The American news channel CNN have selected what they consider to be the ten most important news about natural science and space in 2013. Gladly, planetary systems research top the list, with four different news. Other news concern fundamental physics (two), paleoanthropology and biology (two), cosmology (one) and climate (one). The four planetary news are about a fresh water lake on Mars, the spacecraft Voyager 1 that is leaving the Solar System, the space telescope Kepler looking for exoplanets, and the spectacular meteorite impact in Chelyabinsk, Russia. In four posts I will comment on each of these four top news of 2013.
On February 15, 2013, Earth collided with a small asteroid. It gave rise to an extremely bright light that lit up the sky over the Russian city Chelyabinsk – a superbolide that shone up to 30 times brighter than the Sun. While breaking in the atmosphere an energy was released that corresponded to the detonation of an atomic bomb 25 times more powerful than the one dropped on Nagasaki. The energy was transferred to the air and caused a shockwave called an airburst. The shockwave knocked people off their feet, crushed windows on more than 3,600 buildings in Chelyabinsk alone, blew in doors, and made house walls crack. 1,500 people had to seek medical care, primarily to treat cuts from flying glass. A large number of fragments of the rock fell around the city and are collectively referred to as the Chelyabinsk meteorite. Here, I will describe the fall itself and discuss how common this type of events are.
During a clear night it is not unusual to see one or several “shooting stars”. Such a light phenomenon, called a meteor by experts, happens when a small rock from space collides with Earth and burn in its atmosphere. These rocks are often not bigger than a tenth of a millimeter. Before atmospheric entry they are referred to as meteoroids. These meteoroids orbit the Sun, just like the planets, and collide with Earth since their orbits cross our own, if both bodies happen to be at the same place at the same time.
A really large meteoroid, measuring millimeters or even centimeters across, gives rise to a very bright light when entering the atmosphere. Such an unusually bright meteor is often called a bolide. If a part of the rock survives the passage through the atmosphere and falls to the ground, it is called a meteorite.
There is no official definition of how large a meteoroid can be – but in reality one ceases to talk about meteoroids when the rock in question is about ten meters across. Larger objects are called asteroids. When a small asteroid travels through Earth’s atmosphere, an extremely bright light appear, that sometimes is called a superbolide.
Below I will write about objects with sizes in the range 2-50 meters – for simplicity I call them all (small) asteroids, and if they enter the atmosphere I call the light a meteor.
The size of the asteroid
Since the fall at Chelyabinsk took place in a densely populated area, the meteorite was very well documented, thanks to surveillance cameras mounted on buildings and in cars. Several of the movies can be seen here. However, there were also other types of instruments that registered the meteorite. The most important of these were US military satellites, that accurately measured the brightness and velocity of the meteor. The measurements made public show that the kinetic energy just prior to entry corresponded to the energy liberated when 450-640 kilotons of TNT explode (the Nagasaki atomic bomb explosion released an energy corresponding to about 20-22 kiloton of TNT). The velocity at entry was 19.16 kilometers per second – it is this huge velocity that is responsible for the enormous energy carried by the asteroid. By using a rather simple mathematical relation, the kinetic energy and the velocity can be used to calculate the asteroid mass, which measured 10-15 thousand tons, which corresponds to a boulder with a diameter of 18-20 meters.
When the airburst hit ground, shockwaves formed that were registered by about seventy seismic stations located up to 4,000 kilometers away. Analysis of such seismic data shows that an energy corresponding to 220-630 kilotons of TNT was released. But there is also data from twelve different infrasound stations, facilities that listen for sound with such a low frequency that it cannot be heard by humans. They are part of the International Monitoring System (IMS), whose construction began in 1996, and that will control that the Comprehensive Nuclear Test-Ban Treaty (CTBT) is respected (a ban against nuclear test explosions put in place by the UN general assembly in 1996). These stations can not only hear distant nuclear blasts, but also registered the meteor at Chelyabinsk. These measurements shows that the released energy corresponds to the explosion o f 350-990 kilotons of TNT. A reconstruction of how the meteor brightness varied during the fall, based on films taken from ground, shows that the liberated energy was corresponding to at least 470 kilotons of TNT. Taken together, these measurements give a fairly coherent picture of the fall – it corresponded to about 500 kilotons of TNT and was due to the impact of an asteroid with a size just under 20 meters.
