A comet strike would be one of the most violent events Earth can experience, but what it does depends mostly on how big the comet is. A small one might never reach the ground, while a large one could change the climate of the whole planet. For a big strike, the worst damage would come from what follows, as fire, darkness and cold spread far beyond the crater.
Why Comets Hit Harder Than Asteroids
Asteroids are the most common objects to strike Earth, arriving at about 18 km per second on average.1Lunar and Planetary Institute. Impact Cratering Mechanics Comets that circle the Sun in a few years or decades hit less often, averaging about 30 km per second.2Lunar and Planetary Institute. Impact Cratering Mechanics Long-period comets, the ones that fall in from the far outer solar system, are rarer still and average about 53 km per second.3Lunar and Planetary Institute. Impact Cratering Mechanics Energy grows with the square of speed, so a strike nearly three times faster delivers roughly eight times the energy for the same mass. At 53 km per second, an object would cross the United States in about a minute and a half, and even ice hits with more force than the same weight of explosives.
A comet is a mix of ice, dust and rock, often described as a dirty snowball, while an asteroid is mostly rock or metal. Comets grow glowing tails near the Sun, but the tail is not what would hit us. The solid nucleus is usually a few kilometres across or less. It is thought to be a loose clump of icy pieces, possibly held together only by its own gravity.4Meteoritics & Planetary Science. Structure and density of cometary nuclei That makes comets low in density, and they break apart far more easily than solid rock.
A small fast comet can match the energy of a much heavier, slower asteroid, so size alone does not tell you how bad a strike is. The angle matters too. A head-on meeting adds Earth’s own orbital motion to the comet’s speed, while a comet catching up from behind arrives slower, so the same comet can hit with quite different energy.
Small Comets Explode in the Air
Comets smaller than about 150 metres across tend to break apart completely in the atmosphere, while larger ones survive to the ground.5Monthly Notices of the Royal Astronomical Society. The atmospheric entry of cometary impactors Air pressure flattens the fragile body like a pancake and then tears it into pieces.6Monthly Notices of the Royal Astronomical Society. The atmospheric entry of cometary impactors Most of the comet is lost by breaking up, not by burning away gradually.7Monthly Notices of the Royal Astronomical Society. The atmospheric entry of cometary impactors Its energy is released all at once in an explosion in the air, called an airburst.
On the ground, an airburst begins with a blinding flash followed by a scorching wave of heat. The shock wave moves at the speed of sound, so people would see and feel the heat first and hear it minutes later if they were tens of kilometres away. It flattens trees over a wide area. Windows can shatter and people can be knocked over well beyond the zone where trees come down, so the injury area is larger than the forest damage suggests. There may be little or no crater left to show what hit.
Tunguska shows what this can do. In 1908, the largest explosion from space in recorded history struck over Siberia.8Earth and Space Science. Exploring Potential Impact‐Induced Magnetic Signatures at the Tunguska Event Epicenter Using UAV‐Based Magnetometry It flattened roughly 2,000 square kilometres of forest. Its behaviour cannot be explained by a single solid rock, and some researchers attribute it to a piece of comet.9Kinematics and Physics of Celestial Bodies. Explosion of a comet nucleus fragment in the earth’s atmosphere Soil at the site holds shocked and melted grains formed at temperatures of at least 1,710°C.10Airbursts and Cratering Impacts. New Evidence of High-Temperature, High-Pressure Processes at the Site of the 1908 Tunguska Event: Implications for Impact and Airburst Phenomena
Explosions in the air are the most likely comet event, because small bodies are far more common than large ones. Over empty land, such a blast harms few people.
Fires and Waves From a Large Impact
For a large comet, fire is the most dangerous direct effect, reaching far beyond the crater.11Izvestiya, Physics of the Solid Earth. Assessments of Aperiodic Comet Impact Effects on Earth Areas 3,000 to 14,000 km across could burn, depending on the impact’s energy.12Izvestiya, Physics of the Solid Earth. Assessments of Aperiodic Comet Impact Effects on Earth Even for a rocky body at slower speed, a 3 km impactor could start fires 2,000 km away, and a 0.3 km one 250 km away.13Meteoritics & Planetary Science. Thermal radiation from impact plumes
Much of that heat comes from the sky itself. Vaporised rock thrown above the atmosphere falls back as glowing debris.14PubMed. Ignition of global wildfires at the Cretaceous/Tertiary boundary In the dinosaur-era strike, the sky may have glowed with 50 to 150 times the heat of sunlight for one to several hours.15PubMed. Ignition of global wildfires at the Cretaceous/Tertiary boundary Global wildfires could ignite, and animals caught in the open would be harmed.16PubMed. Ignition of global wildfires at the Cretaceous/Tertiary boundary
Dry plants, leaf litter and wood ignite when radiant heat is strong enough, so fires start in many places at once instead of spreading from one spark. Whether a given place burns depends on how dry its vegetation is, so fire would not be uniform even inside the affected area.
Because the heat comes from above and does not penetrate far, soil and water shield from it. Burrows, caves and deep water offer real protection, and an animal or person with something solid overhead faces far lower risk than one in the open.
An impact at sea blasts a temporary hole in the water that collapses into waves.17Icarus. Asteroid Impact Tsunami: A Probabilistic Hazard Assessment Water absorbs some of the energy but sends a huge plume of vapour upward, and in shallow seas the seafloor can still be cratered. Coastal flooding replaces wildfire as the main local danger. The most likely ocean strike, though, is a body much smaller than the ocean is deep, and its waves die out faster than once assumed.18Meteoritics & Planetary Science. Characteristics of oceanic impact‐induced large water waves—Re‐evaluation of the tsunami hazard Small ocean impacts are therefore less of a coastline threat than earthquake tsunamis, while large ones can still cause huge waves.
Darkness, Cold and Acid Rain
Dust, sulfur and soot from wildfires can block enough sunlight to darken the whole planet.19Reviews of Geophysics. Environmental perturbations caused by the impacts of asteroids and comets High-altitude dust would sharply cool the land.20Global and Planetary Change. Global climatic effects of atmospheric dust from an asteroid or comet impact on Earth All three can drastically cut temperatures and rainfall, a stretch scientists call an impact winter (months of darkness and cold after a strike).21Geophysical Research Letters. Causes and Climatic Consequences of the Impact Winter at the Cretaceous‐Paleogene Boundary Yet only soot from the firestorms dims light below what plants need to grow for many months.22Geophysical Research Letters. Causes and Climatic Consequences of the Impact Winter at the Cretaceous‐Paleogene Boundary
Soot sits high in the atmosphere and absorbs sunlight, so crops and wild plants stop growing. Daylight would fall to a dim twilight for a long stretch, starving photosynthesis even if temperatures were not freezing.
The three materials behave differently. Dust is heavy and tends to fall out of the sky fairly quickly, so its cooling is brief. Sulfur forms tiny droplets that reflect sunlight back to space and can linger in the upper atmosphere for years, which drives longer cooling. Soot absorbs sunlight and heats the air around it, which helps lift it higher and keeps it aloft, making fire smoke a stubborn veil.
Even a 0.3 km carbon-rich asteroid holds five times more sulfur than the entire upper atmosphere today, and such objects hit about once every 10,000 years.23Elsevier (Earth and Planetary Science Letters). Impact-induced perturbations of atmospheric sulfur Larger impacts, about once every million years, could cool the planet by more than 2°C for three years or longer.24Elsevier (Earth and Planetary Science Letters). Impact-induced perturbations of atmospheric sulfur The heat would also load the air with nitrogen compounds that fall as acid rain.25Newsroom. Asteroid devastation could even be worse than feared Acid rain would arrive in the days to months after a strike, harming plants, soils and the surface of the ocean and freshwater. It is a secondary stress on top of darkness and cold, not the main cause of widespread die-off.
Could It Cause a Mass Extinction
Extinction starts at the bottom of the food chain. When plants and plankton die, the loss cascades upward through the animals that depend on them.26Paleobiology. Extinction cascades and catastrophe in ancient food webs Models of the dinosaur era show more species died for the same plant die-off in the youngest communities, including plant-eaters like Triceratops.27University of Chicago News. Ancient ecosystem’s vulnerability to catastrophe contributed to dinosaur extinction Large plant-eaters need a lot of food every day, so a sudden loss of plants hits them first, and the meat-eaters that depend on them follow. In the oceans, plankton production collapsed and food stopped sinking to the deep sea.28Annual Review of Ecology, Evolution, and Systematics. Consequences of the Cretaceous/Paleogene Mass Extinction for Marine Ecosystems
The dinosaur killer was most likely an asteroid about 10 km across, according to most scientists. A comet of similar energy would have much the same effects, because what matters is energy, not the label.
Some species got through. Those that ate dead material, seeds or fungi could still find food when living plants were scarce. Small bodies needed less food, and burrowing or aquatic habits offered protection from heat. No single trait guaranteed survival; it depended on a mix of luck, habitat and diet.
