On February 15, 2013, the million inhabitants of the central Russian city of Chelyabinsk experienced a half-megaton explosion from a disintegrating space rock. What happened, and how did the people of Chelyabinsk react?
Shortly after sunrise on February 15, a projectile entered the atmosphere over the Ural Mountains travelling at more than eighteen kilometers per second.
It was about twenty meters (sixty feet) in diameter, or half the diameter of the famous Tunguska impact of 1908, which flattened a thousand square miles of
Siberian forest. The rocky projectile, which came from the asteroid belt, left a trail of smoky condensation across the sky as it vaporized in the
atmosphere. Its terminal explosion, at an altitude of twenty-three kilometers, released energy equivalent to a couple dozen Hiroshima-sized atom bombs.
When it exploded, the bolide (a very bright meteor that explodes in the atmosphere) was for a few seconds brighter than the sun. About two minutes later
the shock wave reached the ground in Chelyabinsk, breaking windows and injuring about 1,500 people from flying glass and other debris.
There was no advance warning of this asteroid strike. With a diameter of twenty meters (twenty meters is the best current estimate of the diameter, based
on the mass that would produce the ~0.5 MT energy of the bolide), the Chelyabinsk impactor was smaller than most asteroids that have been detected by the
telescopes of the NASA Spaceguard Survey, which focuses on finding asteroids of about one hundred meters or larger. Furthermore, since it approached Earth
from very near the direction of the sun, it could not have been seen by a ground-based optical telescope of any size. It therefore struck without warning,
although the atmospheric explosion was measured by down-looking surveillance satellites. The Chelyabinsk bolide had about a tenth of the energy, and
exploded more than twice as high, as Tunguska. The blast energy was directed more sideways that downward. These factors resulted, thankfully, in much less
damage on the ground.
The explosion also produced a shower of stony meteorites, of a type common among the asteroids (ordinary chondrite). These meteorites, distributed over an
impact region more than one hundred kilometers long, were easily collected by local people because most fell on snow, leaving obvious little “entry
Perhaps most surprising is what did not happen: There was no panic reaction that this was a nuclear attack. As far back as 1981, geologist Gene
Shoemaker and others had warned that an unexpected cosmic impact might be misinterpreted as an attack and trigger a nuclear exchange. This fear has hovered
over all subsequent studies of the impact hazard. Perhaps it was a legitimate concern during the Cold War, and maybe this is still a danger if a cosmic
impact took place over the disputed territory of nuclear-armed antagonists such as India and Pakistan. Chelyabinsk is near the heart of the Soviet defense
industry, surrounded by facilities that design and build nuclear weapons and rocket delivery systems. Those with long memories may remember that this area
was the target of the 1960 U-2 flight of Gary Powers that was shot down by a Soviet missile. Yet apparently neither the Russian military nor the public
associated the event of February 15 with a nuclear attack. For this we can all breathe easier.
The people of Chelyabinsk initially did not know what hit their city. Videos show that the brilliant light passing across the sky was ignored by most
people, with traffic flow unaffected. We would have expected drivers to pull to the side of the road and get out to look at the trail of the bolide
stretching across the sky, but this is not what the videos show. When the shock wave hit it was unexpected. Many people were injured because they went to
the windows to see what was happening. In at least one case a teacher initially told her students to “duck and cover,” the old Cold War–drill. They hid
under their desks, but when nothing happened they got up and went to the window just in time for the shock wave. Photos of the damage showed that in many
office and apartment buildings, only a few windows were blown out, while most remained intact. Many of the videos of the fireball posted on the Internet
were taken from automatic auto dashboard cameras, or dashcams, installed by drivers in Russia to defend against insurance fraud or police corruption in
case of an accident.
The response from the scientific community was quick. Scientists have been studying the impact hazard for two decades, and they are well connected by the
Internet. In addition, many residents of Chelyabinsk began posting YouTube videos within an hour after the event. Within a few hours, the cause of the
explosion was identified as the stratospheric disintegration of an incoming rocky object. The initial Russian news reports speculated that the asteroid was
only a few meters in diameter and the energy was only a few tens of kilotons. However, the data from a worldwide network of atmospheric pressure sensors
and seismic stations quickly established the energy of the explosion as between 300 and 500 kilotons. The response from the U.S. orbital monitoring system
was also remarkably fast. Three days after the event they released the exact time and location, with measurements of the bolide track and its altitude
(about 23 km) and velocity (more than 18 km/s) at peak brightness.
Fortunately, there seem to have been few suggestions that this was an unnatural event such as a UFO, or the product of some secret weapons system. Such
stories continue to circulate about Tunguska, even a century after it happened. For a couple of days there were claims that the object was a comet rather
than an asteroid. In part these came from the misunderstanding that the white trail left by the impactor was made of water droplets analogous to an
airplane contrail, rather than ablation from the rocky object. Also, for the past century many Russian scientists have persisted in claiming that the
Tunguska impactor was a comet, even after most other scientists abandoned that idea in favor of a rocky asteroid.
A curious aspect is the fact that February 15 was also the date of the closest passage by Earth of a 30–40 meter asteroid called 2012 DA14. It flew just
28,000 km above the Earth’s surface. This remarkable coincidence seemed to call for an explanation, and many press stories suggested that the two objects
were traveling together in space or were somehow related. The question of whether a “coincidence” is the result of truly correlated events or just chance
is a familiar theme to skeptics.