Movies from 15 cameras on the ground around Chelyabinsk have been used to reconstruct the behavior of the meteor. According to this reconstruction, the meteor traveled at an angle of only 18 degrees with respect to Earth’s surface. When it first became visible, it was at a height of 97 kilometers and traveled at a speed of 19.16 kilometers per second. At a height of 45 kilometers an extreme erosion started, a process called ablation that leads to heavy mass loss through evaporation. The rock also started to fragment, and when it passed a height of 29 kilometers, it had broken up into about twenty pieces, each weighing about 10 tons. A larger fragment, weighing 20 tones moved a bit ahead of the rest of the swarm. This is when the meteor was brightest. The smaller pieces started to fragment in turn at a height of 25 kilometers. At that point, about 13 seconds had passed since the beginning of the fall, and the velocity was still very high, about 18 kilometers per second.
However, the air density increases steeply close to ground, and the forces acting on the fragments became extreme. During just two seconds the fragments decelerated strongly, and passed the height of 17 kilometers at a velocity of just 6 kilometers per second. As a comparison, passenger jets normally fly at an altitude of 10 kilometers. At this point, the leading fragment had been reduced to a small rock of only 15 kilograms. The meteor left behind a trail of fine rocky dust in the form of a cylindric cloud with a thickness of a couple of kilometers and a length of about 50 kilometers.
One of the pieces that broke off one of the slower fragments at a height of 25 kilometers is called F1 and has an estimated mass of 400-500 kilograms. F1 survived all the way down to the ground, and calculations show that it landed in Lake Chebarkul, located 70 kilometers west of Chelyabinsk. About 300 meters from the calculated impact site is the actual crater – a seven meter large hole in the 70 centimeter thick ice. On October 16, a team of divers recovered a 570 kilogram rock on this site – if this boulder actually is a part of the Chelyabinsk meteorite remains to be seen.
In addition to this large fragment, thousands of smaller ones have been found with a confirmed extra-terrestrial origin, having a total mass in excess of 100 kilograms. The largest fragment found so far has a mass of 3.4 kilograms.
The impact frequency of small asteroids
An asteroid with a two meter diameter releases and energy corresponding to one kiloton of TNT when it enters Earth’s atmosphere. If the diameter increases to five-six meters, about 20 kilotons of TNT is released – as much as the Nagasaki atomic bomb. If the diameter is increased to 50 meters, and energy of 10 megaton TNT, or 500 Nagasaki-bombs. The question is – how common is it that small asteroids with diameters in the 2-50 meter range impact on Earth?
There are several ways of closing in on this issue. The first is to consider the near-Earth asteroids we actually know and calculate theoretically how frequently they should impact Earth. It is necessary to compensate for the fact that we often do not know all near-Earth asteroids of a certain size – but it is often possible to estimate how many that have been missed. Such estimates relies on statistics of how often and for how long asteroid surveys have been active, compared to the number of actual discoveries. For example, we believe that about 90% of all near-Earth asteroids with a size of a kilometer or more have been discovered. For such objects it is relatively easy to calculate impact frequencies. When it comes to asteroids in the size range 10-20 meters, we know of 500 such objects. However, this is nothing compared to the 20 million asteroids of this size we think is lurking around Earth’s orbit. In such cases the compensation is uncertain, the the calculated impact frequency is unreliable.
However, there is another way of investigating the existence of small near-Earth asteroids – by counting the number of small craters on the Moon. Asteroids with sizes of a few tens of meters will cause craters with a size of a few hundred meters on the Moon – these are easily counted thanks to the large number of spacecraft that has mapped the lunar surface. As it turns out, these two methods yield very consistent estimates of the impact frequency.
According to these calculations, there should be about four impacts every year, by asteroids being at least two meters in diameter. Asteroids that are five meters or larger should impact every second year. Asteroids that are ten meters or larger should impact every thirty years. Asteroids as large as the one that fell in Chelyabinsk should impact every 150 years. Asteroids that have diameters of 25 and 50 meters, respectively, who delivers an energy corresponding to 1 to 10 megatons of TNT, should impact on 300 and 3000 year intervals, respectively.
These somewhat uncertain estimates can be compared to actual observations of large impacts made by satellites and infrasound stations. Such facilities have virtually global coverage and detects impact both over land and sea, regardless if the area is populated or not. They have no difficulties in detecting impact by asteroids with sizes as small as a couple of meters. The problem is that these observations only have been running during a fairly short period of time – from 1994 to present, and at a small scale, between 1960-1974. The results may therefore not be entirely reliable, since the statistics is based on a fairly small number of objects. It is still interesting to compare the impact frequency we expect, based on observations of asteroids and lunar craters, with the actual number of impacts, as observed by satellites and infrasound facilities.
