The launch marks the 9th flight of the three-core SpaceX Falcon Heavy booster, and the 7th flight of the US Air Force's (not so) secret unmanned spaceplane, the X-37B (USSF-52).

The launch was previously scheduled for Dec. 10, however it was scrapped due to issues with ground equipment just 30 minutes before liftoff - pushing the event back 18 days.

Officially, the X-37B will enter into various orbits around Earth and serve as a testing ground for NASA's study of the effects of long-duration exposure to space on organic materials, the Epoch Times reports, adding that the mission will also include experiments having to do with "space domain awareness," which the US Space Force defines as the ability to "rapidly detect, warn, characterize, attribute, and predict threats to national, allied, and commercial space systems."