Friday, May 13, 2011

Relativistic Weapon Impact

The logistic computer calculates that we have about a 62% chance of success, should we attempt to destroy the enemy base. Unfortunately, we would have only a 30% chance of survival – as some of the scenarios leading to success involve ramming the portal planet with the Anniversary at light speed.” So speaks the unfortunate starship commander in The Forever War (Haldeman, 1974, p. 110).

An attack by highly relativistic impact weapons is a military planner’s nightmare. The first problem is that the incoming weapon is highly superluminal: it appears to be travelling much faster than light. Suppose such a weapon was launched from interstellar space towards the earth at a velocity of 0.99995c (i.e. 99.995% of the speed of light). Its apparent velocity as observed from the earth is 20,000 times the speed of light (the speed-up factor is c/(c-v) which tends to infinity as the object’s speed approaches that of light). Suppose our military first spot this incoming object out at Jupiter’s orbit, 35 light minutes away. How long do we have before impact? Less than one tenth of a second: there is simply no time to react.

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