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By Marcelo Zurita *

Every week there is new news about an asteroid that will pass close to Earth, but without presenting any risk to the planet. Often, this "next" means several million kilometers away, but on social media and some sensational media, these news are full of conspiracy theories and always predict an apocalyptic impact that never occurs. However, what would happen if a large asteroid hit Earth? What are the chances of this happening one day?

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Asteroid hitting Earth. Credits: The Mirror / Reproduction

First of all, I regret to inform you that this will certainly happen one day. Based on several studies on the population of these bodies in the solar system, it is estimated that there are about 1 million asteroids with the potential to reach Earth. However, with all the efforts made by astronomers in recent decades, only 1% of these asteroids are known. Something that proves our lack of knowledge about asteroids is the global system of infrared sensors administered by the American government.

In 32 years of operation, these sensors have already detected more than 800 impacts of small asteroids with the Earth's atmosphere. Of these, only four were detected in space, just hours before reaching Earth. It sounds worrying, doesn't it? And it is. Especially when we add to this the fact that none of the most energetic impacts detected by this network were from previously detected asteroids.

 

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Asteroid impacts detected by the global network of infrasound sensors. Credits: JPL / NASA

This shows how much our planet is still susceptible to the impact of a large asteroid without any warning. So our question now is another: what will happen when a large asteroid hits Earth?

 

Energy of an impact

Our first and most superficial impression is that the bigger the asteroid, the greater the damage. In fact, the size of the asteroid is an important factor in determining the effects of its impact on Earth. But, to be more exact, the main factor is the impact energy.

In the same way that we determine the capacity of a bomb to destroy from the energy it releases, with an impact the same thing happens. But in a bomb, energy is released from a chemical or atomic reaction, whereas in a meteor, the only energy present in the system is kinetic energy, that is, the energy of the asteroid's movement at the moment it hits the soil of our planet.

Well, nobody needs to look for a physics book to remember how to calculate kinetic energy (E = mv² / 2). The important thing here is to know that it is directly proportional to the mass of the object and the square of the speed. This means that if two asteroids hit the Earth with the same speed, but one of them with twice the mass of the other, it would also have twice the kinetic energy. However, if the two have the same mass, but one with twice the speed, it would have four times more kinetic energy.

Thus, we can conclude that the energy of an impact will depend basically on the mass and mainly on the speed with which the asteroid reaches the Earth.

As an example, let's remember the case that occurred in 2013 in Russia. On the morning of February 15 of that year, an asteroid about 20 meters in diameter and approximately 10 tons of mass reached the atmosphere with an estimated speed of 18,6 kilometers per second. It generated a fireball brighter than the Sun, and exploded 23,3 kilometers above the city of Chelyabinsk, releasing an energy equivalent to 26,7 atomic bombs, like the one that devastated the city of Hiroshima (Japan), at the end of World War II.

 Chelyabinsk Meteor in 2013. Source: Bramon

Despite the enormous destructive capacity, the damage caused by the Chelyabinsk meteor was not great. Only shattered windows and small landslides in more than seven thousand properties, and almost two thousand people injured. All of this as a result of the shock wave generated by the explosion. It looks like a lot, but it just wasn't much worse because the asteroid hit the atmosphere at a very shallow angle (about 18 ° to the surface), allowing the atmosphere more time to slow down and consume part of its mass before it it would explode. So the entry angle it is an important factor in determining the damage potential that an impact can have.

 

Input angle

On June 14, a small asteroid hit the atmosphere over Australia, but at such a shallow angle that it ended up returning to space

Yes, depending on the angle of entry, even a large asteroid can return to space generating only a beautiful meteor, without offering any risk to the planet. This is because they travel at a very high speed, so if one of them reaches the “grazing” atmosphere, the higher and rarer layers will not offer enough resistance to stop it. With that, after traveling a huge distance, he ends up escaping and returning to space.

One of the most spectacular cases occurred on February 9, 1913, an event known as "The Great Meteor Procession of 1913". It was a unique meteoric phenomenon, reported from locations across Canada, the northeastern United States, the Bermuda Islands, and from many ships on the high seas, including eight Brazilian ships and totaling records over a range of 11 kilometers in length.

About 100 people who witnessed this event report a series of 40 to 60 fireballs slowly crossing the sky from northwest to southeast. It is not possible to say whether they returned to space or ended up falling somewhere in the Atlantic Ocean, but it was certainly a large asteroid, which hit the atmosphere at a very shallow angle and possibly fragmented as it approached Earth.

However, if the angle of entry is higher, close to 90 ° in relation to the ground, the asteroid quickly reaches the densest layers of the atmosphere, where the air imposes an enormous resistance force. The compression of the gases in front of the object generates heat so great that a good part of the material is vaporized in a few seconds and the resultant can explode generating a meteor shower. Still, a good part of it can reach the ground at supersonic speed, generating a huge explosion and creating a crater.

Something like this happened about 50 thousand years ago in Arizona (USA). Some theories suggest that the first men who inhabited the Americas arrived on the continent at about that time, which would have been a huge misfortune to be in that region when the event occurred: a large asteroid, about 50 meters, hit the Earth.

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A painting conceived by amateur artist and astronomer Gustav Hahn describes how the 1913 'The Great Procession of Meteors' was observed near Toronto's High Park. Credits: Cientec

In a few seconds, after a dramatic atmospheric passage, the object reached the ground at a speed of about 10 kilometers per second, releasing an energy equivalent to 600 Hiroshima bombs. The impact generated a crater called Barringer, about 200 meters deep and more than 1.200 meters in diameter, which, thanks to the desert climate of the region, remains preserved today. Remains of this asteroid are still buried under the crater and some recovered fragments are called Canyon del Diablo meteorites.

