The latest mystery mission of the U.S. Air Force's robotic X-37B space plane has now passed the 400-day mark .

This mission — known as Orbital Test Vehicle (OTV-5) — was rocketed into Earth orbit on Sept. 7, 2017, atop a SpaceX Falcon 9 rocket from Launch Complex 39A at NASA's Kennedy Space Center in Florida.

The uncrewed space plane is carrying out secretive duties during the X-37B program's fifth flight. [The X-37B Space Plane: 6 Surprising Facts]

Flight duration

Each X-37B/OTV mission has set a new flight-duration record for the program:

OTV-1 began April 22, 2010, and concluded on Dec. 3, 2010, after 224 days in orbit.

OTV-2 began March 5, 2011, and concluded on June 16, 2012, after 468 days on orbit.

OTV-3 chalked up nearly 675 days in orbit before finally coming down on Oct. 17, 2014.

OTV-4 conducted on-orbit experiments for 718 days during its mission, extending the total number of days spent in space for the OTV program at that point to 2,085 days.

What's up?

Most X-37B payloads and activities are classified. The only OTV-5 payload revealed to date by Air Force officials is the Advanced Structurally Embedded Thermal Spreader, or ASETS-II.

Developed by the U.S. Air Force Research Laboratory (AFRL), this cargo is testing experimental electronics and oscillating heat pipes for long-duration stints in the space environment. According to AFRL, the three primary science objectives are to measure the initial on-orbit thermal performance, to gauge long-duration thermal performance and to assess any lifetime degradation.