The First High-Phase Observations of a KBO: New Horizons Imaging of (15810) 1994 JR1 from the Kuiper Belt

Abstract

NASA's New Horizons spacecraft observed (15810) 1994 JR$_1$, a 3:2 resonant Kupier Belt Object (KBO), using the LOng Range Reconnaissance Imager (LORRI) on November 2, 2015 from a distance of 1.85 AU, and again on April 7, 2016 from a distance of 0.71 AU. These were the first close observations of any KBO other than Pluto. Combining ground-based and Hubble Space Telecope (HST) observations at small phase angles and the LORRI observations at higher phase angles, we produced the first disk-integrated solar phase curve of a typical KBO from $\alpha$=0.6-58$^\circ$. Observations at these geometries, attainable only from a spacecraft in the outer Solar System, constrain surface properties such as macroscopic roughness and the single particle phase function. 1994 JR$_1$ has a rough surface with a 37$\pm$5$^\circ$ mean topographic slope angle and has a relatively rapid rotation period of 5.47$\pm$0.33 hours. 1994 JR$_1$ is currently 2.7 AU from Pluto; our astrometric points enable high-precision orbit determination and integrations which show that it comes this close to Pluto every 2.4 million years (10$^4$ heliocentric orbits), causing Pluto to perturb 1994 JR$_1$. During the November spacecraft observation, the KBO was simultaneously observed using HST in two colors, confirming its very red spectral slope. These observations have laid the groundwork for numerous potential future distant KBO observations in the New Horizons-Kuiper Belt Extended Mission.