Recently Öpik and Singer [1959] have developed a theory that gives the ballistic component of the neutral density distribution in a planetary exosphere. Their theory assumes that above the base of the exosphere, collisions may be entirely neglected, at least as far as an approximate calculation of the neutral density profile is concerned. The particles at the base of the exosphere are assumed to be in a truncated Maxwellian distribution, with incoming particles of velocities greater than escape velocity absent. The absence of such incoming particles implies a sink, located at some distance from the planet, that prevents the eventual build-up of a full Maxwellian distribution above the escape level, and hence an extension of the barometric law beyond the base of the exosphere.