Discoverer

As American space launches were not classified until late 1961,^{[8]: 176 [15]} the first CORONA satellites were cloaked with disinformation as being part of a space technology development program called Discoverer. To the public, Discoverer missions were scientific and engineering missions, the film-return capsules being used to return biological specimens. To facilitate this deception, several CORONA capsules were built to house a monkey passenger. Many test monkeys were lost during ground tests of the capsule's life support system.^[8]: 50 The Discoverer cover proved to be cumbersome, inviting scrutiny from the scientific community. Discoverer 37, launched 13 January 1962, was the last CORONA mission to bear the Discoverer name. Subsequent CORONA missions were simply classified as "Department of Defense satellite launches".^{[16]: xiii–xiv}

KH-1

The first series of CORONA satellites were the Keyhole 1 (KH-1) satellites based on the Agena-A upper stage, which offered housing and an engine that provided attitude control in orbit. The KH-1 payload included the C (for CORONA) single panoramic camera built by Fairchild Camera and Instrument with a f/5.0 aperture and 61 cm (24 in) focal length. It had a ground resolution of 12.9 m (42 ft). Film was returned from orbit by a single General Electric Satellite Return Vehicle (SRV). The SRV was equipped with a small onboard solid-fuel retro motor to deorbit the payload at the end of the mission. Recovery of the capsule was done in mid-air by a specially equipped aircraft.^[17]

There were three camera-less test launches in the first half of 1959, none of them entirely successful. Discoverer 1 was a test vehicle carrying no SRV nor camera. Launched on 28 February 1959, it was the first man-made object put into a polar orbit, but only sporadically returned telemetry. Discoverer 2 (14 April 1959) carried a recovery capsule for the first time but no camera. The main bus performed well, but the capsule recovery failed, the SRV coming down over Spitzbergen rather than Hawaii. The capsule was never found. Discoverer 3 (3 June 1959), the first Discoverer to carry a biological package (four black mice in this case) failed to achieve orbit when its Agena crashed into the Pacific Ocean.

The pressure to orbit a photographic surveillance satellite to succeed the Lockheed U-2 was so great that operational, camera-equipped KH-1 launches began 25 June 1959 with the (unsuccessful) launching of Discoverer 4, despite there not having been a successful test of the life-support unit for biological passengers. This proved to be a moot point by this time as the link between the Discoverer series and living payloads had been established by the attempted flight of Discoverer 3.^{[8]: 51–54}

The three subsequent Discoverers were successfully orbited, but all of their cameras failed when the film snapped during loading. Ground tests determined that the acetate-based film became brittle in the vacuum of space, something that had not been discovered even in high altitude, low pressure testing. The Eastman Kodak Company was tasked with creating a more resilient replacement. Kodak developed a technique of coating a high-resolution emulsion on a type of polyester from DuPont. Not only was the resulting polyester-based film resistant to vacuum brittling, it weighed half as much as the prior acetate-based film.^[8]: 56

There were four more partially successful and unsuccessful missions in the KH-1 series before Discoverer 13 (10 August 1960), which managed a fully successful capsule recovery for the first time.^[18] This was the first recovery of a man-made object from space, beating the Soviet Korabl Sputnik 2 by nine days. Discoverer 13 is now on display in the "Milestones of Flight" hall in the National Air and Space Museum in Washington, D.C.

Two days after the 18 August 1960 launch of Discoverer 14, its film bucket was successfully retrieved in the Pacific Ocean by a Fairchild C-119 Flying Boxcar transport plane. This was the first successful return of a payload from orbit, occurring just one day before the launch of Korabl-Sputnik 2, a biosatellite that took into orbit the two Soviet space dogs, Belka and Strelka, and safely returned them to Earth.^[19]

The impact of CORONA on American intelligence gathering was tremendous. With the success of Discoverer 14, which returned 16 lb (7.3 kg) of film and provided more coverage of the Soviet Union than all preceding U2 flights, for the first time the United States had a clear picture of the USSR's strategic nuclear capabilities. Before CORONA, the National Intelligence Estimates (NIE) of CIA were highly uncertain and strongly debated. Six months before Discoverer 14, an NIE predicted that the Soviets would have 140–200 ICBMs deployed by 1961. A month after the flight of Discoverer 14, that estimate was refined to just 10–25.^{[8]: 38–39}

Additionally, CORONA increased the pace at which intelligence could be received, with satellites providing monthly coverage from the start. Photographs were more easily assessed by analysts and political leaders than covert agent reports, improving not just the amount of intelligence but its accessibility.^{[8]: 38–39}

