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New Horizons’ Photometric Survey of Kuiper Belt Objects

Presentation #202.02 in the session TNO Theory and Physical Properties (Oral Presentation)

Published onOct 23, 2023
New Horizons’ Photometric Survey of Kuiper Belt Objects

In addition to its 3500-km flyby of Kuiper belt object (KBO) (486958) Arrokoth in 2019, NASA’s New Horizons spacecraft has observed more than 30 other KBOs and dwarf planets since its launch in 2006. Although these objects only appear to New Horizons as unresolved point sources, the unique geometries at which they were viewed enable the analyses of their surface microphysical properties, shapes, and rotation poles. The large heliocentric distances at which these objects orbit the Sun preclude observations of them at solar phase angles greater than 2° from 1 au; however, New Horizons can access nearly the full range of phase angles. New Horizons has observed KBOs from all dynamical classes, including 14 classical, 5 scattered disk, 3 resonant, and 5 large KBOs, as well as 3 dwarf planets and 2 Centaurs at phase angles between 8° and 153°. KBO rotation curves acquired by New Horizons show that rotation periods range from 5 to 49 hours, and the amplitudes of many of these rotation curves increase with increasing phase angle, likely due to the non-spherical shape of the KBOs. Applying the DAMIT lightcurve inversion code (Durech et al. 2010, A&A 513, A46) to the rotation curves of these KBOs reveals that many KBO shapes are flattened and elongated like Arrokoth and have rotation poles with high obliquities (Porter et al. 2022, LPSC LIII, 2170). Combining the high phase angle observations of these KBOs from New Horizons with Earth-based observations acquired at low phase angles enables the construction of solar phase curves with broad phase angle coverage and fits to them using the Hapke photometric model. We compare these photometric model fits among dynamical classes using parameters including the single scattering albedo, macroscopic surface roughness, and directional scattering properties in addition to parameters that describe the phase curve behavior near opposition at small phase angles. The small, dark KBOs, like Arrokoth, have steep solar phase curves and small phase integrals, much like those of other small, dark asteroids, comet nuclei, and satellites. Larger KBOs and dwarf planets with volatile ices on their surfaces have shallower solar phase curves with large phase integrals. As New Horizons travels beyond the classical Kuiper belt it will observe more KBOs from the scattered disk population and more distant KBO populations.

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