Corona was the name of a series of US military reconnaissance satellites operated under a CIA program with substantial assistance from the US Air Force, used for photographic surveillance of the Soviet Union, China and other areas from June 1959 until May 1972. The project name is sometimes given as CORONA, but it is a codeword, not an acronym.
The satellites were designated KH-1, KH-2, KH-3, KH-4, KH-4A and KH-4B. KH stood for Keyhole, and the incrementing number indicated changes in the surveillance instrumentation, such as the change from single-panoramic to double-panoramic cameras. The KH naming system was first used in 1962 with KH-4 and the earlier numbers were retroactively applied. There were 144 Corona satellites launched, of which 102 returned usable imagery.
The Corona satellites used 31,500 ft (9,600 m) of special 70 mm film with a 24 inch (0.6 m) focal length lens. Initially orbiting at 165 to 460 km, the cameras could resolve images on the ground down to 7.5 m. The two KH-4 systems improved the resolution to 2.75 m and 1.8 m respectively and used a lower altitude pass.
Ironically, the name Corona was more fitting than its originators had ever imagined. The initial missions of the program suffered from many technical problems, among them, mysterious fogging and bright streaks were seen on the returned film of some missions, only to disappear on the next mission. Eventually it was determined by a collaborative team of scientists and engineers from the project and from academia, (among them: Luis Alvarez, Malvin Ruderman, and Sidney Drell) that electrostatic discharges (called corona discharge) caused by rubber components of the camera, were exposing the film. The recommended corrective actions which solved the problem included things like better grounding of spacecraft components and outgassing testing of parts before launch. These recommendations are still used on practically all US reconnaissance satellites to this day.
The initial Corona launches were obscured as part a space technology program called Discoverer. The first test launches were in early 1959. The first launch with a camera was June 1959 as Discoverer 4, which was a 750 kg satellite launched by a Thor-Agena rocket. The key issue with the early satellites was the recovery of the exposed film. Radio link technology did not exist, and so film canisters were returned to Earth in capsules, called "buckets", which were recovered in mid-air by a specially equipped aircraft during their parachute descent. The first camera-fitted Discoverer missions failed to return usable film, but following repeated recovery tests on August 18, 1960 with Discoverer 14, a bucket was successfully retrieved two days later by a C-119.
Discoverer 13 was the first satellite that landed and was recovered on August 11, 1960.
The last launch under the Discoverer name was Discoverer 38 in 1962; after that the launches were entirely secret. The last Corona launch was on May 25, 1972. The best sequence of Corona launches was from 1966 to 1971 when there were 32 consecutive launch-and-film-recoveries.
Corona was officially secret until 1992. On February 22, 1995 an Executive Order declassified the imagery acquired by the Corona and two later programs (Argon and Lanyard).