The properties of the smallest gravitationally-bound dark matter structures can be affected by non-trivial particle physics within the dark sector, and thus could provide the first hints of the non-gravitational interactions of dark matter. As our knowledge of Galactic stellar kinematics grows, we are discovering streams and debris consistent with tidally disruptions of this small-scale substructure. As these objects are no longer self-bound, new techniques are required to extract the properties of their progenitors. I will show that the conservation of phase space density and volume during the tidal disruption may provide the necessary handle to reconstruct the mass and density profile of the original substructure. Measuring the phase space density in real data constitutes a significant challenge, but I will demonstrate that several major sources of error can be dealt with in Gaia data.