Scientists Trace the Seven Stages of a Meteorite's Fiery Fall

By studying 75 meteorites that fell to Earth, scientists have identified seven stages that a space rock passes through as it plunges through the atmosphere. Each stage is shaped by different physical processes. The findings, published in a paper on the Wiley Online Library, show that more than simple evaporation determines how fragments of asteroids and comets lose mass and speed before slamming into the ground as meteorites.

"We used to think that solid rocks would evaporate from the enormous heat and brilliant light generated in the air collision," said team leader Peter Jenniskens of the SETI Institute and NASA Ames Research Center. "We found instead that first melting and then fragmentation control how a rock loses mass."

The rock first becomes a "shooting star" high in the atmosphere as a shock wave heats it. It then brightens into a fireball, where it loses the most mass through melting as fast-moving air pulls away droplets of melted material. By about 40 miles (60 kilometers) above the surface, the rock may have already lost up to 40% of its original mass. It then begins to fragment, often earlier than expected because of cracks left over from collisions in space, before a final burst of breakup sends fragments flying at higher speeds. "That final disruption sends fragments flying at higher relative speeds," Jenniskens said, noting that meteorites larger than about 20 grams tend to scatter more widely. In the last stage, melting and fragmentation stop as the remaining fragment slows enough to stop glowing, leaving a thin crust on its surface.

Meteorites that reach the ground are the ones that underwent more melting and fragmentation, which slows them down enough to land rather than burn up entirely. The resulting fragments are often small and slow enough for Earth's winds to blow them off course. Understanding these processes could help scientists predict what would happen if a larger, hazardous asteroid entered the atmosphere, the researchers said, including the 20-meter-wide asteroid that caused an airburst over Chelyabinsk, Russia, in 2013, which Jenniskens said went through the same phases.