  Hubble Space Telescope observations between 2001 and 2010 resolved the binary
components of the Cold Classical transneptunian object (79360) Sila-Nunam
(provisionally designated 1997 CS29). From these observations we have
determined the circular, retrograde mutual orbit of Nunam relative to Sila with
a period of 12.50995 \pm 0.00036 days and a semimajor axis of 2777 \pm 19 km. A
multi-year season of mutual events, in which the two near-equal brightness
bodies alternate in passing in front of one another as seen from Earth, is in
progress right now, and on 2011 Feb. 1 UT, one such event was observed from two
different telescopes. The mutual event season offers a rich opportunity to
learn much more about this barely-resolvable binary system, potentially
including component sizes, colors, shapes, and albedo patterns. The low
eccentricity of the orbit and a photometric lightcurve that appears to coincide
with the orbital period are consistent with a system that is tidally locked and
synchronized, like the Pluto-Charon system. The orbital period and semimajor
axis imply a system mass of (10.84 \pm 0.22) \times 10^18 kg, which can be
combined with a size estimate based on Spitzer and Herschel thermal infrared
observations to infer an average bulk density of 0.72 +0.37 -0.23 g cm^-3,
comparable to the very low bulk densities estimated for small transneptunian
binaries of other dynamical classes.
