By Alexander LITVAK and Valentin SAZONOV, Candidates of Sc. (Tech.); and Mikhail YAKOVLEV, Dr. Sc. (Tech.), Central Research Institute of Machine Building

The possibility of the earth's clashing with an asteroid or a comet was scientifically validated back in the middle of the 20th century. Now what could the aftermath be? And how should we protect our planet?

Tackling this problem has become possible in the last 10 - 15 years thanks to wide- range interdisciplinary studies. Thus, upgrading an astronomer's instruments allows to identify the structure and parameters of orbits and orbital movements as well as the laws governing the birth and death of comets and asteroids. As to the asteroids, we can now see a connection between their diameter and the mean interval of their downfall, on one hand, and the energy released thereby, on the other.

Analyzing nuclear test results is likewise important, particularly studying the dependence of dust formation intensity on the energy released in a blast.

Yet another side of the problem is elucidated by a theory of one's death risk from dire effects.

WHAT IF IT HAPPENS

Should a large heavenly body hit the earth, the impact may cause global consequences described as a "shock winter" (by analogy with the "nuclear winter"). A terrific explosion will send up clouds of fine dust into the atmosphere and thus cause a temperature decrease on the continents by 10C and even more. The cooling spell may persist for weeks and months; it will ruin crops and bring about mass famine. Millions will be killed and, to compound the disaster, the protective ozone layer will be destroyed, and acid rains will contaminate the soil and subsurface waters... Such is the grim setting should the earth collide with a celestial body. Some scientists blame the sudden death of dinosaurs and other animals just before the Cenozoic on a similar cataclysm.*

This tragic scenario may come true if the diameter of a falling celestial body is 0.6 to 5 km, and the approach velocity is 20 km/s for an asteroid and 42 km/s for a comet (with diameter equal to 0.4 - 3 km). Proceeding from nuclear testing data, experts have calculated: if the

Articles in this rubric reflect the opinion of the author.- Ed.

* See: Yu. Avsyuk et al., "Did Dinosaurs Die Out Suddenly?", Science in Russia, No. 3,2002.- Ed.

Pages. 56

earth collides with 1 - 2 km object hurtling at a speed of 20 km/s, the impact will be tantamount to a nuclear blast of 10 ⁵ - 10 ⁶ megatons. As to the probability of cosmic bodies of that size hitting the earth, its values vary in a broad range-one collision in 7 * 10 ⁴ - 6 * 10 ⁶years (mean value, one collision in 5 * 10 ⁵ years).

Our specialists have computed the death risk factor: multiplying the probability of an individual's death in the course of one's natural life by the collision probability, they have obtained the ratio 1/25,000. This figure is compatible with the other risk factors for an average individual in civilized countries. By American data, the risk of death in an air crash is equal to 1/20,000, and in a flood-1/30,000.

HOW TO PROTECT THE EARTH?

Scores of approaches have now been suggested to shield our planet against the outer threat. All of them proceed from the assumption that a collision could be predicted well in advance, because a probable intruder strays within the solar system. Consequently, one can forecast the time when the space traveler's orbit will cross that of the earth and take preemptive measures.

Such countermeasures fall into two groups: 1) the counteraction energy is initiated by man (active defense); and 2) this energy is natural (passive defense). In this very article the authors consider both- the active mode to ward off the asteroid threat, and the passive one to protect our planet from comets. As we see it, these approaches hold best prospects ecologically and by international law standards.

Now the proposed defenses against asteroids boil down to the following. A shuttle- type (reusable) transportation complex equipped with a liquid-propellant rocket engine puts a rocket-space system (RSS) into circumterrestrial orbit. It carries a power generator, control and measurement devices, a radio station, electrojet low-thrust engines (EJ/LTE) and a working medium essential for their proper

Pages. 57

Pages. 58

performance. It may be lithium, caesium, xenon, argon, bismuth or something else. After the EJ/LTE engines have speeded up the RSS setup to a velocity enabling it to leave the circumterrestrial orbit, RSS will shift into an asteroid's orbit and make a touchdown. The RSS engines, meanwhile, will keep up their work to give the space object a push and thus head it off.

