Time for something a little more hands-on. There’s all sorts of crap hurtling around the solar system — satellites, asteroids, comets and so on – and most of it is travelling at velocities that are, to put it mildly, completely insane. Supersonic jets and rifle bullets travel at about 1 km/s. Ramjets go at around 3 km/s. The average Earth-crossing asteroid, on the other hand, is hurtling through space at 20 km/s. This is Very Fast, and the reason Earth-crossing asteroids (so called because they cross the Earth’s orbit) worry people so much is because the kinetic energy of an object is proportional to the square of its velocity as per the equation
It doesn’t help that asteroids also tend to be rather heavy, and so they tend to possess a rather staggering amount of kinetic energy. Kinetic energy that would, if released on Earth somehow – say, by the asteroid hitting it – just ruin everyone’s day.