Saturn's Irregular Satellites at True Opposition

Abstract

In August 2021, five of Saturn's irregular satellites were perfectly aligned with the Earth and Sun at "true" opposition. We observed Albiorix, Kiviuq, Palliaq, Siarnaq, and Tarvos from SOAR for five nights using the Goodman imaging spectrograph to measure their opposition effects. The opposition effect, or surge, is the non-linear increase in reflectance seen as an object nears opposition and the solar phase (Sun-target-observer) angle decreases. The smallest phase angles are attainable at node crossings when the Earth transits the solar disk as viewed from the object. In this configuration, a solar system body is at "true" opposition. When combined with observations at larger solar phase angles, the resulting measurement can be related to the collisional history and physical properties of the surface. These observations enhance the scientific return from the NASA/ESA Cassini Mission to Saturn and cannot be repeated until 2049.