# The fly and the sphere of iron

When I was a child, I read something like the following in a book whose name I have now forgotten (I tried to find the original quote on the web, but although there are some similar passages the precise source eludes me):

Imagine a solid sphere of iron the size of the Earth. Imagine, then, a fly that, once in a million years, sits down on that sphere, only to leave again the next moment. When the frictional heating from the fly's impact has caused the entirety of the sphere to evaporate, eternity has not yet started.

How long (ballpark estimate) would that actually take? Feel free to use any simplifying assumptions you like (Universe contains nothing but sphere and fly, sphere starts at uniform room temperature, indestructible, spherical fly that is at room temperature at each impact, impact speed 3 m/s etc).

About $10^{42}$ years.