What Would Happen If an Asteroid Hit Earth?

If a 10-kilometer asteroid struck Earth — the same size as the one that killed the dinosaurs — it would release energy equivalent to 100 million megatons of TNT. That's roughly two million times more powerful than the largest nuclear weapon ever detonated. The impact would reshape the planet's surface, alter its climate for decades, and potentially end civilization as we know it. But not all asteroid impacts are created equal. The consequences depend entirely on size, and the range from "broken windows" to "extinction event" is vast.

Small Asteroids (10-50 meters): The Airburst Threat

Small asteroids enter Earth's atmosphere more often than most people realize. Objects in the 10-50 meter range typically don't survive the journey to the surface intact. Instead, they explode in the atmosphere in what's called an airburst — and even that can be devastating.

The most recent dramatic example was the Chelyabinsk event on February 15, 2013. A 20-meter asteroid entered the atmosphere over Russia at roughly 19 kilometers per second. It exploded at an altitude of about 30 kilometers, releasing energy equivalent to 500 kilotons of TNT — roughly 30 times the Hiroshima bomb. The shockwave shattered windows across six cities, injuring nearly 1,500 people, mostly from flying glass. Dashboard cameras captured the event in stunning detail: a blinding flash brighter than the sun, followed by a massive sonic boom.

The Chelyabinsk asteroid arrived with no warning. It approached from the direction of the sun, making it invisible to ground-based telescopes. Had it been slightly larger — say 50 meters — or had it struck at a steeper angle, it could have reached the ground and devastated an entire city.

The famous 1908 Tunguska event in Siberia was likely caused by a 50-60 meter asteroid that exploded in the atmosphere. It flattened approximately 80 million trees over 2,150 square kilometers of remote forest. Had it arrived four hours later, Earth's rotation would have placed St. Petersburg directly in its path.

Medium Asteroids (100-500 meters): City-Killers

An asteroid in the 100-500 meter range would be a genuine catastrophe. These objects are large enough to survive atmospheric entry and strike the ground or ocean with tremendous force.

A 200-meter asteroid hitting land would create a crater roughly 4-5 kilometers wide. The blast would completely destroy everything within a 20-kilometer radius. Beyond that, the shockwave would shatter buildings, ignite fires, and create hurricane-force winds for hundreds of kilometers in every direction. If it struck a major city, the death toll would be in the millions.

An ocean impact would be arguably worse. A 300-meter asteroid hitting the Atlantic Ocean would generate tsunamis hundreds of meters tall near the impact site. As these waves radiated outward, they would diminish in height but remain devastating. Coastal cities thousands of kilometers away could face waves 10-30 meters high — enough to inundate low-lying coastal areas along entire continents. Given that roughly 40% of the world's population lives within 100 kilometers of a coastline, the consequences would be staggering.

The asteroid would also vaporize enormous quantities of seawater, injecting steam and salt into the upper atmosphere, disrupting weather patterns for months or years afterward.

Large Asteroids (1-10 kilometers): Continental Destruction

Once an asteroid reaches the 1-kilometer threshold, the impact becomes a global event regardless of where it strikes. A 1-kilometer asteroid would release energy equivalent to roughly 100,000 megatons of TNT. The impact crater would stretch 15-20 kilometers across. Ejecta — superheated rock and debris — would be launched into the upper atmosphere and rain back down across entire continents, igniting widespread firestorms.

The dust and soot from these fires, combined with pulverized rock launched into the stratosphere, would block sunlight for months. Global temperatures would drop by several degrees, devastating agriculture worldwide. This scenario is sometimes called an "impact winter," and it would trigger widespread crop failure and famine even in regions far from the impact site.

At the 10-kilometer scale, you're looking at a mass extinction event. This is what happened 66 million years ago.

The Chicxulub Impact: What Actually Happened

The asteroid that ended the age of dinosaurs struck what is now the Yucatan Peninsula in Mexico, creating the Chicxulub crater — a structure roughly 180 kilometers in diameter and 20 kilometers deep. The sequence of events was apocalyptic on a scale that's difficult to comprehend.

Within seconds of impact, a shockwave flattened forests for thousands of kilometers. Superheated ejecta launched into space re-entered the atmosphere around the globe, heating the sky to oven-like temperatures and igniting wildfires across most of Earth's landmass. Megatsunamis scoured coastlines worldwide. The impact generated magnitude 10+ earthquakes — far stronger than anything in recorded human history.

In the weeks and months that followed, a blanket of dust and sulfur aerosols blocked out the sun. Photosynthesis shut down. Surface temperatures plummeted. Acid rain poisoned lakes and rivers. The oceans acidified as enormous quantities of carbon dioxide dissolved into them. Roughly 75% of all species on Earth went extinct, including every non-avian dinosaur.

The Chicxulub impact released more energy in a single instant than the entire world's nuclear arsenal detonated simultaneously — by a factor of a million.

Planet-Killers (50+ kilometers): Total Sterilization

At the extreme end of the scale, an asteroid 50 kilometers or larger would be an existential threat to all life on Earth. The energy released would be sufficient to boil the oceans, strip away portions of the atmosphere, and melt the Earth's crust at the impact site. The surface of the planet would become uninhabitable for thousands, possibly millions, of years.

Fortunately, objects this large are exceedingly rare in near-Earth space. The last impact of this magnitude occurred during the Late Heavy Bombardment period, roughly 3.8-4.1 billion years ago, when the inner solar system was still being shaped by collisions between leftover planetary building blocks.

NASA's Planetary Defense: The DART Mission

The good news is that humanity is no longer defenseless against asteroid impacts. In September 2022, NASA's DART (Double Asteroid Redirection Test) mission deliberately crashed a spacecraft into the small asteroid moon Dimorphos. The mission was a resounding success: it altered Dimorphos's orbital period by 33 minutes, demonstrating that kinetic impactor technology can meaningfully change an asteroid's trajectory.

This was the first time humanity intentionally changed the motion of a celestial body — a milestone in planetary defense. The European Space Agency's follow-up Hera mission, launched in 2024, is studying the impact crater and Dimorphos's altered orbit in detail to refine future deflection strategies.

Beyond active deflection, NASA's Planetary Defense Coordination Office tracks near-Earth objects continuously. As of now, astronomers have cataloged over 34,000 near-Earth asteroids. The good news: none of the large, tracked asteroids pose a significant impact threat within the next century. The ongoing challenge is finding the smaller, harder-to-detect objects — the Chelyabinsk-sized rocks that can arrive without warning.

What Are the Odds?

Earth is struck by small debris constantly — roughly 100 tons of dust and sand-sized particles enter the atmosphere every day. A Chelyabinsk-scale event (20 meters) happens roughly once every 50-100 years. A Tunguska-scale impact (50-60 meters) occurs approximately once every 500-1,000 years. A civilization-threatening impact from a 1-kilometer asteroid is estimated to happen once every 500,000 years or so. And a Chicxulub-scale extinction event? Roughly once every 100 million years.

The probability in any given year is vanishingly small, but the consequences are so extreme that planetary defense remains one of the most important long-term investments humanity can make.