Nonvertical ascension or fall of a free sphere in a Newtonian fluid

It is shown that the system represented by a free sphere ascending or falling in a Newtonian fluid under the action of gravity buoyancy undergoes a regular, symmetry breaking bifurcation making the trajectory deviate from the vertical direction. The instability threshold expressed in terms of the asymptotic Reynolds number lies below that of a fixed sphere wake. The instability is shown to saturate and reach a fixed point corresponding to a straight oblique ascension (fall).