Approximately 15% of near-Earth asteroids (NEAs) larger than about 100 m exist within binary pairs, spanning diverse spectral types, sizes and orbits. The upcoming Janus mission to two such systems will explore the processes that lead to binary asteroid formation and constrain geophysical and dynamical parameters of each system. Binary asteroids are thought to progress through several evolutionary pathways that result in various possible end states such as a stable binary system, a contact binary or a pair of separated asteroids . The first system, 1996 FG3, is believed to be a stable binary system . The second binary, 1991 VH, is thought to be in a dynamically chaotic state and is possibly still evolving . A primary goal of Janus is to observe these bodies in the thermal infrared in order to constrain thermophysical properties and the binary YORP effect (BYORP). Through thermophysical modeling, these observations can be used to constrain the magnitude of BYORP and its effects on the dynamical evolution of these systems.
Here we discuss the development of a 3-d thermal model for binary asteroid systems to calculate temperatures and thermal IR fluxes. We couple a 1-d thermophysical model to a 3-d shape model and calculate facet-by-facet temperatures during the Janus flybys. For 1991 VH, using an Itokawa-like thermal inertia of 700 ± 200 J m−2 K−1 s−1/2 we calculate the temperature range over the whole body to be ~130 K to 280 K, with a median diurnal amplitude of ~30 K at the equator. For 1996 FG3, a thermal inertia of 120 ± 50 Jm−2K−1s−1/2 yields a whole-body temperature range of ~50 K to 310 K, with a median equatorial diurnal amplitude of ~110 K. We also ran models for each secondary body using assumed shapes and thermophysical properties, and will study the BYORP effect on orbital evolution of these binary systems, including differences between ordered and disordered systems. We expect to be able to estimate the BYORP effect for 1996 FG3. However, though we will get comparable data and measurements, we do not expect to be able to constrain the BYORP magnitude for 1991 VH due to its unstable orbit. Our predictions are directly testable during the Janus mission, with an anticipated launch date of 2022.
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