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SYFY WIRE Bad Astronomy

Why do asteroids explode high in the atmosphere?

By Phil Plait
Artwork depicting the fireball from the Siberian Tunguska impact of 1908. Credit: Don Davis / used by permission

If you've ever watched a bad movie about an asteroid hitting the Earth (and there are so many that if I have to see another one I swear I'm rooting for the asteroid) then you might have noticed that the asteroid itself always slams into the Earth's surface, and it's the impact itself that does all the damage.

Actually, you may not have noticed, because it happens so often and is such a trope that you might just take it for granted, like the air you breathe.

Oh, but that air is important. If the asteroid is small-ish, say a few dozen to a few hundred meters across, then air is critical. That becomes obvious if you turn away from the silver screen and instead watch an actual asteroid impact … like the one that happened on February 15, 2013, over the city of Chelyabinsk, Russia.

This dashboard cam video shows the entire event. The asteroid, which was about 19 meters across, comes in from the left. As it plows through the air at an initial velocity of some 18 kilometers per second (65,000 kilometers per hour!) it heats up and starts to glow. It leaves behind a trail (technically called a train) of vaporized rock. After a second or two it brightens, then flashes in brightness rapidly; this is when the asteroid fragments from pressure and each smaller piece contributes to the energy released. As it moves on, you can see the train is thicker and brighter where it flashed. Then, a few seconds later, it fades out.

The intense glow comes from the compression of air in front of the asteroid (and not so much from friction, as most people think; when you compress a gas it heats up, and moving at hypersonic speeds will compress the gas a lot). The energy comes from kinetic energy, the energy of motion. Together, a lot of energy is released very rapidly, the definition of an explosion. Indeed, the energy you just saw released in that video was equivalent to half a million tons of TNT, the same as a low-yield nuclear weapon.

But from this comes a mystery. Out of the 12,000-ton mass of the original Chelyabinsk asteroid, only about 4-6 tons were recovered, and those were all small pieces. The largest was only 600 kilograms! Given the going rate for Chelyabinsk meteorites, there's a huge incentive to find them, and even being very generous and saying only half have been found, that still leaves a huge amount of material that must have vaporized in the atmosphere.

Artwork depicting the fireball from the Siberian Tunguska impact of 1908. Credit: Don Davis / used by permission

On top of that, the pieces found have a very high mechanical strength; that is, they're pretty solid. If the whole mass had been that strong it would've lasted much longer and gotten lower in the air before disintegrating. Computer models of how asteroids break up in the atmosphere show that the main mass must have been far weaker overall than the pieces found, by a factor of a hundred or so!

How can this be? New research indicates that the answer is, once again, air. Up until now we haven't been treating it correctly in the physics.

What was thought to happen was that the rock slams into the atmosphere and the pressure in front of it screams up. This huge force is so great it flattens the asteroid in a process literally called pancaking. The stress breaks the rock up into smaller pieces. Each piece now has more surface area, and therefore more area to slam into air and glow. They each pancake, and the process repeats, giving you a rapid cascade into disintegration and energy release. kaBOOM.

But it turns out that the computer code being used didn’t really treat how the air gets into the rock, literally finding its way at high speed and pressure into cracks and voids inside the rock. That’s where the new research comes in. Using a more sophisticated code (developed at Los Alamos National Lab to simulate air flow at high velocities), they were able to add in the asteroid material permeability to see how it changes the impact physics.

What they found is that increasing the permeability increases the amount of pancaking, which increases the efficiency of breaking up the rock. That makes sense; a more permeable material allows air to get inside it and act like a wedge, fragmenting the mass. After that, they found that the porosity (the amount of void spaces in the rock) is important in how explosively the rock disperses, where more porous rock explodes more readily. After that, the principle factor is ablation: how well the air moving rapidly past the rock blows off the heated and melted material.

In the end, they found that the behavior of the Chelyabinsk asteroid makes sense if it was permeable. This is what caused the rock to break apart at a height of 30-40 km above the ground, when pressure was still far too small to shatter solid rock. In fact, permeability was even more important than bulk strength, since the air is slamming into the asteroid at such high speed, acting like a jackhammer on it. When the rock broke apart it created a lot of small pieces, and the toughest ones are the ones that didn’t disintegrate from ablation. They fell to the ground to be discovered, but the vast majority of the asteroid completely vaporized.

This kind of work is pretty important. Scientifically, it’s very hard to understand the physics going on at hypersonic speeds; the equations of how things flow and move is already ridiculously difficult at normal speeds, and new factors pop up at a dozen or more times the speed of sound.

But there's a more down-to-Earth (ha! Haha!) reason we need to study these things, too: There will come a day when we see an asteroid headed toward us, and we’ll have to do something about it. The more we understand the mechanics of asteroids and impacts, the better informed the decisions we make. We're still figuring out a lot of the basics, but we're getting better at it all the time.

Science! It's more than just cool. When applied judiciously it can and will quite literally save the world.