Impacts shaped Earth's history: the Chicxulub impact about 66 million years ago is linked to the mass extinction that ended the dinosaurs. Small objects hit often and harmlessly; city-destroying ones are rare; civilisation-threatening ones are very rare. For the first time in history we can look for them, and in 2022 NASA's DART mission showed one can be nudged.
The tier rates the quality of the evidence, not how likely the event is.
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How it could unfold
1Impact or airburst releases energy (a 20 m object over Chelyabinsk released about 0.5 megatonnes)● strong evidence
2Regional blast damage, or tsunami for ocean impacts● strong evidence
3Only for objects of several kilometres: global dust and soot cooling and mass extinction● strong evidence
Historical anchors
Chicxulub, about 66 million years ago.
Tunguska, 1908: an airburst that flattened forest over about 2,000 km².
Chelyabinsk, 2013: an airburst that injured over a thousand people, mostly from shattered glass.
DART, 2022: a spacecraft changed the orbital period of the asteroid Dimorphos by about 33 minutes.
1 in 1,000,000 chance, per century, of an existential catastrophe from an asteroid or comet (his own estimate)
EXPERT ESTIMATE
Expert estimates are personal judgements, not measurements, and experts disagree. This site never adds its own probabilities.
Caveat
The close-approach list shows known objects that miss Earth. NASA's Sentry system tracks objects with a non-zero impact chance over about a century; most have vanishingly small probabilities.
What you can do
Nothing specific: warning times for known objects are long and the response is official. Follow NASA's planetary-defense updates.
NASA Planetary Defense Coordination Office. link ↗
ILLUSTRATIVE MODELKinetic energy of an impactor, from size, material and speed. Energy only.
Diameter slider runs from 1 m to 1,000 m (log scale); for larger objects see the Chicxulub marker.
Impact energy
9.4 Mt
Hiroshima bomb
Chelyabinsk 2013
Tunguska 1908
Tsar Bomba
Chicxulub
Hiroshima bomb: about 15 kilotonnes
Chelyabinsk 2013: about 0.5 Mt, a 20 m object
Tunguska 1908: roughly 10–15 Mt
Tsar Bomba: largest nuclear test, about 50 Mt
Chicxulub: order of 10⁸ Mt, a roughly 10 km object
This is energy only. What happens on the ground depends on entry angle, whether the object breaks up in the air (small stony ones usually do), and the target. For craters, blast and thermal radius use the Earth Impact Effects Program (Collins et al., 2005).