For decades, the common assumption was that space rocks hurtling into Earth's atmosphere simply evaporated in the heat and light of atmospheric entry. A new study of 75 documented meteorite falls finds that is largely wrong: melting followed by violent fragmentation, not evaporation, controls how much mass a meteor loses, how fast it slows, and where its surviving pieces land.
"We used to think that solid rocks would evaporate from the enormous heat and brilliant light generated in the air collision," said Dr. Peter Jenniskens, a meteor astronomer at the SETI Institute and NASA Ames Research Center who led the study, published in Meteoritics & Planetary Science. "We found instead that first melting and then fragmentation controls how a rock loses mass."
Seven Stages, From Streak to Splash
By analyzing video and photographs of the falls, the team identified seven distinct phases a space rock passes through on its way to Earth's surface. The streak-of-light phase begins high in the atmosphere as a shock wave forms; by the third phase, the object becomes a true fireball, and melting takes over as the dominant process stripping away material — researchers found melting alone can remove up to 40% of a rock's original mass by the time it stabilizes around 60 kilometers of altitude.
Fragmentation then begins earlier than expected, the team found, when air pressure on the rock's leading surface reaches only about a fifth of the strength measured in meteorites that are later recovered on the ground — a sign that heating and pre-existing cracks from earlier collisions in space weaken rocks far more than previously assumed. "In the laboratory, we cannot generate the amount of radiation that occurs in a natural atmospheric entry at those speeds," said co-author Eric Stern, now chief scientist at Hyperspace Technologies.
Where the Pieces Fall
The researchers also found that a rock's shape after breaking apart can determine where its smaller fragments land. If the rear of the main body stays intact, it pulls a low-pressure wake behind it that draws trailing fragments along a narrow strip of ground — a detail co-author Darrel Robertson of NASA Ames said helps explain why meteorite debris fields are often long and thin rather than scattered randomly. The final stage produces a bright flare, typically tinged red because the object has already slowed considerably by that point, before the remaining fragments complete their fall.