Discovery of Tight Binaries in the Kuiper Belt by New Horizons LORRI

In addition to the close flyby (~3500 km) of Kuiper belt object (KBO) Arrokoth on New Year’s day 2019, the New Horizons spacecraft passed within 0.3 au of five other KBOs, enabling searches for binaries at unprecedented spatial resolutions using the high resolution mode of the LOng Range Reconnaissance Imager (LORRI). LORRI observations of the cold classical (CC) KBO 2011 JY31 at a resolution of ~110 km/pixel clearly revealed the presence of two approximately equal brightness objects at two different epochs. Using the astrometry from the high resolution LORRI observations and assuming a circular orbit with tidally locked bodies rotating with the orbital period determined from the nearly concurrent lightcurve observations using the lower resolution LORRI mode, we find that the binary orbit was well fit with a semi-major axis of 198.6 ± 2.8 km, an orbital inclination of 61.34 ± 1.34 deg, and an orbital period of 1.940 ± 0.002 days. LORRI observations of the CC KBO 2014 OS393 at a resolution of ~100 km/pixel show what appears to be two approximately equal brightness objects with an apparent separation of ~150 km. Unfortunately, OS393 could not be detected during a second epoch of observations because the target was near the minimum of its lightcurve. LORRI observations of CC KBO 2011 HZ102 at a resolution of ~130 km/pixel and CC KBO 2014 PN70 at a resolution of ~120 km/pixel showed no evidence for binarity, but PN70 was only marginally detected, which means a slightly fainter companion could easily be missed. LORRI detected the scattered disk KBO 2011 HK103 at two different epochs with a resolution of ~210 km/pixel, but we saw no evidence for binarity. Thus, at least 25%, and probably 50%, of the CC KBOs observed using the LORRI high resolution mode are binaries, reinforcing conclusions discussed by others (e.g., Fraser et al. 2017, Noll et al. 2020) that the cold classical Kuiper belt population has a high proportion of binaries. These New Horizons observations also highlight the unique binary orbital parameter space probed by LORRI, which has resulted in the discovery of the two tightest KBO binaries found to date.

Acknowledgments: This work was supported by the New Horizons project under NASA contract NASW-02008 and NAS5-97271/TaskOrder30. This work is the result of a joint collaboration with the New Horizons Science Team. We thank the NH Mission Operations team (especially Alice Bowman, Omar Custodio, Helen Hart, and Karl Whittenburg), the Science Operations team (Debi Rose, Nicole Martin, Ann Harch,and Emma Birath), and the GNC lead (Gabe Rogers) for their expert planning and execution of these complicated observations.