by Anatoly MIKISHA, Cand. Sc. (Tech.), Mikhail SMIRNOV, Dr. Sc. (Phys. & Math.), RAS Institute of Astronomy
The fall on the Earth of a heavenly body, like an asteroid, big meteorite or a comet, is something that does happen now and then but, fortunately enough, not too often.
Since the dawn of mankind there has been quite a number of such events. In ancient times our forefathers usually took this as divine interference into our earthly life - a punishment likely for our sins. And no one could even dream of resisting this heavenly wrath, or protecting oneself from it.
But all that are things of the past.
And what about now?
Do we have any means and methods which can help us anticipate and prevent a catastrophe of this kind?
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"FALLING STARS", "METEORITE SHOWERS", MINOR PLANETS...
Ancient legends and chronicles always described as miracles the flights of big comets and the falls of "heavenly rocks" on this planet. As for scientists, always preoccupied with facts, and not fiction, they not only watched such events, but tried to systematize them ever since the time of the Renaissance. The astronomical science over the past 200 years has proved that the space between the planets of the solar system is not some empty void, but something filled with objects of different shapes and sizes-from tiny particles to rocks measuring thousands of kilometers across. All of them are minor heavenly bodies which are subdivided into interplanetary dust, meteorites, asteroids and comets. Also identified in recent time are what we call mini-comets.
Objects of any size moving through space can approach our planet and even fall upon its surface. If such an object is small, then, as it hits the Earth's upper atmosphere it is enveloped by a layer of hot plasma and simply evaporates. The scientific name for such bodies is meteors and the common folk call them "falling stars": they suddenly flare up in the night sky, leaving a bright "tail" behind them which rapidly fades away At times there occur "meteor showers" when the Earth runs across whole "streams" of such heavenly bodies. Well known among them are the Perseides, Leonides, Taurides, etc., named after the constellations of their origin. Stellar "showers" associated with the Perseides are usually observed each year during nights approaching August 12, and with the Leonides - around November 17.
But the picture is very different when our planet encounters a bigger body in space. Entering the lower atmosphere, this body evaporates only in part and sometimes disintegrates or blows up, and, having lost its velocity, finally drops on the ground. Such flying objects are called bolides, or fireballs,
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and those reaching the earth's surface - meteorites. Their impact, especially of the bigger ones, can precipitate a catastrophe on a local scale. That is why, with time, scientists became seriously preoccupied with this threat and launched serious studies of such minor heavenly bodies. The main landmarks along this road can be listed as follows:
1577 - Danish astronomer T. Brahe came to the conclusion that comets are genuine celestial bodies.
1682 - Swiss mathematician Jacob Bernoulli suggested for the first time the possibility of a comet colliding with the Earth.
1696 - British astronomer William Wilson stated: the comet which appeared in 1680 must return in 2255 and collide with the Earth, precipitating the end of the world.
1794 - German physicist, R Chladni (Foreign Member of the St. Petersburg Academy) proved that "stones dropping from the sky" (meteorites) are of cosmic origin.
1801 - Italian astronomer G. Piazzi (Foreign Member of the St. Petersburg Academy) discovered the first asteroid; in keeping with a long-established tradition of naming smaller planets after deities, he called it Ceres. Over the past 200 years astronomers have observed dozens of thousands of such objects (some of them "lost" and rediscovered). Asteroids observed on many occasions were given "personal" numbers and their orbits are well known.
1906 - American geologist J. Barringer proved that the Arizona crater in the United States had been formed by a meteorite impact.
1908 - the fall of the Tunguska meteorite in Eastern Siberia.
1924 - American geologist A. Gifford revealed the true nature of the giant craters on the surface of the Moon. In his view they had been formed by meteorite impacts.
1928 - Russian researcher Leonid Kulik began studying the aftermath of the fall of the Tunguska meteorite.
1932 - discovered for the first time was an asteroid crossing the orbit of the Earth (1862 Apollo). Since that time it became clear that asteroids exist not only in their main belt - between Mars and Jupiter, but also outside it.
1984 - purposeful search for asteroids was launched in near-Earth space; experts introduced the notion of "asteroids approaching the Earth".
1992 - astronomers launched observations of smaller bodies beyond the orbit of Neptune (Kuiper zone); studies of their movements proved that they can be migrating towards the Earth.
1994 - authors of this article suggested that meteorites flying towards the Earth are linked with meteor showers.
1995 - observations started of major bodies in meteor showers-a systematic search of meteorites approaching the Earth.
ROCKS FROM THE BLUE?
