it stands heat. The bottom of the shuttle orbitor is painted black, it doesn't make it less effective to resist heat. In atomic level, light reflection is basically the electrons absorbing energy, become excited and jump to higher orbitals. Because it is unstable to stay in higher orbitals, electrons jump back and emits the light they absorbed. When high energy laser shoots at the surface, the mirror may be able to reflect the visible light, but not the infrared which is the heat. The intense heat is enough to break down the molecular structure, and destroy the mirror.