The KH-1 series ended with Discoverer 15 (13 September 1960), whose capsule successfully deorbited but sank into the Pacific Ocean and was not recovered.^[20]

Later KH Series

In 1963, the KH-4 system was introduced with dual cameras and the program made completely secret by then president, John Kennedy. The Discoverer label was dropped and all launches became classified. Because of the increased satellite mass, the basic Thor-Agena vehicle’s capabilities were augmented by the addition of three Castor solid-fueled strap-on motors. On 28 February 1963, the first Thrust Augmented Thor lifted from Vandenberg Air Force Base at Launch Complex 75 carrying the first KH-4 satellite. The launch of the new and unproven booster went awry as one SRB failed to ignite. Eventually the dead weight of the strap-on motor dragged the Thor off its flight path, leading to a Range Safety destruct. It was suspected that a technician had not attached an umbilical on the SRB properly. Although some failures continued to occur during the next few years, the reliability rate of the program significantly improved with KH-4.^[21][22] Maneuvering rockets were also added to the satellite beginning in 1963. These were different from the attitude stabilizing thrusters which had been incorporated from the beginning of the program. CORONA orbited in very low orbits to enhance resolution of its camera system. But at perigee (the lowest point in the orbit), CORONA endured drag from the atmosphere of Earth. In time, this could cause its orbit to decay and force the satellite to re-enter the atmosphere prematurely. The new maneuvering rockets were designed to boost CORONA into a higher orbit, and lengthen the mission time even if low perigees were used.^[23] For use during unexpected crises, the National Reconnaissance Office (NRO) kept a CORONA in "R-7" status, meaning ready for launch in seven days. By the summer of 1965, NRO was able to maintain CORONA for launch within one day.^[24]

Nine of the KH-4A and KH-4B missions included ELINT subsatellites, which were launched into a higher orbit.^[25][26]

Some P-11 reconnaissance satellites were launched from KH-4A.^[27]

At least two launches of Discoverer were used to test satellites for the Missile Defense Alarm System (MIDAS), an early missile-launch-detection program that used infrared cameras to detect the heat signature of launch vehicles launching to orbit.^[28]

The last launch under the Discoverer cover name was Discoverer 38 on 26 February 1962. Its bucket was successfully recovered in midair during the 65th orbit (the 13th recovery of a bucket; the ninth one in midair).^[29] Following this last use of the Discoverer name, the remaining launches of CORONA satellites were entirely TOP SECRET. The last CORONA launch was on 25 May 1972. The project ended when CORONA was replaced by the KH-9 Hexagon program.^[30]

Cameras

The CORONA satellites used special 70 mm film with a 24 in (610 mm) focal length camera.^[31] Manufactured by Eastman Kodak, the film was initially 0.0003 in (7.6 μm) thick, with a resolution of 170 lines per mm (0.04 inch) of film.^[32][33] The contrast was 2-to-1.^[32] (By comparison, the best aerial photography film produced in World War II could produce just 50 lines per mm (1250 per inch) of film).^[32] The acetate-based film was later replaced with a polyester-based film stock that was more durable in Earth orbit.^[34] The amount of film carried by the satellites varied over time. Initially, each satellite carried 8,000 ft (2,400 m) of film for each camera, for a total of 16,000 ft (4,900 m) of film.^[32] But a reduction in the thickness of the film stock allowed more film to be carried.^[34] In the fifth generation, the amount of film carried was doubled to 16,000 ft (4,900 m) of film for each camera for a total of 32,000 ft (9,800 m) of film. This was accomplished by a reduction in film thickness and with additional film capsules.^[35] Most of the film shot was black and white. Infrared film was used on mission 1104, and color film on missions 1105 and 1008. Color film proved to have lower resolution, and so was never used again.^[36]

The cameras were manufactured by the Itek Corporation.^[37] A 12 in (30 cm), f/5 triplet lens was designed for the cameras.^[38] Each lens was 7 in (18 cm) in diameter.^[32] They were quite similar to the Tessar lenses developed in Germany by Carl Zeiss AG.^[39] The cameras themselves were initially 5 ft (1.5 m) long, but later extended to 9 ft (2.7 m) in length.^[40] Beginning with the KH-4 satellites, these lenses were replaced with Petzval f/3.5 lens.^[36] The lenses were panoramic, and moved through a 70° arc perpendicular to the direction of the orbit.^[32] A panoramic lens was chosen because it could obtain a wider image. Although the best resolution was only obtained in the center of the image, this could be overcome by having the camera sweep automatically ("reciprocate") back and forth across 70° of arc.^[41] The lens on the camera was constantly rotating, to counteract the blurring effect of the satellite moving over the planet.^[36]

