Secretary of the Air Force Barbara Barret announced on twitter that the United States Space Force will launch its secretive X37-B Orbital Test Vehicle on May 16th. The orbiter will hitch a ride on a ULA Atlas 501. United States Air Force currently has two of the vehicles in rotation, so it is unknown if the space plane going up on the 16th is the same vehicle that orbited earth for 780 days before returning to Cape Canaveral last October. The space plane is operated by the Air Force's Rapid Capabilities office.
The usually tightlipped operators of the X37-B have been shy to reveal much about their spaceplane in the past. On this go around the United States Air Force and the recently developed United States Space Force have combined forces and are feeling much more open about the mission.
Secretary Barret made a statement that this mission will carry the most experiments of any X37-B Orbital Test Vehicle flight to date, including two from NASA involving seed germination in low gravity.
Official photos were released of the X37-B being integrated into its payload fairing. The USAF abbreviation still appears on the right wing leading some to speculate it is, in fact, the same launched atop a SpaceX Falcon 9 in September of 2017.
The X-37B is useful for orbital experiments and has a wide variety of applications. The orbiter is about the size of a school bus and has a cargo bay. Because it can reenter the atmosphere as the shuttle did, it can land on a runway and return valuable experiments and important data quickly and reliably.
Source: United States Air Force