The nuclei of active comets emit molecules anisotropically from discrete
vents. As the nucleus rotates, we expect to observe periodic variability in the
molecular emission line profiles, which can be studied through mm/submm
spectroscopy. Using this technique we investigated the HCN atmosphere of comet
103P/Hartley 2, the target of NASA's EPOXI mission, which had an exceptionally
favorable apparition in late 2010. We detected short-term evolution of the
spectral line profile, which was stimulated by the nucleus rotation, and which
provides evidence for rapid deceleration and excitation of the rotation state.
The measured rate of change in the rotation period is +1.00 \pm 0.15 min per
day and the period itself is 18.32 \pm 0.03 hr, both applicable at the epoch of
the EPOXI encounter. Surprisingly, the spin-down efficiency is lower by two
orders of magnitude than the measurement in comet 9P/Tempel 1 and the best
theoretical prediction. This secures rotational stability of the comet's
nucleus during the next few returns, although we anticipate a catastrophic
disruption from spin-up as its ultimate fate.Comment: Accepted for publication in ApJ Letter