What Would Happen if an Asteroid Hit the Earth?

Unlike the dinosaurs, we humans have a fairly good idea of what would happen if an asteroid hit the Earth. Let’s explore the space-born apocalypse!

66-million years ago, an asteroid between six and nine miles in diameter hit the sea just off the Yucatan Peninsula in Mexico. In moments, our planet was transformed beyond recognition, dramatically changing the course of evolution in the process. Given the fact we have a rather better understanding of space than the dinosaurs did, many people are asking if we have anything to worry about.

I’ll get this out the way first – fortunately, the chances of a catastrophic impact event occurring within our lifetimes, or even the lifetime of our species, is slim. Looking at the bigger picture, life as a whole has proven incredibly resilient, surviving through six major extinction events, as well as dozens of smaller ones.

Spectacular Light Shows, Flattened Forests and Broken Glass

Alex Alishevskikh

Momentarily brighter than the Sun and visible 62 miles (100 km) away, residents of Chelyabinsk near the Ural Mountains got quite a surprise on the way to work during a frigid winter morning.

Impact events are far more common than most people think, but that’s because few of them are even visible to the naked eye. Tiny rocks from space bombard the Earth all the time, but friction causes all but the largest of them to burn up and disintegrate. We have our atmosphere to thank for this. In fact, one of the reasons the surface of the Moon is so potholed is that it doesn’t have a proper atmosphere to burn up any impactors.

Some impacts, however, are large enough to cause quite a spectacular display, as did the 10-foot-wide asteroid dubbed 2018 LA did when it landed near Botswana on June 2nd this year.

Going a little further back, you might remember the Chelyabinsk meteor in Russia on February 15, 2013. Travelling 42,900 miles per hour (69,000 kph) and being 60 feet in diameter (20 m), it exploded at an altitude of 18.5 miles (30 km). Fortunately, the atmosphere absorbed most of the energy, which was some 30 times greater than that unleashed by the Hiroshima atomic bomb. Nonetheless, it was enough to send a powerful shockwave all the way to the ground, causing $33 million worth of damage and injuring some 1,500 people. The impact was completely unexpected.

The Chelyabinsk meteor was one of the biggest impact events in modern history, but it still pales in comparison to the Tunguska event of 1908. To put it into perspective, the explosion, some 3 to 5 miles (5 to 10 km) above the ground was equivalent to about 1,000 Hiroshima bombs. Scientists estimate that the impactor had been about 200 feet (60 m) in diameter or, if it had been a comet (which are mostly made of ice, rather than rock), up to three times that to cause such damage. Luckily, the Tunguska impactor struck over a largely unpopulated region of Siberia, and there were no confirmed deaths. Nonetheless, it devastated hundreds of square miles of forest and no doubt flattened many thousands of unfortunate woodland critters. Had the impactor exploded over a major city, it would likely have killed hundreds of thousands of people.

The End of Civilisation as We Know It

Pixabay

A large enough impact could cause a rain of fire as debris come hurtling back down to Earth afterwards.

Human civilisation is a fragile thing, even if our species itself has proven to be exceptionally resilient. Although it shouldn’t come as any surprise that we pose a far greater threat to ourselves than anything external ever could, there’s little doubt that a severe impact event could send us hurtling back to the Stone Age. In fact, space agencies and governments around the world consider the threat significant enough to warrant investing large sums into impact-avoidance strategies, such as NASA’s Planetary Defence Coordination Office.

Estimates of the likelihood of a civilisation-destroying impact event occurring within a given timespan vary wildly, with NASA giving a rather vague figure of ‘once every few million years’. While a repeat of the Tunguska event could potentially wipe out a small city in the freak chance it would land right on top of one, any impactor with a diameter of 0.6 miles (1 km) or greater could cause worldwide effects. However, the extent of the effects also depends greatly on the speed and density of the impactor and where it would land. For example, a highly dense, iron-rich asteroid landing in relatively shallow waters would cause far more damage than a much larger chunk of porous rock landing in the middle of a desert. The former could, for example, cause a devastating tsunami.

Depending on its speed, composition and point of impact, an asteroid as small as a mile in diameter could be all it takes to severely impact civilisation, although it would likely not lead to mass extinction on par with what happened 66-million years ago. Nonetheless, a strong enough impact occurring on land or in shallow water could kick up enough dust into the atmosphere to cause average global temperatures to drop significantly. Even a drop of 0.7 degrees in the global average for a single year is enough to lead to severe food shortages, for example, which is exactly what happened during the volcanic winter of 1815, following the eruption of Mount Tambora.

While the so-called Year Without a Summer was hardly an extinction-level event, it does go to show just how fragile modern civilisation is. Given that the world now has a much higher population than it did in the early nineteenth century, it only stands to reason that the consequences of such a change to our climate would be even more serious now than it was then.

Another source suggests that there’s a one in 10,000 chance of a large asteroid of around 1.25 miles (2 km) hitting the Earth during the next century. Although such an impactor would have to be very dense to penetrate the lower atmosphere, it could still cause a large enough firestorm to have a significant impact on the global environment for many years. Furthermore, in addition to the direct consequences, it’s fair to say that the enormous damage to our civilisation and loss of human life would likely lead to severe political and economic consequences too. In other words, an impact event involving an asteroid of this size may be enough to destroy civilisation as we know it, though we’re not quite in mass-extinction territory yet.

