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Why cone shapes are ‘just right’ for meteorites

Chelyabinsk meteorite.(Credit: Jon Taylor via Wikimedia Commons)

Scientists have an explanation for how the physics of flight in the atmosphere leads to cone-shaped meteorites.

The progression, which the researchers discovered through a series of replication experiments, involves melting and erosion during flight that ultimately results in an ideal shape as meteoroids hurl through the atmosphere.

“Slender or narrow cones flip over and tumble, while broad cones flutter and rock back and forth, but we discovered between these are cones that fly perfectly straight with their point or apex leading,” says lead author Leif Ristroph, an assistant professor in the Courant Institute of Mathematical Sciences at New York University.

“Amazingly, these ‘Goldilocks’ cones of the ‘just right’ angles exactly match the shapes of eroded clay resulting from our experiments and of actual conical meteorites. By showing how the shape of an object affects its ability to fly straight, our study sheds some light on this long-standing mystery about why so many meteorites that arrive on Earth are cone-shaped.”

Meteorites in flight

The forces behind the peculiar shapes of meteorites, which are meteors or “shooting stars” that survive the fiery flight through the atmosphere and land on Earth, have long been a mystery.

“The shapes of meteorites are not as they are in space, since they are actually melted, eroded, and reshaped by atmospheric flight,” Ristroph says. “While most meteorites are randomly shaped ‘blobs,’ surprisingly many—some say about 25 percent—are ‘oriented meteorites,’ and complete samples of these look almost like perfect cones.”

To explore the forces that produce cone-shaped meteorites, researchers replicated meteoroids traveling through outer space: clay objects, attached to a rod, served as “mock meteorites” that erode while moving through water.

The clay objects held in the water current eventually carved into cones of the same angularity as conical meteorites—not too slender and not too broad.

Origin tale

The researchers recognize the limitations of their experimental design: unlike the clay objects, actual flying meteoroids don’t have a fixed position and can freely rotate, tumble, and spin. This distinction raised the question: what allows meteorites to keep a fixed orientation and successfully reach Earth?

The team then conducted additional experiments in which they examined how different shaped cones fell through water. Here they discovered that narrow cones flip over while broad cones flutter. Between the two are “just right” cone shapes that fly straight.

“These experiments tell an origin story for oriented meteorites: the very aerodynamic forces that melt and reshape meteoroids in flight also stabilize its posture so that a cone shape can be carved and ultimately arrive on Earth,” Ristroph says.

“This is another interesting message we’re learning from meteorites, which are scientifically important as ‘alien visitors’ to Earth whose composition and structure tell us about the universe.”

Additional researchers who contributed to the work are from NYU and NYU Shanghai. The National Science Foundation funded the research, which will appear in PNAS.

Source: NYU