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.”