  There exists a variety of proposals to transform a conventional s-wave
superconductor into a topological superconductor, supporting Majorana fermion
mid-gap states. A necessary ingredient of these proposals is strong spin-orbit
coupling. Here we propose an alternative system consisting of a one-dimensional
chain of magnetic nanoparticles on a superconducting substrate. No spin-orbit
coupling in the superconductor is needed. We calculate the topological quantum
number of a chain of finite length, including the competing effects of disorder
in the orientation of the magnetic moments and in the hopping energies, to
identify the transition into the topologically nontrivial state (with Majorana
fermions at the end points of the chain).
