NEUTRAL interstellar hydrogen can be prevented from penetrating freely into the heliosphere of solar plasma, because it is partially coupled to the subsonic flow of interstellar plasma. Results from modelling the backscattering of the diffuse Lyman-α radiation are therefore unreliable. The interstellar density of H+ is probably 0.02 cm−3 or less, and the density of neutral hydrogen greater than 0.2 cm−3.