Speed and energy on entry

A meteoroid arrives with enormous speed and kinetic energy. In this lesson you will calculate entry speeds and trace where all that energy goes.

Entry speeds

Meteoroids enter the atmosphere at between about 11 and 72 km/s. The lowest speed is set by Earth's own gravity: anything falling in from far away is speeded up to at least 11 km/s. The highest comes from a meteoroid meeting Earth head on, adding Earth's orbital speed of about 30 km/s to its own speed around the Sun.

Shower meteors from comets are often the fastest. Leonid meteoroids, for example, meet Earth head on at about 71 km/s. Meteorites that reach the ground usually started out slower, at under about 25 km/s, because slower bodies are heated less fiercely.

Where the energy goes

A moving meteoroid has a large store of kinetic energy. As it ploughs into the air it compresses the gas in front of it, which heats it to thousands of degrees. Energy is transferred from the kinetic store to the thermal store of the air and the rock, and away by light and sound. The glowing trail is a meteor; a very bright one is a fireball.

The heated surface melts and is swept away, a process called ablation. Most of the meteoroid's mass is lost this way. Typically only a small fraction of the original body survives as meteorites.

Hot outside, cold inside

In the particle model, heating makes the particles in the outer layer vibrate so strongly that the solid melts. But rock conducts heat poorly, and the hot phase lasts only a few seconds, so the heat does not have time to travel inwards. The melted layer is constantly stripped away, carrying its heat with it. When the meteorite slows and stops glowing, the last molten film freezes into a fusion crust under a millimetre thick, while the interior keeps the low temperature it had in space. Fresh meteorites picked up immediately have been described as cool or even frosty, not hot.

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