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Hubble Space Telescope observations between 2001 and 2010 resolved the binarycomponents of the Cold Classical transneptunian object (79360) Sila-Nunam(provisionally designated 1997 CS29). From these observations we havedetermined the circular, retrograde mutual orbit of Nunam relative to Sila witha period of 12.50995 \pm 0.00036 days and a semimajor axis of 2777 \pm 19 km. Amulti-year season of mutual events, in which the two near-equal brightnessbodies alternate in passing in front of one another as seen from Earth, is inprogress right now, and on 2011 Feb. 1 UT, one such event was observed from twodifferent telescopes. The mutual event season offers a rich opportunity tolearn much more about this barely-resolvable binary system, potentiallyincluding component sizes, colors, shapes, and albedo patterns. The loweccentricity of the orbit and a photometric lightcurve that appears to coincidewith the orbital period are consistent with a system that is tidally locked andsynchronized, like the Pluto-Charon system. The orbital period and semimajoraxis imply a system mass of (10.84 \pm 0.22) \times 10^18 kg, which can becombined with a size estimate based on Spitzer and Herschel thermal infraredobservations 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 transneptunianbinaries of other dynamical classes.

Mutual Events in the Cold Classical Transneptunian Binary System Sila and Nunam