Nature has provided us with several lines of evidence that the two events were not related. Immediately following the event, meteor expert Peter Brown of
the University of Western Ontario checked radar meteor data to see if the Earth had experienced unusual meteor activity due to passage through a stream of
debris associated with either 2012 DA14 or the Chelyabinsk bolide, and found none. Richard Binzel of MIT, who studies physical characteristics of
asteroids, noted that the spectrum of 2012 DA14 implies a composition of the asteroid radically different from the composition of the recovered meteorites
from the Chelyabinsk event. William Bottke of the Southwest Research Institute in Boulder, an expert on the evolution of asteroid orbits, noted that 2012
DA14’s orbit is probably the result of numerous close encounters with Earth and other inner planets, whereas the orbit of the Chelyabinsk asteroid was a
relatively “fresh” transfer orbit from the main belt. Last and perhaps most compelling, the direction of arrival of the two objects was radically
different. 2012 DA14 approached Earth from an extremely southerly direction, and it would be impossible for another object in a similar orbit to hit Earth
in the far northern hemisphere over Russia. The Chelyabinsk asteroid approached from the east.
Humans are hardwired to see patterns and causal relationships, and when we witness such an unlikely coincidence it can be hard to accept it as such. Within
a few hours of the Russian event, there was speculation on Facebook that NASA was at fault, that 2012 DA14 had undiscovered companions, and that as the
Earth rotated we could expect more impacts in Europe. To NASA’s credit, this speculation was immediately countered in a February 15 early-morning press
conference, in which Planetary Science Director Jim Green stated unequivocally that there was no connection. Unfortunately, the usual suspects are now
accusing NASA of a cover-up. Blogs dedicated to bashing climate researchers and scientists who don’t accept neocatastrophism now have another
pseudoscientific conspiracy theory to promote.
We are left to ask just how unlikely is the coincidence of the two events happening on the same day? Is the improbability of that coincidence sufficient to
overwhelm the strong observational evidence that they are unrelated? As is often the case in assessing the chances of such coincidences, the estimate of
probability depends to a great degree on how one frames the question. One can imagine three ways the question might be asked: (1) What is the chance that
these two events would occur on a specific day, say Valentine’s Day or some other specific “predicted” day, like the peak of a known meteor shower? (2)
What is the chance that two such events would occur within a day of one another, on whatever day one or the other chanced to occur? (3) What is the chance
that on the same day that one of these events occurred, say the Chelyabinsk bolide, another uncommon and newsworthy event also happened? Each of these
probabilities can be estimated, and the numbers are radically different.
The flyby of an asteroid like 2012 DA14 has about the same probability as the Chelyabinsk impact to within our uncertainties, which are at least a factor
of two. An impact somewhere on Earth of an object the size of the Chelyabinsk projectile, like the pass of an asteroid of the size and at the distance of
2012 DA14, happens about once every century on average, or about once in 40,000 days, to one significant digit. If you start by selecting a specific date,
like February 15, 2013 (case 1 above), the chances of both events happening are extraordinarily small, one chance in 40,000 squared, or about one in a billion. But a more realistic question to ask is, as in case 2 above, given that one of the events, say the Chelyabinsk event, did happen,
what is the chance of the other event, the 2012 DA14 flyby, occurring on the same day? The answer to that is, of course, just the chance of the other event
on any given day, or 1 in 40,000.
Even this estimate is highly “framed” by the specifics of the particular events. Suppose we asked what are the odds of a fifty meter diameter asteroid
passing within 30,000 km of the Earth the same day, or of a 20 meter diameter asteroid passing within 15,000 km? The answers would be “Who cares? They
didn’t.” By sizing a “box” specifically to just include the events in question, we radically affect the apparent coincidence of the events.
As a final example, consider case 3 above. Given one of the events, again take Chelyabinsk, we can ask, “What is the chance of another newsworthy event of
a kind that might plausibly be related, occurring on the same day?” An event “that might plausibly be related” could include another actual impact, or the
close fly-by of the Earth by another small asteroid, or perhaps a major earthquake. Looking ten or fifteen years into the past or future, there have been a
number of real or imagined events. There was a very large bolide over Indonesia in 2009, a large meteorite that notoriously failed to break up and actually
hit the ground whole in Peru in 2007, and the asteroid 2008 TC3, that was discovered in space before it hit the Earth in October 2008. Although the 2012
DA14 close flyby is claimed to be roughly a once-per-century event, the much larger asteroid (99942) Apophis will make a similarly close flyby in only
sixteen years, in 2029. Still other asteroids flew by that failed to make headlines because they were discovered only after their close flybys, but
presumably would have raised a question of correlation if they had occurred on the same day as another event. In the same time frame, we have experienced
two of the most destructive earthquakes in history (Indian Ocean in 2004 and Japan in 2011).
Thus some newsworthy event, such as an actual atmospheric entry, a close asteroid flyby, or a major earthquake, occurs every few years, say about once in a
thousand days. So the answer to case 3 appears to be about one chance in a thousand that a second event, of a kind that could raise a question of
correlation, might occur within a day of the event in question.
The probability of the coincidence of the two events can range from one in a billion to one in a thousand, depending on how we frame the question. It is
noteworthy that no reasonable analysis suggests that such a coincidence is “expected”; even one in a thousand is a rare event. But on any given day, one
can make an advance list of many thousands of possible events, each of which has a one-in-a-thousand chance of happening. With that many chances, it is
virtually certain that some of them will occur. So raising the question of correlation is not frivolous, but the odds are not so rare as to seek extreme
explanations rather than conclude that it was no more than a curious coincidence.