Awaiting the Asteroid

What would happen if a giant rock were to hit our planet?

What would happen if an asteroid hit Earth? It depends.

Before a rock actually hits Earth, it has to go through the atmosphere, which gives us some protection. The particles of air drag on the incoming rock and slow it down, like a cushion that absorbs its impact. Think of a bullet fired into a pool of water, or a bowling ball dropped into a huge vat of Jell‑O. The air particles can’t get out of the way fast enough, and the energy of the space rock compresses them into a shock wave. When air—or anything—gets com‑ pressed, it gets hot. In this case, the temperature at the shock front can get up to 3000°F. This is why our space shuttles and landing modules heat up on reentry from orbit, and it’s why we put advanced ceramics and cooling systems in front of them to deflect and absorb the heat created by this air drag.

Rocks from space don’t usually come with fancy shields to keep them cool, so they just get hot. Very hot. Depending on how hot they get, they might shatter in the atmosphere, exploding into smaller fragments that rain debris on the surface, or they might hang together and deliver most of their energy directly to the surface of Earth.

Small rocks (up to about one meter wide) actually hit Earth all the time but burn up in the atmosphere as shooting stars. If you catch them on a clear night, they are even beautiful to watch.

But as the rocks get bigger, they start to get more dangerous, and not even our atmosphere can stop them. To get a sense of scale, the following table compares the amount of energy that asteroids of different sizes would have with the explosive power of the bomb dropped on Hiroshima in World War II.

Rocks that are five meters wide have about the same energy as the bomb dropped on Hiroshima. This sounds bad, but actually, scientists aren’t too worried about them. These rocks often strike somewhere in the ocean or explode in the very upper atmosphere, usually far from populated areas.

Stepping up to 20 meters in size (about five elephants wide) means a rock carrying the same energy as 30 Hiroshima bombs. That’s a huge explosion. If we get really unlucky and a rock that size makes it through the atmosphere and hits somewhere like Manhattan, it would be an enormous disaster. Millions of lives would be lost. But it wouldn’t necessarily mean the end of the human race. In fact, a 20-­meter asteroid exploded in our atmosphere very recently.

In 2013 over Chelyabinsk, Russia, a 20-­meter-­wide rock from the asteroid belt hit our atmosphere at 60,000 kilometers per hour. It was midmorning, but the light from the explosion was reportedly brighter than the Sun and could be seen up to 100 kilometers away. Around 1,000 people were injured. It was spectacular enough to cause panic and widespread religious reawakenings, but not spectacular enough to end humanity’s time on Earth.

Above that size (in the kilometer range) is where the danger zone for our species really begins. Scientists believe that the last arrival of a multiple-kilometer-­size rock was 65 million years ago, and that it might have been what caused the extinction of the dinosaurs. (Interestingly, scientists think the rock that killed the dinosaurs (about 10 kilometers wide) flew by Earth years before it actually hit our planet, which should have given the dino-scientists some warning.)

You might be asking yourself: If Earth is thousands of kilometers wide (12,742 kilometers, to be precise), how can a relatively small rock of a few kilometers cause so much destruction? Let’s consider the case of a humble, five-kilometer rock.

A five-­kilometer-­wide rock falling to Earth would carry something on the order of 1023 joules of energy. For comparison, the average American uses about 3 x 1011 joules of energy in a year, and all of humanity uses about 4 x 1020 joules. So this one collision would carry a thousand years’ worth of human energy, all concentrated and delivered rapidly in a single spot. In nuclear-­weapon units, that’s two billion kilotons, or about 100,000,000 times the energy in the Hiroshima bomb.

That much energy released on land would create an explosive shock wave that would travel rapidly from the impact site, carrying with it enough heat and wind to destroy anything within thousands of miles. It would also cause earthquakes that shatter all the land around it, and trigger enough volcanoes to soak the entire area in hot lava.

If you are anywhere near that impact, your fate is simple: you are toast. Charred, blackened toast that can’t be improved no matter how much you butter it. How close do you have to be? In this scenario, Los Angeles is probably not far enough away from an impact that happens in New York.

But even if you are far from the impact area (say, on the other side of the world), you probably won’t survive for very long. You might avoid the immediate blast, but you would still suffer the earthquakes and reignited volcanoes that the impact causes. Your larger problem, though, will be the cloud of superheated dust, ash, and rock fragments that will be thrown up into the atmosphere. Some of this superhot dust will drift away, roasting Earth’s surface and torching forests. And it will hang out in the skies for a long, long time. This cloud would shroud Earth in darkness for years, or decades, or longer, which is probably what killed the dinosaurs.

You might wonder what would happen if the asteroid hits water rather than land. Unfortunately, things don’t get much better. First, a lot of the initial energy would be absorbed by the water, creating a mega-­tsunami with waves several kilometers high. Imagine yourself looking up at a wave that’s four or five times bigger than the Empire State Building. A wave that big means that Denver would suddenly become beachfront property, and Australia and Japan would get completely wiped off the map.

And that’s just the immediate aftermath. A giant cloud of dust would probably kill most of our ecosystem, making life as we know it pretty unsustainable. And if the asteroid hits water, the impact would also put enough water vapor in the atmosphere to cause an accelerated greenhouse effect. This greenhouse effect would trap energy on Earth and heat the planet up to unlivable temperatures.

That’s just what a five-­kilometer rock would do. Now imagine what an even bigger asteroid would do!

This is an excerpt from Frequently Asked Questions About the Universe by Jorge Cham and Daniel Whiteson, published by Riverhead, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2021 by Jorge Cham and Daniel Whiteson. Published in Arc Digital with permission.

A guest post by
Creator of PhD Comics. Co-creator of the animated series "Elinor Wonders Why." Co-author of "We Have No Idea." Co-host of the show "Daniel and Jorge Explain the Universe."
A guest post by
Particle physicist. Professor of physics at UC Irvine. Co-creator of the animated series "Elinor Wonders Why." Co-author of "We Have No Idea." Co-host of "Daniel and Jorge Explain the Universe."