  Motion of a rigid body immersed in a semi-infinite expanse of gas in a
$d$-dimensional region bounded by an infinite plane wall is studied for free
molecular flow on the basis of the free Vlasov equation under the specular
boundary condition. We show that the velocity $V(t)$ of the body approaches its
terminal velocity $V_{\infty}$ according to a power law $V_{\infty}-V(t)\approx
Ct^{-(d-1)}$ by carefully analyzing the pre-collisions due to the presence of
the wall. The exponent $d-1$ is smaller than $d+2$ for the case without the
wall found in the classical work by Caprino, Marchioro and Pulvirenti~[Comm.
Math. Phys., \textbf{264} (2006), pp. 167--189] and thus slower convergence
rate results from the presence of the wall.
