A basic concept of mechanics is that of momentum, the product of a body's mass and velocity. It has not only size, but also direction (that of its velocity). It follows immediately from Newton's Second Law (force is the rate of change of momentum) by an operation called integration that the sum of all momenta in the universe is constant, or that the total momentum of a closed system (just like its energy) is always conserved.
You can make your own little universe or closed system, provided you do not let external forces come in. For example, you and the canoe that you are standing in. You try to jump to the shore, and instead you push away the canoe and fall into the water. Why? Because the sum of all moments is constant. It was zero to begin with, and must remain zero afterward. Your forward momentum had to be canceled by the backward momentum of the canoe; the center of mass stayed where it was before, which left you in the water.
For exactly the same reason you have no difficulty jumping ashore from a battleship. The center of mass remains where it was before; your forward momentum is still canceled by the backward momentum of the battleship, but the velocity you have imparted to it is, say an inch per century.
To say that the center of mass must stay where it was before
¾or continue on its previous path¾is really the same, when you think about it as saying the sum of all moments is constant. So when we have "the bombs bursting in air," the center of mass of the fragments of each bomb continues along the same parabola as if it had not exploded, or at least that would be the case in vacuum (air resistance is an external force).Which tells us how not to prevent an asteroid hitting us. There will of course, be plenty of time to prepare for a threatening collision with an earth-crossing asteroid
¾many years, perhaps even decades; for once the object is spotted, the predictions of celestial mechanics are extraordinarily precise. If we sent a spacecraft to plant multi-megaton nuclear bombs on it and blow it up, that would not do the job, for one of the fragments might still travel toward the Earth, hitting it even sooner than the asteroid, and possibly still infliciting considerable damage.But we are getting dose: there are several strategies of fragmenting or deflecting the approaching asteroid by nuclear bombs. The most promising appears to be the detonation of a neutron bomb at a height, perpendicular to its path, at which the bomb's neutron and gamma radiation will deliver the maximum "dose" (largest area, highest intensity) to the asteroid. For a sphere, this height turns out to be 40% of the radius. If the surface area visible from the detonation point (the "shell," about 30% of the total area) is irradiated to a depth of about 20 cm, it is heated, expands, and sends a stress wave through the remainder of the asteroid, resulting in the ablation or blow-off of the shell toward the irradiation side; conservation of momentum demands that the remainder of the asteroid move off in the opposite direction.
As we have seen, the center of mass of the asteroid will still hit the earth, but quite painlessly, for the main part of the asteroid is deflected to one side of it and the fragments of the shell to the other.
For an object 50 m in radius, the recoil velocity of the asteroid away from its original path is minute, about 11 cm per second per kiloton exploded. Although 11 cm/s is very much slower than a leisurely walk, when a snail crawls day and night for a decade, it covers quite a distance. If an object threatening to collide with the earth isdetected early enough to allow a lead time of one decade, a deflection velocity of about 1 cm/s is needed to prevent a collision. For asteroids 100 m, 1km and 10 km in diameter, this can be accomplished with nuclear detonations of 0.1 kt (less than Hiroshima), 0.1 Mt, and 0.1 Gt respectively. All of these are well within man's capabilities even now. (Not just within his capabililites, but within his arsenals, except that
¾thanks to Carter and the appeasers¾they are not neutron bombs.)So the prevention of the one credible and preventable threat to the environment requires nuclear bombs.
Am I still talking like a Green?
More: T.J. Ahrens, A.W. Harris, "Deflection and fragmentation of near-Earth asteroids," Nature 12/3/92, pp. 429-433.]
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Vol. 20, No. 6
Newsletter: Access to Energy Newsletter Archive Volume: Volume 20 Issue/No.: Vol. 20, No. 6 Date: February 01, 1993 11:08 AM Title: The Ascendance of the Lie
Copyright © 2004 - Access to Energy Newsletter Archive
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