Lightcurves are powerful tools for investigating the spin rates and gross shapes of bodies throughout the Solar System, providing insights on the physical make-up of bodies without visits by spacecraft. Mutual events – eclipses and occultations – of binary bodies are doubly powerful in that their changing and interacting geometry map object shapes, sizes and mutual orbit configurations with added precision. With datasets spanning growing observing time baselines the phase space of possibilities converges, and with observations in multiple filters the distribution of surface ices can also be traced. However, interpretation of these datasets also requires modeling of both object interactions due to the individual object geometry, our Earth-based perspective, as well as the nature of the objects themselves. We present the development of such a model which uses ray-tracing to estimate the lightcurve of a binary system, including shadow-casting and bidirectional reflectance to generate comparative samples which can be iteratively referenced to actual observations of binary systems experiencing mutual events. We use the large dataset collected on (79360) Sila-Nunam from 2010-2017 both inside and outside of the mutual events to test the robustness of the model and to fully interpret this binary Cold Classical trans-Neptunian object. We also explore the predictive nature of the model for planning future observations of binary systems based on limited initial inputs.
Support for this work comes from NASA Grant/Contract/Agreement No. NNX15AE04G issued through the Solar System Observations Program.