Channel 1 of Russian TV showed on February 22, 2013 a documentary film "Chelyabinsk Meteorite: Seven Days Later", describing in detail a shocking state of a great number of citizens of this city after a bright flash in the blue morning sky, which had taken place in the morning of February 15. Then followed an explosion, accompanied by howling of car sirens, clinking of broken glass, switching off of mobile phones and rapidly forming traffic-jams on the roads.

The film contains a lot of emotional assessments, including the opinion of director of the Ural Mechanical Engineering Plant, the shops of which looked like after an air raid: "We have never experienced anything like that in peace time. People are gradually coming to their senses, but the feeling that we are not protected as we have thought before still remains."

According to the report of the Ministry of Emergencies, as a result of the meteorite explosion over the biggest industrial center of Southern Ural there suffered 1,240 people, 52 were hospitalized, 4,715 buildings were damaged, 190,000 m² of glass was broken, material damage exceeded 1 bln rubles. In elimination of aftereffects were involved 4,660 men of the staff and 1,047 technical devices.

But actually Chelyabinsk had the good luck--should the body flight direction turn to be 30 km to the north of the place and the angle of the entry to the atmosphere a bit steeper, the consequences for the city with a million population could have been extremely dramatic.

The size of the falling body is estimated as 18-20 m, and the mass--approximately 10 thous. tons. According to usual human standards this is too much, but for the Solar system--only one of those cosmic grains of sand, several millions of which are revolving around our star in the asteroid belt, located between Mars and Jupiter orbits. When the next such "grain" will emerge on the way of the Earth and where will it fall--unfortunately nobody can yet say.

The flight and explosion over the Chelyabinsk region caused a lot of comments in mass media. One of them was mentioned in the Science in Siberia newspaper: on its pages Vyacheslav Gusyakov, Dr. Sc. (Phys. & Math.), head of the laboratory for studies of tsunami of the Novosibirsk Institute of Computing Mathematics and Mathematical Geophysics (RAS Siberian Branch), told about his attitude to such abnormal phenomena. The interest to the "celestial" subject was initiated by his teacher Acad. Anatoly Alexeev, a specialist in the sphere of theoretical and computing geophysics--last years of his life were devoted to the problem of threats coming from outer space. Several works were carried out under his supervision to assess comet-asteroid danger and possible measures of counteraction.

Alexeev was interested in the possibility of rousing tsunami in case of falling of cosmic bodies into the ocean. In such context were analyzed instrumental notes (they are made only for approximately last 100 years), concerning the problem of origination of such destructive waves in the Pacific Ocean, but such analysis turned out to be fruitless. Later on it became evident that a century was a rather short time interval for revealing anomalies the scientists were interested in. However, the situation began essentially changing when the researchers, on the basis of historical and geological data, came upon intervals in thousands and tens of thousands of years. It turned out that there exist a lot of facts of cata-

strophic events, which took place already in the historical time (last 2-5 thous. years) possibly and rather probably due to either a close flight or a fall of cosmic objects to the Earth (comets, asteroids) as well as near (according to cosmic standards) explosions of supernova stars.

The international conference "Comet and Asteroid Danger and Mankind's Future", carried out under the ICSU auspices, was held in December 2004 on Canary Islands. It presented reports of leading world experts dealing with all aspects of this problem, from astronomical to socio-political and economic ones. Gusyakov also participated in its work as a representative of the Commission for Georisks of the International Geodesic and Geophysical Union. According to him, it was there that scientists from Australia, USA, Russia and Northern Ireland decided to combine their efforts and create an International Working Group for impact events in Holocene (HIWG), which intended to collect facts on the reality of some biggest climatic catastrophes, which had occurred approximately 11,900 and 4,350 years ago, as well as in 536-540 A.D., and their connection with cosmic factors.

At present the group consists of several dozens of specialists, who are confident that the striking action of cosmic bodies exerted greater influence on climate variations of the Earth and the development of civilization than it is commonly acknowledged. The collected facts confirm the thesis that such threats are quite real and cannot be ignored during integral assessments of risks of natural disasters. The incidence of cosmic objects is regular and not once had heavy consequences.

Every day several tons of cosmic dust gets on our planet in the form of smallest particles. Bigger ones (from 1 mm and over) seldom leave fiery traces in the night firmament. When the number of flashes reaches several hundreds per minute, they speak about meteorite rain. And if falling fragments reach the surface of the planet, such "rain" is called meteoritic. Here are two well-known examples. On April 23, 1803, not far from the French town of L'Egl in Normandy, there fell over 2.5 thous. stone splinters. The heaviest in the 20th century meteorite rain fell in 1976 in the Chinese province of Jilin. The total mass of the then collected fragments made up more than 4 tons; the biggest found fragment weighed 1,770 kg. In case of increasing of parameters of the falling body to several meters and more, the visible route in the atmosphere is also increased, while braking in the air more often ends in thermal explosion at the height of several kilometers. Such body is called bolid. Its fragments can fall on the earth after the explosion, but more often it is fully destroyed and in that case the search for its traces becomes fruitless. However, the "newcomer" bigger than 150-200 m manages to reach the Earth--and as a result of the blow and explosion there is isolated main part of energy and a meteorite crater is formed. Almost 200 of them have already been found and a majority of them are included in the Canadian database of impact structures of the Earth, supported by the Center for Planetary Studies of the Broonsvik University. At present it contains 184 structures, the diameter of which varies from several tens of meters to 200 km and the age of the oldest crater (Vre-defort, South Africa) is 2.2 bin years.

