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The YORP Effect on Asteroid Spin

The Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect is a tiny torque on an irregular asteroid caused by anisotropic reflection and thermal re-emission of sunlight. Over years to decades, it can measurably change an asteroid’s spin rate and pole orientation.

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Core idea: treat the spin rate $\omega$ as evolving under a net torque $\tau$ with a simple angular-momentum balance:

For quick back-of-the-envelope work, a common scaling writes the radiative torque as a solar-pressure scale times a dimensionless shape coefficient $C_{\mathrm{Y}}$:

Here $\Phi_\odot$ is the solar flux at the asteroid’s distance from the Sun, $c$ is the speed of light, and $R$ is a characteristic size. The sign and magnitude of $C_{\mathrm{Y}}$ depend on shape, obliquity, and thermal physics.

Observed consequence: a slowly changing period

If $\dot{\omega}$ is roughly constant over an observing baseline, then:

Inline LaTeX quick demo: density $\rho$, scientific notation $10^{-6}$, units $\mathrm{m\,s^{-1}}$, and subscripts $\tau_{\mathrm{YORP}}$.

Takeaway: YORP is small per rotation, but persistent. That makes it one of the rare astrophysical effects where “sunlight pushing on rock” can be detected as a secular change in an asteroid’s lightcurve-derived spin state.