I kept wondering where this 72 km/s maximum is coming from, and I figured it out! This is the calculation:
Why? Firstly, it's obvious that the Earth is traveling at 30 km/s in its orbit. But what possible directions can the asteroid hit from? The most logical approach is to hit it moving the exactly opposite direction. That means we demand an asteroid in a retrograde orbit. Normally this won't happen for orbits in the inner solar system around the sun, but this came come from the Ort cloud or somewhere far away.
The idea is that an object very from the sun is disturbed and begins a highly elliptical orbit. These can be retrograde. It would also need to intersect with our orbit at its closest point to the sun (this is why we sum the two velocities).
The kinetic energy of a body in a circular orbit is half of its gravitational potential energy. Since the far point of the object's orbit (formally a comet I think) is nearly infinity, that means its kinetic energy at the close approach will be exactly equal to its potential energy at 1 AU. That means it's specific kinetic energy (just 1/2 v^2) will be twice that of Earth. That means it will be traveling at the square root of 2 times faster than Earth.
Obviously this would be rare, but the principle is that anything moving faster than this when it hits the atmosphere obviously came from somewhere outside our solar system. This is my brief illustration of the concept. Earth is green, sun is yellow, and the object is grey.