  The Lagrangian, the Hamiltonian and the constant of motion of the
gravitational attraction of two bodies when one of them has variable mass is
considered. This is done by choosing the reference system in one of the bodies
which allows to reduce the system of equations to 1-D problem. The trajectories
found in the space position-velocity,(x,v), are qualitatively different from
those on the space position-momentum,(x,p).
