USA-261, also referred to as Orbital Test Vehicle 4 (OTV-4) or AFSPC-5, is the second flight of the second Boeing X-37B, an American unmanned vertical-takeoff, horizontal-landing spaceplane.

It was launched to low Earth orbit aboard an Atlas V rocket from Cape Canaveral on May 20, 2015.

The spaceplane was operated by the Air Force Space Command, which considers the mission classified and as such has not revealed the objectives.

However, the Air Force did reveal that the X-37 would be carrying an experimental hall-effect thruster for testing ahead of potential use in future AEHF satellites.

[2] Additionally, NASA announced it was flying a materials science payload similar to MISSE called METIS.