Impact Scenarios

What would happen if
an asteroid hit Earth?

Size determines almost everything. A 20-metre rock produces a shockwave. A 1-kilometre object produces a global winter. Here is what the science says at each scale.

Compare asteroid sizes →

What would it take?

The outcome depends almost entirely on size and speed. Try a few combinations and compare them with events that actually happened.

Diameter 150 m

10 m10 km

Speed 17 km/s

11 km/s72 km/s

Composition

Impact energy

Hiroshima Chelyabinsk Tunguska Tsar Bomba Chicxulub

Energy is kinetic only (half the mass times the speed squared) and assumes the whole object reaches the ground, so small sizes overstate the surface damage: most objects under about 25 metres break up in the atmosphere. No known object of any size is on a collision course with Earth.

The atmosphere as first shield

For objects under about 25 metres, Earth's atmosphere does most of the work. As the rock decelerates from tens of kilometres per second to subsonic speeds, the mechanical stress exceeds the rock's structural strength. It breaks apart. The energy releases as a shockwave - an airburst - at altitude.

Chelyabinsk in 2013 was a ~20-metre object that never reached the ground intact. Its airburst still shattered windows across a wide area and sent 1,500 people to hospital with glass injuries. Every joule of that energy came from a rock the size of a terraced house, travelling at roughly 19 kilometres per second when it entered the atmosphere.

Once objects grow beyond about 25 metres, the outcome changes. The rock is large enough to retain some integrity through the atmosphere - and whatever reaches the surface arrives at catastrophic velocity.

Effects by size class

Under 25 m

Atmospheric airburst - no ground impact

Frequency: Every few years Example: Chelyabinsk 2013 (~20 m)

25–140 m

Local to regional destruction if it reaches the surface

Frequency: Every few hundred years Example: Tunguska 1908 (~50–80 m)

140 m – 1 km

Regional to continental devastation

Frequency: Every few thousand years Example: Apophis (~370 m)

1–5 km

Continental to global effects; nuclear winter-like climate disruption

Frequency: Every few hundred thousand years Example: None in recorded history

Over 5 km

Global catastrophe; potential mass extinction

Frequency: Every tens of millions of years Example: Chicxulub (~10–15 km, 66 million years ago)

What happens on the ground

For objects large enough to reach the surface, the physics are extreme. The impactor compresses rock faster than it can mechanically respond. Rock flows like a fluid. The energy - kinetic energy = ½mv² - releases almost instantaneously.

A 140-metre asteroid travelling at 20 km/s strikes with an energy comparable to tens of thousands of nuclear weapons. The crater is typically 10-20 times the impactor's diameter, excavated in seconds. The compression and rebound creates a transient cavity that can be several kilometres across even for a 140-metre object.

Beyond the crater, the ground shockwave travels outward, collapsing structures. Ejecta - rock and debris launched at high velocity - rains down across a wide radius. The thermal pulse from the fireball ignites fires within the line of sight of the impact. For objects above about 300 metres, the combined effects extend hundreds of kilometres.

Ocean impacts

About 70% of Earth's surface is ocean. A 300-metre asteroid striking deep water would generate a large tsunami. Models suggest waves several tens of metres high could reach coastlines hundreds of kilometres away. The crater - if any forms - would be on the ocean floor. Coastal populations face the greatest risk from mid-ocean impacts in this size range.

An ocean impact does not eliminate the thermal and blast effects close to the strike point. The energy still releases almost instantaneously. A large steam explosion accompanies the impact, and the seismic wave travels through the seafloor in all directions. The difference from a land impact is that the immediate zone of devastation is ocean - but the downstream effects on coastlines can be severe.

Global effects above 1 kilometre

At 1 kilometre and above, the energy released begins to have global consequences. Ejecta lofted into the upper atmosphere blocks sunlight. Nitrogen oxide production acidifies rain. Temperatures fall. Crop failures become possible across entire continents.

At 10 km and above - the Chicxulub scale - the effect is comparable to a global nuclear winter lasting years to a decade. Sulphur aerosols from vaporised rock and sediment spread globally within weeks. Photosynthesis slows or stops across much of the planet. Food chains collapse from the base up. That is the mechanism behind the Cretaceous mass extinction 66 million years ago.

The current situation

No known asteroid is on a collision course with Earth. The scale of consequences described here is precisely why early detection matters: with decades of warning, a kinetic impactor - as demonstrated by the DART mission in 2022 - can deflect a threatening object long before it arrives. Finding objects early turns a potential catastrophe into an engineering problem.

Will an asteroid hit Earth?

Current impact risk and how NASA calculates the odds.

Asteroid size comparison

How asteroid dimensions translate to real-world scale.

The DART mission

How NASA proved asteroid deflection works in practice.

Planetary defence

How Earth's detection and deflection programmes work.

Common questions

What would happen if a small asteroid hit Earth?

It depends on size. Objects under about 25 metres typically break apart in the atmosphere before reaching the ground - their energy is released as an airburst at altitude. The 2013 Chelyabinsk event involved a ~20-metre rock that never touched the surface, yet its shockwave still shattered windows and sent 1,500 people to hospital. Objects in the 25-140 metre range can cause local to regional damage if they reach the surface, depending on composition and impact angle.

How big does an asteroid need to be to destroy a city?

Roughly 50-150 metres, depending on composition, impact angle, and whether the strike is land or water. The Tunguska object, estimated at 50-80 metres, flattened approximately 2,000 km² of Siberian forest in 1908 - an area larger than Greater London. A similar object striking a densely populated urban area would cause catastrophic damage. Objects in the 140-metre range carry energy equivalent to tens of thousands of Hiroshima bombs.

What would a 1-kilometre asteroid impact do?

A 1-kilometre impactor would cause regional to global effects. The impact energy - roughly 100,000 megatons of TNT - would excavate a crater tens of kilometres wide. Ejecta lofted into the upper atmosphere would begin blocking sunlight globally. Nitrogen oxide production would acidify rain. Temperatures would fall. Agricultural disruption across large regions would follow. The precise effects depend on where it strikes - land versus deep ocean - and at what velocity.

How likely is a large asteroid impact in the next 100 years?

No known asteroid carries a meaningful probability of impact in the next century. NASA tracks all known objects above 140 metres and none are on collision courses. The statistical frequency of such events - once every few thousand years for objects in the 140m-1km range - means the baseline probability in any given 100-year window is low. For objects above 1 km, the frequency drops to once every few hundred thousand years.

Could we survive a Chicxulub-scale impact today?

A 10 km or larger impactor would cause civilisation-level effects: global firestorms, years-long climate disruption, and mass extinction of species. Modern infrastructure and agriculture would not survive the prolonged loss of sunlight. The reassuring fact is that all near-Earth objects of this scale have been found and none are on a collision trajectory. NASA estimates more than 95% of kilometre-scale and larger near-Earth objects have been catalogued.

What protects Earth from most incoming asteroids?

Earth's atmosphere acts as the first line of defence for small objects. An incoming rock decelerates from tens of kilometres per second to subsonic speeds within seconds. The mechanical stress exceeds the rock's strength and it breaks apart, releasing its energy as a shockwave - an airburst - at altitude. Only objects large enough to survive atmospheric deceleration intact, roughly 25 metres and above, have a chance of reaching the surface in any significant form.