  We investigate the drag force on a moving impurity in a spin-orbit coupled
Bose-Einstein condensate. We prove rigorously that the superfluid critical
velocity is zero when the impurity moves in all but one directions, in contrast
to the case of liquid helium and superconductor where it is finite in all
directions. We also find that when the impurity moves in all directions except
two special ones, the drag force has nonzero transverse component at small
velocity. When the velocity becomes large and the states of the upper band are
also excited, the transverse force becomes very small due to opposite
contributions of the two bands. The characteristics of the superfluid critical
velocity and the transverse force are results of the order by disorder
mechanism in spin-orbit coupled boson systems.
