Throughout the history of life on earth, there has always been a shadow cast upon it, that at any moment an asteroid or comet could enter the atmosphere and detonate with such force as to cause a mass extinction, such as the one that killed off most of the dinosaurs.
Reconstruction of the T. rex type specimen (CM 9380) at the Carnegie Museum of Natural History
Think! Hundreds of millions of years of evolution were needed to reach an amazing animal like a Tyrannosaurus Rex, and then it was snuffed out, just like that, by the very thing that may have originally seeded the chemical building blocks of life on earth. T. Rex met with bad luck indeed. But that moment of a bad spin of the cosmic roulette wheel led to the rise of the mammals, and ultimately us, the first species on this planet that can understand, detect and deflect asteroids and mitigate this great threat to life on earth. And, we’re getting better at it.
Humans and dangerously large meteorites have a long history. Perhaps the most famous impact crater on earth, Barringer crater in Arizona, or at least the best preserved, is only about 50,000 years old. While there probably wasn’t anyone in North America at the time to witness it, humans were already living on planet earth when that crater was formed. Had someone saw it, they would have witnessed the equivalent of a ten megaton nuclear blast where the 160-foot wide iron asteroid fragment that created the crater would have entered the atmosphere amid blinding light, struck the earth and exploded vaporizing most of the meteorite in the process. The vaporized metal would have then fallen to earth as iron rain which to this day can still be picked up in the area with a magnet. Anything alive within miles of the impact would have died instantly.
But this is not the only case of a large meteorite smacking earth and doing damage. Two Siberian events during the 20th century stand out, not so much for the damage they did, but the damage they could have done if they hadn’t struck remote areas.
1. Tunguska blast
Fig 1. Location of the event in Siberia
The Tunguska event was a large explosion that occurred near the Stony Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russia, on the morning of 30 June 1908 (NS). This blast remains somewhat of a mystery since it was an airburst and not a lot of material has been recovered that could shed light on the nature of the object. As a result, it might have been larger or smaller than the object that caused Barringer crater depending on the density of the object, and current estimates put the size of the blast at about 5 megatons. Had this happened over a city, the effect would have been many times worse than the atom bombs dropped on either Hiroshima or Nagasaki.
2. Sikhote-Alin meteorite
Fig 2. A 1.7kg individual meteorite from the Sikhote Alin meteorite shower (coasrsest octahedrite, class IIAB). This specimen is about 12cm wide. Sikhote Alin meteorite shower fell on 1947 February 12 in the dense forest of eastern Siberia, and over 23 tons of meteoritic material has been recovered.
The second event also happened in Russia. In 1947 the 200,000-pound Sikhote-Alin iron meteorite entered the atmosphere and basically went off like a grenade, peppering a mountainside with twisted iron shrapnel fragments. Again, had this happened over a city, it would have been devastating.
Fig 3: Chelyabinsk is located in Chelyabinsk Oblast
More recent examples would be the Chelyabinsk meteorite from 2013. This 12,000 metric ton asteroid fragment shattered windows across its path and caused numerous injuries, again over Russia, which is due to Russia’s enormous land mass. It’s a big target but the oceans present an even bigger target and it’s estimated that Hiroshima class explosions from asteroid fragments happen more than once a year on earth, typically over water.
Credits: Barend Swanepoel and Vicus van Zyl
Meteorologists: Timothy Cooper
So fast forward to a few weeks ago on June 2, a boulder-sized, six-foot rock barreled into the atmosphere over Botswana at an estimated 38,000 miles per hour or 17 kilometers per second at about 16:44 UTC (9:44 a.m. PDT, 12:44 p.m. EDT, 6:44 p.m. local Botswana time) and disintegrated several miles above the surface, creating a bright fireball that detonated in the evening sky. Now, six-foot rocks falling to earth from space is nothing unusual, it happens all the time, but this one stands out because it was detected before it entered the atmosphere. Scientists at the Catalina Sky Survey spotted the object hours before when it was about the same distance away as the moon.
Scientists quickly realized that it was going to collide with earth and were able to make predictions of just where it was going to hit, somewhere in Southern Africa. It turned out to be Botswana and was caught on camera, and no doubt meteorite hunters are on the scene as I speak combing that country looking for chunks.
So while asteroids and comets remain a very real threat to human civilization, we’re also getting better at detecting them, spotting a 6-foot rock at the distance of the moon is pretty amazing, though more funding is needed to truly set up a method of detecting and deflecting dangerous asteroids. But one thing is clear, if we’re smart, it won’t be long before the earth is no longer subject to mass extinctions by asteroids.