Recovery ran on different clocks. On land, plant growth may have recovered within about a decade, with forests regrowing over 60 to 80 years.29Earth and Planetary Science Letters. Rapid (10-yr) recovery of terrestrial productivity in a simulation study of the terminal Cretaceous impact event The ocean food supply to the deep sea stayed suppressed for up to 3 million years.30Earth and Planetary Science Letters. Rapid (10-yr) recovery of terrestrial productivity in a simulation study of the terminal Cretaceous impact event Life returned to the crater itself within years, with a thriving ecosystem in 30,000 years.31PubMed Central. Rapid Recovery of Life at Ground Zero of the End Cretaceous Mass Extinction
Has a Comet Hit Before
Rocks keep the evidence. Impact layers contain shocked quartz, tiny diamonds and metals typical of space rocks.32Australian Journal of Earth Sciences. Geochemical and isotopic signatures of Archaean to Palaeoproterozoic extraterrestrial impact ejecta/fallout units Shocked quartz has fine parallel cracks inside the crystals, created when a huge pressure wave passes through in an instant. Ordinary geology, such as volcanoes, rarely produces that exact pattern, so it is accepted as evidence of impact.
Comets leave an extra sign, a rare form of helium.33Australian Journal of Earth Sciences. Geochemical and isotopic signatures of Archaean to Palaeoproterozoic extraterrestrial impact ejecta/fallout units Because it is scarce here normally, a jump in its amount in ocean sediment stands out. Ocean rocks show a surge in this helium over 2.5 million years about 35 million years ago, suggesting a shower of comets.34USGS. Geochemical evidence for a comet shower in the late Eocene
One idea remains disputed. Some researchers argue a comet struck about 12,900 years ago, pointing to nanodiamonds and charcoal layers at sites in North America and Mexico. Many scientists reject it, finding contrary evidence for each clue, and the question is still open.
We have also watched a strike happen. In 1994 the pieces of comet Shoemaker-Levy 9 slammed into Jupiter.35Icarus. UKIRT Observations of the Impact and Consequences of Comet Shoemaker–Levy 9 on Jupiter Within five minutes, a plume about 6,500 km across was heated to around 1,100°C.36Icarus. UKIRT Observations of the Impact and Consequences of Comet Shoemaker–Levy 9 on Jupiter The comet had already been torn into pieces by Jupiter’s gravity, showing how fragile comets are.37Meteoritics & Planetary Science. Structure and density of cometary nuclei
Would We See It Coming
Most comets from the outer solar system aren’t discovered until they are already heading inward.38The Planetary Science Journal. How Meteor Showers Can Guide the Search for Long-period Comets They are faint and dark while distant, brightening only once the Sun warms them. Telescope surveys scan the sky repeatedly, but a dim object far away can be easy to miss until it moves closer. Comet Siding Spring went from discovery to a close pass by Mars in under 22 months.39PubMed Central. DEFENDING THE EARTH FROM LONG-PERIOD COMETS AND SNEAKY ASTEROIDS: SHORT TERM THREAT RESPONSE REQUIRES LONG TERM PREPARATION That leaves little time to prepare any mission, since spacecraft need years to design, build and launch.
Stopping a comet is harder still. Gentle methods like ramming or towing need more than 10 years of lead time even for a small object.40Acta Astronautica. Hypervelocity nuclear interceptors for asteroid disruption They change an object’s speed by a tiny amount, which only becomes a large shift in position after many years. Yet most real warnings would be shorter than 10 years.41Acta Astronautica. Hypervelocity nuclear interceptors for asteroid disruption A nuclear explosion in space may be the only option for objects larger than about 100 metres found late.42Journal of Spacecraft and Rockets. Interception of Comets and Asteroids on Collision Course with Earth But hitting a fragile body risks breaking it into pieces that still head for Earth.43Journal of Spacecraft and Rockets. Interception of Comets and Asteroids on Collision Course with Earth An object under about 70 metres could be nudged to splash into an ocean instead of a city, if reached early enough.44Journal of Spacecraft and Rockets. Interception of Comets and Asteroids on Collision Course with Earth
NASA’s DART test in 2022 hit a small asteroid with a spacecraft and changed its orbit. It showed deflection works for rocky bodies with years of warning. A fast comet with months of warning would be a much harder case.
For an ordinary person, the odds are small. Large comet strikes come roughly every million years or more.