  We compare different examples of spherical accretion onto a gravitating mass.
Limiting cases include the accretion of a collisionally dominated fluid and the
accretion of collisionless particles. We derive expressions for the accretion
rate and density profile for semi-collisional accretion which bridges the gap
between these limiting cases. Particle crossing of the Hill sphere during the
formation of the outer planets is likely to have taken place in the
semi-collisional regime.
