  Anisotropic outgassing from comets exerts a torque sufficient to rapidly
change the angular momentum of the nucleus, potentially leading to rotational
instability. Here, we use empirical measures of spin changes in a sample of
comets to characterize the torques and to compare them with expectations from a
simple model. Both the data and the model show that the characteristic spin-up
timescale, $\tau_s$, is a strong function of nucleus radius, $r_n$.
Empirically, we find that the timescale for comets (most with perihelion 1 to 2
AU and eccentricity $\sim$0.5) varies as $\tau_s \sim 100 r_n^{2}$, where $r_n$
is expressed in kilometers and $\tau_s$ is in years. The fraction of the
nucleus surface that is active varies as $f_A \sim 0.1 r_n^{-2}$. We find that
the median value of the dimensionless moment arm of the torque is $k_T$ = 0.007
(i.e. $\sim$0.7\% of the escaping momentum torques the nucleus), with weak
($<$3$\sigma$) evidence for a size dependence $k_T \sim 10^{-3} r_n^2$.
Sub-kilometer nuclei have spin-up timescales comparable to their orbital
periods, confirming that outgassing torques are quickly capable of driving
small nuclei towards rotational disruption. Torque-induced rotational
instability likely accounts for the paucity of sub-kilometer short-period
cometary nuclei, and for the pre-perihelion destruction of sungrazing comets.
Torques from sustained outgassing on small active asteroids can rival YORP
torques, even for very small ($\lesssim$1 g s$^{-1}$) mass loss rates. Finally,
we highlight the important role played by observational biases in the measured
distributions of $\tau_s$, $f_A$ and $k_T$.
