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The HST Lightcurve of (486958) 2014 MU69

We report HST lightcurve observations of the New Horizons (NH) spacecraft encounter KBO (486958) 2014 MU69 acquired near opposition in July 2017. In order to plan the optimum flyby sequence the NH mission planners needed to learn as much as possible about the target in advance of the encounter. Specifically, from lightcurve data, encounter timing could be adjusted to accommodate a highly elongated, binary, or rapidly rotating target. HST astrometric (Porter et al. 2018) and stellar occultation (Buie et al. 2018) observations constrained MU69's orbit and diameter (21-41 km for an albedo of 0.15-0.04), respectively. Photometry from the astrometric dataset suggested a variability of $\ge$0.3 mags, but they did not determine the period or provide shape information. We strategically spaced 24 HST orbits over 9 days to investigate rotation periods from approximately 2-100 hours and to better constrain the lightcurve amplitude. Until NH detected MU69 in its optical navigation images beginning in August 2018, this HST campaign provided the most accurate photometry to date. The mean variation in our data is 0.15 mags which suggests that MU69 is either nearly spherical (a:b axis ratio of 1:1.15), or its pole vector is pointed near the line of sight to Earth; this interpretation does not preclude a near-contact binary or bi-lobed object. However, image stacks do conclude that MU69 does not have a binary companion $\ge$2000 km with a sensitivity to 29th magnitude. We report with confidence that MU69 is not both rapidly rotating and highly elongated. We note that our results are consistent with the fly-by imagery and orientation of MU69 (Stern et al. 2019). The combined dataset also suggests that within the KBO lightcurve literature there are likely other objects which share a geometric configuration like MU69 resulting in an underestimate of the contact binary fraction for the CC Kuiper Belt.