  We calculate orbits for the Milky Way dwarf galaxies with proper motions, and
compare these to subhalo orbits in a high resolution cosmological simulation.
We use this same simulation to assess how well are able to recover orbits in
the face of measurement errors, a time varying triaxial gravitational
potential, and satellite-satellite interactions. We find that, for present
measurement uncertainties, we are able to recover the apocentre r_a and
pericentre r_p to ~ 40%. However, even with better data the non-sphericity of
the potential and satellite interactions during group infall make the orbital
recovery more challenging. Dynamical friction, satellite mass loss and the mass
evolution of the main halo play a more minor role.
  We apply our technique to nine Milky Way dwarfs with observed proper motions.
We show that their mean apocentre is consistent with the most massive subhalos
that form before z=10, lending support to the idea that the Milky Way dwarfs
formed before reionisation.
