Meteor entry and fusion crust

A meteoroid hits the atmosphere at many kilometres per second, far faster than sound. The air in front of it cannot get out of the way; it is squeezed and heated intensely, so the rock and the air around it glow as a meteor, or as a fireball if it is very bright.

The heat melts the rock's outer surface. The molten layer is so runny that it is swept off almost as soon as it forms, and fresh rock melts beneath it. This loss of surface material is called ablation. Corners and points go first, so surviving meteorites are often rounded. Most of the original mass never reaches the ground.

When the rock has slowed enough that melting stops, the last film of melt cools into a thin, glassy, usually dark coating, the fusion crust. On stony meteorites it is seldom more than 1 or 2 millimetres thick. Because the fiery part of the fall lasts only seconds, the heat has little time to soak into the interior.

Many meteoroids break apart under the strain. The pieces then land scattered over an oval area called a strewn field, with the heaviest fragments, which keep their speed best, usually carried farthest along the path.

Some objects meet the air at such a shallow angle that they skim through the upper atmosphere and leave it again, like a stone skipping across a pond.

See it in action

Premieres 19 NovHow the Solar System Was Born4,567 million years ago · Allende, Mexico, 1969Coming soonThe Night It Rained Stars on America1833 · Eastern United States

Coming up in future stories

• The Meteorite That Hit a $300 Chevy 1992 · Peekskill, New York
• The Meteorite They Chained Up in Church 1492 · Ensisheim, Alsace
• The Space Rock Argentina Refused to Lend c. 2500 BC / 2012 · Campo del Cielo, Chaco, Argentina
• The Space Rock That Bounced Off Earth 1972 · Utah to Alberta, North America

Sources

• Fusion crust – Washington University in St. Louis, Meteorite site