|From the electronic version of the New York Times,|
February 15, 2013.
Reports from Russia today tell of hundreds of people hurt during the passage of a meteor. By coincidence or not, this has happened on the very day that a known asteroid, 2012 DA14, is to pass within 17,000 miles of the earth, a widely-publicized happening for which the public has been reassured that there is no danger of the asteroid actually hitting the earth! It will be interesting to follow the speculations: Was this meteor an undetected fragment of the bigger one (it is about 45 meters in diameter)? How big was it? The Russians have given a preliminary estimate of 20 tons. People weren’t hurt by the meteor itself, but by a “blast” from its passage.
What was the “blast?” When an object such as your car, or a bullet, or a meteor, moves through air, a signal also moves through the air “alerting” the air around the object that something is moving. The effect is analogous to ripples spreading away from a pebble dropped in a pond, but this signal is an acoustic wave, just like the waves that carry the sound of your voice across a room. At low velocities, the effect is barely noticeable and the air adjusts to the incoming object by moving away. However, when the object is moving “fast,” the air near the object can’t “get out of the way.” The compression waves emanating from the object pile up through nonlinear effects and form a shock wave.
Pressure, temperature and density all rise across a shock. When the overpressure is only 1.5-2 pounds per square inch (psi), the effect is annoying; when it is 2-10 psi (about one-tenth of normal atmospheric pressure at sea level), minor structural damage can occur. Sonic booms generate pressures on the order of 20-150 psi (up to about 10 times normal atmospheric pressure) and humans have survived these without injury. Ear damage occurs when the overpressures are over 700 psi (about 50 times atmospheric pressure), and lung damage occurs at about 2000 psi (about 140 times atmospheric pressure). Presumably the pressures from the shock of this meteor were on the order of a few hundred psi when it the ground and broke the windows. It was glass from the broken windows that caused most of the injuries.
How fast is “fast”? This is determined by the speed with which sound propagates through air, about 330 meters per second, about 740 miles per hour. Typical impact velocities for asteroids are 17 km/second and for comets, 51 km/second, much higher than the speed of sound in air. This is the reason that all meteors, if they are big enough, generate sonic booms.
According to the New York Times, the Russians deployed seven airplanes to search for meteorite fragments. More than 20,000 people were sent to search the area on foot for fragments. An impact crater has been reported on the outskirts of a city “50 miles west of Chelyabinsk,” an area of many military industrial complexes. Since the Cold War days, it has been a concern that an event like this could be mistaken by nervous governments for a bomb attack. If this had happened a few decades ago, the Russian response might have been different, and if it happened over North Korea or Iran in the middle of the night when the contrail in the sky couldn’t be seen, one has to wonder what might have happened. Scary thought…