Nearly once in a century the Earth is struck by a sufficiently big meteorite capable of causing appreciable material damage and loss of lives, especially if it hits a densely populated area. One Chinese chronicle, for example, reports the fall of a large rock in the Shanxi Province in 1490
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which took a toll of 10,000 lives. We also know that large pieces of interplanetary debris, similar to the Tunguska meteorite, had been "landing" on our planet even before. For example, some 800 years ago on the Southern Island (New Zealand) a catastrophic event took place similar to the 1908 calamity in Eastern Siberia. The blast and the fire it caused wiped out forest on an area of tens of square kilometers.
Known from the latter half of the 19th century in Arizona, USA, is a crater called "Devil's Canyon" with a diameter of 1,240 m and depth of 170 m. Different hypotheses about its origin were suggested with some experts considering it to be of volcanic origin, while others thought it was produced by an explosion of steam, and others still calling it a karst hole. And only in 1906 it was proved that the crater is of an impact origin because found there were some 12 tons of meteorite matter. The giant crater appeared 50 thousand years ago after the fall of a ferro-nickel body from space of about 60 m in diameter. Due to a high velocity of its approach to the Earth (20 km/s) the energy of the explosion which produced the crater must have been of 10-20 Mt (the biggest nuclear blast on the surface was of 50 Mt).
Naturally enough, the Arizona crater is not the only one of its kind on the Earth's surface. More than 230 similar ones are known now which are also called astroblems, or "stellar wounds". And far from all of the Earth's surface has been investigated from this point of view, especially the bottom of the ocean. On the other hand, although all of such craters are believed to be of meteoritic origin, it could very well be that our planet was "hit" in the past by much more massive celestial objects like asteroids and comets. As for meteorites, scientists regard them only as bits and pieces, or fragments thereof. The thing is that practically all of the mentioned meteor streams are linked with some con-
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crete comets: the Leonides stream with the Tempel-Tuttle comet, and Taurideswith Encke. Incidentally, the radiant (vertex) of the latter matches that of the Tunguska meteorite which strongly suggests a "cometary" origin of the East Siberian phenomenon.
In short, the falls of meteorites on our planet as well as its collisions with asteroids and comets pose a direct threat. At the same time detailed studies of the movements of asteroids approaching the Earth have led scientists to conclude that not all of them pose a threat, but only those which cross the Earth's orbit. It is also important to bear in mind that the orbits of asteroids approaching the Earth are not constant in time, but change gradually over tens of thousands of years. These heavenly bodies can also change their course (for example, under the influence of large planets like Jupiter or Saturn) and even stop approaching the Earth. And such changes can be dramatic. For example, some 23 thousand years ago an asteroid approached Venus at a distance of 47,000 km and the bigger semi-axis of its orbit was reduced to such an extent that in 1932 astronomers were able to discover it (the already mentioned 1862 Apollo). In general scientists recognized the possibility of collisions of asteroids with the Earth in the 1980s. Observations of their close fly-bys near this planet produced a situation that, first the experts, and later the public at large began to share the common anxiety about this threat from such passing heavenly bodies - the "asteroid threat" so-called.
Passing through near-Earth space are also many comets, although they seldom come as close as asteroids. At the same time it is a rather difficult task to determine the number of comets approaching the Earth. Their orbits, as a rule, are unstable and changing rapidly. This is connected with the usual gravitational disturbances and also with the fact that, passing time and again near the Sun, a comet gradually "ages" ("slims down"), losing some of its matter, it changes its orbit and, finally, disintegrates. But its place is then taken by new cometary bodies, and a lot depends on their origin (how they are born), including their contribution to what we call the asteroidalcometary threat. Unfortunately, we are still in the dark as to whence these comets come.
Most of these heavenly bodies move around the Sun along elongated elliptical orbits, and the circle comes full swing either in less than 200 years (short-period comets) or in more than 200-year periods (long-period comets). The orbit inclinations of the latter to the ecliptic plane of the Earth's movement are distributed randomly; whereas the inclinations towards it of "short-period" comets (some 150 are known now) are small as a rule. Astronomers are studying such smaller bodies, and not only in theory, but also experimentally (remember the flight of a space probe to Halley's Comet in the 1980s).
At the present level of observation technology it is quite possible to identify in near- Earth space practically all asteroids, or at least the bigger ones - from tens of meters in diameter - as posing a certain threat. It goes without saying that organizing such monitoring will require lots of effort and money And what is more, one can set up an asteroid tracking service which will be able to pre-calculate their movements along changing orbits.
