  Binaries have played a crucial role many times in the history of modern
astronomy and are doing so again in the rapidly evolving exploration of the
Kuiper Belt. The large fraction of transneptunian objects that are binary or
multiple, 48 such systems are now known, has been an unanticipated windfall.
Separations and relative magnitudes measured in discovery images give important
information on the statistical properties of the binary population that can be
related to competing models of binary formation. Orbits, derived for 13
systems, provide a determination of the system mass. Masses can be used to
derive densities and albedos when an independent size measurement is available.
Angular momenta and relative sizes of the majority of binaries are consistent
with formation by dynamical capture. The small satellites of the largest
transneptunian objects, in contrast, are more likely formed from collisions.
Correlations of the fraction of binaries with different dynamical populations
or with other physical variables have the potential to constrain models of the
origin and evolution of the transneptunian population as a whole. Other means
of studying binaries have only begun to be exploited, including lightcurve,
color, and spectral data. Because of the several channels for obtaining unique
physical information, it is already clear that binaries will emerge as one of
the most useful tools for unraveling the many complexities of transneptunian
space.
