Spin-Axis Precession and the Diurnal Yarkovsky Effect

The Yarkovsky effect, arising from the anisotropic re-emission of absorbed solar radiation, is the key driver of semimajor-axis evolution for sub-kilometer asteroids (e.g., Bottke et al. 2006; Vokrouhlicky et al., 2015a). The diurnal component, driven by the body's rotation, is particularly sensitive to the spin-axis obliquity γ. Classically, the drift is approximated as da/dt ∝ cos γ, a relation valid in the limit of rotation around a fixed spin axis. However, some real asteroids exhibit spin-axis precession and may be found in the so-called tumbling state (Pravec et al., 2005), which appears to be increasingly common among small near-Earth asteroids (Alarcon et al., 2026).

This precession modulates the effective obliquity on timescales typically comparable to, or shorter than, their orbital evolution timescale. Modulations of the instantaneous obliquity, in turn, modulate the diurnal Yarkovsky acceleration. At the same time, on eccentric orbits, the diurnal Yarkovsky effect is strongly concentrated near perihelion, so the orientation of the spin axis at perihelion carries disproportionate weight in the orbit-averaged drift.

Although spin-axis precession could have a significant impact on the Yarkovsky effect-driven orbital semi-major axis mobility (e.g., Vokrouhlicky et al. 2015b), the issue has not been systematically addressed to date.

Using a thermophysical model developed by Marceta et al. (2025) and implementing a time-dependent spin-axis geometry, we examine how the interplay among orbital eccentricity, spin-axis precession amplitude, and spin-orbit commensurability affects the long-term Yarkovsky drift. We map the dependence of the secular Yarkovsky drift on eccentricity, mean obliquity, precession cone amplitude, and the precession-to-orbital-period ratio. 

Our preliminary results suggest that spin-axis precession can be an important component of long-term orbital evolution for small asteroids on highly eccentric trajectories. We identify regimes where the drift can be significantly altered by precession. An example includes enhanced drift for low mean obliquity when resonant phasing keeps the spin axis near its most efficient orientation at perihelion.

Acknowledgments: This research is supported by The Science Fund of the Republic of Serbia through Project No. 7453, Demystifying enigmatic visitors of the near-Earth region (ENIGMA)

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