# Minimum size of an asteroid to actually impact earth

From what I understand, an object entering the atmosphere will start to burn up from the tremendous resistance of the atmosphere. Presumably, for asteroids under a certain size, they will burn up completely and never impact the surface of the earth.

Do we have a way of determining the minimum size needed for actual impact?

If so, roughly what is the size and how does it compare to the average size of asteroids that pass by us regularly?

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The answer will depend strongly on the composition of the object... – dmckee Dec 28 '12 at 1:28
I understand this, but neglected to mention it to keep the question short. I'm assuming that there are a few ''typical'' compositions of asteroids, and its these most common compositions that I'm interested in. – Todd R Dec 28 '12 at 1:34
Original poster should also clarify what does “impact” mean. Obviously, a millimetre-sized grain falling at terminal velocity doesn’t make an impact, but what does, how quick it must be to match OP’s undeclared criterion? – Incnis Mrsi Aug 23 '14 at 11:21

Consider a specific example. The leonids arrive at the top of the atmosphere at $72$ km/s with a maximal mass of around $0.5$ g. According to the article these particles are $0.01$ m across. They reach the ground. If such a particle strikes the $30$ km high atmosphere at $45$ degrees it must travel around $4 \times 10^4$ m before it hits the ground, mostly burning up. If it started at rest at the top of the atmosphere it would only accelerate to about $1$ km/s. As you can see it very much depends on azimuth, latitude, meteor composition, and speed.