Water covers about 71% of Earth's surface, so if a large asteroid ever does hit our planet, the most likely target is the ocean. Movies usually show a wall of water rolling over a city within minutes. Real research paints a more complicated picture: some ocean impacts are dangerous, but many are much less dramatic than fiction suggests.
An asteroid entering the atmosphere at tens of kilometers per second carries enormous kinetic energy. If it reaches the water, that energy goes into three things: vaporizing and heating water, digging a temporary cavity in the sea, and throwing material upward. A deep-water impact first forms a bowl-shaped cavity in the water, and the walls of that cavity collapse inward and rebound, sending ripples outward.
Simulations from Los Alamos National Laboratory in the early 2000s, led by Galen Gisler and colleagues, followed impacts of asteroids from about 250 meters to 10 kilometers across. They showed huge plumes of water and vapor thrown high into the sky. They also showed that a large share of the impact energy is spent on vaporizing water and building the crater, rather than on making waves.
Most tsunamis we know, such as the 2004 Indian Ocean event, come from sudden movement of the seafloor. They have very long wavelengths, often hundreds of kilometers, and lose little energy as they cross an ocean. An impact is different: it is a concentrated disturbance at one point. The waves it makes are generally shorter, and shorter waves fade faster as they spread out in a widening ring and as they break.
That is the main reason many researchers now think a small or medium asteroid landing far out at sea is much less dangerous to a coastline than an earthquake of comparable energy. Distance matters a great deal: a splashdown near land is a very different problem from one in the middle of an ocean.
In 2016, NASA and NOAA held a workshop on asteroid ocean impacts, with the results summarized in NASA technical documents. According to those summaries, the participants broadly agreed that the tsunami threat is smaller than earlier studies had suggested. They concluded that asteroids smaller than about 250 meters landing far from shore do not endanger coastal populations the way a large nearby earthquake could, and that a 2003 NASA-commissioned study had overestimated the hazard from ocean impacts. NASA Ames researchers noted that, for coastal communities, waves from a distant deep-ocean impact would not be much worse than a severe storm surge.
There is still no complete agreement. Published estimates of wave size for the same scenario vary widely, and asteroids of roughly 200 to 800 meters were flagged as needing further study. The honest summary is: lower risk than Hollywood, but not zero risk, and still an active research field.
The 66-million-year-old Chicxulub impact, the one linked to the extinction of the non-avian dinosaurs, struck shallow sea at the edge of what is now Mexico. A 2022 study by Molly Range and colleagues, published in AGU Advances, simulated the first minutes of the event and then followed the waves around the globe with NOAA models. It found a powerful global tsunami, with waves reaching most coastlines within about two days, and compared the results with geological evidence from many sites. Figures for the wave heights differ between summaries of the work, so check the paper itself if you need exact numbers. For the size comparison, see our article on Apophis and the dinosaur asteroid.
A deep-ocean case is the Eltanin impact, thought to have happened roughly 2.5 million years ago in the South Pacific, off the southern tip of South America. It left no confirmed crater, and its date and the size of the asteroid are debated, with estimates in the range of about one to four kilometers across. Some studies suggest a large tsunami hit the Chilean coast, but this is an ongoing scientific discussion rather than settled fact.
None of this means Apophis threatens any ocean. NASA's calculations rule out an impact for at least the next century, as explained in Is Apophis dangerous?. The tsunami question is simply part of how planetary defense planners think about the full range of possible impacts. Teams that run the risk tables, described in how NASA tracks asteroids, consider whether a given path could end over land or sea. That is one reason finding objects early matters: it gives time to deflect them, as in the DART test covered in our planetary defense guide.
Real events that we have seen, such as the airbursts at Tunguska and Chelyabinsk, exploded in the atmosphere instead of forming a large crater. You can read what they did in Tunguska and Chelyabinsk.