Now playing: Watch this: Secretive space plane lands after 718 days circling Earth

Sunday marks the first day since May 2015 that an unmanned US Air Force "space plane" isn't circling the Earth above us doing who knows what.

A sonic boom heard over parts of central Florida marked the mysterious X-37B Orbital Test Vehicle's return to NASA's Kennedy Space Center Sunday morning after 718 consecutive days in space.

The military space drone was launched by an Atlas 5 rocket from nearby Cape Canaveral on May 20, 2015, for the fourth mission to use one of the Boeing-built craft that looks somewhat similar to a small version of NASA's old space shuttles.

Altogether, the program, which has at least two of the planes, has spent 2,085 days in space over the course of four missions. The fourth mission that ended Sunday breaks the endurance record set by the previous flight of 614 days between 2012 and 2014.

"This mission once again set an on-orbit endurance record and marks the vehicle's first landing in the state of Florida," the Air Force said in a statement offering few other details about the mission. "The ability to land, refurbish, and launch from the same location further enhances the OTV's ability to rapidly integrate and qualify new space technologies."

According to a fact sheet from the Air Force, the X-37B is being used to test technologies including "advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, advanced propulsion systems, advanced materials and autonomous orbital flight, reentry and landing."

The secretive Air Force planes won't stay on the ground for long. The fifth X-37B mission is set to launch later this year.

Technically Literate: Original works of short fiction with unique perspectives on tech, exclusively on CNET.

Crowd Control: A crowdsourced science fiction novel written by CNET readers.