Letter: Cold meteorites
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When a natural object enters the atmosphere from space, its minimum velocity is about mach 40. Friction with the air causes the surface to melt and, as above Greenland, a bright fireball is produced. The melt on the surface is carried into the atmosphere and takes the heat with it, and the inside keeps the cold of space.
If the object breaks up or is small, it decelerates, the fireball goes out and it falls to Earth fairly gently, under gravity. It would make a shallow pit in soil or ice. If the object is large - over about 100 tonnes - and stays intact, it is hardly braked by the atmosphere and may strike the surface at hypersonic velocity to produce an explosion crater. In this case the meteorite is essentially destroyed.
ROBERT HUTCHISON.
Hemel Hempstead, Hertfordshire
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