Cometary Science Newsletter

April 2021
Michael S. P. Kelley (

Europlanet funding for Workshops and for (Remote) Observations with the Europlanet Telescope Network

Open call, rolling deadline: Funding to support observations at the Europlanet Telescope Network. This network currently provides access to 16 different telescope facilities distributed all over the world, with telescopes up to 2 meters in diameter, 14 of which are either robotic or provide service observations, and continue to operate despite COVID-19. Proposals are invited on any topic broadly related to Solar System or exoplanet observations. Short proposals (max 3 pages) and can be submitted at any time; decisions on funding are made on a bi-monthly basis.

Further details on the call and on how to apply can be found at the call website; our telescope table holds a broad range of information and contact data on all the facilities in the network. To apply, please contact the relevant facilities first, agree on observing time, and then submit your funding proposal. If you have further questions on the call or if you are interested in organizing a coordinated observations campaign that would like to make use of the Europlanet Telescope Network, please contact

Proposals are also invited for support to host workshops on any topic related to observational planetary science. Europlanet is particularly keen to support meetings bringing together professional and amateur astronomers, and/or on observations supporting space missions, but all relevant topics will be considered. Contact to propose ideas or for more information.

Comets postdoc position at the University of Edinburgh

We are hiring a Postdoctoral Research Associate in the field of cometary science. The successful candidate will join the research group led by Dr Colin Snodgrass and Dr Cyrielle Opitom at the University of Edinburgh.

The research goal of this position is to advance our understanding of the formation, evolution, and composition of comets. The successful candidate will work together with Dr Snodgrass and Dr Opitom on a variety of projects in this area, depending on their experience and research interests. Potential areas of interest include (but are not restricted to):

  • The study of comet composition at infrared wavelengths
  • Cometary science with the Rubin Observatory LSST
  • The analysis of archival data from the ESA Rosetta mission
  • Theoretical modelling of any aspect of comet formation or activity processes
  • Studies relevant to the development of the ESA Comet Interceptor mission

This position is initially for a fixed term of two years, but may be extended, subject to available funding.

We welcome applications from all qualified candidates, regardless of nationality.

Please include your CV, a research proposal (2 pages) and the names of two referees.

See for more information, or contact and/or

Refereed Articles

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

Cometary Activity Begins at Kuiper Belt Distances: Evidence from C/2017 K2

  • Jewitt, D 1
  • Kim, Y. 2
  • Mutchler, M. 3
  • Agarwal, J. 2
  • Li, J. 1
  • Weaver, H. 4
  1. UCLA
  2. Technical University at Braunschweig
  3. Space Telescope Science Institute
  4. Johns Hopkins University Applied Physics Laboratory

We study the development of activity in the incoming long-period comet C/2017 K2 over the heliocentric distance range 9 to 16 AU. The comet continues to be characterized by a coma of sub-millimeter and larger particles ejected at low velocity. In a fixed co-moving volume around the nucleus we find that the scattering cross-section of the coma, C, is related to the heliocentric distance by a power law, C ~ r_H^(-s), with heliocentric index s = 1.14+/-0.05. This dependence is significantly weaker than the inverse square variation of the insolation as a result of two effects. These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving particles ejected long before the observations were taken. A Monte Carlo model of the photometry shows that dust production beginning at r_H ~ 35 AU is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution. Mass loss rates in dust at 10 AU are of order 1000 kg/s, while loss rates in gas may be much smaller, depending on the unknown dust to gas ratio. Consequently, the ratio of the non-gravitational acceleration to the local solar gravity may, depending on the nucleus size, attain values ≲ 1e-7 to 1e-5, comparable to values found in short-period comets at much smaller distances. Non-gravitational acceleration in C/2017 K2 and similarly distant comets, while presently unmeasured, may limit the accuracy with which we can infer the properties of the Oort cloud from the orbits of long-period comets.

Astronomical Journal (Published)

DOI: 10.3847/1538-3881/abe4cf NASA ADS: 2021AJ....161..188J arXiv: 2102.06313

First Comet Observations with NIRSPEC-2 at Keck: Outgassing Sources of Parent Volatiles and Abundances Based on Alternative Taxonomic Compositional Baselines in 46P/Wirtanen

  • Bonev, B. P. 1
  • Dello Russo, N. 2
  • DiSanti. M. A. 3
  • Martin, E. C. 4
  • Doppmann. G. 5
  • and 16 co-authors
  1. American University, Washington, DC 20016, USA
  2. Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
  3. NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
  4. University of California Santa Cruz, Santa Cruz, CA 95064, USA
  5. W. M. Keck Observatory, Kamuela, HI 96743, USA

