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Oscillatory variation in Binary YORP revealed by a FEM-based BYORP model

Presentation #202.03 in the session Asteroid Dynamics.

Published onOct 20, 2022
Oscillatory variation in Binary YORP revealed by a FEM-based BYORP model

Binary YORP (BYORP) is an extension of the YORP theory to a binary asteroid system, whereby the mutual orbit is modified due to a thermally induced torque acting on a synchronous secondary [1]. For a system with a fast-spinning primary and a tidally-locked secondary, BYORP can expand or contract the semimajor axis a. Contrastingly, tidal dissipation in the primary always expands the orbit. If the BYORP and tidal torque are opposed, then the system may be driven to a long-term stable condition, namely the BYORP-tide equilibrium [2]. However, past studies were typically limited to analytical modeling using averaged equations or simplified numerical approach in which neither highly coupled mutual dynamics (known as the full two-body problem, or F2BP) nor detailed thermal condition of the bodies were considered. Here, we introduce a Finite Element Modeling based BYORP model which simultaneously simulates the F2BP dynamics and 3-dimensional thermal condition of the secondary [3]. Eclipse events (i.e., in the primary’s shadow) are simulated explicitly by using a ray-tracing technique, while a simplifying assumption is used to approximate the mutual heating between the bodies due to scattered sunlight. Using the model, we investigate BYORP on the binary asteroid system (61839) Didymos. We prepare two simulation runs: with and without the BYORP torque, and compare a from the two runs to constrain BYORP-induced a evolution. Table 1 summarizes the physical and material properties used in this work.

We find that BYORP induces oscillation of a (Figure 1). The instantaneous expansion/contraction rate varies over the heliocentric orbit (from -2.43⨉10-5 to + 2.87⨉10-5 cm s-1). On average, a expands at +0.003 cm yr-1. This value is likely to be smaller than the reported value [4], because our approach here is different from how a evolution rate is typically estimated through telescopic observations. However, this result suggests that BYORP-induced evolution is complex due to the highly coupled mutual dynamics of irregular shapes and thermal evolution.

RN acknowledges support from NASA/FINESST (NNH20ZDA001N).

References: [1] Čuk and Burns (2005) Icarus 176 [2] Jacobson and Scheeres (2011) ApJL 736 [3] Nakano and Hirabayashi (2022) DDA 54(4) [4] Scheirich and Pravec (2022) PSJ 163

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