's Kennedy Space Center in Florida on Sept. 7, 2017.

As of today (April 30), the space plane has been aloft for 601 days, on a mission known as Orbital Test Vehicle 5 (OTV-5) because it's the fifth flight of the X-37B program.

Related: The X-37B Space Plane: 6 Surprising Facts

It's unclear what exactly the spacecraft is doing up there. X-37B missions are classified, and Air Force officials tend to speak of project goals in general terms, as this excerpt from the X-37B fact sheet shows: "The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth."

An artist's depiction of the U.S. Air Force's unmanned X-37B space plane in orbit with its solar array deployed and payload bay open. (Image credit: © United Launch Alliance/Boeing)

Still, the Air Force does divulge some payloads flying on X-37B missions. For example, we know that OTV-5 includes the Advanced Structurally Embedded Thermal Spreader experiment (ASETS-II), which is measuring the performance of electronics and oscillating heat pipes in the space environment.

The Air Force has at least two X-37B vehicles, both of which were built by Boeing. Each space plane is 29 feet (8.8 meters) long and 9.6 feet (2.9 m) tall, with a wingspan of almost 15 feet (4.6 m). The solar-powered spacecraft have payload bays about the size of a pickup-truck bed.

The X-37B launches vertically and lands horizontally on a runway, like the space shuttle orbiters did.

OTV-5 is not, so far, the longest-duration X-37B mission, though the outing will earn that distinction if the established pattern holds: Each OTV flight has lasted longer than its predecessors:

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OTV-1 launched on April 22, 2010, and ended on Dec. 3, 2010 (224 days in space).

OTV-2 began March 5, 2011, and landed on June 16, 2012 (468 days).

OTV-3 launched on Dec. 11, 2012, and came down on Oct. 17, 2014 (675 days).

OTV-4 lifted off on May 20, 2015, and landed May 7, 2017 (718 days).

Mike Wall's book about the search for alien life, "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), is out now. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook.