A comet striking Earth would release far more energy than any asteroid of the same size, because comets travel much faster. Short-period comets hit at an average of 30 km/s, while long-period comets average 53 km/s. Since kinetic energy scales with the square of velocity, a comet could deliver four to ten times more destructive force than a similarly sized asteroid. The results would range from regional devastation to a global extinction event, depending on the comet’s size and where it lands.
Why Comets Hit Harder Than Asteroids
Most people picture asteroid impacts when they think of cosmic collisions, but comets are a different beast. They originate from the outer solar system and pick up tremendous speed as they fall inward toward the Sun. A typical near-Earth asteroid strikes at around 17 km/s. A short-period comet, one that orbits relatively close to the Sun, averages 30 km/s. Long-period comets, which swing in from far beyond Jupiter, average 53 km/s.
That speed difference matters enormously. Doubling an object’s velocity quadruples the energy it delivers on impact. A long-period comet traveling at 53 km/s carries roughly ten times the kinetic energy of an asteroid the same mass traveling at 17 km/s. In practical terms, a comet doesn’t need to be especially large to cause catastrophic damage.
What Happens in the First Minutes
A comet entering the atmosphere would compress the air in front of it into a superheated shockwave. If the comet were a kilometer or more across, the atmosphere wouldn’t slow it down meaningfully. It would reach the surface (or ocean) in seconds, and the impact itself would vaporize both the comet and a massive volume of rock or water at the strike point.
A land impact would excavate a crater many times the diameter of the comet itself and eject billions of tons of pulverized rock into the atmosphere. Some of this material would be launched on ballistic trajectories, re-entering the atmosphere across entire continents and radiating enough heat to ignite widespread fires. The shockwave would flatten forests and structures for hundreds of kilometers in every direction.
An ocean impact brings a different set of problems. The comet would punch a transient cavity thousands of meters deep into the water column. As that cavity collapses, it generates tsunami waves hundreds of meters high near the impact site. Modeling of a deep-ocean strike shows that even after spreading 800 kilometers outward in about 30 minutes, waves can still be 175 meters tall. They weaken significantly crossing open ocean but amplify again as they reach coastlines, arriving at distant shores at 25 to 30 meters, enough to inundate coastal cities far from the impact zone. Water velocity at the deep ocean floor would exceed 1 meter per second out to 800 kilometers, powerful enough to scour the seabed.
The Impact Winter
The longer-term threat from a large comet impact isn’t the blast itself. It’s what happens to the climate afterward. Three types of material get injected into the upper atmosphere: fine rock dust, sulfur compounds from vaporized minerals, and soot from the fires that ignite across the planet. Of these, soot is the most dangerous because it absorbs sunlight efficiently and stays aloft the longest.
Research modeling the extinction event 66 million years ago found that soot from global fires suppressed sunlight reaching Earth’s surface to below 1% of normal levels for over 20 months. At that threshold, photosynthesis essentially stops. Plants on land die. Phytoplankton in the ocean collapse. The entire food chain, from the bottom up, breaks apart. Midlatitude regions (where most of the world’s population and farmland sit today) endure an especially prolonged period of near-total darkness.
Surface temperatures drop dramatically in every scenario. The magnitude of cooling tracks directly with how much sunlight gets blocked and for how long. Temperatures would plunge well below freezing across most of the planet, even in the tropics, for months to years. The atmosphere doesn’t begin to clear meaningfully until around 70 months (nearly six years) after impact. At that point, the soot layer thins enough that it stops heating the stratosphere, allowing moisture to condense and rain the remaining particles out.
Damage to the Ozone Layer
A comet slamming into the atmosphere at tens of kilometers per second generates extreme temperatures in the shockwave, hot enough to force nitrogen and oxygen in the air to combine into nitrogen oxides. These compounds are efficient destroyers of ozone. The same chemistry happens on a much smaller scale when spacecraft re-enter the atmosphere, and estimates show that even that modest heating produces significant nitrogen oxide emissions.
After a large comet impact, the massive pulse of nitrogen oxides injected into the stratosphere would strip away a substantial portion of the ozone layer. With the ozone shield weakened, ultraviolet radiation reaching the surface would spike once the dust and soot finally cleared. For any surviving organisms, the end of the impact winter wouldn’t mean safety. It would mean intense UV exposure capable of damaging crops, killing exposed marine life near the surface, and increasing mutation rates. Recovery of the ozone layer would take years to decades.
The Chicxulub Comparison
The closest real-world analogy is the impact that ended the age of dinosaurs 66 million years ago. A roughly 10-kilometer object struck what is now the Yucatán Peninsula in Mexico, creating the Chicxulub crater and triggering a mass extinction that killed about 75% of all species on Earth. Recent ruthenium isotope analysis of the impact deposits, published in Science, confirmed that the Chicxulub impactor was a carbonaceous asteroid that originally formed beyond Jupiter’s orbit, not a comet.
A comet of equivalent size would have been even worse. Because comets travel faster, a 10-kilometer comet would deliver several times the energy of the Chicxulub asteroid. The crater would be larger, the ejecta more widespread, the fires more extensive, and the impact winter deeper and longer. The Chicxulub event already pushed life on Earth to the edge. A comet of the same size could push it further.
How Likely Is a Comet Impact?
The good news is that comet impacts are far rarer than asteroid impacts. An object 1 kilometer or larger (asteroid or comet) hits Earth roughly once every 500,000 years on average. Comets make up a small fraction of that total risk because there are far fewer of them crossing Earth’s orbital path at any given time.
The bad news is that comets are much harder to detect in advance. Asteroids in near-Earth orbits can be cataloged and tracked for decades. Long-period comets, by contrast, spend most of their existence in the dark outer solar system, invisible to telescopes until they get close enough for sunlight to illuminate them. Current detection capabilities often provide only months of warning. The upcoming Vera Rubin Observatory (LSST) is expected to improve this significantly, finding roughly 40% of long-period comets at least five years before they reach their closest approach to the Sun, and at double the distance of current discovery. That’s a major improvement, but five years is still a short window to mount a deflection mission for an object traveling at 53 km/s.
What Size Comet Would Be Catastrophic
Not every comet impact would be an extinction-level event. A comet under about 100 meters across would likely break apart in the atmosphere, producing an airburst similar to the 1908 Tunguska event but more energetic. Damaging, certainly, but localized.
A comet between 100 meters and 1 kilometer would cause regional to continental-scale destruction. If it hit an ocean, the tsunamis alone could devastate coastlines thousands of kilometers away. A land impact would destroy an area the size of a small country and inject enough dust to cool the climate noticeably for a year or two.
Once you reach 1 kilometer and above, the consequences go global. Fires, dust, and soot block sunlight worldwide. Agriculture collapses. At 10 kilometers, you’re looking at a mass extinction comparable to or worse than the one that ended the Cretaceous period. Civilization as we know it would not survive a 10-kilometer comet impact intact, and recovery, for whatever species remained, would take millions of years to restore anything resembling the biodiversity that existed before.