No snow show

CONTRARY to a once popular theory, the Earth is not being pelted with snowballs the size of houses. Stephen Knowles and his colleagues at the Naval Research Laboratory in Washington DC scanned the skies with the Naval Space Surveillance System’s powerful radar to search for mini-comets thought to be dumping water into the atmosphere (“Not a snowball’s chance…”, 12 July 1997, p 24). The researchers found none, although they would have expected to see between 800 and 5000 if the snowball theory were correct (Journal of Geophysical Research, vol 104, p 12 367). The researchers are confident that the radar would have detected any comets approaching low Earth orbit. “We would have been able to see these things,
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