Every year, for two months - from the end of June to the end of August - we can watch a dazzling sight in the sky, the meteor showers, also described as star showers. This spectacular natural phenomenon is examined by Yuri Voloshchuk, Dr. Sc. (Phys. & Math.), an expert on meteors and research scientist at the Kharkov Technical University of Radio Electronics (Ukraine). His name was recently given to an asteroid itemized in the international catalog under No. 13009.

Meteor showers are a flash, shortlived phenomenon observed in the middle layers of the atmosphere upon the entry of solid cosmic particles. Zooming downward, they become white hot (incandescent) and burn out within split seconds, producing the effect of "shooting stars". That is, if their mass is less than 10 grams. But if the mass is larger than that, a cosmic particle may reach the earth surface. In this case the cosmic voyager is called a meteorite.

Every twenty-four hours over 20 million meteors invade our atmosphere, their total weight approaching 5 tons a year.

Back in 1957 the Soviet Union's first laboratory was set up in Kharkov for conducting regular observations of this phenomenon.

And ten years after, it was developed into an automatic radar system, MARS, without peer in its sensitivity and functional potential. An immense database has been collected by now- on more than a quarter million of cosmic particles and their orbits. The Kharkov database is five-or even sixfold as large as all the world catalogs combined.

They are quite different, these particles, each having a life and biography of its own- much longer than the human life span. There are two different types of meteors- sporadic (single) and those occurring in streams, or stream meteors;. Sporadic meteors are a motley lot-these may be fragments of asteroids and comets of the solar system, or else "fugitives" from the moon and Mars; and they may be small hyperbolic bodies hurtling through the Galaxy's interstellar space. The meteor streams, however, are born of asteroids and comets only. The interest in meteors goes beyond scholarly science alone. These heavenly travelers may endanger life on our planet. True, not all of them but those shooting in a stream, in millions of particles of different size, including larger parental bodies drawn into their orbit. Thus far only a few tens of meteoric streams are known to be carrying such kind of bodies. For instance, the famous stream Hymenid, mentioned back in the ancient chronicles, visits the earth every year. And yet its orbit was determined but a hundred years ago, and its parental

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body, known as the asteroid Phaeton, was discovered only in 1983. Meanwhile the danger of collision is estimated to be rather high. As many as 4810 meteor streams (and their parental bodies, too) cross the earth's path, and of these 90 percent pose a collision threat!

Unfortunately no reliable methods are available yet for pinpointing a parental body's coordinates in a meteor swarm's orbit. In their forecasts scientists proceed from two assumptions: first, that a body like that is always in the middle, and second, that its orbit shows no significant aberrations. But even if there are exceptions to this rule and even if in some of the observable streams the "parents" have fallen apart, the probability of such type asteroids colliding with our planet is hundreds of times higher than for the small planets of the Aton-Apollo-Amor group.

By observations of Yu. Voloshchuk, many of the parental bodies are about 1 km large. Assessing the collision hazard, we have to take them into account. The point is that there is but little likelihood of their being discovered by conventional methods. A meteoric parental body heading for the earth will look like a small dot against the background of myriad stars, and might not be identified with the aid of ordinary optical devices. But using a radio telescope operating in the millimeter wave band (like RT-70 at Eupatoria), we can locate bodies just inches large.

Voloshchuk Yu. I., "The Swan Song of Celestial Bodies" (abridged). Izvestia, Aug. 9, 2003.

Digest prepared by Vladimir GOLDMAN

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