Janus is a NASA SIMPLEx dual-spacecraft mission that will perform the first flybys of two binary asteroids in 2026 . The mission targets represent two distinct configurations of binary asteroid systems: (175706) 1996 FG3 is in a stable, relaxed configuration with the secondary in a 1:1 spin-orbit resonance , while (35107) 1991 VH exists in a more chaotic state with the secondary tumbling in its orbit , , . Radar and light curve measurements have been made of both binary systems but using this data to estimate orbits in the fully coupled non-Keplerian dynamic environment of the binary asteroids is difficult, particularly for the more chaotic 1991 VH system. Using the General Use Binary Asteroid Simulator and polyhedral shape models of both bodies we numerically simulate a variety of possible orbits for the two binary asteroid systems in a fourth degree and order gravity field . Since the density of both bodies is known with the least certainty, we iterate the bulk density of each system until a close match is obtained with the measured orbital period, which is the easiest measurement to obtain and known with the highest degree of accuracy. Several viable solutions are found for 1996 FG3 that closely match the measurements. The 1991 VH solution offers an additional degree of difficulty as the secondary’s shape is not well constrained, and the resulting dynamics are strongly dependent on this shape. Additionally, the secondary moving from uniform spin about its major principal axis to tumbling about all three axes could provide one explanation for the observed change in orbital period between the 2003 and 2020 measurements. This presentation will detail our methodology and best solutions for the fully coupled mutual orbits of both binary systems.
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