If a meteorite streaking through Earth’s atmosphere is too thin, it’ll tumble about. Too wide and it will flutter wildly. But if it’s taken on just the right cone shape, it’ll plummet straight to the ground.
Roughly a quarter to a third of meteorites that make it to Earth converge on this “Goldilocks” cone shape and, until recently, scientists haven’t been able to explain why. New findings published last month in the journal PNAS show that the answer has to do with fluid mechanics, the way forces interact with liquids, gasses, and plasmas.
The story starts back in 2012, with a team of researchers from the Courant Institute at New York University holding balls of clay in a water tunnel to study erosion. The team observed that “if you hold an object fixed and let it erode, it makes a cone,” says study author Leif Ristroph, a mathematician at the Courant Institute who was also part of the team that made the initial observations.
More specifically, he says, erosion always sculpted these cones so their tips, which pointed toward the oncoming water, formed a V with an angle of about 90 to 100 degrees. This tip angle happens to be very similar to those of cone-shaped meteorites found on Earth.
A ball of clay stuck in water may not seem to have much in common with a meteorite crashing to Earth, but the forces at work on their surfaces aren’t all that different, Ristroph says. Meteorites entering the Earth’s atmosphere encounter intense heat that melts and reshapes them in a process similar to water eroding clay.
With that in mind, the researches decided to use water to take a closer look to determine if this cone shape might be favorable for meteorite flight: They dropped solid aluminum cones into water and used a video camera to track their motion.
This is where they witnessed the Goldilocks principle at work. Cones with tip angles that were too slim or too wide moved in unstable paths through the water. But cones that hit a sweet spot between 60 and 100 degrees oriented themselves so that their tips pointed down and fell straight—just as a similarly shaped meteorite would fall through air.
The reason for this stability is the way forces from the surrounding fluids—water in the 2012 experiment, and air in the case of meteorites—act on the cones as they fall, Ristroph says. As fluid is displaced by the cones, it flows around them, only to come back together in the cones’ wakes, like the turbulent surf behind a fast-moving boat. While narrow or wide cones get pushed around by these fluid forces, Goldilocks cones are just the right shape to stay stable.
When to team looked back at their data, they realized the same angles kept popping up and theorized that these erosion and stability effects might be related, says study author Jun Zhang, a physicist and mathematician at the Courant Institute and NYU Shanghai.
Erosion leads to a cone shape, which leads to stability, which enables further erosion. “These two mechanisms reinforce each other,” says Zhang.
But there may yet be more to the story.
Meteorites aren’t just made of clay, Maitrayee Bose, an isotope cosmochemist at Arizona State University who was not involved in the study, said in an email. Meteorites can be made of harder materials, like iron and stone, which could respond very differently to erosion stresses when falling through the atmosphere, she says.
And it’s not just what meteorites are made of that needs to be considered.
At supersonic speeds—like those of a falling meteorite—flight dynamics get very complicated, and surface pressure and temperatures can skyrocket, Ed White, an aerospace engineer at Texas A&M University who was not involved in the study, said in an email. A water tunnel experiment may not be enough to properly capture everything that’s going on, he says.
Ristroph acknowledges these limitations as well. “We’ve sort of solved two parts of the problem individually,” he says, “but how that all works out in the full problem is not yet solved.”
Given the complexities of meteorite flight, it may be difficult to replicate every aspect of it in a lab. “We have our work cut out for us,” Ristroph says.