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This interesting, archived page https://www.webcitation.org/618QHms8h?url=http://www.fai.org/astronautics/100km.asp which I found in this answer, says:

Later in the same decade (or very early in the next; Soviet information at the time was very scanty) the Soviet Union put in orbit an unmanned satellite, in very low orbit, whose attitude was controlled by aerodynamic forces. The real reason of such an experiment is not yet known. It is known however that it successfully described a few orbits just above the 100 km line (how much higher I do not know), but collapsed rapidly shortly after he crossed, or got too much close to, the 100 km. Karman line.

Question: Did this really happen?


The question Have spacecraft ever dipped below the Karman line and then safely continued spaceflight? cites earlier proposals by Johnathan McDowell that the height of "orbit" be lowered from 100km to 80km, and says:

The item in Science Alert's A Harvard Astrophysicist Says Outer Space Is Actually Closer Than We Think (see also Science; Outer space may have just gotten a bit closer) talks about the recent Acta Astronautica article by Jonathan McDowell The edge of space: Revisiting the Karman Line, and says:

So back to McDowell. He chose for his proposed boundary the 80-kilometre mark, just below the mesopause - the boundary between the lower mesosphere and the upper thermosphere, and the coldest point in the Earth's atmosphere.

And this is because of the satellites. McDowell analysed over 90 million points of orbital data from 43,000 satellites dating back to 1957, using archives maintained by the North American Air Defence Command.

Most of the satellites fly pretty high, but he identified 50 that flew below the 100-kilometre mark, down as low as the 80-kilometre mark, over two or more complete revolutions of Earth.

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  • $\begingroup$ Reading such a news, I wouldn't close out that it had been in fact an unsuccessful launch. Like the space probes "flying 4000 km beside the Moon" $\endgroup$ – peterh Oct 17 '18 at 19:49
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I found a few references to the Zenit 4 M/MK/MKM series of reconnaissance satellites, which may have been equipped with stabilizers to help them maintain attitude in tenuous atmosphere, but the perigee appears to have been around 170km, not 100km. The motivation for the low perigee was to minimize distance to photographic targets.

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http://www.svengrahn.pp.se/histind/Recces/third.htm

https://space.skyrocket.de/doc_sdat/zenit-4m.htm

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    $\begingroup$ Gunter's space page(s) list over sixty separate launches of these spacecraft, so it seems quite possible that one of them explored orbits below 170km, where "explored" may involve a variable amount of intent. $\endgroup$ – uhoh Oct 17 '18 at 21:24

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