  We introduce a parameter, X, to predict the changes in the rotational period
of a comet in terms of the rotational period itself, the nuclear radius, and
the orbital characteristics. We show that X should be a constant if the bulk
densities and shapes of nuclei are nearly identical and the activity patterns
are similar for all comets. For four nuclei for which rotational changes are
well documented, despite the nearly factor 30 variation observed among the
effective active fractions of these comets, X is constant to within a factor
two. We present an analysis for the sungrazing comet C/2012 S1 (ISON) to
explore what rotational changes it could undergo during the upcoming perihelion
passage where its perihelion distance will be ~2.7 solar radii. When close to
the sun, barring a catastrophic disruption of the nucleus, the activity of ISON
will be sufficiently strong to put the nucleus into a non-principal-axis
rotational state and observable changes to the rotational period should also
occur. Additional causes for rotational state changes near perihelion for ISON
are tidal torques caused by the sun and the significant mass loss due to a
number of mechanisms resulting in alterations to the moments of inertia of the
nucleus.