The first CORONA satellites had a single camera, but a two-camera system was quickly implemented.^[42] The front camera was tilted 15° aft, and the rear camera tilted 15° forward, so that a stereoscopic image could be obtained.^[32] Later in the program, the satellite employed three cameras.^[42] The third camera was employed to take "index" photographs of the objects being stereographically filmed.^[43] The J-3 camera system, first deployed in 1967, placed the camera in a drum. This "rotator camera" (or drum) moved back and forth, eliminating the need to move the camera itself on a reciprocating mechanism.^[44] The drum permitted the use of up to two filters and as many as four different exposure slits, greatly improving the variability of images that CORONA could take.^[45] The first cameras could resolve images on the ground down to 40 ft (12 m) in diameter. Improvements in the imaging system were rapid, and the KH-3 missions could see objects 10 ft (3.0 m) in diameter. Later missions would be able to resolve objects just 5 ft (1.5 m) in diameter.^[46] 3 ft (0.91 m) resolution was found to be the optimum resolution for quality of image and field of view.^{[citation needed]}

The initial CORONA missions suffered from mysterious border fogging and bright streaks which appeared irregularly on the returned film. Eventually, a team of scientists and engineers from the project and from academia (among them Luis Alvarez, Sidney Beldner, Malvin Ruderman, Arthur Glines,^[47] and Sidney Drell) determined that electrostatic discharges (called corona discharges) caused by some of the components of the cameras were exposing the film.^[48][49] Corrective measures included better grounding of the components, improved film rollers that did not generate static electricity, improved temperature controls, and a cleaner internal environment.^[49] Although improvements were made to reduce the corona, the final solution was to load the film canisters with a full load of film and then feed the unexposed film through the camera onto the take-up reel with no exposure. This unexposed film was then processed and inspected for corona. If none was found or the corona observed was within acceptable levels, the canisters were certified for use and loaded with fresh film for a launch mission.

Calibration

CORONA satellites were allegedly calibrated using the calibration targets located outside of Casa Grande, Arizona. The targets consisted of concrete arrows located in and to the south of the city, and may have helped to calibrate the cameras of the satellites.^[50][51][52] These claims about the purpose of the targets, perpetuated by online forums and featured in National Geographic and NPR articles, have since been disputed, with aerial photogrammetry proposed as a more likely purpose for them.^[53]

Recovery

Film was retrieved from orbit via a reentry capsule (nicknamed "film bucket"), designed by General Electric, which separated from the satellite and fell to Earth.^[54] After the fierce heat of reentry was over, the heat shield surrounding the vehicle was jettisoned at 60,000 ft (18 km) and parachutes deployed.^[55] The capsule was intended to be caught in mid-air by a passing airplane^[56] towing an airborne claw which would then winch it aboard, or it could land at sea.^[57] A salt plug in the base would dissolve after two days, allowing the capsule to sink if it was not picked up by the United States Navy.^[58] After Reuters reported on a reentry vehicle's accidental landing and discovery by Venezuelan farmers in mid-1964, capsules were no longer labeled "SECRET" but offered a reward in eight languages for aerial footage return to the United States.^[59] Beginning with flight number 69, a two-capsule system was employed.^[48] This also allowed the satellite to go into passive (or "zombie") mode, shutting down for as many as 21 days before taking images again.^[35] Beginning in 1963, another improvement was "Lifeboat", a battery-powered system that allowed for ejection and recovery of the capsule in case power failed.^[21][60] The film was processed at Eastman Kodak's Hawkeye facility in Rochester, New York.^[61]

The CORONA film bucket was later adapted for the KH-7 GAMBIT satellites, which took higher resolution photos.

Launch

CORONA were launched by a Thor-Agena rocket, which used a Thor first stage and an Agena as the second stage of the rocket lifting the CORONA into orbit.

The first satellites in the program orbited at altitudes 100 mi (160 km) above the surface of the Earth, although later missions orbited even lower at 75 mi (121 km).^[36] Originally, CORONA satellites were designed to spin along their main axis so that the satellite would remain stable. Cameras would take photographs only when pointed at the Earth. The Itek camera company, however, proposed to stabilize the satellite along all three axes—keeping the cameras permanently pointed at the earth.^[39] Beginning with the KH-3 version of the satellite, a horizon camera took images of several key stars.^[43] A sensor used the satellite's side thruster rockets to align the rocket with these "index stars", so that it was correctly aligned with the Earth and the cameras pointed in the right direction.^[62] Beginning in 1967, two horizon cameras were used. This system was known as the Dual Improved Stellar Index Camera (DISIC).^[45]