Our planet is actually a moving target for a great many asteroids. Say, if a giant body 10 km in diameter hits the earth, it will wreak havoc and, at best, throw humankind back into the Stone Age. Even a 50 m object will cause much destruction-like what is described as the Tunguska meteorite did early in the 20th century, wiping out hundreds of square kilometers of taiga forestland in Siberia. Now, what is the likelihood of a like disaster? Experts say the probability factor is 1/5,000-th at is, the same as for an air accident.

The threat is ever present. One asteroid, 70m in diameter, was discovered on the fourth day(!) of its fly-by past the earth on March 8, 2002. It acted like an ace flyer in an air battle and approached the earth from the sun, thus beating telescopic observation.

The world's best minds are attacking the problem of a cosmic shield for our planet. The United States is planning to set up a coordination center for collecting every kind of outer space "reconnaissance data".

No "space defense doctrine" has been developed yet. The present scenarios of preemptive measures look just as grand as staged events a la Hollywood. At any rate, the blitz battle is gonna be too tough even for automatic vehicles or missiles. They will have to turn back the intruder or else blow it to smithereens by a nuclear charge or by focusing a sunbeam.

Claudio Maccone of Italy - he's a researcher at the Astrodynamics Center-reasons like a boxer: it's pretty hard, he says, to beat off a direct hit in boxing. It is best to step back and punch with the hook. This martial scientist suggests launching several rockets for a preventive strike from different points spaced far apart. If need be, they can hit the invader at an angle and turn it aside. Next, in the distant future, squadrons of patrol spaceships

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could be protecting the earth, with a moon-based stronghold besides. Should spaceships be powerful enough, they could catch and tug an asteroid as "was booty" and even extract minerals from it.

Smaller asteroids could be handled alright. Joseph Spitale, an American planetologist, thinks they could be deflected by means of repainting, through the Jarkovsky effect (discovered in 1900 by Ivan Jarkovsky of Russia): this would change the intensity of solar radiation reflected from their surface as well as heat transfer and other parameters, and push them into a different orbit. Even though the magnitude of this effect is relatively small, it can accumulate in time to be able to divert even 300 m space objects.

But if things come to a battle, the response should be quick and error-free. Imagine a sharp-shooter hitting a button a few kilometers away. The same applies to hitting the mark in the case of asteroids. That's why "anti-asteroid" execises will be needed-with potential attack, retaliatory strike, and all.

One experiment is already in the making. Maybe space defense systems will have to enter the lists like real knights and engage the "stone guests". Anyway, the Spanish company Deimos-Space has a scenario dubbed Don Quixote, after the knight of the rueful countenance glorified by de Cervantes. The famous hidalgo attacked los molinos de viento; but his space counterpart will challenge a worse monster, a rotating stone giant in space. Selecting a target asteroid, the space hidalgo will rush at it at a rate of 1 km/s. His faithful escudero,Sancho, will in the meantime be watching by to keep tabs on the skirmish and report on the enemy's behavior.

Well and good, but beware if you miss the mark, or still worse, if your maladroit hit sends the asteroid to collide head-on with the earth. Therefore a good deal of preliminary calculations will be needed-precise measurements of the target asteroid's orbit and of other parameters.

By way of experiment, "irregular" rotating objects could be sent up. Turning now this, now that side to the earth, they will produce different luster effects due to changes in the reflectivity of sunbeams. All that is much important for early detection and warning.

The small planets so-called- which have been formed as a result of numerous collisions-have taken on rather fantastic forms. Changes in the intensity of reflected light alone make it difficult to identify the structure of space debris. But one can employ dummies instead-artificial objects with pre-assigned characteristics-and, with optical instruments attuned properly, keep watching the developments. Such asteroid twins could be quite helpful for learning the ways of space vagrants.

They are fickle and unpredictable in their ways, these space vagabonds. And they are well disguised to baffle detection. Some may prove to be real malefactors poised for attack. So we had better simulate the face-off-and see what's gonna happen next. We should be at the ready to preclude a tragedy. Here a lot will depend on our own selves in the final count.

R. Kagirov, "War Between Men and Stones", Izvestiya, No. 236, 2002

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