A major upgrade to the NIRSPEC instrument at the Keck II telescope was successfully completed in time for near-infrared spectroscopic observations of comet 46P/Wirtanen during its exceptionally close flyby of Earth in 2018 December. These studies determined the abundances of several volatiles, including C2H2, C2H6, CH3OH, NH3, HCN, H2CO, and H2O. Long-slit spatial distributions of gas rotational temperature and column density are diagnostic for the presence of icy grains in the coma and understanding if different volatiles are associated with common or distinct outgassing sources. These spatial distributions suggest that C2H2, C2H6, and HCN have a common outgassing source, whereas H2O and CH3OH have additional, more extended sources. The synergy of these findings with observations by space missions (Rosetta and EPOXI) motivates continuing studies to address whether or not C2H6, C2H2, and HCN have a common source of release (plausibly associated with CO2) in a larger sample of comets and whether systematic differences exist in the release of these species compared to H2O and CH3OH. Abundances of volatiles are reported relative to H2O, as traditionally done, as well as C2H6. While not unique, the choice of C2H6 demonstrates the value of extending the chemical taxonomy of parent volatiles in comets toward additional compositional "baselines" and, importantly, closer integration between coma abundances and the underlying volatile associations as revealed by spatial distributions. Our findings on composition and sources of outgassing include information relevant to future evaluations of 46P/Wirtanen as a prospective spacecraft target.

The Planetary Science Journal (Published)

DOI: 10.3847/PSJ/abd03c NASA ADS: 2021PSJ.....2...45B

Rapidly Varying Anisotropic Methanol (CH3OH) Production in the Inner Coma of Comet 46P/Wirtanen as Revealed by the ALMA Atacama Compact Array

  • Roth, N. X. 1,2,14
  • Milam, S. N. 1
  • Cordiner, M. A. 1,3
  • Bockelée-Morvan, D. 4
  • DiSanti, M. A. 5,14
  • Boissier, J. 6
  • Biver, N. 4
  • Crovisier, J. 4
  • Dello Russo, N. 7,14
  • Bonev, B. P. 8,14
  • Qi, C. 9
  • Remijan, A. J. 10
  • Charnley, S. B. 1
  • Gibb, E. L. 11,14
  • de Val-Borro, M. 12
  • Jehin, E. 13
  1. Solar System Exploration Division, Astrochemistry Laboratory Code 691, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
  2. Universities Space Research Association, Columbia, MD 21046, USA
  3. Department of Physics, Catholic University of America, Washington DC, USA
  4. LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, F-92195 Meudon, France
  5. Solar System Exploration Division, Planetary Systems Laboratory Code 693, NASA-GSFC, Greenbelt, MD 20771, USA
  6. Institut de Radioastronomie Millimetrique, 300 rue de la piscine, F-38406 Saint Martin d’Heres, France
  7. Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD, 20723 USA
  8. Department of Physics, American University, 4400 Massachusetts Avenue, NW Washington, DC 20016 USA
  9. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Mail Stop 42 Cambridge, MA 02138, USA
  10. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
  11. Department of Physics & Astronomy, University of Missouri-St.Louis, 1 University Boulevard, St. Louis, MO 63121 USA
  12. Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719, USA
  13. Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liège, Belgium
  14. Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration

We report the detection of CH3OH emission in comet 46P/Wirtanen on UT 2018 December 8 and 9 using the Atacama Compact Array (ACA), part of the Atacama Large Millimeter/Submillimeter Array (ALMA). These interferometric measurements of CH3OH along with continuum emission from dust probed the inner coma (<2000 km from the nucleus) of 46P/Wirtanen approximately one week before its closest approach to Earth (Δ = 0.089–0.092 au), revealing rapidly varying and anisotropic CH3OH outgassing during five separate ACA executions between UT 23:57 December 7 and UT 04:55 December 9, with a clear progression in the spectral line profiles over a timescale of minutes. We present spectrally integrated flux maps, production rates, rotational temperatures, and spectral line profiles of CH3OH during each ACA execution. The variations in CH3OH outgassing are consistent with Wirtanen’s 9 hr nucleus rotational period derived from optical and millimeter wavelength measurements and thus are likely coupled to the changing illumination of active sites on the nucleus. The consistent blue offset of the line center indicates enhanced CH3OH sublimation from the sunward hemisphere of the comet, perhaps from icy grains. These results demonstrate the exceptional capabilities of the ACA for time-resolved measurements of comets such as 46P/Wirtanen.

The Planetary Science Journal (Published)

DOI: 10.3847/PSJ/abdd3a NASA ADS: 2021PSJ.....2...55R arXiv: 2103.06684

The Volatile Composition of the Inner Coma of Comet 46P/Wirtanen: Coordinated Observations Using iSHELL at the NASA-IRTF and Keck/NIRSPEC-2

