: how are comets , meteorites and asteroids so powerful that they are able to completely annihilate their target in a matter of seconds?

Question

: how are comets , meteorites and asteroids so powerful that they are able to completely annihilate their target in a matter of seconds?

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Answer 1

They’re going very, very fast.

An object that hits Earth from deep space can’t be going slower than Earth’s escape velocity. Orbits are reversible, so any outward trajectory that could take a rocket to deep space (and necessarily starts with reaching escape velocity) has a corresponding inward trajectory with an object coming in and *reaching* escape velocity.

Earth’s escape velocity is, by human standards, incredibly fast. It’s about 11 km/s or, if you prefer, about 40,000 kilometers per hour (or, if you prefer, more than 20,000 miles per hour). That’s much faster than any object you see in everyday life. Even a bullet typically leaves a gun barrel at something like 1 km/s, about ten times slower. Even anti-tank weapons usually don’t break 2 km/s. And because kinetic energy goes with the *square* of speed, ten times faster means 10^2 = 100 times more energy per mass than a bullet.

So what we’re talking about here is an object much bigger than a bullet, striking at a speed that makes it hit 100 times harder per mass. Bullets weigh a few grams – let’s say about 4 for the sake of argument – but even a baseball-sized meteor weighs ~600 grams (using 3 g/cm^3 for a density, which is a reasonable number for a rocky meteor; iron meteors are much heavier).

So in sum, a baseball-sized meteor is like getting hit by (600/4) = 150 bullets, each of which is hitting 100 times harder than a normal bullet. Or, if you prefer, being hit by 15,000 bullets at once. In energy terms, a 600 gram meteor going at 11 km/s has a kinetic energy of 36 MJ, or about 100 times the energy of a speeding car.