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Comment by dmckeon | Hacker News Reader

People tend to focus on the size of projectiles from space, but their great speed is often overlooked - from a simulator at: http://impact.ese.ic.ac.uk/ImpactEffects/

>Impact Velocity - This is the velocity of the projectile before it enters the atmosphere. The minimum impact velocity on Earth is 11 km/s. Typical impact velocities are 17 km/s for asteroids and 51 km/s for comets. The maximum Earth impact velocity for objects orbiting the sun is 72 km/s

For comparison, the velocity of the ISS is about 7.7 km/s, about equal to the speed of detonating cord.

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The minimum impact velocity on Earth is 11km/s??? I beg to differ -- bodies can impact the Earth at arbitrarily low velocities. It depends on where they originate. There are clearly some assumptions being made there that aren't spelled out.

Starting from rest at a distance infinity, any object (regardless of it's own mass) necessarily undergoes a minimum increase in kinetic energy due to the gravitational potential of the mass of the target object (in this case the Earth, which has a known mass and so this is calculable). So I think the assumption is that the object isn't propelling itself away from the earth, and the object hasn't experienced any major competing gravitational forces inside the radius of the Moon's orbit.

Also, 11km/s isn't terribly quick on the celestial scale, I'd imagine.

What you say is true but most objects are not starting from rest at infinity. Things thrown up from the Earth, for example.

It also neglects that there is an atmosphere...

>Impact Velocity - This is the velocity of the projectile before it enters the atmosphere. The minimum impact velocity on Earth is 11 km/s.

I think it's implied that the impacting objects come from outside the Earth-Moon System. Otherwise there would obviously be no minimum impact velocity.

No kidding. Shoot an 8-foot 2x4 at the Earth at 100km/sec and guess how fast the carbon-fluff will be going when it hits. The pieces of anything that breaks up as it descends will quickly slow to their terminal velocity at that height.

NASA says the February Russian fireball was going about 11.6 miles/sec when it blew up (14 miles up) but there's a lot of denser atmo in that 14 miles.

http://neo.jpl.nasa.gov/news/fireball_130301.html

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