  • Roth, N. X. 1,2,12
  • Bonev, B. P. 3,12
  • DiSanti, M. A. 4,12
  • Dello Russo, N. 5,12
  • McKay, A. J. 3,4,12
  • Gibb, E. L. 6,12
  • Saki, M. 6,12
  • Khan, Y. 6,12
  • Vervack, R. J., Jr. 5,12
  • Kawakita, H. 7,12
  • Cochran, A. L. 8
  • Biver, N. 9
  • Cordiner, M. A. 1,10
  • Crovisier, J. 9
  • Jehin, E. 11
  • Weaver, H. A. 5
  1. Solar System Exploration Division, Astrochemistry Laboratory Code 691, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
  2. Universities Space Research Association, Columbia, MD 21046, USA
  3. Department of Physics, American University, 4400 Massachusetts Avenue NW, Washington, DC 20016, USA
  4. Solar System Exploration Division, Planetary Systems Laboratory Code 693, NASA-GSFC, Greenbelt, MD 20771, USA
  5. Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
  6. Department of Physics & Astronomy 1 University Boulevard, University of Missouri-St.Louis, St. Louis, MO 63121, USA
  7. Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto, 603-8555, Japan
  8. University of Texas at Austin/McDonald Observatory, 2512 Speedway, Stop C1402, Austin, TX 78712, USA
  9. LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, F-92195 Meudon, France
  10. Department of Physics, Catholic University of America, Washington DC, USA
  11. Space sciences, Technologies & Astrophysics Research (STAR) Institute, University of Liège, Belgium
  12. Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract NNH14CK55B with the National Aeronautics and Space Administration.

The 2018 perihelion passage of comet 46P/Wirtanen afforded an opportunity to measure the abundances and spatial distributions of coma volatiles in a Jupiter-family comet with exceptional spatial resolution for several weeks surrounding its closest approach to Earth (Δmin ∼0.078 au on UT December 16). We conducted near-infrared spectroscopic observations of 46P/Wirtanen using iSHELL at the NASA Infrared Telescope Facility on UT 2018 December 18 in direct coordination with observations using the newly upgraded NIRSPEC-2 instrument at the W. M. Keck Observatory, and securely detected fluorescent emission from CH3OH, C2H6, and H2O. This coordinated campaign utilizing the two premier near-infrared facilities in the northern hemisphere enabled us to sample distinct projections of the coma into the plane of the sky simultaneously, and provided an unprecedented view into the inner coma of 46P/Wirtanen near closest approach. We report rotational temperatures, production rates, and abundance ratios (i.e., mixing ratios) for all sampled species and compare our iSHELL results to simultaneous (or near-simultaneous) measurements taken with NIRSPEC-2. We demonstrate the extraordinary synergy of coordinated measurements using iSHELL and NIRSPEC-2, and advocate for future cometary studies that jointly leverage the capabilities of these two facilities.

The Planetary Science Journal (Published)

DOI: 10.3847/PSJ/abd706/54 NASA ADS: 2021PSJ.....2...54R

Tails: Chasing Comets with the Zwicky Transient Facility and Deep Learning

  • Dmitry A. Duev 1
  • Bryce T. Bolin 1,2
  • Matthew J. Graham 1
  • Michael S. P. Kelley 3
  • Ashish Mahabal 1,4
  • Eric C. Bellm 5
  • Michael W. Coughlin 6
  • Richard Dekany 7
  • George Helou 2
  • Shrinivas R. Kulkarni 1
  • Frank J. Masci 2
  • Thomas A. Prince 1
  • Reed Riddle 7
  • Maayane T. Soumagnac 8,9
  • Stéfan J. van der Walt 10
  1. Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
  2. IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
  3. Department of Astronomy, University of Maryland, College Park, MD 20742, USA
  4. Center for Data Driven Discovery, California Institute of Technology, Pasadena, CA 91125, USA
  5. DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA
  6. School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
  7. Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
  8. Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
  9. Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
  10. Berkeley Institute for Data Science, University of California, Berkeley, Berkeley, CA 94720, USA

We present Tails, an open-source deep-learning framework for the identification and localization of comets in the image data of the Zwicky Transient Facility (ZTF), a robotic optical time-domain survey currently in operation at the Palomar Observatory in California, USA. Tails employs a custom EfficientDet-based architecture and is capable of finding comets in single images in near real time, rather than requiring multiple epochs as with traditional methods. The system achieves state-of-the-art performance with 99% recall, 0.01% false positive rate, and 1-2 pixel root mean square error in the predicted position. We report the initial results of the Tails efficiency evaluation in a production setting on the data of the ZTF Twilight survey, including the first AI-assisted discovery of a comet (C/2020 T2) and the recovery of a comet (P/2016 J3 = P/2021 A3).

Tails code is available at DOI: 10.5281/zenodo.4563226.

The Astronomical Journal (In press)

NASA ADS: 2021arXiv210213352D arXiv: 2102.13352

The Visual Lightcurve of Comet C/1995 O1 (Hale–Bopp) from 1995 to 1999

  • M. Womack 1,2,3
  • and 24 co-authors
  1. National Science Foundation
  2. Florida Space Institute
  3. University of Central Florida Physics Department

The great comet C/1995 O1 (Hale-Bopp) presented a remarkable opportunity to study its long-term brightness over four years. We used 2240 observations published in the International Comet Quarterly from 17 observers during 1995 July to 1999 September to create a secular lightcurve. In order to account for observer differences, we present a novel algorithm to reduce scatter and increase precision in a lightcurve compiled from many sources. It is implemented in a publicly available code, ICQSPLITTER, which uses a self-consistent statistical approach. To first order, the comet's lightcurve approximates an r-4 response for both pre- and postperihelion distances. The preperihelion data are better fit with a fifth-order polynomial with inflection points at 4.0, 2.6, 2.1, and 1.1 au, some of which are associated with physical changes in the coma. