Researchers have identified seven distinct stages that describe how meteorites transform as they fall through Earth's atmosphere, revealing that melting and fragmentation play far larger roles than previously thought in determining mass loss, deceleration, and landing sites.
Dr. Peter Jenniskens of the SETI Institute and NASA Ames Research Center led the study, which examined video recordings and photographs of 75 meteorite falls. The findings appear in Meteoritics & Planetary Science.
"We used to think that solid rocks would evaporate from the enormous heat and brilliant light generated in the air collision," Jenniskens said. "We found instead that first melting and then fragmentation controls how a rock loses mass."
The seven phases begin high in the atmosphere when the rock generates a shock wave (Phase 1) and intensifies in brightness (Phase 2). By Phase 3, the rock becomes a fireball with melting as the dominant mass-removal process. Phase 4 occurs around 60 kilometers altitude, where melting reaches a stable state and can remove as much as 40 percent of the original rock's mass.
Fragmentation begins in Phase 5 when increasing air pressure fractures the rock. Researchers found that rocks fragment when air pressure reaches only about one-fifth of the strength measured in recovered meteorites, likely because heating and previous collision damage weaken them.
"Our modeling shows that as long as the back of the space rock remains intact, that rock pulls a vacuum in its wake into which fragments tend to flow," said co-author Darrel Robertson of NASA Ames. "Those small meteorites fall in a narrow strip on the ground."
Phase 6 occurs when the rear portion breaks apart, producing bright flares and scattering fragments at higher speeds. Phase 7 continues until pieces slow enough to stop producing visible light and develop a fusion crust.
Jenniskels noted that solid asteroids up to tens of meters undergo these same processes, offering insights into larger, potentially dangerous asteroid airbursts like the 20-meter object that produced an airburst over Chelyabinsk, Russia, in 2013.