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What happens when an asteroid hits Earth

Size, speed, what it's made of and where it lands decide whether it's a fireball over the sky or a global disaster.

Earth is hit by space rock every day, almost all of it as dust and tiny meteors that burn up harmlessly. Larger impacts are rare, but they happen. In 2013, a rock about 20 metres across exploded over Chelyabinsk, Russia. It blew out windows across the city and injured around 1,500 people, mostly from flying glass. When you send something at the Earth in Gravity Lab's sandbox, it works out what happens using the same physics as the Earth Impact Effects Program (Collins, Melosh & Marcus, 2005), a model widely used to estimate the effects of impacts.

1. Entry: does it reach the ground?

An asteroid hits the atmosphere at somewhere between 11 and 72 kilometres per second. Air piles up in front of it faster than it can get out of the way. The pressure rises until a stony body cracks, spreads out and breaks apart. Smaller stony objects usually release their energy as an airburst high above the ground: a blinding flash and a powerful shock wave, but no crater. That's what happened at Chelyabinsk, and in 1908 at Tunguska in Siberia, where an object about 60 metres across flattened around 2,000 square kilometres of forest without leaving a crater.

Bigger or stronger objects, especially iron ones, survive to the ground. The model works out whether the object breaks up, how far its fragments spread, and whether the cloud of debris bursts in the air or reaches the surface.

2. The effects on the ground

• The crater. An impact excavates a crater roughly 10 to 20 times wider than the object. Rock thrown out of it falls back as an ejecta blanket around it.
• The thermal pulse. The fireball is hot enough to set fires and burn skin far beyond the crater.
• The air blast. A shock wave spreads outward and knocks down buildings and trees. Its strength falls with distance in a well-studied way.
• The earthquake. Large impacts shake the ground like a major earthquake.
• Tsunamis. An ocean impact opens a cavity in the water, and the collapse sends out waves. How high they run up depends heavily on the local coastline.

For impacts on land, Gravity Lab estimates the toll using where people actually live, applying the same damage rates used in official studies of large explosions. These are estimates. Real outcomes depend on warning time, building types and many details no model can know.

3. Global effects

Above a certain size, an impact's effects spread worldwide. Clouds of dust, soot from fires, and sulphur from vaporised rock rise into the stratosphere and block sunlight for months or years. Harvests fail around the world. Researchers Clark Chapman and David Morrison (1994) estimated that the threshold for a global catastrophe is an impactor around 1 to 2 kilometres across.

The Chicxulub impact 66 million years ago, by an asteroid about 10 kilometres across, is the extreme case. It left a crater about 180 kilometres wide on the Yucatán coast and caused the mass extinction that ended the age of the dinosaurs.

Could we stop one?

Yes, given enough warning. In 2022, NASA's DART spacecraft deliberately crashed into Dimorphos, a small moon of the asteroid Didymos, and shortened its orbit by about 32 minutes. It was the first time humans changed the motion of a natural object in space. The earlier a threatening asteroid is found, the smaller the nudge needed to make it miss. That's why surveys that find and track near-Earth objects matter so much.

The asteroid Apophis, about 340 metres across, will pass within about 32,000 kilometres of Earth on 13 April 2029. That's closer than the satellites that sit in geostationary orbit, but it's a certain miss. Gravity Lab's "What if Apophis hit?" scenario shows what would happen if it didn't.

Try it yourself

In Sandbox Mode, open the Library for the Tunguska, Apophis and Chicxulub scenarios, or pick an asteroid or comet from the tray and throw it. Slow time down just before it arrives to watch the entry, then read the report to see why it turned out the way it did.