Cygnus NG-21 is the twenty-first planned flight of the Cygnus robotic resupply spacecraft and its eighteenth flight to the International Space Station (ISS). It is planned to launch in August 2024. It is contracted to Northrop Grumman under the Commercial Resupply Services II (CRS-2) contract with NASA. The capsule is scheduled to be launched aboard a SpaceX Falcon 9 rocket.

Cygnus is an expendable American cargo spacecraft developed by Orbital Sciences Corporation but manufactured and launched by Northrop Grumman Space Systems as part of NASA's Commercial Resupply Services (CRS) program. It is usually launched by Northrop Grumman's Antares rocket from the Wallops Flight Facility, although three flights were on ULA's Atlas V and three are planned for SpaceX's Falcon 9, in both cases launching from Cape Canaveral Space Force Station. It transports supplies to the International Space Station (ISS) following the retirement of the American Space Shuttle. Since August 2000, ISS resupply missions have been regularly flown by the Russian Progress spacecraft, as well as by the European Automated Transfer Vehicle, and the Japanese H-II Transfer Vehicle. With the Cygnus spacecraft and the SpaceX Dragon, NASA seeks to increase its partnerships with domestic commercial aviation and aeronautics industry.