E SSM The atmospheric density at an altitude of $2500 \mathrm{~km}$ is about $1 \mathrm{molecule} / \mathrm{cm}^3$. (a) Assuming the molecular diameter of $2.0 \times 10^{-8} \mathrm{~cm}$, find the mean free path predicted by Eq. 19.5.1. (b) Explain whether the predicted value is meaningful.