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.
In World War II the Allies relied upon airborne reconnaissance to see what the Germans were up to and many of the secret weapons like the V2 rocket and a V1 flying bomb were uncovered by aerial photos. But Western Europe is small in comparison to the Soviet Union. You could drop it in the middle of a Soviet Union and not know it was there at all. Although the U.S. started aerial reconnaissance along the Soviet borders in 1946, it was the start of a Korean War in 1950 which brought home the need for more information on the Soviet Air Force and its capabilities and if it could mount a surprise bomber attack with nuclear weapons on the US. High-altitude overflights were gradually built up first with a Boeing B-47, a predecessor to the B-52 and later with the Lockheed U-2 spy plane.
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.
In 1960 Gary Powers U-2 spy plane was shot down over the Soviet Union causing a major diplomatic incident and forcing the U.S. to suspend overflights but the problem had also been anticipated. The CIA who also ran the U-2 spy planes headed up a project called Corona and have been working on put in a camera into a satellite in a low Earth orbit 160 kilometers above the earth. There it would be safe from any Soviet defenses and of a speed at which it traveled some 27,000 km/h they could image huge tracts of land in a very short space of time.
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.
Now it might sound like a crazy idea but catching it with a plane was actually the easy part and they've done it before with the Genetrix surveillance balloons, the difficult part was getting the film bucket to be in the same area as the waiting planes. To keep the program secret and stop people from asking too many questions about the number of test flights from a Vandenberg Air Force Base it was initially called Discoverer, the cover story being that the satellites were carrying small animals into orbit for research and then being dropped back to earth to see how they were affected by the launch and being in space but the only things we were really carrying were cameras.
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.
Unlike the satellites of today which stay in orbit for years, the corona ones were only intended to be there for maybe a few weeks most. Once the film has been exposed and returned the rest of a satellite was no longer needed and they couldn't refill it. So it became the world's most expensive disposable camera system. Each corona satellite used to panoramic cameras each with 610-millimeter focal length lenses and they used 70-millimeter film that had a resolution of 170 lines per millimeter, twice that of the best film used for World War II reconnaissance.
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.
The lenses themselves were made from the finest materials and at a time were the most perfectly ground lenses ever made. The satellites operated in a nearly polar orbit meaning they traveled almost north to south where the orbit offset just enough so that it would move a few degrees further around the globe with each orbit.
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.
By the end of the program, each satellite had two separate film buckets each containing up to 4900 meters of the film allowing one to be dropped off whilst the other was still in use. Once the film had been exposed and the mission objectives had been covered it will be ejected from a satellite protected by a detachable heat shield at around 60,000 feet. A drogue parachute was deployed before then the main chutes carried it down to around 15,000 feet where it will be captured by planes trailing an airborne claw which when winched the bucket onboard the plane. This method of airborne recovery became so successful that it continued to be used on subsequent reconnaissance systems well into the late 1980s and the Chinese were still using a similar system for their spy satellites up until the 2000s.
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.
By 1972 Corona had done a 167 successful recoveries and photograph over 920 million square kilometers of land. The photos, the program took affected every major US overseas military policy of the 1960s and beyond and stopped much of the overreaction but had caused much of mistrust between the U.S. and the Soviets in the 1950s. Satellite reconnaissance became the cornerstone of nuclear disarmament treaties for both sides with chopped-up bombers left in the open for the other side to see from space. After it was Declassified in 1992 its archives revealed much more about the natural world and our ancient history that had been seen from the air before and even now they are still used to see the effect that we have had on the world over the decades since these photos were taken.