9. As shown in the figure, in the cube $A B C D-A_{1} B_{1} C_{1} D_{1}$ with edge length 2, $E$ is the midpoint of $B C$, and point $P$ moves along the line segment $D_{1} E$. Then, the minimum distance from point $P$ to the line $C C_{1}$ is $\qquad$.
Answer
As shown in the figure, construct the plane $D E F D_{1} / / C C_{1}$, then the minimum value of the distance from point $P$ to the line $C C_{1}$ is the distance from $C C_{1}$ to the plane $D E F D_{1}$.

Draw $C G \perp D E \Rightarrow C G \perp$ plane $D E F D_{1}$, and $C G \cdot D E = C D \cdot C E = 2 \Rightarrow C G = \frac{2}{\sqrt{5}} = \frac{2 \sqrt{5}}{5}$. Therefore, the minimum value of the distance from point $P$ to the line $C C_{1}$ is $\frac{2 \sqrt{5}}{5}$.