The US Space Force’s mysterious X-37B spaceplane is almost ready for its seventh flight. Space Force and SpaceX are “making final preparations” for the scheduled launch of the robotic X-37B on Sunday evening (Dec. 10), Space Force officials said in an updated email today (Dec. 7).

The spacecraft is scheduled to lift off atop a Falcon Heavy rocket from NASA’s Kennedy Space Center in Florida over a 10-minute period, starting at 20:14 ET (01:14 GMT on December 11).

The Space Force is believed to have two X-37B vehicles, both manufactured by Boeing. Spaceplanes look very similar to old NASA orbiters but are much smaller; Both X-37Bs can fit in the payload bay of a single space shuttle.

To date, the two X-37Bs have flown a total of six missions, each longer and more ambitious than the last. The final vehicle, known as OTV-6 (Orbital Test Vehicle-6), touched down in November 2022 after 908 days in Earth orbit.

It is unclear how long the upcoming OTV-7 flight will last; Space forces have released few details about the X-37B missions because most of their payloads are classified. Some of this equipment is likely to be new intelligence tools; Military officials have long said the X-37B was used primarily as a testbed for new technologies.

However, the X-37B also carries some civilian research cargo. For example, one of the unclassified experiments being conducted on OTV-7 is Seeds-2, a NASA project that will test how long-term exposure to space radiation affects seeds.