  We present new results on the Eris/Dysnomia system including analysis of new
images from the WFC3 instrument on the Hubble Space Telescope (HST). Seven HST
orbits were awarded to program 15171 in January and February 2018, with the
intervals between observations selected to sample Dysnomia over a full orbital
period. Using relative astrometry of Eris and Dysnomia, we computed a best-fit
Keplerian orbit for Dysnomia. Based on the Keplerian fit, we find an orbital
period of 15.785899$\pm$0.000050 days, which is in good agreement with recent
work. We report a non-zero eccentricity of 0.0062 at the 6.2-$\sigma$ level,
despite an estimated eccentricity damping timescale of $\leq$17 Myr.
Considering the volumes of both Eris and Dysnomia, the new system density was
calculated to be 2.43$\pm$0.05 g cm$^{-3}$, a decrease of $\sim$4% from the
previous value of 2.52$\pm$0.05 g cm$^{-3}$. The new astrometric measurements
were high enough precision to break the degeneracy of the orbit pole
orientation, and indicate that Dysnomia orbits in a prograde manner. The
obliquity of Dysnomia's orbit pole with respect to the plane of Eris'
heliocentric orbit was calculated to be 78.29$\pm$0.65$^{\circ}$ and is in
agreement with previous work; the next mutual events season will occur in 2239.
The Keplerian orbit fit to all the data considered in this investigation can be
excluded at the 6.3-$\sigma$ level, but identifying the cause of the deviation
was outside the scope of this work.
