In the late 1980s astronomer Louis Frank noted an large number of mysterious spots appearing in satellite images of Earth's atmosphere. This led to the hypothesis that they might be small comets -- chunks of loose ice about the size of a house that quickly fell apart upon entering the atmosphere. This would explain why they did not persist and were not seen as bursts or lines of light, as are bolides and meteors. For about 15-20 years this idea was referenced in textbooks, science websites, and university courses.
"From the number of atmospheric holes, it’s possible to calculate that the atmosphere is being hit by a 20 to 40-tonne snowball comet every 3 seconds....this is enough to add an inch of water to the Earth’s surface every 20,000 years. Over the 4.5-billion-year lifetime of the Earth they could have provided all the water in the oceans."
New Scientist, Sept 14th, 2002
Sadly, this turns out not to be the case. The 'comets' were just imaging static. We know this because we don't see the correct proportions of argon, krypton, and xenon in the atmosphere, or the expected amount of deuterium in earthly water, we don't see icy bursts of ex-comets splattered across the moon; and perhaps most of all, as our ability to observe near-Earth space has improved, we no longer see thousands of small comets.
BQ24