Can charge drag explain the Pioneer anomaly?

According to precise measurements of Doppler shift for the signals from Pioneers 10 and 11, these two spacecraft have been experiencing a drag-like force of about 2×10−7 Newtons since they passed into the outer solar system. Recently, Nieto et al. [M.M. Nieto, et al., Phys. Lett. B 613 (2005) 11] have estimated the drag on these craft that would be caused by impacting dust grains in the region beyond 20 AU. They conclude that the amount of dust required to explain the anomaly is excessive, about 3×10−19 g/cc. However, if the two spacecraft carry a large electric charge, then charge drag against the dusty plasma in this region could be significant. In the present Letter, estimates of this force are made, and conditions are found under which the observed level of drag is obtained. A density of charged dust of around 5×10−22 g/cc at a kinetic temperature of 105 K suffices, provided that the dust grains are very small (mass ∼100 amu) and the spacecraft charge is near its reported maximum (Z∼1012).