Image: Don Davis.

Here’s a cheery thought to send you into the weekend: New research suggests that the greatest danger posed by an incoming asteroid is not from the cataclysmic impact of it striking the Earth—but from the enormous shockwave it produces when it enters the atmosphere.

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A study accepted for publication in Meteoritics and Planetary Science (preprint available here) suggests that most deaths from a sufficiently large asteroid will be caused by the airburst produced on atmospheric entry, regardless of whether the asteroid disintegrates in the air or hits the surface. The slightly less terrible news is that the killer shockwave would have to happen near a highly populated urban area to have such an effect. The best news is, large asteroid impacts are very, very rare.

A typical asteroid orbits the sun at about 67,100 mph (30 km/s). At this speed, an asteroid will hit the Earth’s atmosphere with great force, expelling a hideous amount of energy. The resulting shockwave, says study lead author Clemens Rumpf from the University of Southampton in the UK, would deliver tornado-like winds to the surface, along with a trail of fiery debris. In some cases, the asteroid will completely disintegrate, but if it survives the journey through the atmosphere, it will strike the Earth, forming a crater, tossing debris for miles, and triggering large earthquakes. Simply put, it would be a very bad day on Earth, especially for those poor souls living near the impact site.

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To assess the mortality risk from asteroids, Rumpf considered three possible impact scenarios, namely the effects of an asteroid strike where the object burns up before it hits the ground, an event where it strikes the Earth’s surface, and the effects of an ocean hit with its resulting tsunami.

Flow chart from hell: Impact effect flow diagram showing how an airburst or surface impact is determined and the corresponding impact effects. (Image: C. M. Rumpf et al., 2017)

In the study, Rumpf sketched out a flow chart showing the various possible ways an impact event could unfold (pictured above). He also expanded on two scenarios in particular.

In the first of those scenarios, he considered what would happen if a 650-foot-wide (200 meters) asteroid struck the Atlantic ocean 80 miles (130 km) off the shore of Rio de Janeiro. Such an event, says Rumpf, would result in approximately 50,000 deaths. His calculations show that 75 percent of these deaths would be the result of the incoming tsunami, and the remaining 25 percent would be killed by the airburst winds.

Previous research has warned about the disastrous effects of a tsunami, but Rumpf’s study shows that continental shelves will likely play an important buffering role, dissipating the waves at its steep edge and along the sloping beach.

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In the second scenario, Rumpf provided casualty estimates for events in London and Berlin. He considered asteroids of two sizes, 165 feet and 650 feet (50 meters and 200 meters), and impact events involving either an airburst or an airburst plus impact. The chart below conveys his findings.

Image: C. M. Rumpf et al., 2017

Chillingly, he projects upwards of millions of deaths, the vast majority (~85 percent) caused by the airburst winds—even if the asteroid were to hit the ground. Some 15 percent of the population would be killed by the heat generated by an airburst. Other causes of death—pressure waves, heat, earthquakes, the impact, debris, and tsunami—account for the rest.

Scary stuff. But keep in mind that an impact of this magnitude happens about once every 40,000 years. What’s more, most of the planet’s surface is uninhabited, so the odds of an asteroid exploding over your city is very slim. Even so, scientists should probably keep working on how to nuke a deadly asteroid out of the sky, just in case.

[arXiv via New Scientist]