On the nights immediately following the mysterious Tunguska event, which destroyed a tract of Siberian wilderness in 1908, eyewitnesses reported seeing noctilucent clouds -- brilliant night-visible clouds made up of ice particles that form rarely and only at very high altitudes. Recently, noctilucent clouds have been observed on the nights following launches of rockets that release large amounts of water vapor into the upper atmosphere. This shows that it was a comet impact and not the impact of an asteroid that caused the destruction in Siberia.
The fact that noctilucent clouds are made of ice particles in the upper atmosphere was only recently discovered is an assumption required by the argument.