In the Cold War era one of the biggest problems for the US and the West, in general, was just not knowing what was going on in the Soviet Union and to a similar extent, Communist China. Not for nothing did Churchill say that an Iron Curtain has descended across Europe.
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There had also been other aerial surveillance methods like project gen tricks, which used helium balloons to carry cameras at heights of up to a 100,000 feet and blown by the westerly winds across the Soviet Union China. But only around 6% of these were recovered with usable images, the rest were either shot down or blown off-course. Clearly a better safer method was needed and quickly.
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The problem with putting a camera in space was up until then no one had actually launched something into orbit and then safely recovered it back to earth. Now you may well ask why didn't they just use video cameras and beam the images back but that kind of technology just wasn't ready and it wouldn't be until the late 1970s almost 20 years later before high-resolution digital imagery would be good enough for intelligence gathering. So the idea they came up with was to drop the exposed film from orbit in a heat shield his bucket back to earth over the Pacific Ocean and then catch its parachute over planes at about 15,000 feet.
The idea of taking pictures from space and then getting them back was one thing but in the late 1950s just getting the newly developed Thor-Agena rocket safely off the launch pad was another. It took 12 attempts before on August the 10th 1960 Discoverer 13 became the first man-made object to be safely recovered from space nine days before the Soviets did the same with the Korabl Sputnik 2. After the testing period which lasted up until Discoverer 39, the program's name reverting back to Corona and it was classified as top secret and remained that way until 1992.
Two cameras enabled stereographic imaging to be done allowing the image technicians to better gauge the depth and size of objects seen on the ground. Instead of taking just simple snapshots with the cameras looking straight down from orbit, the lenses exposed the filmstrip as it moved through a 70 degree arc. This moving lens was to avoid the movement blur caused by the speed of a satellite and to get an almost continuous image strip of the ground below making the maximum use of a film available. To stop the torque reaction of the lens as it returned to the starting position from upsetting the satellite's orientation, though lower heavier part of the lens on later models continuously rotated to act as a counterbalance.
In order to accurately judge the size of objects a set of concrete calibration targets were created on the ground around Casa Grande in Arizona that could be easily seen from space. Each one was a shape of a maltese cross and about 18 meters in diameter. 256 of them were placed exactly one mile apart or 1.65 kilometers in a 16/16 mile grid. Although they were abandoned in 1972 when the program came to an end, some 143 of them were still in position as of 2018.
At the beginning of the Corona program, the best resolution that could be seen was around 7 meters but with continual updates and improvements in both the film and the cameras by the program's end, it was down to 1.5 meters. The amount of film also increased as new thinner polyester based films were developed that were also much more tolerant of harsh conditions in space. Something which had plagued the earlier acetate films with breakages.
If a plane missed the film bucket or for some reason, it was off course and landed in the sea, it was fitted with a salt plug which would dissolve after two days and sink the bucket rather let it float around in the sea and possibly be captured by a foreign power. But Corona was more than just a spy satellite, it became a testbed for some of the key technologies that will be used in programs like Gemini, Mercury and Apollo, from the re-entry of the Earth's atmosphere at a specific point to splashdown and recovery from the sea at a predefined area in the ocean.