  In high energy heavy ion collisions, the directed flow of particles is
conventionally measured with respect to that of the projectile spectators,
which is defined as positive $x$ direction. But it is not known if the
spectators deflect in the "outward" direction or "inward" -- toward the center
line of the collision. In this Letter we discuss how the measurements of the
directed flow at mid-rapidity, especially in asymmetric collision such as
Cu+Au, can be used to answer this question. We show that the existing data
strongly favor the case that the spectators, in the ultrarelativistic
collisions, on average deflect outwards.
