The HST Lightcurve of (486958) 2014 MU69

To optimally plan the fly-by sequencing of (486958) 2014 MU₆₉ for the New Horizons spacecraft it is critical to determine, to the best of our ability, if the object is binary (as is the case for ~20% of cold classical KBOs in this size range), the rotation period, size and shape of the body. Existing HST astrometric datasets placed constraints on its diameter (21-41 km for an albedo of 0.15-0.04) and orbit, and early photometry suggested that a lightcurve with an amplitude of up to ~0.6 mags could be hidden within the measurement uncertainties. However, the sampling interval of this dataset made it impossible to further refine those estimates. We therefore designed an HST lightcurve program to be executed near its opposition in July 2017 (GO 14627, PI Benecchi) when 486958 would be brightest and provide the highest S/N data. We collected data using the WFC3 camera in the F350LP filter using an exposure time of 367 seconds and tracking on the object. 5 images were collected during each HST orbit and orbits were scheduled in groups of six. The 1st two sets of 6 orbits were separated by 0.6 days, the 2nd and 3rd by 1.4 days and the 3rd and 4th by 5.5 days. This allowed us to search for a range of periods from a few to a few tens of hours; combined with the astrometric photometry even longer periods can be investigated.The data were analyzed using two different PSF fitting techniques (an MCMC model and a TinyTim matching algorithm) which gave similar results. The lightcurve amplitude was found to be <0.15 magnitudes for any period that we could fit to the data. This places significant constraints on the axis ratio of 486958 to <1.14 assuming an equatorial view. This means that the timing of the fly-by does not need to be adjusted to look at the "larger" axis of the object, simplifying the engineering of the fly-by significantly. The small amplitude makes it difficult to uniquely identify the rotation period at this time. Stacking all of the images from this campaign allows us to search for binary companions to a depth of >29th magnitude. At first analysis we do not identify any companions. This work was made possible through a STScI grant under NASA contract NAS5-26555.