Data from satellites and infrasound stations turn out to agree very well with estimates based on lunar craters and asteroid observations, concerning the smallest objects. But the larger the objects become, the larger the difference between estimates and observations seems to be. The actual number of impactors being five meters or larger, is double compared with expectations. Objects that are ten meters or larger impacted every fifth year, which is about six times more frequently than expected.
We may now as if the Chelyabinsk meteor was an unusual event or not. The answer depends on how the comparison is made. We have had an essentially global surveillance of Earth’s “airspace” during the last twenty years. Based on the expected impact frequency, an event like that in Chelyabinsk only has a 13% probability of happening during such a short time span. If this is correct, the meteor over Chelyabinsk could be classified as a rather rare event. In almost nine cases out of ten should a twenty year long hunt for impacts of this magnitude come up with nothing – yet we hit the jackpot at the first attempt. This could simply be due to luck – or that the impact frequency actually is higher than previously assumed. However, if one uses the higher impact frequency obtained by extrapolation from the actual impacts of smaller objects, it is no longer surprising that the Chelyabinsk event took place – such impacts should happen every twenty years.
If Chelyabinsk had been an isolated case, it had been easier just to blame chance. But there are another two events that also seem out of place. During the period 1960-1974, infrasound surveillance of nuclear tests took place. On August 3, 1963, a large impact was registered that passed rather unnoticed since it happened over the ocean, outside South Africa. During this event, the energy equivalent of at least a megaton of TNT was released, which is twice as much as in Chelyabinsk. The probability that such an impact shall take place during such a fourteen period is just 3% – according to expectations.
To this list one can add the large impact at Tunguska in 1908, which is the largest we know of in historical times. During this event, up to 15 megatons of TNT could have been released, which is 30 times more than at Chelyabinsk. This impact was registered both by seismographs and meteorological stations around the globe. If we assume that we have had the possibility to detect such events during the last 150 years, when we have had seismographs, there is only a 5% probability that such an impact will take place during such a time period, according to expectations.
One may therefore ask if our expectations simply are wrong. Is it possible that impacts by 2-50 meter bodies in fact happens ten times more often, than we would expect from studying lunar craters and asteroids. If this is the case – what does it mean, and why is this kind of questions important?
The craters on the Moon constitute an archive that shows how the population of near-Earth asteroids has looked like throughout lunar history. Studies of the Moon shows that this population has looked pretty much the same during the last three billion years. Throughout all of this time, the number of asteroids has been fairly constant and the size distribution, i.e., the number of objects within any given size interval, has been more or less the same. This tells us that the population is a system in equilibrium. The number of near-Earth asteroids surely decreases steadily through collisions with the Sun and the planets, but since a re-population takes place due at the same rate, through transfer from the main asteroid belt located between Mars and Jupiter, the number of near-Earth asteroids at any given moment remains fairly constant. Asteroids of a certain size may disappear when they collide and smash each other to pieces, but they get replaced when even larger asteroids break up. During such a collisional equilibrium, a particular type of size distribution is formed. For example, for every asteroid with a 100 meter diameter, there should be about 150 asteroids with 10 meter diameter. Such a collisional equilibrium is clearly maintained in the main asteroid belt.
However, this kind of balance is never exact – there are natural fluctuations over time. For example, a collision between two main belt asteroids may cause a temporary increase in the number of smaller fragments, and it takes a while before they grind each other down and the balance is regained. It means that the influx of small asteroids to our region of space may increase temporarily as well. If the suspected elevation of the impact frequency is real, it is possible that we now experience such a temporary deviation from equilibrium.
Natural science aims at developing an understanding of the world around us and to describe it in detail. It is a matter of finding out how things really work and function, so that one do not have to guess or rely on unfounded assumptions. This urge to acquire knowledge is as old as humanity itself – the better we know our surroundings and environment, the easier it is for us to adapt, and the higher are our chance of survival. The more we know about actual conditions in nature, the better decisions we can make – decisions that affects our future life. Our thirst for knowledge is hardwired into our genes and is a prerequisite for our existence.
The efforts to characterize the properties of near-Earth asteroids aim at understanding the processes that are responsible for these properties. We want to know what happens when asteroids collide with each other, and we want to know how these fragments leave the main asteroid belt and find their ways onto Earth-crossing orbits. We want to know how often asteroids of different size collide with Earth, and we want to know what happens during such a collision. The meteor over Chelyabinsk is important in this context since it fills out gaps in our knowledge, it gives us opportunity to test models and theories, and it reminds us that we do not live in isolation on our planet – we are parts of an interplanetary environment that affects us, and we must get to know it.
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