What happens when an asteroid hits Earth depends almost entirely on its size. A rock the size of a car burns up harmlessly in the atmosphere. A rock the size of a football stadium could flatten a city. And a rock 10 kilometers (6 miles) across, like the one that killed the dinosaurs, could end civilization. The energy of any impact comes down to a simple relationship: half the object’s mass multiplied by its velocity squared. Because asteroids travel at tens of kilometers per second, even small ones carry enormous destructive power.
Small Impacts: More Common Than You Think
Asteroids about one meter across (roughly three feet) strike Earth a few times every year. You never notice because they disintegrate high in the atmosphere, producing nothing more than a bright streak of light. These are the “shooting stars” people wish on.
Scale up to about 19 meters (62 feet), and you get something like the Chelyabinsk event of 2013. That asteroid entered the atmosphere over Russia at 18.5 kilometers per second and released roughly 0.2 megatons of energy, about 10 to 15 times the force of the Hiroshima bomb, at an altitude of around 25 kilometers. It never reached the ground. The shockwave alone shattered 20,000 square meters of glass windows and damaged over 3,000 buildings, injuring more than 1,600 people. Nearly all those injuries came from flying glass, not from the asteroid itself. Events this size happen roughly every 60 to 80 years.
At 50 meters across, you get something like the 1908 Tunguska event, which flattened 2,000 square kilometers of Siberian forest. That’s an area roughly the size of London. Had it arrived four hours later, Earth’s rotation would have placed St. Petersburg directly underneath it. Objects this size strike every 200 to 300 years on average.
City-Killers and Region-Destroyers
Once an asteroid reaches a few hundred meters in diameter, it carries enough energy to devastate an entire region. The immediate impact zone experiences temperatures hotter than the surface of the sun, vaporizing rock and soil instantly. A fireball expands outward, followed by a pressure wave that levels structures for tens of kilometers in every direction. The ground shakes with the force of a major earthquake, and the ejected debris rains back down over a wide area.
If an asteroid this size lands in the ocean instead of on land, the problem shifts. The impact punches a temporary cavity thousands of meters deep into the water, and as that cavity collapses, it generates massive tsunami waves. One modeled scenario for a deep-ocean Atlantic impact projects waves 175 meters high within 30 minutes, decaying as they spread outward but still reaching 100 to 120 meters when they hit the U.S. East Coast about two hours later. Within four hours, virtually the entire eastern seaboard would experience waves 60 meters or taller. Europe and Africa would see 15 to 20 meter waves within 12 hours. These waves would have short periods of only a couple of minutes, so they might only penetrate 3 to 4 kilometers inland before withdrawing, but that’s still enough to destroy every coastal city in their path.
What a Kilometer-Wide Asteroid Does
A one-kilometer asteroid strikes Earth roughly every 500,000 years. At this scale, the effects go global. The impact itself creates a crater 10 to 20 kilometers wide. Millions of tons of pulverized rock and dust launch into the upper atmosphere, where they spread around the planet within days. Fires ignite across a continent-sized area, adding soot to the atmospheric mix.
The dust and soot block sunlight, dropping global surface temperatures by several degrees. Crops fail. Food chains collapse. This “impact winter” isn’t a single bad season. Depending on how much material reaches the stratosphere, temperatures can stay below normal for years. An asteroid this size wouldn’t necessarily end human civilization, but it would trigger famine, economic collapse, and potentially billions of deaths from secondary effects rather than the impact itself.
Extinction-Level Impacts
The asteroid that struck what is now Mexico’s Yucatán Peninsula 66 million years ago was roughly 10 kilometers across. It created the Chicxulub crater, about 180 kilometers in diameter, and wiped out approximately 75% of all species on Earth, including every non-avian dinosaur.
The chain of destruction unfolded in stages. The impact itself generated earthquakes far stronger than anything in recorded human history and sent a wall of superheated debris into the upper atmosphere. Within hours, that debris began re-entering the atmosphere worldwide, heating the sky like a broiler and igniting wildfires across multiple continents. The impact hurled an estimated 35 billion to 770 billion tons of sulfur into the atmosphere, along with enormous quantities of dust and soot.
After three to four days, most of the larger debris had settled back to Earth. But the atmosphere was now choked with fine particles, soot, and sulfur-based aerosols. Larger particles (around 250 microns) fell out within hours to days, but submicron dust and stratospheric soot lingered for many months. This blanket of material caused surface temperatures to plunge by several degrees to a few tens of degrees below normal, a drop that persisted for 5 to 10 years.
The sulfur posed a separate, longer-lasting threat. It combined with water vapor to form sulfuric acid, which fell as acid rain. Oceans absorbed the acid, dropping pH levels enough to dissolve the shells of marine organisms near the surface. Photosynthesis collapsed on land and in the sea simultaneously. The organisms that survived tended to be small, adaptable, and capable of surviving on decaying matter rather than fresh plant growth. It took millions of years for ecosystems to fully recover.
Could We Stop One?
For the first time in history, humans have actually tested a method. In 2022, NASA’s DART spacecraft deliberately crashed into Dimorphos, a small asteroid moonlet orbiting a larger asteroid. The impact shortened Dimorphos’s orbital period by 33 minutes and 15 seconds, far exceeding the minimum expectations. The collision also changed the shape of Dimorphos’s orbit from circular to slightly elongated.
This matters because deflection doesn’t require destroying an asteroid. You just need to nudge it enough, years or decades in advance, so that its slightly altered path misses Earth entirely. A change of millimeters per second, applied early enough, translates to thousands of kilometers of difference by the time the asteroid reaches Earth’s orbit. The DART results confirmed that a kinetic impactor (essentially ramming a spacecraft into the asteroid at high speed) works as a real deflection strategy, not just a theoretical one.
The catch is time. Deflection only works if you spot the threat years in advance. A large asteroid on a known collision course could be redirected with current or near-future technology. A surprise impact from a comet or an asteroid approaching from a blind spot near the sun would leave little time to respond. Ongoing sky surveys have cataloged the vast majority of asteroids one kilometer and larger, putting the odds of an undetected civilization-threatening impact within any given human lifetime at extremely low levels. The greater remaining risk comes from smaller, harder-to-find objects in the 50 to 300 meter range, large enough to cause catastrophic regional damage but small enough to escape detection until relatively close to Earth.