military's X-37B robotic space plane is closing in on a mission-duration record.

The Space Force minishuttle has now been aloft for 773 days.

Related: 10 surprising facts about the US military's X-37B space plane.

"Delta 9 Detachment 1 oversees operations of the X-37B Orbital Test Vehicle, an experimental program designed to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S.

— X-37B: The Space Force's mysterious space plane.

— The US Space Force's secretive X-37B space plane: 10 surprising facts.

— Space Force's X-37B robotic space plane wings past 500 days in Earth orbit!

The Space Force is thought to have two X-37B vehicles in its fleet, both of which were built by Boeing.

The X-37B launches vertically atop a rocket and lands horizontally on a runway, like NASA's old space shuttle orbiter.

The military space plane looks a lot like the now-retired shuttle, in fact, but it's much smaller — just 29 feet (8.8 meters) long, compared to 122 feet (37 m).

There's another key difference as well: NASA's shuttle was crewed, whereas the X-37B is robotic.

Boeing has noted that the X-37B features many elements that mark a first use in orbit for a space plane, including fully automated de-orbit and landing functions, flight controls and brakes that use all electro-mechanical actuation (no hydraulics) and a body made of a relatively light composite structure, rather than traditional aluminum.

"The vehicle is the first since the space shuttle with the ability to return experiments to Earth for further inspection and analysis.

This United States [Space] Force unmanned space vehicle explores reusable vehicle technologies that support long-term space objectives."!