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The subject of today's article is impact risk and concludes the three-part series looking at 'fireballs falling to Earth'. Risk is the index used to estimate the dangerousness of a potential catastrophe. In the case of meteorite crashes, risk depends on the potential harm caused by the asteroid and the on probability of impact. So, for impact risk to be significant, it is necessary that the combination of both impact probability and effects is high enough.
Impact probability
In the previous article (part 2) we saw that near-Earth objects are very sparsely distributed in space. In fact, in this case “near-Earth” means that the nearest point of the orbit of the space body to the sun is less than 1.3 times the diameter of the Earth's orbit. In consequence the probability of an impact is very low, and on average it takes about 625 million years for a near-Earth asteroid or comet to hit the Earth. However, not all near-Earth objects have the same impact probability, those passing nearer to the Earth have a higher impact probability. In fact, for potentially hazardous bodies, those that will pass at less than 7.5 million kilometers (5% of our planet's orbit diameter) from the Earth's orbit sometime in the next 100 years, the probability of a collision is more than 5 times higher and in average it takes an individual member of this class about 112 million years to impact the Earth.
To estimate the probability of a collision with a meteorite of a given size class, we need to weigh individual impact probability by the expected number of meteorites in that size class. In the table below you can see the results for potentially hazardous bodies. For example, we would expect an impact of a meteorite in the size class of the Tunguska event, between 35 m and 44 m, once every 500 years approximately.
Table: Impact frequency and time recurrence for different sizes of potentially hazardous objects. Source: NASA's Study to Determine the Feasibility of Extending the Search for Near-Earth Objects to Smaller Limiting Diameters.
Impact effects
As a rule of thumb, smaller meteorites (less than tens of meters in diameter) cause no harm. Somewhat bigger meteorites, up to about 1 km in diameter, cause local effects. Finally, meteorites above 1.5-2 km can cause global effects. For bigger space bodies, such as rogue planets, even a close miss can be dangerous, since gravitational interactions can pull the Earth and/or the Moon out of their orbits with – let's put it like this – very bad consequences. (Fortunately, rogue planets are nowhere near, they're so big we would have already seen them).
If you want more detailed estimations, you can try the online program Earth Impact Effects, developed by G. Collins, J. Melosh, and R. Marcus. For example, you might want to try what the program says for the fireball observed in Russia about one month ago. In that case, you'll need to know that according to NASA, the Chelyabinsk meteorite had a diameter of 17 m, a mass of 10,000 tons, and entered the atmosphere at a speed of 40,000 mph (64,000 km/h)1.
Impact risk
To calculate risk, we multiply the expected effects of a collision (for example in casualties) by impact probability or rate (e.g. in impacts per year). By doing this for different size classes we can estimate which is the meteorite size we have to pay more attention and down to which size it is reasonable that we extend our search.
For example, if the Moon were to impact the Earth the effects would be cataclysmic, but since the Moon is in a stable orbit around the Earth it is unimaginable that such impact could occur (impact probability is zero) and so the Moon poses no risk. On the other side, in a year an impact with a meteorite smaller than 3-4 m in diameter is almost sure. But there is no risk here either since they're so small they disintegrate during their entrance into the atmosphere as common shooting stars.
Maximum risk is actually due to space bodies of sizes between 1 km and 3 km. Although such impacts are estimated to occur about once in a million years, they are able to create planet-wide havoc and eliminate the complete human population of billions of human beings. In consequence, their annualized risk is on the order of a thousand casualties per year. And regarding the minimum size we should worry about, the Near-Earth Object Definition Team suggests a size of 140 m, since for smaller meteorites the expected impact effects do not outweigh the cost of monitoring them.
Vigilance of risk
So, it is worth being vigilant on potential risks coming from space. And that's what NASA's Near Earth Object program does. It looks at the orbits of potentially hazardous bodies to refine the estimation of impact probability for individual bodies, their estimated impact effects, and consequently their risk. For most of them the risk is negligible, either because the probability of a collision is very low, either because they are small. In fact, at the moment of writing only one of the identified potential impactors presents a minimal risk (a risk of 1 in a scale of 10). Its name is 2007 VK184, it is 130 m wide and according to present calculations will pass at less than 5000 km from the Earth on June 3rd, 2048 (the Earth's radius is 6370 km). We don't expect it to impact the Earth then, but if it were it would produce a crater about 2 km wide and an earthquake of magnitude 5.8 on the Richter scale (same magnitude as L'Aquila earthquake). That's why its risk evaluation indicates that “merits careful monitoring” to account for possible present errors in calculating its orbit or possible unexpected deviations from its course. In that case, though, it might be possible to use more accurate and up-to-date orbital data to calculate the impact time and location in advance and, if necessary, take preventive and/or adaptive measures such as evacuation.
Figure: Orbit and position of asteroid 2007 VK184 on March 21th, 2013. Source: JPL Small-Body Database Browser.
So, to sum up, about 90% of near Earth objects larger than 1 km have already been identified and for the next 100 years no impact is expected. For smaller objects down to 140 m in diameter, the list is no near complete yet, but impact probability is very low and the list will likely reach the 90% level of completion in a few years. Finally, once or twice in a century we expect an impact of meteorites the size of Chebarkul one, but their effects are limited and actually less risky than other natural disasters we have to put up with, such as earthquakes, hurricanes or floods. Of course, it is still worth keeping an eye on the sky, just in case. Just remember that even if the probability of a life-destroying impact is very low, it will eventually happen sometime in the future. We just don't know when, it could happen in two hundred years or in a million years. Being alert is worth the effort. The cost is not that big, and the potential benefits are huge.
  1. In case you tried giving this input to the program, you'll see it does not predict the airblast that actually took place. As any model, Earth Impacts Effects has a certain degree of uncertainty in the value of the parameters used, which results in a certain degree of uncertainty (imprecisions) of the predictions. On the other side, natural variability (shape, inhomogeneous composition, etc.) cannot be taken into account by the model and this results also in uncertainties in the predictions. And there are also uncertainties in the estimation of the characteristics of the meteorite made by the NASA… Luckily there are statistical methods to account for such uncertainties. But that's another story…

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