But the picture is far more complicated as regards comets. They are far more difficult to detect because of the great distances involved. And the appearance of "long-period", and especially of a periodic ones (which have no constant period) is unpredictable in principle. And if we want to gain time for taking appropriate measures well in advance, we should be able to detect such objects at the very approaches to the solar system. And this must be long before a comet develops its bright "tail" of cosmic gas and dust. All of these things are also important because, due to certain dynamic and kinematic features, they have a prominent role to play in the potential space menace. Say, "long- period" and near-parabolic comets (with orbits
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close to parabola) in proximity to the Earth's orbit travel at greater velocities than the "short-period" ones, to say nothing of asteroids. What is more, orbits of comets are oriented in space in a disorderly way, at random with respect to the ecliptic plane in which our planet is moving. And that means that there can be head-on collisions with the Earth when the summary velocity of the two approaching bodies reaches 72 km/s. And it turns out that a good quarter (and even more than a half, according to the latest data) of such potential "attackers" can realize upon collision with the Earth an amount of energy of the order of 10 5 Mt of the TNT equivalent. And these are the "long- period" comets.
Finally, it should be stressed that practically all such heavenly bodies (with periods of several hundred years and more) were first discovered rather recently, although they had appeared near the Sun centuries ago. But then they could not be detected by optical means. And today comets are viewed as unexpected "guests" crashing in from very remote, perhaps even interstellar space. One can mention byway of example the recently observed Hale-Bopp (discovered in 1995) and Hyakutake (1996) comets. In assessing their likely threat, however, one has to admit that they are "rare guests" appearing about once a year only. And the Earth is a rather small target for being hit by one of them.
OBSERVATIONS DECIDE EVERYTHING
How can one assess the likely menace coming from space? The answer depends, above all, on our knowledge of the density of the "population" of smaller bodies in the solar system. Within this general context there are several problems facing the astronomical science today.
To begin with, it is necessary to identify all major asteroids and keep a catalogue of their orbits with high accuracy. This should make it possible to decide in due time which of them may be a threat to the Earth, and whether or not these bodies would collide with our planet or bypass it in direct proximity. Special monitoring and tracking equipment is being developed now for a purposeful search and catalogization of asteroids; and there is every reason to believe that by the year 2010 almost all of the dangerous celestial objects from the main asteroid belt will be identified and listed in catalogues.
Next on the agenda is a systematic search for comets and their tracking. Needless to say, it would be impossible to list all of them, but when it becomes clear where they come from, we shall be able to determine their orbits with confidence and predict their appearance in the solar system or near the Earth.
The third of these tasks is connected with the detection of dangerous celestial objects on their approach to the Earth. Those of them measuring less than hundreds of meters, be it asteroids, meteorites or mini-comets, become visible only in the immediate surroundings of our planet. They can be detected in a matter of weeks or days. But as regards objects of smaller dimensions - measuring in decameters (such as the Tunguska meteorite) the time of their approach to the Earth after their detection will not exceed tens of hours only. And the smallest of such bodies are studied in a more or less systematic way only in the process of their incineration in the Earth's atmosphere (meteors and bolides) or - in some very rare cases - in the form of meteorites reaching the ground surface.
And there is yet one more version of detection of hazardous objects - the search for and tracking of all bodies moving along trajectories of possible collisions. Ground telescopes are
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capable of detecting them 10 to 15 days in advance of any such events, that is within several million kilometers away from the Earth. Therefore we should organize "patrolling" of the whole sky with a large number of apparatuses. However, even then it would be impossible to detect objects flying from the Sun and visible only against the bright background of daytime sky. In a word, the proposed system of detection of dangerous objects can be implemented only with the help of special tracking systems from space.
Today successful studies are in progress in many countries related to the methods of monitoring of meteor and bolide streams for the purpose of the earliest possible identification of any dangerous cosmic objects. So far this is being done only at great distances from the Earth, and the problem of their detection in direct proximity to this planet remains absolutely unresolved. It may well be that this problem defies any solution in principle with the help of ground monitoring only, and what we really need is space observation technology.
And one more thing in conclusion. Unfortunately, the currently available accuracy of trajectory tracking of space objects is not sufficient for any active countermeasures from the Earth (such as fracturing asteroids by powerful sources of energy). But there is still hope that more detailed astronomical studies will help us in the not too distant future to translate the unpredictable collisions of comets, asteroids and other celestial bodies with the Earth from the category of random events into that of events which can be predicted on the basis of observations and theoretical calculations. This will open up the way to developing methods of protection of this planet from cosmic threats.
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