Clinging on to Survival in a Post-Apocalyptic World

Pixabay

Depending on its speed and consistency, an impactor the size of a small city could be severe enough to cause severe climate change and mass extinctions on a world-transforming scale.

Ever since life first appeared on Earth some 3.5-billion years ago, mass extinctions have been one of the main drivers of evolution. After all, had it not been for the extinction of the dinosaurs, we wouldn’t be here today. All extinction events so far have ultimately been a result of sudden climate change, which results in massive disruption to the food chain and the consequent drop in biodiversity.

Impact events are no exception, although we need to go all the way back to the Cretaceous-Palaeogene boundary 66-million years ago to find the most recent mass extinction caused by an impact event. The impactor in question was a city-sized rock that crashed into the shallow sea off the coast of Mexico at an estimated speed of 40,000 miles per hour (64,000 kph). The resulting firestorm would have obliterated everything for hundreds of miles around, while a tidal wave 1,000 feet (305m) high ripped up the sea floor and devastated the entire Caribbean and most of what’s now the southern US, Central America and Venezuela. An air blast, travelling faster than the speed of sound, may have knocked down trees over a thousand miles way.

The immediate effects of the Chicxulub impact would have been felt over much of the western hemisphere. Nonetheless, it wasn’t the consequent firestorm, tsunami and rain of fire and ash that devastated the surface of our planet and started one of the most severe extinction events of the past 3.5-billion years. It was the longer-term consequences – namely a decades-long impact winter that saw global temperatures plummet and entire orders of plants and animals wiped out. Only the strongest and most resilient survived, and biodiversity took millions of years to recover.

Fortunately, events such as the dinosaur-killing asteroid only happen once every half a billion years. Nonetheless, if such an event were to occur today, we can safely assume that civilisation would cease to exist, and humanity would be pushed to the brink of extinction. Given our adaptability, however, we probably wouldn’t die out completely, but the long-term damage to the Earth’s environment would see us struggling to survive for the rest of our existence. We’d most likely make it in the form of small, isolated communities, but our world wouldn’t be a pretty place to live in.

The End of All Life – How to Completely Sterilise a Planet

Tim Bertelink

An impactor a couple of hundred miles in diameter could boil away the Earth’s oceans and sterilise the planet in short order.

As we’ve seen, mass extinction events are a driving force of evolution to the extent we wouldn’t exist without them. However, that’s because they always leave behind a foundation upon which to start again and thrive in a new world with a very different environment. If every trace of life were to disappear completely, including the most menial of microbes, then we can assume the story of life on Earth will be finished for good. Fortunately, the chances of that happening are vanishingly low, yet not impossible.

For an impact event to result in the complete sterilisation of the planet, we’d need something severe enough to cause the oceans to boil away, the ozone layer to vanish and radiation to bombard the surface. Even that might not be enough to kill off the hardiest of extremophile organisms living hundreds of feet beneath the surface of the Earth. In fact, that’s exactly where there’s still a chance that Mars isn’t yet completely dead, despite its heavily irradiated and arid surface.

To annihilate every trace of life on Earth, including those hardy animals known as tardigrades, we’d need a vastly bigger asteroid than the one that killed the dinosaurs. Consider something with a mass of more than 3.75 quintillion pounds (1.7 quintillion kg). To give some examples, either an asteroid like 2 Pallas or 4 Vesta, both of which are well over 300 miles (482 km) in diameter, would be large enough to boil away the Earth’s oceans and leave the planet as sterile as the surface of Mercury.

Even in the cataclysmic scenario we present above, some of the hardiest organisms on the planet might be able to survive for a short time. Nonetheless, the Earth itself would be practically reverted to the primordial Hadean aeon, which takes us to the next category of impact event – the planet splitter.

Not Even a Rock to Live On

Pixabay

4.5 billion years ago, a Mars-sized planet hurtled into the primordial Earth. This is the most widely accepted explanation for the formation of our unusually large moon.

Planetary destruction due to an impact event is incredibly unlikely in our case, but it’s commonplace in young solar systems in which chaos begets order as planets form from a primordial cloud of dust and space rocks. Such was the setting in our own solar system some 4.5-billion years ago. At that time, Earth as we know was utterly unrecognisable. For a start, there wasn’t any moon.

The giant-impact hypothesis, also known as the Theia Impact, is the by far the most widely accepted explanation of the formation of the Moon. 4.5-billion years ago, the state of the solar system was somewhat analogous to a horde of very drunk drivers heading home after a big party. Massive impact events were so commonplace that the era has been dubbed the Heavy Bombardment. The hypothesis suggests that a protoplanet the size of Mars collided with Earth, splitting it in two and creating a ring of debris that eventually formed into what is now the Moon. This event would have been catastrophic enough to produce a global ocean of magma while dramatically reducing the mass of our planet.

Although it’s unlikely that even the building blocks of life existed on Earth that far back, it’s safe to say that such an event happening today would extinguish every last microbe (and tardigrade).

*

Perhaps the most remarkable fact of all is that, even if the Earth were to be wiped out, traces of human civilisation would still live on. Our robotic space probes, for example, will still be sailing endlessly through the great void, and there’s a good chance that Voyager 1, for example, will still be around for long after we’re gone as a testament to the great achievements of our species. That’s something that even the complete destruction of our planet, star and solar system couldn’t take away from us!

Further Reading