The prospect of a deadly asteroid impact on the Earth has gripped the public's imagination ever since it was realized that such a thing was possible, but, until now, no one has taken a detailed look at what type of damage one would cause. Using 50,000 simulated Earth-impacting asteroids, scientists from Southampton University have concluded that 60 percent of casualties would be caused by wind blasts and shock waves, while land impacts would be much more dangerous than asteroids that hit the sea.

If you look at the Moon through a telescope, you can see just how much damage an asteroid strike can cause. Without an atmosphere, weather, or any tectonic activity, the scars of the cosmic bombardment that the solar system suffered in its early epochs are recorded on our satellite for all time. There are craters up to 2,000 mi (3,000 km) across and the basins of the great lunar maria may have been created by asteroids of titanic size.

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Fortunately, asteroid impacts are now relatively rare, but they do occur, including the famous Tunguska event of 1908 and the near-miss over Chelyabinsk, Russia, in 2013, which exploded in midair. According to Clemens Rumpf, a senior research assistant at the University of Southampton, our planet is struck by an asteroid 60 m (190 ft) in diameter about every 1,500 years, and one 400 m (1,300 ft) every 100,000 years.

But while scientists have made many calculations of how much energy would be released by an asteroid strike and worked out what would generally happen, no one has made a detailed study of the various dominant factors of such an impact – like wind blast, shock waves, heat blast, cratering, earthquakes, flying debris, and tsunami effects – or which would cause the most casualties for asteroids of different sizes.

In this study, what the scientists discovered was that the most dangerous factors in an impact are wind blast and shock waves, which would account for 60 percent of all fatalities. This is because shock waves would cause a sudden spike in air pressure, which would rupture lungs and other internal organs, and the wind blasts would be so massive that people would be hurled with a force strong enough to flatten a forest.

The next most dangerous factor would be heat, which would kill 30 percent of those lost, though, with sufficient warning, people could take shelter underground from this. Meanwhile, tsunamis would only account for 20 percent of deaths, and land-based impacts were found to be an order of magnitude more dangerous than sea strikes. This is because, though dangerous to coast lines, the wave's energy through the ocean would dissipate as it traveled and be broken up on reaching the continental shelves.

Next lowest would be cratering and flying debris, which would cause one percent of casualties. The latter is also avoidable, as the Chelyabinsk event demonstrated. During this, many people were injured by flying glass because they looked out the window to see what caused the flash, only to be hit by glass shards when the shockwave hit. This goes to show that the much-derided idea of "duck and cover" from old US civil defense films is still very relevant.

The least damaging factor turned out to be earthquakes, taking only 0.17 percent of the death-toll.

Overall, the Southampton team says an asteroid would have to be at least 18 m (60 ft) in diameter to cause fatalities. In comparison, the Chelyabinsk meteor was about 17 to 20 m (55 to 65 ft) wide and caused more than 1,000 injuries including burns and temporary blindness.

The team says that the purpose of the study isn't just an exercise in ghoulish speculation. Its intent is to help emergency services better plan how to respond to such an unlikely event by warning people to take shelter, evacuating small towns near the site of impact, or starting the more complicated actions needed to protect an urban population

"If only 10 people are affected, then maybe it's better to evacuate the area," says Rumpf. "But if 1,000,000 people are affected, it may be worthwhile to mount a deflection mission and push the asteroid out of the way."

The research was published in Geophysical Research Letters.

Source: American Geophysical Union

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