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Barringer crater, Arizona, USA. Credits: USGS / D. Roddy

A determining factor for this asteroid to reach the ground with so much energy was its composition basically of iron and nickel, which guaranteed an enormous density and mechanical resistance, making half of its original mass reach the ground. THE composition of the asteroid is then one more factor that we need to consider.

 

Composition

Practically 95,6% of the meteorites that reach Earth are rocky, that is, they give rise to rocky asteroids, which have an average density slightly higher than a terrestrial rock. This was the type of asteroid that hit Chelyabinsk in 2013, and because it is less dense, they are more likely to fall apart before they even hit the ground.

Metallic meteorites are much rarer, about 3,8%. However, its density is much higher and, with this, it resists more to the atmospheric passage, as was the case of the asteroid that generated the Barringer Crater. Another 2,5% of meteorites make up an even rarer class - carbonaceous. Most of them have low density and low mechanical resistance, and are easily destroyed and consumed by the atmosphere.

Um a very curious case that has not yet been fully explained occurred on June 30, 1908, in the Siberian region of Tunguska. An asteroid measuring between 50 and 100 meters, reached the atmosphere and exploded at about 10 kilometers of altitude. The light generated by the explosion was so intense that people could not see, and the thermal radiation instantly heated everything that was within a radius of 100 kilometers from the place. A few minutes later, the shock wave sent people back.

The explosion released energy equivalent to a thousand bombs from Hiroshima and below the location where the meteor exploded. More than two thousand square kilometers of forest have been completely cleared, cut and set on fire. Fortunately, Tunguska was one of the most remote regions on the planet and there were no fatalities, only a few injured in nearby villages.

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Trees felled and burned by the explosion at Tunguska - Kulik, 1927. Credits: Wiki Commons

Something intriguing in this case is that even after several expeditions to the site, until today no meteorites have been found resulting from this impact. Thus, it is not possible to say what the asteroid's composition would be and neither its approximate size. The asteroid probably had low mechanical resistance and was completely destroyed in the explosion. However, there are also those who believe that the Earth was hit by a comet that day. A huge stone of ice that was completely vaporized in the atmosphere.

In short, the greater the density of the asteroid, the greater the damage caused by it. But, as we saw in the case of Tunguska, the place where he falls is also decisive.

 

Fall location

Fortunately, 71% of the planet's surface is covered by oceans, and the rest still has huge regions completely uninhabited. So, the chances that a fall from a large asteroid will cause fatal victims is still small. On the other hand, imagining an event like Tunguska's taking place over a large metropolis is chilling. Millions of lives could be lost instantly.

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 Demographic density map - darker areas are the most populated. Source: Columbia University

We must also not forget that, if the asteroid is large enough, it could cause devastating effects regardless of where it falls. It is believed that asteroids over 140 meters in size could affect entire countries. Already a little larger, they could release into the atmosphere such a large amount of particles that they could interfere with the climate of the entire planet, causing the so-called impact winter.

The good thing is that if the asteroid falls into the ocean, the chances of generating a tsunami are much less than previously thought. Recent studies indicate that the deeper the waters, the more easily the impact will be absorbed and dissipated by the ocean. Evidently, huge waves will occur in the vicinity of the impact site. But as they move, their energy is dissipated and the wave loses strength. This means that coastal cities are only at risk if the impact is close to the coast - about 50 to 60 kilometers away.

Something like this happened recently, on December 18, 2018. An asteroid about 10 meters hit Earth over the Bering Sea, between Russia and Alaska. It exploded at a height of 25 kilometers, with energy equivalent to 12 Hiroshima bombs. It was the second largest impact event detected by the global network of infrasound sensors. However, none of the nearest coastal cities, 500 kilometers away, experienced any changes in the waves. In other words, the risk that an impact on the ocean will cause a tsunami is very small. Unless the asteroid is truly large, like the one that extinguished dinosaurs and most species 65 million years ago.

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3D simulation of the impact of an asteroid on the ocean. Credits: LANL

Asteroid Day

Being aware that impact events can occur at any time and knowing the type of damage they can cause, a small group of scientists and personalities proposed the creation of a global mobilization to raise awareness among the population and the authorities about the threat of a catastrophic event of an asteroid.

The United Nations (UN) established June 30 as International Asteroid Day or simply Asteroid Day. The date was chosen in reference to the Tunguska event, the biggest impact event in recent human history.

Increasing our ability to discover and track asteroids potentially dangerous to Earth is one of the main objectives of the Asteroid Day, which promotes events and activities worldwide, aiming to increase the population's knowledge and interest on this topic.

We do not yet know which will be the next asteroid to hit Earth or when it will occur, but we do know that major impacts are common, even in the recent history of our planet. So, if we want to prevent this from occurring, or at least, reduce the damage it may cause, we need to study these celestial bodies further.

First of all, we need to discover and monitor asteroids that may pose a risk to us. In addition, it is necessary to discuss strategies, plan containment actions, joining efforts in the development of technologies that allow us to mitigate these threats, thus guaranteeing the future of humanity on this planet.

 

Marcelo Zurita is president of the Paraibana Astronomy Association - APA; member of SAB - Sociedade Astronômica Brasileira; technical director of Bramon - Brazilian Meteor Observation Network - and regional coordinator (Northeast) of Asteroid Day Brasil