Needless to say, this can be achieved only if the jet propulsion thrust is high enough. Its magnitude depends on the mass flow rate of the working medium (substance) per second and the jet exhaust rate. In many types of EJ/LTE engines this value attains to 100 km/s, while in liquid-propellant rocket engines it is as low as 3 - 4 km/s. That is why the electrojet engine suits best.

Here's the principle of its operation: a current, taking the path through electrodes, generates Jou-lean heat; it heats the working medium to a plasma state. As a result, one or several electrons "come off" from an atom's electron shell, and this atom becomes an ion, that is, it suffers the magnetic field action. Now the interaction of the azimuthal magnetic field with the current produces a Lorentz force which accelerates each volume unit of the plasma "ejected" at a definite rate (that of the jet exhaust). In keeping with the third law of mechanics, there is a counterforce of the same value as the Lorentz force, or the EJ/LTE thrust.

The suggested antiasteroid method provides for two routes of approach. The first-that of the reusable transportation complex-links the earth with the artificial satellite orbit. It is to this orbit that the working medium is brought and, if necessary, these may be units for the RSS setup (or for a new RSS after the expiry of the service life of the old one). The other route-for the RSS-is meant for taking the working medium from satellite orbit to the EJ/LTE perched on the asteroid. That is to say, only the working medium (substance) could be sent into orbit instead of so many engines. This means much economy in the number of RSS launchings from the satellite's orbit.

This very method, however, is too laborious and time-consuming. The point is that the mass of a working medium launched into orbit is rather small, 100 tons, so many more consignments will have to be sent up. It will take a few months to speed up the RSS to an escape velocity and make it leave the artificial satellite orbit. The safe distance where one can have no fear of colliding with an asteroid, according to our estimates, is 10 ⁶ km, or several times more than that between the earth and the moon. Covering this distance and thus accomplishing the objective could take dozens and even hundreds of years.

As we see it, this time can be cut down considerably. Say, by increasing the payload mass in orbit (to several hundred tons instead of 100 t) as well as the number of shuttle flights, we can reduce this time sev-eralfold, even tenfold. It is likewise possible to combine this technique with one of the other ones providing for the use of a space body's material as working medium for EJ/LTE.

Protecting the earth against comets needs a different approach due to their specific physico-chemical structure. The central part of a comet is an ice core with impregnations of refractory substances; this core carries a coat of dust. If this coat is destroyed, the comet mass will become accessible to solar radiation and start evaporating under its action.

This may be achieved if a man-made space vehicle hits a comet. That would not involve great energy expenses. The approach rate should be around 20 - 30 km/s to turn the collision into a blast. The vehicle's design should provide for a nonuni-form distribution of the mass concentrated in definite nodal points, and their number may be as high as 5,000. At a proper time powder charges are exploded there to send heavy fragments to the cornet and smash its dust coat. This heavenly body will shrink as a result of evaporation, and a jet thrust produced thereby will divert it into a different trajectory. Such a diversion maneuver should be started as early as 4.7 years before the possible encounter with the earth if we want to deflect a comet 1 mln km off.

COSMIC-SCALE PROBLEM

The cosmos gives us sufficient time to brace ourselves for the encounter with unwelcome guests. We are still at the initial stage of this work. But there are signs that a space defense system is being taken in good earnest. An international scientific conference held at Snezhinsk, Chelyabinsk administrative region, in 1994 considered protecting the earth against dangerous space objects and mapped out a broad program of research taking in asteroids and comets as well as the consequences of their collisions with the earth and other planets. This conference outlined steps toward setting up early detection systems and for preemptive measures in the event of a real threat. Ecological safety considerations are an important part of this project too.

It offers a good chance for international cooperation in tackling the most peaceful of all problems and in fostering the progress of human civilization.

Illustrations supplied by the authors.

Permanent link to this publication: