Cometary Science Newsletter

46P/Wirtanen

46P/Wirtanen was recovered on May 8, 2018 in images obtained by Nick Moskovitz (Lowell Observatory), in collaboration with Tony Farnham and Matthew Knight (University of Maryland). Images of the comet were obtained at the Discovery Channel Telescope operated by Lowell Observatory during astronomical twilight at 5 airmasses.

At the time of the recovery, Wirtanen was at a heliocentric distance of 2.61 AU, geocentric distance of 2.80 AU, and had an r' magnitude of 20.3. There is no obvious coma in the images, but the brightness suggests that it is already active.

The updated orbit solution confirms that Wirtanen will make its close approach to the Earth at a distance of 0.078 AU on December 16, 2018, but the exact time of close approach has an uncertainty of several hours. Additional astrometry is requested to refine the orbit and constrain the effects of non-gravitational forces.

For more information about the comet, visit the campaign website at http://wirtanen.astro.umd.edu/.

The Sublimative Torques of Jupiter Family Comets and Mass Wasting Events on Their Nuclei

• Steckloff, J.K. ^1,2,3,4
• Samarasinha, N.H. ¹

1. Planetary Science Institute, Tucson, AZ
2. University of Texas at Austin, Austin, TX
3. Massachusetts Institute of Technology, Cambridge, MA
4. Purdue University, West Lafayette, IN

Sublimative outgassing of comets produces torques that alter the rotation state of their nuclei. Recently, parameterized sublimative torque models have been developed to study rotation state changes of individual comet nuclei and populations of cometary bodies. However, these models simplify the interactions between the escaping gas and cometary surface into only a few parameters that hide the details of these complex interactions. Here we directly compare the X-parameter model (Samarasinha & Mueller, 2013) with the SYORP model (Steckloff & Jacobson, 2016) to tease out insights into the details of the gas-surface interactions driving sublimative torques. We find that, for both of these models to accurately model sublimative torques, the number of sublimating molecules that contribute to the net torque is largely independent of the detailed shape and activity of the nucleus, but rather depends primarily on the size of the nucleus and the effective heliocentric distance of the comet. We suggest that cometary activity must be largely restricted to regions of steep gravitational surface slopes (above the angle of repose), where mass wasting can refresh activity by shedding mantles of refractory materials and exposing fresh volatiles. We propose a new classification scheme for comets based on the frequency of this mass-wasting process (relative to the timescale of activity fading): quasi-equilibrium, episodic, quasi-dormant, and extinct.

Icarus (In press)

DOI: 10.1016/j.icarus.2018.04.031 arXiv: 1804.10232