8. For a regular triangular prism $A B C-A_{1} B_{1} C_{1}$ with all edges of length 3, a line segment $M N$ of length 2 has one endpoint $M$ moving on $A A_{1}$ and the other endpoint $N$ moving on the base $A B C$. Then, the trajectory (surface) of the midpoint $P$ of $M N$ and the three faces of the regular triangular prism sharing vertex $A$ enclose a geometric body with a volume of $\qquad$
8. $\frac{\pi}{9}$.

From the problem, we know that the distance from the midpoint $P$ of $M N$ to point $A$ is always 1. Therefore, the locus of point $P$ is the part of the sphere with $A$ as the center and 1 as the radius that lies within the triangular prism.
Thus, the volume of the enclosed geometric body is $\frac{1}{6} V_{\text {hemisphere }}=\frac{\pi}{9}$.