WEST LAFAYETTE, Ind. — Our atmosphere is a better shield from meteoroids than researchers thought, according to a new paper published in Meteoritics & Planetary Science.

When a meteor comes hurtling toward Earth, the high-pressure air in front of it seeps into its pores and cracks, pushing the body of the meteor apart and causing it to explode.

“There’s a big gradient between high-pressure air in front of the meteor and the vacuum of air behind it,” said Jay Melosh, a professor of Earth, Atmospheric and Planetary Sciences at Purdue University and co-author of the paper. “If the air can move through the passages in the meteorite, it can easily get inside and blow off pieces.”

Researchers knew that meteoroids often blew up before they reached the Earth’s surface, but they didn’t know why. Melosh’s team looked to the 2013 Chelyabinsk event, when a meteoroid exploded over Chelyabinsk, Russia, to explain the phenomenon.

The explosion came as a surprise and brought in energy comparable to a small nuclear weapon. When it entered Earth’s atmosphere, it created a bright fire ball. Minutes later, a shock wave blasted out nearby windows, injuring hundreds of people.

The meteoroid weighed around 10,000 tons, but only about 2,000 tons of debris were recovered, which meant something happened in the upper atmosphere that caused it to disintegrate. To solve the puzzle, the researchers used a unique computer code that allows both solid material from the meteor body and air to exist in any part of the calculation.

“I’ve been looking for something like this for a while,” Melosh said. “Most of the computer codes we use for simulating impacts can tolerate multiple materials in a cell, but they average everything together. Different materials in the cell use their individual identity, which is not appropriate for this kind of calculation.”

This new code allowed the researchers to push air into the meteoroid and let it percolate, which lowered the strength of the meteoroid significantly, even if it had been moderately strong to begin with.

While this mechanism may protect Earth’s inhabitants from small meteoroids, large ones likely won’t be bothered by it, he said. Iron meteoroids are much smaller and denser, and even relatively small ones tend to reach the surface.

This research was supported by NASA’s Office of Planetary Defense under grant NNX14AL15G.

Writer: Kayla Zacharias, 765-494-9318, kzachar@purdue.edu

Source: Jay Melosh, 765-494-3237, jmelosh@purdue.edu

Note to journalists: A copy of the paper is available here.

ABSTRACT

Air penetration enhances fragmentation of entering meteroids M. E. Tabetah, H. J. Melosh

The entry and subsequent break-up of the ~ 17-20 m diameter Chelyabinsk meteoroid deposited approximately 500 kT of TNT equivalent energy to the atmosphere, causing extensive damage that underscored the hazard from small asteroid impacts. The break-up of the meteoroid was characterized by intense fragmentation that dispersed most of the original mass. In modeling of the entry, the apparent mechanical strength of the meteoroid during fragmentation, ~ 1-5 MPa, is two orders of magnitude lower than the mechanical strength of the surviving meteorites, ~ 330 MPa. We implement a two- material computer code that allows us to fully simulate the exchange of energy and momentum between the entering meteoroid and the interacting atmospheric air. Our simulations reveal a previously unrecognized process in which the penetration of high- pressure air into the body of the meteoroid greatly enhances the deformation and facilitates the break-up of meteoroids similar to the size of Chelyabinsk. We discuss the mechanism of air penetration that accounts for the bulk fragmentation of an entering meteoroid under conditions similar to those at Chelyabinsk, to explain the surprisingly low values of the apparent strength of the meteoroid during break-up.