Meteors Don't Just Burn: Seven Violent Stages Decide Their Fate

Why do the rocks shatter?

Phase 5 starts when thickening air pressure begins to fracture the descending rock, sending pieces off the main body in repeated bursts of light. Here the team found one of the study's most surprising results: rocks can start breaking apart when the air pressure on their leading faces reaches only about one-fifth of the strength measured in meteorites actually recovered on Earth. The researchers suggest that entry heating, combined with cracks from earlier collisions in space, weakens the rocks enough to explain the gap.

During this stage the rock shrinks rapidly and decelerates hard. The more it fragments, the faster it loses speed. Co-author Stu Pilorz of the SETI Institute said: "We were able to tie that slow-down from fragmentation to the previous mathematical descriptions based on ablation" — ablation being the older, evaporation-centered model of mass loss.

The shape of the surviving rock also matters for where fragments land. If the rear section stays intact, the main body drags a zone of reduced pressure behind it, and smaller pieces get sucked along in its wake.

"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."

How does the fall end?

Phase 6 arrives when that rear portion finally breaks apart. The last major disruption produces a bright flare and scatters fragments at higher relative speeds. Because the meteor has already slowed considerably by then, these final flashes tend to look red rather than the green hues often seen earlier in the descent.

"In past falls, we noticed that meteorites larger than about 20 grams tended to be scattered wider, and many came from close to the surface of the original space rock, which must have been its backside," Jenniskens said.

The survivors begin Phase 7 as relatively large fragments that keep melting, breaking and slowing. Once they decelerate enough to stop glowing, melting ceases and a thin dark coating — the fusion crust — settles on their surfaces. Winds can then nudge them off their original trajectories before they land as meteorites.

Why can't labs reproduce this?

The findings rest on observation partly because no laboratory can match the real thing. "In the laboratory, we cannot generate the amount of radiation that occurs in a natural atmospheric entry at those speeds," said Eric Stern, formerly of NASA Ames and now chief scientist at Hyperspace Technologies, Inc. "Rock could aggressively fragment and erode instead, but then we would not expect the observed systematics in how fireballs brighten."

What does this mean for dangerous asteroids?

The study's reach extends beyond small meteorites. The same seven-phase framework, the researchers argue, applies to car-sized and city-block-sized asteroids that explode in the atmosphere — events known as airbursts.

"Asteroids up to tens of meters in size are also solid rocks because they tend to spin faster than do the larger rubble pile asteroids," Jenniskens said. "The 20-meter diameter asteroid that caused the airburst over Chelyabinsk, Russia, in 2013 went through the same phases."

Measuring how different solid rocks slow down and fragment on entry, the team concludes, gives scientists a better handle on the physics governing those potentially dangerous explosions — and a clearer picture of why meteorites land where they do.

The findings are preliminary in the sense that they describe observed patterns across 75 falls rather than controlled experiments, and the authors note that heating and pre-existing space cracks remain a proposed explanation for the early fragmentation, not a directly measured one.