Despite the superfluid1 character of 4He below the lambda transition temperature, there are two distinct mechanisms by which an object moving through the liquid dissipates kinetic energy: (1) it can scatter the excitations (rotons, phonons, 3He isotopic impurities) that constitute the normal fluid component; and (2) for speeds in excess of the Landau critical velocity vL it can create rotons, apparently in pairs. The first process has been studied extensively, mostly by measurements of the zero-field mobilities of positive and negative ions. The second process, which is much rarer, has also been investigated in considerable detail by studies of the motion of negative ions in isotopically pure 4He at elevated pressures. Here, we report an attempt to extend the latter type of investigation to lower pressures. This yields unexpected results that can be accounted for satisfactorily only if we postulate the existence of a third dissipation mechanism.