The Milky Way's dwarf spheroidal (dSph) satellites have been tentatively identified as the dark-matter subhalos that form in lCDM N-body simulations. Yet their estimated mass number distribution falls well below what is predicted; the so-called "missing satellite problem." One key to understanding and possibly resolving the discrepancy is accurate estimates of the satellites' masses which in turn are dominated by their own dark-matter halos, the present-day extent of which will depend on tidal interactions as they orbit the Milky Way. A proper accounting of these tidal effects depends on accurate characterizations of their orbits.
Alternatively, some fraction of the dSphs may share a different origin; coalescence from a past tidal interaction between the Milky Way and another massive galaxy. In this case as well, it is their orbits, and presumably shared angular momentum, that would reveal the secret of their origin. Similarly, questions regarding the origin and formation of the Milky Way's system of globular clusters (GCs) will remain unresolved until an accurate characterization of their orbits can be made.
Finally, full phase-space data, including tangential velocites, are needed to quantitatively understand the process of tidal disruption that is responsible for the cold stellar streams and overdensities found to be inhabiting the halo.
The determination of accurate absolute proper motions for these systems is the key to providing their 3-D velocities and orbits. The required astrometric accuracy, over fields tens of arcminutes in size, which implies ~0.05 mas/yr (per system) is beyond the capability of current proper-motion programs. However, the recent development of large-format orthogonal transfer (OT) CCDs makes it possible to achieve deep exposures with high astrometric precision; high enough to allow the ground-based determination of absolute proper motions at the requisite precision for the faint systems described above over the course of just two to three years.
With the ODI survey we plan to observe on the order of 10 dSph systems, 30 globular clusters and 3 to 4 of the major Milky Way halo streams and overdensities.