We humans tend to take our existence for granted. Some of us have even become comfortably numb: In the middle of the night we can get something to eat at a nearby frituur or kebab shop. Even though it’s raining fiercely, we sleep underneath a roof and a warm blanket. Even if we lose our job, the government (or at least our friends) can help us out. Current conditions for life belong to the most stable since the existence of mankind. All of this gives us a blind assurance that the future will not differ much from today. Physicist Lenka Zychova would like to rid us of this illusion with this eight part series on the different ways life on Earth could be suddenly extinguished. The first part tackled mass extinctions. This second part will address asteroids and meteorite impacts.
“I would not mind if an asteroid fell on my head. I have a band aid. A blue one.” I was staring at the five-year-old Matthias for a while, and after a few seconds of thoughts crisscrossing my brain from side to side I reacted: “I have never seen a blue band aid.”
An unlikely event such as an iron meteorite falling on a head is a real and probably fun possibility for kids. However, I am afraid that Anne Hodges never heard about this. At least before that day. That day, a beautiful autumn day in 1954, this lady living in Alabama lay in her living room on a couch and relaxed. It was a calm day, leaves were turning into different colours when suddenly, a meteorite hit her.
Wait a moment… A meteorite? In a living room? I suppose the five-year-olds would not be surprised, but Mrs. Hodges freaked out. A piece of an interplanetary body swooped through the Earth’s atmosphere, and with the elegance of a freely falling 5 kg rock, fell on the roof of her house. With a noise, it crashed into the living room, bounced off the commode, and hit Mrs. Hodges into her hip. If the little Matthias was in her place, he would say, “I knew it,” while looking for a band aid. A blue one.
The impact of a meteorite on Earth is not a rare event. Meteorites are constantly visiting us. In addition, every day, up to 40 tons of interplanetary rubble, small grains of dust and larger stones penetrate the Earth’s atmosphere. It looks like a huge amount, but in mass, it roughly corresponds to the amount of rice that can be stuffed into a large lecture room. If we would spread it over the entire surface of the Earth, the dreaded meteor shower will leave only about 30 grains of rice on the area of the size of Ghent. Moreover, a vast majority of this space gravel evaporates during its movement through the Earth’s atmosphere, and only a tiny fraction reaches the surface.
On the other hand, every now and then the Earth becomes the target for a passionate slap from a larger body. The Barringer Crater in Arizona is a beautiful example of this. This 1200-meter-wide pit was created by a 300 000-ton asteroid about 50 000 years ago. With the speed of 65 000 km/h, the asteroid dived into the Earth’s atmosphere and hit the Earth’s surface with the energy of 20 tons of TNT. The impact was so strong that the iron meteorite melted and splashed completely into the surrounding area. The crater is named after poor Mr. Barringer, who spent his life looking for a vast supply of iron at the bottom of the crater. He did not find anything, because all meteoric material had been mixed with the ambient rock long ago, causing him to go bankrupt. Not just asteroids, even craters can be cruel.
If a similar event took place today in an inhabited area, most of the (remaining) newspapers would have the same front page: “Asteroid impact kills tens of thousands of people!”. Just remember the sunny, but cold day of February 15, 2013. A bright meteor flew over the Russian city of Chelyabinsk.
It was such a huge meteor that it was visible even in the daylight (such bright meteors are called bolides). The people of Chelyabinsk and the surrounding area watched as the 10 000-ton body protruded through the Earth’s atmosphere, exploded and tore into smaller pieces. But there was something weird about it: this violent event of explosions was happening in total silence! It took time for sound to travel to the Earth surface. But people did not realize that and continued to stare with open mouths at this explosive death of a cosmic body. After two minutes of freezing silence, a deafening sound wave struck. The shock wave shattered windows of almost 7200 buildings. Nearly 1500 people were injured. If the body were heavier, the consequences would have been much worse.
Astronomers have managed to locate 93% of the extremely dangerous asteroids. These have a diameter of more than 1 km, and their orbits are crossing that of the Earth. Such bodies can cause a massive extinction of animals, including us. For proof, we do not have to look that far in the past – 65 million years ago, cheerful, gingko chewing dinosaurs were surprised by a 10 km wide asteroid. The lack of T-rexes stomping about today is a direct consequence of this impact. However, the real danger is actually not in the largest asteroids but in the smaller ones. There are many more of them, they are more difficult to observe, but still, they are able to erase a whole city from the map. For example, we have only been able to locate 46% of the interplanetary bodies with a diameter of 300-500m. If a 500 m diameter body with a speed of more than 100 000 km/h would hit Brussels, a crater with a diameter of almost 20 km would be excavated. The debris that would be flung into the air upon impact would bombard not only people in Antwerp but also citizens of Paris and Frankfurt! Smaller bodies, with a diameter of 100-300 m, capable to charmingly turn Gent into one large crater, are mapped only for less than 12%. And what about the even smaller ones, like the Chelyabinsk meteor (about 15 m in diameter)? How many of them are crossing Earth´s orbit? 50 000? 100 000? We do not know. We have just mapped only a few thousands of them.
These statistics look scary; therefore, it would be good to relate them to the likelihood that the asteroid would strike at all. The body that hit Earth 65 million years ago and caused one of the greatest extinction can be expected once in every 100 million years. A smaller body, with a diameter of 1 km, which would be able to immensely weaken global agriculture over the years, appears in Earth´s neighbourhood about once in a million years. Once in a thousand years we can expect a body that would turn a city into a tremendous hole, and every century we can look for an object that, if exploding in the atmosphere, would wipe out the area of Tokyo (as happened in 1908 over central Siberia, today known as the Tunguska event).
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” Smaller asteroids are more dangerous than the largest ones, because they are more abundant and difficult to observe. A meteorite with a diameter of 500 meters would leave a crater with a diameter of 20km and the debris would reach Paris and Frankfurt. “
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” The composition, mass, angle of impact, speed and the impact area all determine the consequences of a meteor strike.”
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However, asteroids do not wait behind a corner for a thousand or a million years to come. There may be some impact events happening soon after one another. Or not. Additionally, two asteroids with the same diameter may have quite different effects (and probabilities of impact). If the asteroid is rich in iron, then it has a higher mass, and therefore the results can be more catastrophic. On the other hand, the smaller the angle of impact, the less significant the consequences are. It also depends on the speed of the body and the impact area.
For example, a porous, low-iron asteroid with a diameter of 1 km entering the Earth’s atmosphere at the angle of 10 degrees at a speed of 36,000 km/h, would actually slow down and, for the most part, it would break down. Only a 5-kilometer crater would be left. On the contrary, an equally large, iron-rich asteroid, entering the Earth’s atmosphere perpendicularly, at the unprecedented speed of 180 000 km/h, would create a 42 km wide crater! The impact energy would be about 200 times higher than in the previous case. However, asteroids do not fly in the Solar System like wasps around your picnic area. They orbit the Sun on known orbits, therefore some collision angles are more common than others. For example, the first scenario is much more likely (once in 160,000 years) than the more threatening second case (once every 6 million years).
Yes, the Earth is under constant threat, so it’s no surprise that the monitoring of asteroids receives a great deal of attention (and adequate funding). There are several surveys that make a significant contribution to finding new asteroids every year. Some of the most important are Catalina, Pan – STARRS, NEOWISE or LINEAR. The great work of scientists and engineers involved in finding potentially dangerous asteroids is reflected in the following numbers: In 2000, we knew less than a thousand near-Earth asteroids. In 2018 we know almost 18 000! And this number is growing every day.
Stay tuned for the next article in this series on solar eruptions, geomagnetical storms and other threats from the sun!
