Question about stokes law?

Stokes law can be used to determine the velocity that a sphere will fall or rise when placed in a fluid. An object can't fall faster than terminal velocity. However, are there restraints on how fast it can rise? For instance, I used the equation to find out how fast a 2 meter diameter low density sphere would rise if placed under water. The velocity I got was unreal, ~4km/s which is really fast. Are there limitations for large spheres?