ROTATION STATE OF COMET 103P/HARTLEY 2 FROM RADIO SPECTROSCOPY AT 1 mm

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 * Based on observations carried out with the IRAM 30-m, JCMT 15-m, and CSO 10.4-m telescopes. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).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 ± 0.15 min per day and the period itself is 18.32 ± 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.

Rotation-stimulated structures in the CN and C3comae of comet 103P/Hartley 2 close to the EPOXI encounter

“…for the middle moment of our run. Compared to other results obtained for similar dates, our solution is the same as the 18.32 ± 0.03 h periodicity detected in the HCN data (Drahus et al 2011), agrees within the Fig. 4.…”

“…The nucleus is found to precess about the longest axis of inertia with the period of 18.34 ± 0.04 h and roll about the shortest axis of inertia (longest nucleus extent) with the probable period of 27.79 ± 0.31 h (both at the epoch of the EPOXI encounter). The two periodicities are nearly commensurate in 2:3 resonance, which causes the activity pattern to repeat every three precession cycles -a property also observed in ground-based HCN data (Drahus et al 2011).…”

“…This means that both molecular environments behaved in very similar ways. To analyse the coma evolution with time, we introduced the nomenclature used earlier by Drahus et al (2011). Each residual frame corresponds to a given precession phase (changing from 0 to 1) and precession cycle (denoted as A, B, and C) of the threefold precession period (the threecycle period).…”

“…We also determine the spin axis orientation of the nucleus. Our campaign was realized simultaneously with the HCN monitoring at IRAM 30-m (Drahus et al 2011(Drahus et al , 2012, which makes it possible to investigate the connection between HCN and CN. Although the HCN molecule is assumed to be the main donor of cometary CN (e.g.…”

The Active Asteroids

“…An object losing mass through sublimation can, depending on the rate and angular dependence of the mass loss, experience a torque orders of magnitude larger than torques due to either collisions or YORP, quickly being driven toward rotational breakup (cf. Drahus et al 2011). While, in principle, rotational instability can affect any asteroid regardless of its size, in practice the spin-up times vary inversely with radius and the objects most likely to experience rotational bursting are the smallest (exactly as observed in the near-Earth population).…”