KH-9 (BYEMAN codename HEXAGON), commonly known as Big Bird or KeyHole-9, was a series of photographic reconnaissance satellites launched by the United States between 1971 and 1986. Of twenty launch attempts by the National Reconnaissance Office, all but one were successful. Photographic film aboard the KH-9 was sent back to Earth in recoverable film return capsules for processing and interpretation. The best ground resolution achieved by the main cameras was better than 0.6 m (2 ft 0 in).
The satellites were an important factor in determining Soviet military capabilities and in the acquisition of accurate intelligence for the formulation of U.S. national policy decisions as well as deployment of U.S. forces and weapon systems. The satellites were instrumental in U.S. National Technical Means of Verification of Strategic Arms Limitation Talks (SALT) and the Anti-Ballistic Missile Treaty (ABMT).
The KH-9 was declassified in September 2011 and an example was put on public display for a single day on 17 September 2011 in the parking lot of the Steven F. Udvar-Hazy Center of the National Air and Space Museum.
The KH-9 was originally conceived in the early 1960s as a replacement for the CORONA search satellites. The goal was to search large areas of the Earth with a medium resolution camera. The KH-9 carried two main cameras, although a mapping camera was also carried on several missions. The photographic film from the cameras was sent to recoverable re-entry vehicles and returned to Earth, where the capsules were caught in mid-air by an aircraft. Four re-entry vehicles were carried on most missions, with a fifth added for missions that included a mapping camera.
Between September 1966 and July 1967, the contractors for the Hexagon subsystems were selected. Lockheed Missiles and Space Company (LMSC) was awarded the contract for the Satellite Basic Assembly (SBA), Perkin Elmer for the primary Sensor Subsystem (SS), McDonnell for the Reentry Vehicle (RV), RCA Astro-Electronics Division for the Film Take-Up system, and Itek for the Stellar Index camera (SI). Integration and ground-testing of Satellite Vehicle-1 (SV-1) were completed in May 1971, and it was subsequently shipped to Vandenberg Air Force Base in a 70 ft (21 m) container. Ultimately, four generations ("blocks") of KH-9 Hexagon reconnaissance satellites were developed. KH9-7 (1207) was the first to fly a Block-II panoramic camera and SBA. Block-III (vehicles 13 to 18) included upgrades to electrical distribution and batteries. Two added tanks with ullage control for the Orbit Adjust System (OAS) and new thrusters for the Reaction Control System (RCS) served to increase KH-9's operational lifetime. In addition, the nitrogen supply for the film transport system and the camera vessel was increased. Block-IV was equipped with an extended command system using plated wire memory. In the mid 1970s, over 1,000 people in the Danbury, Connecticut area worked on the secret project.
A reentry vehicle from the first Hexagon satellite sank to 16,000 ft (4,900 m) below the Pacific Ocean after its parachute failed. The USS Trieste II (DSV-1) retrieved its payload in April 1972 after a lengthy search, but the film had disintegrated during the nine months underwater, leaving no usable photographs.
Over the duration of the program, the lifetime of the individual satellites increased steadily. The final KH-9 operated for up to 275 days. Different versions of the satellite varied in mass; most weighed 11,400 or 13,300 kg (25,100 or 29,300 lb).
The Satellite Control Section (SCS), which forms the aft part of the SBA, started as Air Force Project 467. SCS was intended as a more capable replacement for the on-orbit propulsion, which had been provided by the Agena upper stage for previous generations of reconnaissance satellites. The SCS featured an increased diameter of 10 ft (3.0 m) (compared to 5 ft (1.5 m) for the Agena) and a length of 6 ft (1.8 m). It incorporated a secondary propulsion system inherited from the Agena. SCS was equipped with deployable solar panels and an unfurlable parabolic antenna for high data rate communication.
The main camera system was designed by Perkin-Elmer  to take stereo images, with a forward looking camera on the port side, and an aft looking camera on the starboard side. Images were taken at altitudes ranging from 90–200 mi (480,000–1,060,000 ft; 140–320 km). The camera optical layout is an f/3.0 folded Wright camera, with a focal length of 60 in (1,500 mm). The system aperture is defined by a 20 in (510 mm) diameter aspheric corrector plate, which corrects the spherical aberration of the Wright design. In each of the cameras the ground image passes through the corrector plate to a 45°-angle flat mirror, which reflects the light to a 0.91 m (3 ft 0 in)-diameter concave main mirror. The main mirror directs the light through an opening in the flat mirror and through a four-element lens system onto the film platen. The cameras could scan contiguous areas up to 120° wide, and achieved a ground resolution better than 2 ft (0.61 m) during the later phase of the project.
Missions 1205 to 1216 carried a "mapping camera" (also known as a "frame camera") that used 9 in (230 mm) film and had a moderately low resolution of initially 30 ft (9.1 m), which improved to 20 ft (6.1 m) on later missions  (somewhat better than LANDSAT). Intended for mapmaking, photos this camera took cover essentially the entire Earth with at least some images between 1973 and 1980. Almost all the imagery from this camera, amounting to 29,000 images, each covering 3,400 km2 (1,300 sq mi), was declassified in 2002 as a result of Executive order 12951, the same order which declassified CORONA, and copies of the films were transferred to the U.S. Geological Survey's Earth Resources Observation Systems office.
Scientific analysis of declassified KH-9 satellite images continues to reveal historic trends and changes in climate and terrestrial geology. A 2019 study of glacial melt in the Himalayas over the past half-century used data collected by KH-9 satellites throughout the 1970s and 1980s to demonstrate that melt rates had doubled since 1975.
Missions 1205 to 1207 carried Doppler beacons  to help map the atmospheric density at high altitudes in an effort to understand the effect on ephemeris predictions. The measurements of the atmospheric density were released through NASA.
Missions 1203, 1207, 1208, 1209, and 1212 to 1219 included Ferret ELINT subsatellites, which were launched into a high Earth orbit to catalogue Soviet air defence radars, eavesdrop on voice communications, and tape missile and satellite telemetry. Missions 1210 to 1212 also included scientific subsatellites.
|Name||Block ||Mission no.||Launch date||NSSDC ID
|Other Name||Launch vehicle||Orbit||Decay date|
|KH9-1||I||1201||15 June 1971||1971-056A 
|OPS 7809||Titan IIID||184.0 km × 300.0 km, i=96.4°||6 August 1971 |
|KH9-2||I||1202||20 January 1972||1972-002A 
|OPS 1737||Titan IIID||157.0 km × 331.0 km, i=97.0°||29 February 1972 |
|KH9-3||I||1203||7 July 1972||1972-052A 
|OPS 7293||Titan IIID||174.0 km × 251.0 km, i=96.9°||13 September 1972 |
|KH9-4||I||1204||10 October 1972||1972-079A 
|OPS 8314||Titan IIID||160.0 km × 281.0 km, i=96.5°||8 January 1973 |
|KH9-5||I||1205||9 March 1973||1973-014A 
|OPS 8410||Titan IIID||152.0 km × 270.0 km, i=95.7°||19 May 1973 |
|KH9-6||I||1206||13 July 1973||1973-046A 
|OPS 8261||Titan IIID||156.0 km × 269.0 km, i=96.2°||12 October 1973 |
|KH9-7||II||1207||10 November 1973||1973-088A 
|OPS 6630||Titan IIID||159.0 km × 275.0 km, i=96.9°||13 March 1974 |
|KH9-8||II||1208||10 April 1974||1974-020A 
|OPS 6245||Titan IIID||153.0 km × 285.0 km, i=94.5°||28 July 1974 |
|KH9-9||II||1209||29 October 1974||1974-085A 
|OPS 7122||Titan IIID||162.0 km × 271.0 km, i=96.7°||19 March 1975 |
|KH9-10||II||1210||8 June 1975||1975-051A 
|OPS 6381||Titan IIID||157.0 km × 234.0 km, i=96.3°||5 November 1975 |
|KH9-11||II||1211||4 December 1975||1975-114A 
|OPS 4428||Titan IIID||157.0 km × 234.0 km, i=96.7°||1 April 1976 |
|KH9-12||II||1212||8 July 1976||1976-065A 
|OPS 4699||Titan IIID||159.0 km × 242.0 km, i=97.0°||13 December 1976 |
|KH9-13||III||1213||27 June 1977||1977-056A 
|OPS 4800||Titan IIID||155.0 km × 239.0 km, i=97.0°||23 December 1977 |
|KH9-14||III||1214||16 March 1978||1978-029A 
|OPS 0460||Titan IIID||172.0 km × 218.0 km, i=96.4°||11 September 1978 |
|KH9-15||III||1215||16 March 1979||1979-025A 
|OPS 3854||Titan IIID||177.0 km × 256.0 km, i=96.3°||22 September 1979 |
|KH9-16||III||1216||18 June 1980||1980-052A 
|OPS 3123||Titan IIID||169.0 km × 265.0 km, i=96.5°||6 March 1981 |
|KH9-17||III||1217||11 May 1982||1982-041A 
|OPS 5642||Titan IIID||177.0 km × 262.0 km, i=96.4°||5 December 1982 |
|KH9-18||III||1218||20 June 1983||1983-060A 
|OPS 0721||Titan 34D||163.0 km × 224.0 km, i=96.4°||21 March 1984 |
|KH9-19||IV||1219||25 June 1984||1984-065A 
|USA 2||Titan 34D||170.0 km × 230.0 km, i=96.5°||18 October 1984 |
|KH9-20||IV||1220||18 April 1986||1986-F03||Launch failed ||Titan 34D||—||—|
(NSSDC ID Numbers: See COSPAR)
The total cost of the 20 flights KH-9 program from FY1966 to FY1986 was US$3.262 billion in respective year dollars (equivalent to 14.66 billion in 2019, with an average reference year of 1976).
The reaction control system of a KH-9, showing the propellant tanks.
A KH-9 in scaffolding, being prepared for launch.
A technician despools one of the re-entry modules which contained the film used by the cameras.
A diagram showing the assembly, testing and launch preparations of a KH-9.
The forward section of a KH-9.
A KH-9 re-entry module hanging from its parachute, ready to be retrieved.
Another view of the KH-9's reaction control system.
A KH-9 being transported by truck in a special canister.
The shroud and base of a KH-9 undergo vibration tests in an acoustic chamber.
This painting of a KH-9 shows the two different camera types that the satellite carried.
Basic specifications and diagram of the KH-9.
This diagram shows the various sections of the KH-9 and their dimensions.
A KH-9 at the National Museum of the United States Air Force (front view).
A KH-9 at the National Museum of the United States Air Force (forward section).
A KH-9 at the National Museum of the United States Air Force (film recovery unit).
A KH-9 at the National Museum of the United States Air Force (center section).
A KH-9 at the National Museum of the United States Air Force (left camera).
A KH-9 at the National Museum of the United States Air Force (rear view without solar panels).
Other U.S. imaging spy satellites:
|Wikimedia Commons has media related to KH-9 HEXAGON.|