If an actual extraterrestrial spacecraft had crashed or landed on the Earth, which I doubt because I do not believe in UFO's. I highly doubt that more than one alien species from different solar systems/galaxies would find the Earth as UFO sitings have reported saucer shaped (some with and others without lights), cigar shaped or many other shaped "alien craft." Humans have produced, regardless of nation, very similar shaped aircraft and spacecraft (except for the Space Shuttle of course) so that's why I doubt alien craft have ever come to Earth. In any case, if one or even two extraterrestrial intelligent life were to come to Earth and view our primitive and hostile life (wars betwen nations, etc.) I doubt they would attempt to land on the Earth. As far as reverse enginering their spacecraft, their technology would probably be so far advanced than ours that we would not be able to comprehend the technology (as seen in several Star Trek TV show episodes especially when the Starship Enterprise went back in time to 20th century Earth, about 300 years from today. It would be like sending a smart phone or many modern devices back in time to the 18th century. Simply impossible to reverse engineer a device from 300 years in the future.
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