A great number of researchers widen the list of craters at the expense of not only proved but also of presupposed structures. The Expert Database on the Earth Impact Structures (EDEIS), supported at Gusyakov's laboratory, includes 1,118 structures, among them, according to

him, 213 can be regarded as proved, 187--probable, 499--perspective and 99--presupposed. It is very difficult to establish the age of their formation, though just these data are necessary to get assessments of the expected frequency of falls of cosmic bodies, which is very important for the contemporary stage of the Earth's geological history, i.e. for the Holocene.

There are approximately 10 proved craters for that epoch and discovery of each new one essentially changes the indicated assessments. That's why the abovemen-tioned group of specialists has concentrated on the studies of impact events, which took place for the past 10-12 thous. years. In 2006 and August 2012, the expeditions to the island of Madagascar and Australian Grut-Island were carried out to study the so-called long dunes: the mechanisms of their formation is still arbitrary. According to Gusyakov, they could have formed as a result of splashes of gigantic waves of tsunami on the coastline, the source of generation of which in both cases could be only comet or asteroid blow in the Indian Ocean and Gulf of Carpentaria.

The basic countries with the programs of systematic discovery of dangerous cosmic objects are the USA, Great Britain, Italy, Japan and Chili. The leading organization engaged in search of asteroids and small planets is the Center of Small Planets, located in Cambridge, Massachusetts (USA); it is financed and controlled by the NASA. It is a component part of the Smithson Astrophysical Observatory and the Smithson State Research Institute. Almost 10,000 objects have been found during the years of its existence (from 1995 up till now).

At present the International Consortium is formed to create an Alpine observatory in Chili: One of its basic devices will be LSST (Large Synoptic Survey Telescope) with an 8 m diameter mirror: it will allow to find a majority of objects of the size of more than 200 m in a ten-day period. Russia was invited to join this consortium, but such participation requires some financial contribution.

Unfortunately, notwithstanding all efforts and investments, no hundred percent guarantee of timely discovery of dangerous asteroids exists yet. The rather big such object 2012DA14 with a diameter of around 45 m and a mass of 130 thous. t (its trotyl equivalent in case of incidence could have made up approximately 2.5 Mt), which passed at a distance of 27 thous. km from the Earth in early hours of February 15, 2013, had been discovered only a year ago by a Spanish amateur astronomer. Only after that NASA and other cosmic agencies got interested in it.

But asteroids from Kuiper belt (this is how astronomers call the space between orbits of Mars and Jupiter, where millions of fragments of different sizes are moving, representing, according to one of the hypotheses, remains of the planet once existing there and destroyed also as a result of cosmic cataclysm) represent only a part of the problem. According to a group of British astronomers, non-periodic comets can be more dangerous, suddenly emerging in the inner part of the Solar system from the so-called Oort cloud, which surrounds it and travels with it along the expanses of the Galaxy; it consists of approximately 10 bln comets, chaotically moving along its own orbits. From time to time one of these "wanderers" for some unknown reasons leaves such orbit and heads for the Sun, takes a turn around it and again disappears in cosmic expanses. The mass of these cosmic "wanderers" can be enormous (billions of

tons), and even hitting the tail part is fraught with danger for the Earth's biosphere.

Comets are not always loose formations, consisting of snow and ice. Some of them as a result of repeated passing near the Sun are losing their flying components and turn into rather compact structures, consisting of almost pure carbon (i.e. actually of graphite). Such "wanderers" are invisible in any optical range and can be detected only by means of radars.

So, what is to be done? Simply sit and wait till the next asteroid falls on us from the sky? No, of course, not, assumes Gusyakov. "We must admit the reality of cosmic threat for our planet together with other natural disasters, adequately assess a risk level and possible consequences and begin preparing programs for reduction of risks and training the population to act properly in case of threat from cosmos. Without such preliminary measures, all urgent warnings and notifications are fruitless. On the contrary, they themselves can become a source of danger, material and human losses. The problem specifics lies in the fact that probability of such disaster in visible future (10-50 years) is very small, but consequences for the whole civilization can be too great to ignore its possible appearance."

At present it is commonly acknowledged: the threshold size value of the asteroid capable of arousing a global climatic disaster with the most serious consequences for our planet is 1 km diameter. The trotyl equivalent of its explosion in case of falling on the Earth will make up approximately 50-70 Gt. According to the available estimates, the total number of such bodies in the Solar system makes up around 1 thous. It is considered that today the number of detected asteroids of this class make up around 90 percent. At this moment there are no bodies located in the trajectory of dangerous proximity with the Earth in the following 50-100 years among them. The number of heavenly bodies in the subkilometer range (100-1,000 m) makes up already many tens of thousands. Their complete detection will require several decades of continuous observations.

In conclusion Gusyakov pointed out: "Studies of the past falls of cosmic bodies on the Earth and consequent events is a complex scientific task requiring involvement of different specialists. There are all necessary conditions for such integral studies at the RAS Siberian Branch. However, conservatism and prejudice of the certain part of scientific fellowship is rather difficult to overcome. In 2009, we tried to form a joint project 'Major Natural Disasters in the Holocene and Their Impact on Climate Variations and State of Ecosystems in Northern Eurasia', which was submitted in the name of five institutes to the competition of integration projects of RAS Siberian Branch. But the project was not supported by the High Academic Commission, due to the fact that representatives of too many different scientific subjects--from mathematics and geology to dendrochronology and history--were included in it."

And as a postscript: Secretary of the RF Security Council Nikolai Patrushev spoke out for the fastest creation of an international anti-asteroid program, in which Russia should take the most active part.

Chelyabinsk--15.02.13: What Has Happened?--"Science in Siberia" newspaper, Nos. 8, 9, 2013

Illustrations from Internet-site Chelyabinsk.ru

Prepared by Sergei MAKAROV