Impact Physics
How much energy does
an asteroid impact release?
A 20-metre rock shattered windows across a Russian city. A 10-kilometre object ended the age of dinosaurs. The numbers in between - measured in Hiroshima equivalents - explain the full scale of the threat.
See impact effects by size →Try it: size, speed and energy
Set a diameter, a speed and a composition, then watch where the energy lands against real events. Doubling the size multiplies the energy by eight; doubling the speed multiplies it by four.
Impact energy
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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 physics of impact energy
An asteroid's destructive potential comes from its kinetic energy: KE = ½mv². The velocity matters as much as mass. Near-Earth asteroids typically approach at 15-30 km/s - that is 54,000 to 108,000 km/h. A commercial aircraft flies at roughly 900 km/h.
At those speeds, even a 20-metre rock weighing perhaps 10,000 tonnes carries the energy of hundreds of Hiroshima bombs. The energy releases almost instantaneously at impact or, for smaller objects, as an airburst at altitude. There is no slow release - all of it arrives in a fraction of a second.
Impact energy compared
| Event | Size | Hiroshima comparison |
|---|---|---|
| Chelyabinsk 2013 | ~20 m | 33× Hiroshima bomb (15 kt) |
| Tunguska 1908 | ~50–80 m | ~700× Hiroshima |
| 140-metre asteroid (typical PHA) | 140 m | ~20,000–33,000× Hiroshima |
| 1-kilometre asteroid | 1,000 m | ~6.7 million× Hiroshima |
| Chicxulub impactor | ~10–15 km | ~6.7 billion× Hiroshima |
Why speed matters more than size
If you double an asteroid's diameter, its mass increases eightfold - assuming similar density and shape. At the same velocity, its kinetic energy also increases eightfold. If instead you double its velocity, its kinetic energy increases fourfold. A slow-moving large asteroid can carry less energy than a fast-moving smaller one.
Most close-approach data tables include velocity for this reason. A 200-metre asteroid approaching at 30 km/s is a different threat from an identically sized rock at 15 km/s - the faster one carries four times the energy. The combination of size and speed determines what the impact would actually do.
Chelyabinsk: 500 kilotons from 20 metres
The 2013 Chelyabinsk event released roughly 500 kilotons of TNT equivalent - 33 times the yield of the Hiroshima bomb. The object was only 20 metres across and weighed perhaps 10,000-12,000 tonnes. That is roughly the weight of two Eiffel Towers.
All of that energy was released in a single second at 30 kilometres altitude. The shockwave broke windows in buildings 100 kilometres away. Around 1,500 people were hospitalised, mostly from flying glass. The fireball was briefly brighter than the Sun, visible across a wide region of western Siberia.
The 140-metre threshold in context
A 140-metre asteroid - the minimum size for a potentially hazardous asteroid (PHA) designation - carries roughly 300-500 megatons of energy at typical impact speeds. That is 20,000 times the Hiroshima bomb, and comparable to the entire nuclear arsenal of a major power detonated simultaneously in one location.
An impact in a densely populated area would destroy a major city outright and cause severe damage across hundreds of kilometres. The 140-metre threshold is not arbitrary - it marks the scale at which an impact becomes a regional catastrophe rather than a local incident.
What the largest impacts look like
Chicxulub released an estimated 100 million megatons - roughly 6.7 billion times the Hiroshima bomb. Most of that energy deposited into the atmosphere and ground, triggering wildfires across a hemisphere within hours, and lofting dust and sulphur aerosols into the stratosphere.
Those aerosols blocked sunlight globally for years to a decade. Photosynthesis slowed. Food chains collapsed. That sequence of events - not the impact itself - is what caused the Cretaceous-Paleogene mass extinction 66 million years ago.
A useful reference point
The Hiroshima bomb released approximately 15 kilotons of TNT. The largest nuclear weapon ever detonated - the Soviet Tsar Bomba in 1961 - released around 57 megatons, comparable to a small Tunguska-scale event. The Chelyabinsk asteroid, at 500 kilotons, sits between those two. A 1-kilometre asteroid carries 100,000 megatons - 1,750 times the Tsar Bomba.
Related pages
What would happen if an asteroid hit Earth?
Effects at each size class, from airbursts to mass extinctions.
Asteroid size comparison
How asteroid dimensions compare to everyday objects.
Notable historical approaches
The asteroids that came closest and what we learned.
Will an asteroid hit Earth?
Current impact risk and what NASA is tracking.
