Discovery of the Closest Saturnian Irregular Moon, S/2019 S 1, and Implications for the Direct/Retrograde Satellite Ratio

Abstract

We present a tracked orbit for a recently discovered 25th magnitude irregular moon of Saturn, using Canada-France-Hawaii Telescope imaging. Our 2 yr of observational arc on the moon leads to an orbit with a semimajor axis of 11.2 million kilometers and an inclination of 44 deg. This makes it one of the smallest Saturnian irregular moon orbits known and puts the moon in the Inuit group. This moon is also a magnitude brighter than the faintest known Saturnian irregulars. We show that the moon’s small semimajor axis results in it spending most of the time lost in the glare of the often-nearby planet, thus explaining how it escaped detection in previous surveys. We postulate that the disparity in the known inventory with more retrograde than direct irregular moons is partly due to the selection bias against finding the direct moons (whose groupings have smaller semimajor axis).