Collision Hazards

The largest ocean going ships are about half a million tons. I would expect that any interstellar ships we or others might build would have to be at least that size in order to survive a journey lasting decades or centuries. Let's estimate a million tonnes - 10^9 kg - roughly twice the mass of the supertanker the Seawise Giant.

I remember the first time I saw a piece of armor plate that had been struck by a hypervelocity projectile. Even though the projectile was only a bit larger than a beebee, it had drilled a hole right through two inches of armor plate. Lower velocity projectiles spread their energy over larger areas. The operative factor is the speed of sound. If the projectile is moving significantly faster than the speed of sound in the target material, there isn't enough time for the forces to be transmitted laterally, and the projectile just keeps boring a hole until it has piled up enough mass in front of it to slow the whole procession, including the shock wave at the front, below the speed of sound.

It's a bit more complicated than that of course.

Suppose you had a large interstellar space craft moving a something like half the speed of light. The kinetic energy would be pretty large:

m*c^2(gamma - 1), where gamma = 1/sqrt(1-v^2/c^2) = 1/sqrt(3/4) = 1.1555, so that available kinetic energy is 1.4 * 10^16 J/kg, or 1.4 * 10^25 J for a million tonne behemoth like that described in the first paragraph. That amounts to the equivalent of 3 billion megatons of TNT - not enough to blow a planet to bits, death star style, but possibly enough to blast a hole right through it. So you might not want to aim your interstellar ship directly at a habitable planet, just in case you had trouble slowing down.

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