  We propose an intrinsic maximum speed for dissipationless E cross B drift of
Skyrmion quasiparticles in quantum Hall ferromagnets. When this speed is
exceeded, Skyrmions can radiate spin-waves by making internal excitations which
allow total spin to be conserved. Our proposal is illustrated by a
time-dependent Hartree-Fock approximation calculation of the excitation
spectrum for a Skyrmion bound to an impurity.
