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An illustration of a space rock rocketing to Earth; when does it become a meteorite?. | Credit: Robert Lea (created with Canva)
Studying 75 meteorites falling to Earth, scientists have identified the seven stages a space rock goes through as it crashes through our atmosphere, each of which is determined by different physical processes.
The research found that there are more factors than mere evaporation that determine how fragments of asteroids and comets lose mass and speed passing through the atmosphere before slamming into the ground as meteorites. As well as just "burning up" in the atmosphere, space rocks also go through periods of melting and fragmentation that occur at different altitudes.
Aside from shining light on the life cycles of meteorites, the team's research could help us better understand what would happen if a large, potentially hazardous asteroid were to enter Earth's atmosphere.
"We used to think that solid rocks would evaporate from the enormous heat and brilliant light generated in the air collision," team leader Peter Jenniskens of the SETI Institute and NASA Ames Research Center said in a statement. "We found instead that first melting and then fragmentation control how a rock loses mass."
Seven stages to Earthfall
The first stage, phase 1, identified by the team, begins high in
Earth's atmosphere. The falling space rock creates a shock wave in front of it that heats it and the gas surrounding it. This turns the falling space rock into a "
shooting star" streaking across the sky.
Phase 2 begins when the fragment reaches denser air, and the meteor begins to brighten. At this stage, astronomers can determine how the rock is spinning based upon changes in brightness.
This brightening continues until phase 3, at which point the space rock becomes a
fireball. The team found that it is at this stage that the falling rock loses the most mass via melting. This happens as air flows rapidly around the rock and pulls away melted matter, leaving a trail of droplets that continue to evaporate.
An illustration of a fireball streaking across the sky. | Credit: Shutterstock
At around 40 miles (60 kilometers) over the surface of Earth phase 4 begins, which sees the fireball reaching a melting equilibrium. By this point, the initial space rock may have lost up to 40% of its original mass.
Phase 5 marks the point at which the rock starts to fragment, with individual pieces often flaring as they break away from the main body. The team found that this breaking apart often occurs earlier than expected, putting that down to cracks that may have already existed in the rock due to collisions in space.
What remains of the rock by this stage shrinks rapidly, also slowing significantly, with this slowdown occurring faster if the rock has fragmented more successfully.
"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," team member Darrel Robertson of NASA Ames Research Center said. "Those small meteorites fall in a narrow strip on the ground."
A stony meteorite in NASA Ames Research Center's Arcjet Interaction Heating Facility. | Credit: NASA Ames/SETI Institute, courtesy of Zev Hoover and Ron Dantowitz, Dexter Southfield Schools/MARS Scientific.
The fireball flares for the final time during phase 6, as pieces of the space rock break away more rapidly.
"That final disruption sends fragments flying at higher relative speeds," said Jenniskens. "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."
At the start of phase 7, melting and fragmentation are still occurring, but what is left of the space rock has slowed down enough to stop glowing. Melting then ceases, leaving a thin crust on the surface of the fragment.
The team determined that meteorites that reach terra firma ironically went through more melting and fragmentation, which slowed them down enought to actually land. The fragments are often small and slow enough at this stage to allow Earth's winds to carry them off course and determine where they land as meteorites.
A fragment of the Hillsborough meteorite. | Credit: SETI Institute
Of the 75 meteorites investigated in this research, the team identified several different types of space rock composed of different materials. They were able to determine the altitudes at which these different types of space rock went through each of the seven identified phases.
Identifying the processes that occur to solid objects passing through the atmosphere could help scientists better understand what would happen to larger, more dangerous objects about to hit Earth. This includes objects that cause damage when they erupt as "
airbursts" or potential "
planet-killer" asteroidsthe size of city blocks.
"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," said Jenniskens. "The 20-meter [66 feet] diameter asteroid that caused the airburst over
Chelyabinsk, Russia, in 2013 went through the same phases."
The team's findings appear in a paper published on the
Wiley Online Library.
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