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SYFY WIRE Bad Astronomy

Chelyabinsk-sized Asteroid Impacts May Happen More Often Than Previously Thought

By Phil Plait

On Feb. 15, 2013, a small asteroid collided with Earth. It came in over Russia at a low angle, slamming into our atmosphere, violently compressing the air in front of it. That created a vast amount of heat and pressure, which simultaneously melted and broke up the asteroid into smaller fragments. Within seconds, the huge energy of motion of the rock was suddenly and violently dissipated, creating an explosion equal to about 500,000 tons of TNT detonating. The resulting shock wave swept down over the nearby city of Chelyabinsk, shattering windows and injuring well over a thousand people.

This was the largest asteroid impact* the Earth has suffered in many decades, and scientists have understandably rushed to study it. We’re learning quite a bit about the asteroid itself, its behavior, and how much we should worry about future impacts.

Two new studies of the Chelyabinsk event have just been released (here and here), and they both came to some very interesting conclusions. One is that the asteroid itself was probably a single rock that had previously undergone some stress in space — most likely a collision with another asteroid — that caused it to have fractures inside. The second is that impacts in this size range may happen more often than previously thought. It’s not like the sky is falling (in other words, don’t panic! — see “What, Me Worry?” below), but it may mean we need to be even more aware of what’s going on in the heavens above us.

Stress Fracture

Much of what we’ve learned about the impact comes from video recordings of the event. There were quite a few, due to the oddly high number of people in Russia who have dashboard cams — insurance fraud is rampant in Russia, so people have cameras in their cars running all the time in case someone fakes a collision and injury. In a little twist of fate, the scientists in the first study used the video to reconstruct an astronomical crime scene, a case of cosmic trespassing. Or maybe “breaking and entering” might be more apt.

First, the size of the asteroid could be measured from the energy released and its speed. It was a little bit bigger than previously estimated, about 19 meters (62 feet) in diameter (rather than 17 meters), giving it a mass of about 12,000 tons. As this main mass plummeted through our atmosphere at a speed of 20 kilometers per second — dozens of times faster than a rifle bullet — the huge pressure it generated compressed the air in front of it, heating it up. This caused the asteroid to heat up in turn, and its outer surface started to melt. While it was still around 68 kilometers (42 miles) above Earth’s surface, this material started getting blown off by the asteroid’s high-speed passage in a process called ablation. Within seconds, the pressure ramped up hugely, and the ablation accelerated. The most severe material loss happened as the asteroid fell from 40 to 30 kilometers up. Because the asteroid came in at a shallow angle, this process took several seconds.

At a height of just under 40 km, the intense pressure started to fragment the asteroid, cracking it into 11 large pieces. This breakup of the main mass was extremely violent, blasting the pieces outward at 400 meters per second (900 miles per hour), scattering them. By the time they dropped to 29 km further breakups had split the rocks into 20 boulder-sized pieces, each weighing about 10 tons, and these themselves broke up as they fell. One bigger piece, probably twice the mass of the others, completely broke up at a height of 22 km — amazingly, it disintegrating so thoroughly that the largest fragment left from it was only about 15 kilograms, smaller than a basketball!

The trail of dust left by the asteroid as it plowed through our air was dozens of kilometers long and 1 – 2 km thick. It was also hot, of course, and hot material rises when it’s surrounded by cooler material — this is called convection, and it’s why hot air balloons float. In this case, though, it had an amazing effect: The hottest material in the dust trail was down the middle (like the lead in a pencil), so this stuff rose quickly, breaking out of the cooler dust around it. This drew in air from the sides, causing the outer part of the trail to spin horizontally, like two tornadoes on their sides (or like in cloud streets). This actually split the dust trail in two! I was particularly fascinated to read that; the videos and pictures clearly show two trails, and I wondered at the time why that was.

From Whence It Came

All of this data led the scientists to conclude that the Chelyabinsk asteroid was initially a single, monolithic rock. Some asteroids are thought to be rubble piles, literally collections of rocks held together by their own gravity; these are created when an asteroid suffers multiple slow-speed collisions. It gets pulverized but the pieces stay in place, like a car windshield getting hit by a big rock or a sack full of broken glass. However, the Chelyabinsk asteroid was more likely one rock that had deep fractures running through it, possibly from previous impacts.

