  Topological defects such as vortices, dislocations or domain walls define
many important effects in superconductivity, superfluidity, magnetism, liquid
crystals, and plasticity of solids. Here we address the breakdown of the
topologically-protected stability of such defects driven by strong external
forces. We focus on Josephson vortices that appear at planar weak links of
suppressed superconductivity which have attracted much attention for electronic
applications, new sources of THz radiation, and low-dissipative computing. Our
numerical simulations show that a rapidly moving vortex driven by a constant
current becomes unstable with respect to generation of vortex-antivortex pairs
caused by Cherenkov radiation. As a result, vortices and antivortices become
spatially separated and accumulate continuously on the opposite sides of an
expanding dissipative domain. This effect is most pronounced in thin film edge
Josephson junctions at low temperatures where a single vortex can switch the
whole junction into a resistive state at currents well below the Josephson
critical current. Our work gives a new insight into instability of a moving
topological defect which destroys global long-range order in a way that is
remarkably similar to the crack propagation in solids.
