Scientists have developed a clearer picture of what happens as rocks from space enter Earth’s atmosphere and survive to reach the ground. By analyzing video recordings and photographs of 75 meteorite falls, researchers identified seven stages that describe how these objects become meteorites. Their findings show that melting and fracturing play a much larger role than previously thought in determining how much mass a rock loses, how quickly it slows, and where its fragments land. The results were published in the journal Meteoritics & Planetary Science.
“We thought solid rocks would be vaporized by the tremendous heat and bright light produced by midair collisions,” says meteor astronomer and lead author Dr. Peter Jenniskens of the SETI Institute and NASA Ames Research Center. “Instead, we found that first melting and then fragmentation control the loss of rock mass.”
The researchers determined that rocks entering from space pass through seven distinct stages as they travel through the atmosphere. Different physical processes dominate each stage, influencing the rock’s appearance, behavior and eventual fate.
How space rocks become bright meteor fireballs
The journey begins in Phase 1, high above Earth. At that altitude, the atmosphere becomes dense enough for a fast-moving rock to create a shock wave in front of it. Collisions with air molecules heat both the rock’s surface and the surrounding gas, causing them to glow. The resulting streaks of light are commonly called meteors or “shooting stars.”
In Phase 2, the meteor travels through increasingly dense air and grows brighter. Some meteors repeatedly change brightness, revealing how quickly they rotate. Among the fastest-rotating objects in the study, a complete rotation took between 0.5 and 5 seconds.
This study investigated one of the recorded meteorite falls: a fireball produced by asteroid 2023 CX1 over Normandy, France, on February 13, 2023. The meteorite, known as Saint-Pierre-le-Viget, reached Earth that day and was later recovered. Credit: Image courtesy of G. de Reijke.
During Phase 3, the meteor becomes brighter still and develops into a fireball. Melting becomes the primary process removing material from the rock. A powerful air current strips molten material from the surface, forming droplets that continue to evaporate after separating from the main body.
Eric Stern, formerly of NASA Ames and now principal investigator at Hyperspace Technologies, said, “In the laboratory, we cannot generate the amount of radiation that occurs at such rates during natural atmospheric entry,” adding, “The rock could actively fragment and erode, but in that case we would not expect the systematic changes in the brightness of the fireball that we observed.”
The bolide enters Phase 4 at an altitude of about 60 kilometers, or about 40 miles, when the melting process reaches a relatively stable balance. During this stage, the fireball’s brightness remains approximately constant or increases at a constant rate. The researchers say that melting alone could eventually remove as much as 40 percent of the rock’s original mass.
Why meteors break apart as they approach Earth
As the fireball descends into denser layers of the atmosphere, rising pressure causes the rock to begin breaking apart. This marks the start of Phase 5, when pieces separate from the main body and can produce repeated bursts of light.
One particularly surprising finding was how early this separation began. The researchers found that fractures can form when the air pressure acting on the front of the rock reaches about one-fifth of the intensity measured in meteorites recovered on Earth. They suggest that heating during atmospheric entry, combined with weakening from cracks created by earlier collisions in space, could explain this unexpected behavior.
At this stage, the remaining rock begins to contract rapidly and undergoes significant deceleration. The more extensive the fragmentation, the faster the rock loses speed.
“We were able to connect fragmentation-induced slowing with previous mathematical descriptions based on ablation,” said co-author Stu Pilorz of the SETI Institute.
The researchers also found that the shape of the remaining rock can influence where small fragments land. If the rear of the rock remains intact, a decompression area forms in its wake, drawing small debris backward.
“Our modeling shows that as long as the back surface of a space rock remains intact, it tends to draw a subsequent vacuum and debris flows into it,” says co-author Darrell Robertson of NASA’s Ames Research Center. “Those small meteorites fall in a narrow strip of the ground.”
The final explosion and journey to the surface
Phase 6 begins when the rear portion of the main rock finally fractures. This last major rupture produces a bright flare and scatters debris at a greater relative velocity. Because the meteor has already slowed substantially, these final flashes generally appear red, unlike the bright green color often seen earlier in the descent.
“That final disruption causes debris to fly off at a higher relative velocity,” Jenniskens said. “We noticed that in past falls, meteorites that weighed more than about 20 grams tended to be scattered over a wide area, and many of them came close to the surface of the original space rock, which would have been the back side.”
The debris that eventually survives to reach Earth’s surface begins as relatively large pieces. However, it continues to melt and fragment while losing speed before landing.
During Phase 7, these processes continue until the remaining debris slows enough to stop producing visible light. When melting ends, a thin outer coating known as fusion crust remains on the surface. The blackened debris then completes its descent, with atmospheric winds carrying it away from its original trajectory, where it may fall as a meteorite.
What meteorite falls reveal about dangerous asteroids
The researchers examined 75 recorded meteorite falls involving several different types of space rocks. By comparing their behavior, they were able to determine the altitudes at which different materials enter each of the seven stages.
These observations do more than explain how small meteorites reach the ground. They also offer clues about the behavior of much larger asteroids, including objects ranging in size from cars to entire city blocks that can produce powerful explosions in the atmosphere.
By measuring how different solid rocks slow down and break apart during atmospheric entry, scientists may better understand the processes governing potentially dangerous asteroid airbursts.
“Asteroids that reach tens of meters in size also tend to rotate faster than larger debris-pile asteroids, so they are also solid rocks,” Jenniskens said. “The 20-meter-diameter asteroid that caused an atmospheric explosion over Chelyabinsk, Russia, in 2013 went through the same stage.”
Source: www.sciencedaily.com
