Early today a huge aerial explosion rocked the Siberian city of Chelyabinsk, collapsing or damaging buildings and shattering windows throughout the city. Slivers of window glass accelerated by the blast wave from the explosion sent at least 500 people to hospitals for treatment, with many more injured less severely. The media are trumpeting a “meteor” explosion and speculating about a link to this afternoon’s flyby of the Near-Earth Asteroid 2012 DA14. I am being barraged with requests for information, even though the amount of solid quantitative date now available is minimal. Nonetheless, there are several points that can confidently be made.
1. This was not a meteor. A meteor is an optical phenomenon, a flash of light seen in the sky when a piece of cosmic debris (usually dust- or sand grain-sized) enters Earth’s upper atmosphere, converts its huge kinetic energy into heat, and “burns up” (vaporizes), usually at an altitude of at least 100 km. The Chelyabinsk object was a fragment of asteroidal or cometary origin, probably several meters in diameter, properly called a “meteoroid” or, more loosely, a “small asteroid”. A brilliant fireball seen in the atmosphere is called a bolide. Some bolides, caused by entry of large pieces of hard rock, drop meteorites on the ground: a meteorite is a rock of cosmic origin that reaches the ground in macroscopic pieces (not dust or vapor). Some bolides are cometary fluff, of which nothing is strong enough to survive as a meteorite. This body was fairly strong, and is therefore more likely to be an asteroid-derived meteoroid. Indeed, some Russian sources are claiming that a meteorite from the blast fell in a lake in nearby Chebarkul, Russia, but this has not been verified. Such judgments are tricky because the distance to the fireball is usually wildly underestimated (“it cleared my barn, so it must have been at least 50 feet up”).
2. The path of 2012 DA14 is well understood. It is in a generally Earth-like orbit, except that its orbit is inclined relative to the plane of Earth’s orbit around the Sun. To first approximation, it is neither “catching up with Earth” or “being swept up from behind by Earth”: Its motion relative to Earth it basically at right angles to the direction of our orbital motion. It will pass us from south to north. Think of two cars on the freeway traveling in the same direction at the same speed, one of them in lane 2 and the other switching from lane 1 to lane 3. Chelyabinsk is basically “behind the Earth” as seen by the approaching asteroid. In other words, the Chelyabinsk object is not associated with 2012 DA14.
3. There is also speculation about 2012 DA14 being accompanied by debris and even small satellites. This is well founded, but these fragments, produced by collisions of small rocks with the asteroid, must follow paths that are closely similar to that of the parent asteroid. If they exist, and if they hit Earth, they will do so near or to the south of the Equator. Incidentally, the orbits of satellites of NEAs are usually close in, simply because distant satellites will be stripped away by the tidal forces of the Sun (and now, during a close flyby, by Earth also), and their orbital speeds are tiny (centimeters to meters per second).
4. There was an early report of Russia scrambling jet fighters to intercept the object. Here’s how that works: suppose the bolide is traveling at the absolute minimum entry speed of about 10 km/second and radar picks it up at a range of 1000 km. This radar detection tells them the speed of the bolide. From detection to arrival they have 100 seconds, tops. Then they have the interesting task of intercepting something moving 10 (or 20) km/s with an airplane that has a top speed of, say, Mach 2.5. That’s about 0.75 km/s. See the problem? The real military significance of impact airbursts is not that it is impossible to intercept them with jet aircraft: it is the danger of a completely unpredicted high-yield aerial explosion occurring over a major city in a heavily armed, politically unstable region: think, Tel Aviv, Tehran, etc. Instant World War III.
5. There’s a lot of talk and speculation about how rare such events are. Any meaningful statistics would require that we know how big it really was (the bigger the rarer). But a reasonable first guess is that this is a decadal object: ten per century hitting Earth, of which typically nine are in sparsely populated or unpopulated areas, such as the Tunguska Event of 30 June 1908 and the two Brazilian events around 1930. We’ll know more about the size and blast energy soon. So my take is that these events are not rare, but having one over a city is unusual.
In the 1997 edition of my book Rain of Iron and Ice I included a lengthy table of reports from public media and scientific journals documenting injuries, deaths, property damage, and near-misses due to cosmic impact events, ranging from a meteorite knocking off a girl’s hat to a powerful airburst showering a city in China with tens of thousands of stones and killing over 10,000 people [Ch’ing-yang, Shansi, 1490 AD; source: Kevin Yao, Paul Weissman, and Don Yeomans, Meteoritics 29, 864-971 (1994)]. My Monte Carlo models of the long-term effect of impact events in my 2000 book Comet and Asteroid Impact Hazards on a Populated Earth provide quantitative estimates of the events occurring in hundreds of 100-year computer models. In it, Model H89 generates a low-altitude airburst of 83 megatons yield at an altitude of 19 km. A random location generator placed this blast over the city of Orleans, France, killing 40,000 people and igniting a firestorm. After this model was published, Pete Worden, who was then Commandant of Falcon AFB in Colorado Springs, sent me an account that he had found in Bishop Gregory of Tours’ History of the Franks: “580 AD In Louraine, one morning before the dawning of the day, a great light was seen crossing the heavens, falling toward the east. A sound like a tree crashing down was heard over all the countryside, but it could surely not have been any tree, since it was heard more than fifty miles away… The city of Bordeaux was badly shaken by an earthquake… The city of Orleans also burned with so great a fire that even the rich lost almost everything.”