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The nuclei of active comets emit molecules anisotropically from discretevents. As the nucleus rotates, we expect to observe periodic variability in themolecular emission line profiles, which can be studied through mm/submmspectroscopy. Using this technique we investigated the HCN atmosphere of comet103P/Hartley 2, the target of NASA's EPOXI mission, which had an exceptionallyfavorable apparition in late 2010. We detected short-term evolution of thespectral line profile, which was stimulated by the nucleus rotation, and whichprovides 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 perday and the period itself is 18.32 \pm 0.03 hr, both applicable at the epoch ofthe EPOXI encounter. Surprisingly, the spin-down efficiency is lower by twoorders of magnitude than the measurement in comet 9P/Tempel 1 and the besttheoretical prediction. This secures rotational stability of the comet'snucleus during the next few returns, although we anticipate a catastrophicdisruption from spin-up as its ultimate fate.

Rotation State of Comet 103P/Hartley 2 from Radio Spectroscopy at 1 mm