9. $A B C D-A_{1} B_{1} C_{1} D_{1}$ is a unit cube, then the distance between the line $A C$ and $A_{1} D$ is $\qquad$ .
9. $\frac{\sqrt{3}}{3}$

Notice that $C B_{1} / / A_{1} D, A_{1} C_{1} / / A C$.
Therefore, the required distance is the distance between the planes $A C B_{1}$ and $D A_{1} C_{1}$, which is $\frac{\sqrt{3}}{3}$.