Plugging some numbers into wolfram alpha, a 90,000 kg object (about 12 cubic meters of iron, for example) moving at the reentry velocity we'd expect from lunar

Plugging some numbers into wolfram alpha, a 90,000 kg object (about 12 cubic meters of iron, for example) moving at the reentry velocity we'd expect from lunar orbit (10 km/s) would have about the equivalent kinetic energy of a 1kT nuke, which is quite small. You'd have to lob a really big rock at those speeds to get to the megaton range. (Or from interstellar distances, since energy scales with v^2.) Seems infeasible for a near-future lunar mining op. But it also demonstrates how powerful nuclear weapons are.