| OA-4 | |
| Names List: | Orbital-4 (2008–2015) |
| Mission Type: | ISS logistics |
| Operator: | Orbital ATK |
| Spacecraft: | S.S. Deke Slayton II |
| Spacecraft Type: | Enhanced Cygnus[1] |
| Launch Mass: | [2] |
| Launch Date: | UTC (4:44:57pmEST) |
| Launch Rocket: | Atlas V 401 (AV-061) |
| Launch Site: | Cape Canaveral, SLC41 |
| Launch Contractor: | United Launch Alliance |
| Disposal Type: | Deorbited |
| Decay Date: | UTC |
| Orbit Reference: | Geocentric orbit[3] |
| Orbit Regime: | Low Earth orbit |
| Orbit Inclination: | 51.64° |
| Apsis: | gee |
| Docking: | |
| Insignia: | Orbital Sciences CRS Flight 4 Patch.png |
| Insignia Caption: | NASA insignia |
| Programme: | Commercial Resupply Services |
| Previous Mission: | SpaceX CRS-7 |
| Next Mission: | OA-6 |
| Programme2: | Cygnus flights |
| Previous Mission2: | Orbital-3 |
| Next Mission2: | OA-6 |
OA-4, previously known as Orbital-4, was the fourth successful flight of the Orbital ATK uncrewed resupply spacecraft Cygnus and its third flight to the International Space Station (ISS) under the Commercial Resupply Services (CRS-1) contract with NASA.[6] [7] With the Antares launch vehicle undergoing a redesign following its failure during the Orb-3 launch, OA-4 was launched by an Atlas V launch vehicle. Following three launch delays due to inclement weather beginning on 3 December 2015, OA-4 was launched at 21:44:57 UTC on 6 December 2015. With a liftoff weight of, OA-4 became the heaviest payload ever launched on an Atlas V.[8] The spacecraft rendezvoused with and was berthed to the ISS on 9 December 2015. It was released on 19 February 2016 after 72 days at the International Space Station. Deorbit occurred on 20 February 2016 at approximately 16:00UTC.[9]
See main article: Cygnus (spacecraft).
OA-4 was the fourth of eight flights by Orbital ATK under the Commercial Resupply Services (CRS-1) contract with NASA and the inaugural flight of the larger Enhanced Cygnus PCM. The mission was originally scheduled for 1 April 2015.[10] The Atlas V launch vehicle launched in the 401 vehicle configuration with a four-meter fairing, no solid rocket boosters and a single-engine Centaur upper stage.
In an Orbital ATK tradition, this Cygnus spacecraft was named Deke Slayton II after Deke Slayton, one of NASA's original Mercury Seven astronauts and Director of Flight Operations, who died in 1993. This spacecraft reuses the name Deke Slayton, originally applied to the Orb-3 spacecraft which was lost in an Antares rocket explosion in October 2014.[11]
The mission was the first flight of the enhanced variant of Orbital ATK's Cygnus spacecraft, capable of delivering more than of essential crew supplies, equipment and scientific experiments to the International Space Station (ISS).
Total cargo: [12]
Total cargo with packing material: