Dec 10 — The U.S. military’s secretive X-37B spaceplane which was poised for launch on Sunday on the robot craft’s seventh mission to orbit atop a SpaceX Falcon Heavy rocket has been delayed to Monday, SpaceX said on Sunday.

“SpaceX is targeting Monday, December 11 at 8:14 p.m. ET for Falcon Heavy’s launch of the USSF-52 mission to orbit from NASA’s Kennedy Space Center in Florida,” the rocket manufacturing company said in a statement.

Since its debut more than a decade ago, the X-37B has been a source of intrigue within the space community, mostly owing to the mysterious nature of its activities in low Earth orbit. Despite not knowing its true purpose or location, skywatchers have occasionally spotted and photographed the space plane in the night sky using telescopes.

The U.S. Space Force typically does not disclose the classified aspects of X-37B missions. As such, little is known about the types of activities the robotic vehicle carries out in orbit.