Asteroid collision: How to defend Earth, II

an object such as Innoculatus — a hypothetical asteroid striking the Earth after just three days’ warning, which scientists used as a modeling example — plunges into the atmosphere and makes its final approach to Washington, D.C. The compression of the atmosphere in front of the asteroid and friction with the air would cause rapid heating. At lower altitudes, where the air is denser, the heating becomes so intense that the asteroid vaporises and explodes. For the 1908 Tunguska event in Siberia (see 16 July 2008 HSNW), this happened at about eight kilometers above ground.

Supersonic shock wave
If you were unfortunate enough to be looking up from directly below, the explosion would be brighter than the sun. The visible and infrared radiation would be strong enough to make anything flammable ignite, says Mark Boslough of Sandia National Laboratory in Livermore, California. “It’s like being in a broiler oven,” he says. Anyone directly exposed would quickly be very badly burned.

Even before the sound of the blast reaches you, your body would be smashed by a devastating supersonic shock wave as the explosion creates a bubble of high-pressure air that expands faster than the speed of sound. Planetary scientist Jay Melosh of Purdue University in Indiana once experienced a shock wave from an experiment that exploded 500 tons of TNT, a tiny blast in comparison with the blast from an asteroid. “I was standing on top of a hill about 1.5 kilometers away wearing earplugs,” he recalls. Melosh says you would see the shockwave in the air due to the way it refracts light. “It’s a shimmering bubble,” he says. “It spreads out in complete silence until it reaches you, then you hear a double boom.”

Melosh was at a safe distance, but at ground zero below an exploding asteroid, the shock wave would be powerful enough to knock down buildings. It would arrive about 30 seconds after the blazing hot flash of light, and could also knock any nearby planes out of the sky, Boslough says. Any surviving buildings would be pummeled by raging winds blowing faster than any hurricane can muster.

Shiga notes that two-thirds of Earth’s surface is ocean, so that while our atmosphere is likely to protect us from asteroids smaller than 100 meters across, anything larger hitting the ocean — including chunks of Innoculatus’s rubble pile (the hypothetical asteroids consisted of two parts: a pile of rubble 270 meters across which