In fact, the scientists were able to get a decent trajectory for the asteroid, which they then backtracked to determine its orbit (this had been done before, but apparently not accurately enough to get a good orbit despite earlier claims). They found the orbit was very similar to that of the 2.2 kilometer-wide (1.4 mile) near-Earth asteroid 1999 NC43. It’s possible that NC43 got hit by a smaller rock millennia ago, chipping off the smaller 19-meter chunk that hit us in 2013. The data aren’t conclusive, but are certainly consistent with that idea.

Blast from the Pass

The other scientific study released today looked at the explosion of the asteroid itself, and used it to estimate how many such impacts the Earth suffers over time.

They found that if you were 100 km (62 miles) from the Chelyabinsk impact, at peak brightness — and this stuns me — the asteroid was 30 times brighter than the Sun! I haven’t heard any anecdotes about this specifically, but the thermal pulse, the flash of heat, must have been palpable, even from that distance. Incredible.

They also found the gigantic shock wave that pummeled the city of Chelyabinsk was probably generated when the asteroid was at a height of 24 – 30 km above the ground, when the 20 or so big pieces were rapidly disintegrating. I was surprised to read that the overpressure, the extra pressure generated by the blast wave, was only about 3% above sea level air pressure! But that was enough to shatter windows and cause glass to fly everywhere. That much pressure over an area the size of a window adds up to a decent force, clearly, especially when applied suddenly.

The video here shows the vapor trail described above (again, note how it's split down the middle) and has the shock wave in it at the 27 second mark. It's pretty amazing.

What, Me Worry?

Finally, using the energy of the blast, together with data from other atmospheric impacts (from satellite and infrasound data), they compared the number of impacts we’ve seen in the past two decades from what is estimated using telescopic surveys and by counting lunar craters. Weirdly, they found that the rate of impacts from objects between 10 and 50 meters in diameter over the past 20 years is several times higher than previously estimated. According to their work, statistically speaking, we should expect a Chelyabinsk-sized impactor every 25 years or so.

That surprised me. The scientists who ran the study say this rate is much higher than you’d expect if we were hit by a steady rate of rocks over geologic times. It’s possible we’re in a period of unusually common impacts, although they don’t offer an explanation as to why this might be (they are just reporting it apparently exists). It could be that some largish asteroid in space got hit and broke up some time long ago, increasing the number of rocks crossing our orbit. It could simply be coincidence, and we’re just randomly getting more impacts recently. Or it may be that we just don’t have good enough statistics yet to nail these numbers down — I’ll note the way these numbers are derived involves a lot of steps where estimates are made (like determining the asteroid size from the energy of its blast when it impacts Earth).

If you’re wondering how impacts like these would go unnoticed, remember that the Earth is 70% or so covered in water, so most of these wouldn’t have eyewitnesses. We have to detect them indirectly with technology (satellite or infrasound) that’s relatively new. Also, rocks as small as 20 meters are very hard to detect, so sky surveys haven’t been sensitive enough to turn up very many rocks this size.

While this may or may not change our ideas of how often we’re hit by small rocks, my own feeling is the same as it was before: Impacts large enough to cause damage on the ground are worth our concern, but not panic. They happen rarely enough that we needn’t run around in circles screaming, but they are something we need to take seriously. This is a lottery we don’t want to win.

Obviously, we need more and bigger/better telescopes surveying the heavens for impactors. Given the calculated orbit for the Chelyabinsk asteroid, in the weeks before it hit us it was in a part of the sky close to the Sun and was difficult to observe. Coupled with the fact that it was small and faint, I’m not at all surprised it was missed.

But it needn’t have been. We need more telescopes observing, but we also need more in space, above the Earth’s atmosphere, so the sky is black and fainter asteroids can be found more easily, even when they are in parts of the sky difficult to observe from the ground. NASA has a mission like this planned, called NEOCam, and work has already started on it. It will revolutionize our understanding of potentially hazardous asteroids, and is an excellent first step in cataloging them (as will be the B612 Foundation’s Sentinel probe).

The next step would be preventing the impacts, which is a whole ‘nuther can of worms. In a way, I’m glad the Chelyabinsk impact happened; it really opened the eyes of a lot of people to the dangers of these space rocks. We need to keep our eyes open, figuratively and literally. In the long run, asteroid impacts are one of the more serious threats to our civilization, and one of the every few dangers we can actually do something about. Impacts can be very nearly 100% preventable, if we choose to make them so.

* Although the asteroid blew up high in the atmosphere and didn’t strike the Earth directly (it did rain down small meteorites), astronomers refer to this event as an impact. It still hit us, and still exploded, so the term is relevant.

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