As a cometary nucleus, regarded as an admixture of ices and dust (Whipple, 1950), approaches the sun, the ices sublimate and the resulting gases freely expand dragging along some of the dust.
For a typical medium bright comet (e.g., P/Halley) at 1 AU, the cometary atmosphere is collision dominated within a radius Rc, within which the collision mean free path is equal to the radial distance itself. Within this radius, the cometary atmosphere may be considered as a continuum, and described by hydrodynamic equations. Admittedly, this definition of the regime of hydrodynamic flow is rather crude, and what one has is a gradual transition from pure hydrodynamic flow when r << Rc to slip and transitional flow when r ≈ Rc, to free molecular flow when r > Rc. Depending on whether one considers neutral-neutral collisions or neutral-ion collisions Rc ~ 5 × 103 – 104 km for a medium bright comet at a heliocentric distance of 1 AU.