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Motion of many comets is affected by non-gravitational forces caused byoutgassing from their surfaces. Outgassing also produces reactive torquesresulting in cometary spin evolution. We propose that the two processes arecorrelated and show that the change of cometary spin rate over its heliocentricorbit scales linearly with the amplitude of its non-gravitational acceleration.The proportionality constant depends on the comet size and orbital elements(semi-major axis and eccentricity) and on the (dimensionless) lever armparameter $\zeta$ that relates the outgassing-induced torque and acceleration.We determine $\zeta$ for 7 comets for which both non-gravitational accelerationand change of spin period $\Delta P$ were measured and verify this relation.This sample spanning almost 4 decades in $\Delta P$ yields $\log\zeta=-2.21\pm0.54$, surprisingly small value and spread. We then apply our framework to 209comets with measured non-gravitational accelerations and determine the objectsmost likely to exhibit large spin period changes, $\Delta P\gtrsim 20$ min perorbit assuming rotation period of 10 hr and $\zeta$ comparable to our controlsample. These objects should be primary targets for future studies of cometaryspin variability, further constraining distribution of $\zeta$. Lack of cometswith very high expected spin rate changes (which is not equivalent to havingthe highest non-gravitational acceleration) suggests that (1) cometary fissiondue to outgassing-driven spin-up must be an important process and (2) thedistribution of $\zeta$ has a lower limit $\sim 10^{-3}$.

Non-Gravitational Forces and Spin Evolution of Comets