When a space rock plunges into Earth’s atmosphere, it creates a brilliant streak of light before some pieces may eventually reach the ground as meteorites.
Scientists have now discovered that this dramatic journey is more complicated than previously thought.
Researchers studied 75 meteorite falls that had been recorded on video or in photographs and identified seven distinct stages that space rocks go through as they descend.
The findings, published in Meteoritics & Planetary Science, suggest that melting and breaking apart are the main ways these rocks lose mass—not simply evaporating or “burning up.”
“We used to think that solid rocks would evaporate from the enormous heat and brilliant light generated in collisions with air,” said lead author Dr. Peter Jenniskens of the SETI Institute and NASA Ames Research Center.
Instead, the researchers found that melting begins first, followed by increasingly important fragmentation.
The first stage begins high in the atmosphere. As the rock races through the thin air, it creates a shock wave. Collisions with air molecules heat both the rock and surrounding gas until they glow, producing what we commonly call a meteor or shooting star.
As the rock reaches thicker air, it becomes brighter. Some rocks also spin rapidly, completing a rotation every 0.5 to 5 seconds.
During the third stage, the meteor becomes much brighter and develops into a fireball. At this point, melting becomes the main cause of mass loss. Fast-moving air strips molten material from the rock’s surface, leaving droplets behind.
At around 60 kilometers above Earth, the rock enters a more stable melting stage. Researchers found that a space rock can lose as much as 40% of its mass through melting alone.
Deeper in the atmosphere, increasing air pressure begins to break the rock apart. Surprisingly, this fragmentation can start when the pressure is only about one-fifth of the strength measured in meteorites recovered on Earth. Scientists think previous collisions in space, combined with intense heating during atmospheric entry, may weaken the rocks.
As pieces break away, the main rock slows rapidly. If its rear surface remains intact, it can create a low-pressure wake that draws smaller fragments behind it. This helps explain why meteorites from some falls land in relatively narrow strips.
Eventually, the back of the rock can also break apart, producing a final bright flare and throwing fragments outward. These late flares are often red because the rock has already slowed considerably.
During the seventh and final stage, the remaining pieces continue melting and fragmenting until they become slow enough to stop glowing. Melting then ends, leaving the characteristic thin, dark “fusion crust” found on many meteorites. Winds can carry the fragments away from their original paths before they finally reach the ground.
The findings could also help scientists understand larger and potentially dangerous asteroids. According to the researchers, the roughly 20-meter-wide asteroid that exploded over Chelyabinsk, Russia, in 2013 went through the same basic stages—making small meteorites useful natural laboratories for understanding much bigger space rocks.
