The problem comet hazard analyzed in detail in numerous publications. It should be noted that the greatest danger is a massive long-period comets, their appearance is often unexpected, because of arbitrary orientation of the planes of the orbits and large or very large orbital period. Moreover, many of these comets is aperiodic, i.e move in non-closed trajectories (parabolic or hyperbolic) and therefore are really new. These comets possible over a high speed collision with the Ground - up to 72 km/s (on a collision trajectory)that could lead to a global disaster. The possibility of such catastrophic events is proved by many facts. First, to date, on the surface of the Earth are found more than 230 large impact craters

Of course, most of these craters, most likely, were formed in the fall on the earth's surface rocky bodies that can penetrate the earth's atmosphere almost without being destroyed. It is likely that some part of the craters were formed by and large comet nuclei or bodies of intermediate composition. But collision with comets may lead to catastrophic consequences. Some scientists believe that immediately after its formation at high temperatures and cooling the earth's surface was very dry (for example, as now the moon), and that almost all the water and other volatile compounds were delivered by the flow of comets that hit at a time on the Earth. By the way, the comet could deliver not only water, but also complex organic compounds which occurrence in the earth conditions, as some believe, was unlikely, and thus created the basis for the emergence of the simplest organisms. Although it is still and hypotheses, but also Tunguska phenomenon, there are other facts confirming the fall of comet nuclei in the past on the Ground. For example, one of the most mass extinction of flora and fauna over the last 230 million years occurred 65 million years ago (between the Mesozoic and Cenozoic of biological eras or on the boundary of the Cretaceous and tertiary geological periods)when the disappeared about 2/3 of all living organisms, including the dinosaurs. With the same time in geological deposits of the earth surface is associated layer with a high content of extremely rare on Earth element iridium

Scientists L. Alvarez and S. Vandenberg showed that the content of this element in that period on the earth's surface could greatly increase as a result of the fall of a large comet's nucleus (with the diameter about 10 km)with high content of iridium. There was even found a crater with a suitable age and relevant morphological features, which might arise during such event. The crater, named Chicxulub, has a diameter of 180 km and is located on the Yucatan Peninsula in Mexico. But the reason of extinction of living organisms could not be increased concentration of iridium and explosion caused by the collision of the comet's nucleus with the earth's surface, which led to the release into the atmosphere (including in its upper layers) huge amounts of dust. Global dusty atmosphere inevitably leads to a sharp drop in temperature of its lower layers (10 degrees or more), as the dust screens solar radiation. This change in average temperature can be stored up to 1 year - the so-called effect of "nuclear winter" (it is also inevitable in the mass use of nuclear weapons, and so it was appropriate name). It is likely that this effect is caused by falling of a large comet's nucleus (but it could be an asteroid) on the earth's surface 65 million years ago, and led to the catastrophic death of living organisms.

Another confirmation of the reality of cometary nuclei collisions with planets is a unique event, which occurred "in front of" the whole mankind. Mean the fall of fragments of comet shoemaker-levy 9 with Jupiter in July 1994, the comet was discovered around Jupiter in early 1993, after split into 20 parts, which were distributed along its orbit in the form of a luminous "heavenly necklace". As shown by simulation of motion of the comet "back", it was either ripped o" remote icy moon of Jupiter, or previously captured by the planet-giants ordinary comet. Most likely, the comet nucleus was torn apart by the tidal forces when passing close to Jupiter. The fall of fragments of the comet's nucleus with sizes from 1 to 10 km at a speed of about 60 km/s took place from 16 to 22 July 1994 on the reverse side of the southern hemisphere of Jupiter. It is not possible to directly observe the effects of collisions. But the consequences of falling was observed on the visible hemisphere of Jupiter after 40-50 minutes because of its rapid rotation. They were great. Traces of explosions in the form of huge dark spots and diverging from them ring shock waves (of diameter comparable with the Earth) on a background jupiterians atmosphere was observed in all the observatories of the world. But the best quality images were obtained using the orbital telescope "Hubble" working outside the earth's atmosphere.

But before to start discussing the question of the protection of Land from large comets and asteroids, let us consider briefly discuss more "simple" problem - and it really is great danger of collision of the Earth with a comet?

Speaking mathematical language, we are talking about finding the probability of crossing orbits of the Earth and comets almost at one point. Intuitively it seems that this probability must be extremely small, but what is the quantitative assessment of this smallness? To answer the question it is enough to note that the observations about 200 long-period comets we know that about 5 comets per year cross the plane of the orbit of the Earth (remember that we are talking only about known comets-type comets Charles V or comet Halley).

For the collision is necessary that the trajectory of at least one of these comets intersected with the position of the Earth, or rather would have passed through the circle area in 1/4 of the earth surface area. Calculations show that the probability of such event, as expected, extremely low - about one event in 50-100 million years. But the geological age of the Earth is close to 4.5 mgle. So, during this time we "visited"at least several tens of comets? And when you consider that each year, astronomers discovered another 3-5 new comet, comet factor is becoming one of the most important elements of the earth's history! This conclusion is also confirmed by the observations of craters of the moon, many of which were formed under the influence of comets, asteroids and meteorites.

Linking "optimistic" and "pessimistic" perspective on comet threat to Earth, you can specify the most reliable interval for such events - about once every 10-20 million years. Is it much or little? Whether it is necessary to take into account the comet hazard and to take some preventive measures? For those, who is "optimistic" (in my lifetime), note that the above estimates characterize only the frequency of events, but in principle can not give an answer to the question when the event happens. It should be clear that in everyday life we intuitively assess the risk factor. Even without any special explanations, each of us is aware that the danger from moving car is much higher than the danger from falling from the roof of a brick or accident of the plane. Therefore, the first thing "insure" potentially the most likely events. But as to this logic to the man who brick fell! We understand that the probability of an accident at the nuclear power plant is extremely small. And, despite this, we have witnessed the horror of Chernobyl. Consequently itself to myself, the probability of some event or other important but insufficient to characterize the severity of the phenomenon.

Scientists have long understood this fact in the development and design of complex systems. To compare different probabilistic properties and exposure of human events, it was suggested to use the concept of "hazard"equal to the product of the probability of the event on the number of potential victims. For example, every year on the globe in the aviation disasters killed about a thousand people. Statistics show that the probability of a catastrophe is about one event per month. In this case, the risk of fatal accidents for humanity is about 80-90. Given that the major collision of a comet with the Earth will be accompanied by the death of all living beings, the rating of this threat exceeds aviation almost 5-7 times! The probability of the effect is small, but the consequences of global!