What Will Happen If a Giant Meteorite Hits the Earth?

In science fiction films and literature, colliding asteroids or comets with Earth almost always has catastrophic consequences. In reality, small meteoroids fall often and mostly go unnoticed [1]. So, which cosmic bodies threaten the planet and humanity, and how real is the risk of a collision with a giant meteorite?

The destructiveness of a collision between Earth and a celestial object, like a meteorite or a comet, depends on several factors [2], primarily its size, composition, and the angle at which it hits the planet. 

The main damaging factors are the shockwaves in the atmosphere, tsunamis (if the impact occurs in a body of water), earthquakes, acid rain, fires caused by thermal radiation, and accompanying phenomena such as ozone layer depletion and dust clouds that could block sunlight from reaching Earth’s surface. This would make photosynthesis impossible and lead to global cooling. Scientists note that their estimates are approximate, and conclusions are tentative: many factors that could also impact life on the planet remain unaccounted for.

A collision with an object with less than 10 megatons (Mt) of kinetic energy poses minimal threat globally. However, local damage can be significant, as was the case with the Tunguska meteorite in 1908 (50-100 meters in size, with energy around 10 Mt) [3] and the Chelyabinsk meteorite in 2013 (20 meters in size, with energy about 0.5 Mt) [4]. The impact of a body with kinetic energy between 10 and 10,000 Mt (such as comets less than 400 meters in diameter and asteroids less than 650 meters in diameter) would result in a shockwave, earthquakes, and fires affecting an area between 10,000 and 100,000 km². Tsunamis would be particularly dangerous, inundating coastal plains across entire ocean basins.

When colliding with a body with energy between 10,000 and 100,000 Mt (comets less than 850 meters in diameter and asteroids less than 1.4 km), the effects, such as ozone layer depletion, would be felt on a planetary scale. In one possible scenario, clouds of dust and pulverized rock would rise into the stratosphere, leading to global cooling. Cataclysms of this magnitude have likely not yet occurred in human history.

If Earth collides with a body with energy between 100,000 and 1 million Mt (comets less than 1.8 km in diameter and asteroids less than 3 km), the consequences would shift from regional to global. Stratospheric dust sulfates from the asteroid and crater emissions and soot from large-scale forest fires triggered by thermal radiation would impact the climate worldwide. Additionally, the collision could generate enough nitrogen oxides (NOx) to destroy the ozone layer.

A collision with a body with energy between 1 million and 10 million Mt (comets under 4 km and asteroids under 6.5 km) would produce dust and sulfate, reducing sunlight at the surface to levels insufficient for photosynthesis. Fires covering an area of 10 million km² would produce smoke that further blocks sunlight from reaching Earth.

With a collision involving a body of over 10 million Mt, entire regions would be destroyed by earthquakes and shockwaves, and tsunamis up to 100 meters high would flood up to 20 km of coastal areas. The smoke, dust, and sulfates would make it so dark that visibility would be nearly zero. An asteroid around 15 km in size, which collided with Earth 66 million years ago at the end of the Cretaceous period (creating the Chicxulub crater, about 200 km in diameter), is believed to have caused mass extinctions of living organisms.

A collision with a comet or asteroid with energy around 1 billion Mt could turn the entire surface of the world’s oceans acidic due to sulfur carried by the celestial body. A dense layer of sulfate aerosols could form around Earth, significantly impacting the climate. Combined with other physical effects, the consequences for the biosphere would be devastating.

The probability of Earth colliding with a giant asteroid or comet is low. Asteroids about 10 meters in size strike Earth roughly every 10 years, 100-meter asteroids every 10,000 years, and 10-km asteroids approximately every 100 million years. To reduce asteroid risk, an intensive search for near-Earth asteroids has been underway since 1998. About 28,000 such bodies have been detected, with none posing an imminent collision threat. So far, 889 asteroids with diameters of 1 km or more have been identified, estimated to be about 90% of their total population, to locate 90% of asteroids over 140 meters in diameter.

References:

G.W. Evatt, A.R.D. Smedley et al. (2020). The spatial flux of Earth’s meteorite falls found via Antarctic data. Geology. 48 (7): 683–687

Owen B. Toon, Kevin Zahnle et al. (1997). Environmental perturbations caused by the impacts of asteroids and comets. AGU. 35 (1): 41-78

Jenniskens P.  et al. (2019). Tunguska eyewitness accounts, injuries, and casualties. Icarus 327: 4-18

Popova O.P. et al. (2013). Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization. Science. 342: 1069-1073