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Types of meteorites When found meteorites often have a fusion crust, then a glassy surface layer. The surface is usually partly ablated away by the frictional heating due to passage through the atmosphere, the fusion crust being the remains of this melted material. The heat causes a heat pulse as well as vaporization. One can estimate the effect by seeing how much heat one gets by conversion of the Kinetic energy to heat. The maximum encounter velocity v max is given by (see section on cometary encounters): v max 2 = (2.41 v 0 ) 2 + v e 2 = c. 73UP>2 = c. 73.28 km s -1 where v 0 is the Earth's orbital velocity and v e is the Earth's escape velocity. Most bodies actually enter with a smaller relative velocity - around 40 km s -1 . At v max the kinetic energy density is 5.2 10 9 J kg -1 . The average energy needed to melt rock is about 1.2 10 6 J kg -1 , while the energy to crush a rock is only about 10 9 J kg -1 . The fastest ones therefore tend not to survive. But "typical" 30 km s -1 arrivals at a low angle of entry can survive
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