The forces acting on solar Coronal Mass Ejections ( CMEs ) in the interplanetary medium have been evaluated so far in terms of an empirical drag coefficient C _ { D } \sim 1 that quantifies the role of the aerodynamic drag experienced by a typical CME due to its interaction with the ambient solar wind .
We use a microphysical prescription for viscosity in the turbulent solar wind to obtain an analytical model for the drag coefficient C _ { D } .
This is the first physical characterization of the aerodynamic drag experienced by CMEs .
We use this physically motivated prescription for C _ { D } in a simple , 1D model for CME propagation to obtain velocity profiles and travel times that agree well with observations of deceleration experienced by fast CMEs .