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My character learned that an alien crash-landed on earth some time ago. He has no idea how large the space ship was, but he knows that just one person survived the crash landing (it is possible that it was a one-person-ship in the first place).

He assumes that programs for detection of near-earth objects will detect a crash-landing shuttle eventually. So he starts to wonder how humankind missed the crash landing. What theories may he come up with?

In the end, I want the alien to crash-land long before any programs for detection of near-earth objects starts. How long ago does the landing need to be?

He is working for the government, so he is pretty sure that the government did not know about the alien. A cover-up of the crash landing is no a theory he believes in.

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closed as off-topic by Renan, L.Dutch, Bellerophon, sphennings, Vincent Apr 2 '18 at 14:04

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    $\begingroup$ A "shuttle" is pretty small. Even today it may slip through undetected, depending on its trajectory -- there are not many military radar stations watching central Africa or the South Pacific, for example. (I'm not saying it will slip undetected, only that it might.) Consider for example the sad and tragic history of the Malaysian Airlines flight 370, a Boeing 777, 64 meters long with a wingspan of 60 meters; for comparison, the Space Shuttle was 37 meters long with a wingspan of 24 meters. $\endgroup$ – AlexP Apr 2 '18 at 10:55
  • $\begingroup$ he learned from someone? If he works for the govt and know about the crash, then the govt know about the crash. $\endgroup$ – Vincent Apr 2 '18 at 14:04
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  • When the spacecraft enters the atmosphere, it might get rather hot. So the best date might be before the DSPS were launched, that is before the 1960s.
  • Alternatively, the arrival might correspond with one of the Cold War false alarms. (It turns out that the alarm was not false, but because there was no reasonable explanation it was labeled as such. A tiny little government conspiracy to cover "we don't know what happened.")
  • Another option (not exactly new) would be to take a historical large meteorite event and use the alien craft as an explanation. If your character considers this, he cannot exclude an arrival as late as 2013.
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There are several aspects to this.

  1. Nuclear Test Ban Safeguards

To detect the unauthorized testing of nuclear weapons, there is a global network of monitoring stations. These would have detected and pinpointed a crash site if it was severe enough, along with every meteorite impact of any size. Only really big signals are looked at. If there was some reason to go back and look at the data, it would probably be there. So, by this method, never unless there's something extra.

  1. Astronomy

Probably wouldn't see anything. Even if something was seen, it would be ruled an artefact (technically correctly) as it's not what they're interested in.

  1. The Southern Hemisphere

As we know from assorted missing aircraft and lost expeditions throughout history, the southern hemisphere is big, mostly empty and mostly devoid of monitoring. And if a kakapo saw anything, they're wise enough not to talk about it.

So here's a theory your character might come up with:

If the ship crashed in the southern hemisphere near the ring of fire, nobody would see it, no radar would pick it up, most ground-based detectors would either be confused by the tremors from the ring of fire or programmed to filter any such tremors out, nobody would stumble across the wreckage and even if they did there are only a small number of societies the outside world would take seriously.

If you can be 100% certain that no system has that data (recognized or not), that's one of the few ways it can be done.

If you think a system may have recorded it but left the information unprocessed, you have a broader scope. Eastern Europe is secretive and paranoid, so an explosion is likely to go uninvestigated. It'll be picked up on seismographs, but if it's in an area with illicit fireworks factories or suspected burial sites for excess WW2 munitions, scientists would write it off as an accident and won't look.

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