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

TL;DR

This study used mm/submm spectroscopy to observe the rotation and deceleration of comet 103P/Hartley 2's nucleus, revealing rapid spin-down and stability implications during its 2010 apparition.

Contribution

It provides the first detailed measurement of the comet's rotation period change and deceleration rate using radio spectroscopy, highlighting differences from theoretical predictions.

Findings

Measured rotation period of 18.32 hours.

Detected a spin-down rate of +1.00 min/day.

Found lower spin-down efficiency than models predict.

Abstract

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.