Phase Curves from the Kuiper Belt: Photometric Properties of Distant Kuiper Belt Objects Observed by New Horizons

Prior to its close encounter with the Kuiper Belt object (KBO) (486958) 2014 MU69 on 2019 January 1, NASA's New Horizons spacecraft observed other KBOs from distances greater than 0.1 au at solar phase angles far larger than those attainable from Earth. The expanded range in phase angle afforded by these distant KBO (DKBO) observations enables comparisons between their phase functions and those of other solar system objects. Here we present extended New Horizons phase angle coverage of plutino (15810) Arawn (1994 JR1) to 131 degrees, resonant KBO 2012 HE85 to 64 degrees, scattered disk KBO 2011 HK103 to 124 degrees, hot classical (515977) 2012 HZ(84) to 73 degrees, and cold classical KBOs 2011 HJ(103) and 2011 JY(31) to 27 degrees and 122 degrees, respectively. In general, DKBO solar phase curves have slopes (i. e., phase coefficients) and shapes (with corresponding phase integrals q) similar to those of other dark, small solar system objects including comet nuclei, asteroids, and satellites. Until stellar occultations by these DKBOs provide information about their size, geometric albedos p (and Bond albedos A = pq) must be inferred from the median albedos measured by thermal radiometry for each dynamical class. Bond albedos for these DKBOs range from 0.01 to 0.04. Cold classical JY(31) has a slightly lower slope and higher phase integral than the other DKBOs, and its slope and phase integral come closest to matching those of cold classical MU69, suggesting that cold classical KBOs share surface scattering characteristics that are distinct from those of other KBOs.