We consider a body moving in a rarefied homogeneous medium composed of point particles.The body is a bounded connected domain with piecewise smooth boundary.It is assumed that the mass of particles is much smaller than the mass of the body.The particles are initially at rest.They do not interact with each other and collide with the body in the perfectly elastic fashion.This means that the particles move freely between consecutive reflections from the body boundary and all the reflections obey the billiard law: if in a reference system attached to the body v+ and v_ denote the velocities of a particle before and after the impact and nt is the outer unit normal to the body boundary at the point of impact, then v+ =v_-2(v_, n) n.