CAPE CANAVERAL, Florida—A SpaceX Falcon Heavy rocket is poised for launch as soon as Tuesday night from the Kennedy Space Center in Florida, and the US military’s mysterious X-37B spaceplane is fastened atop the heavy-lifter for a ride into orbit.

Although the Space Force is keeping details about the military spaceplane’s flight under wraps, we know it’s heading into an unusual orbit, probably significantly higher than the X-37B’s previous sojourns that stayed within a few hundred miles of Earth’s surface.

SpaceX’s launch team called off a launch attempt Monday night “due to a ground side issue” and reset for another launch opportunity as soon as Tuesday night. When it lifts off, the Falcon Heavy will light 27 kerosene-fueled engines to power the rocket off its launch pad overlooking the Atlantic coastline.