Cometary Science Newsletter

June 2018
Michael S. P. Kelley (

Comet Notes

Brief observational reports or other notes related to specific comets. Limited to 1000 characters. The CSN is not intended to replace telegram services or other breaking news outlets.


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

Refereed Articles

Abstracts of articles in press or recently published. Limited to 3000 characters.

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

The Reactivation and Nucleus Characterization of Main-Belt Comet 358P/PANSTARRS (P/2012 T1)

  • Hsieh, H. H. 1,2
  • Ishiguro, M. 3
  • Knight, M. M. 4
  • Micheli, M. 5,6
  • Moskovitz, N. 7
  • Sheppard, S. S. 8
  • Trujillo, C. A. 9
  1. Planetary Science Institute, USA
  2. Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan
  3. Seoul National University, South Korea
  4. University of Maryland, USA
  5. ESA SSA-NEO Coordination Centre, Italy
  6. INAF - Osservatorio Astronomico di Roma, Italy
  7. Lowell Observatory, USA
  8. Carnegie Institution for Science, USA
  9. Northern Arizona University, USA

We present observations of main-belt comet 358P/PANSTARRS (P/2012 T1) obtained using the Gemini South telescope from 2017 July to 2017 December, as the object approached perihelion for the first time since its discovery. We find best-fit IAU phase function parameters of HR=19.5±0.2 mag and GR=-0.22±0.13 for the nucleus, corresponding to an effective radius of rN=0.32±0.03 km (assuming an albedo of pR=0.05). The object appears significantly brighter (by >1 mag) than expected starting in 2017 November, while a faint dust tail oriented approximately in the antisolar direction is also observed on 2017 December 18. We conclude that 358P has become active again for the first time since its previously observed active period in 2012-2013. These observations make 358P the seventh main-belt comet candidate confirmed to exhibit recurrent activity near perihelion with intervening inactivity away from perihelion, strongly indicating that its activity is sublimation-driven. Fitting a linear function to the ejected dust masses inferred for 358P in 2017 when it is apparently active, we find an average net dust production rate of 2.0±0.6 kg/s (assuming a mean effective particle radius of 1 mm) and an estimated activity start date of 2017 November 8±4 when the object was at a true anomaly of 316±1 deg and a heliocentric distance of R=2.54 AU. Insufficient data is currently available to ascertain whether activity strength has changed between the object's 2012-2013 and 2017 active periods. Further observations are therefore highly encouraged during the object's upcoming observing window (2018 August through 2019 May).

The Astronomical Journal (In press)

arXiv: 1805.09353

C/2016 R2 (PANSTARRS): A comet rich in CO and depleted in HCN

  • Wierzchos, K. 1
  • Womack, M. 1
  1. University of South Florida

We observed comet C/2016 R2 (PANSTARRS) with the ARO 10-m SMT, and report the first detection of CO emission from this comet with amounts high enough to be the primary driver of activity. We obtained spectra and maps of the CO J=2-1 rotational line at 230 GHz between 2017 December and 2018 January. We calculated an average production rate of Q(CO)=(4.6+/-0.4)x1028 mol/s at r ~ 2.9 au and Delta ~ 2.1 au. The CO line is thin, FWHM ~ 0.8 km/s with a slight blue-shift of ~ -0.1 km/s from the ephemeris velocity, and we derive a gas expansion velocity of Vexp= 0.50 +/- 0.15 km/s. This comet produced approximately half the CO that comet C/1995 O1 (Hale-Bopp) did at 3 au. If CO production scales with nucleus surface area, then the radius need not exceed R ~ 15 km. The spectra and mapping data are consistent with CO arising from a combination of a sunward-side active area and an isotropic source. For HCN, we calculated a 3-sigma upper limit production rate of Q(HCN) < 8x1024 mol/s, which corresponds to an extraordinarily high abundance ratio limit of Q(CO)/Q(HCN) > 5000. We inferred a production rate of molecular nitrogen of Q(N2) 2.8x1027 mol/s using our CO data and the reported N2/CO column density ratio (Cochran & McKay 2018a,b). The comet does not show the typical nitrogen depletion seen in comets. The CO-rich, N2-rich and HCN-depleted values are consistent with formation in an environment of T < 50 K that may have provided significant N2 shielding.

The Astronomical Journal (In press)

NASA ADS: 2018arXiv180506918W arXiv: 1805.06918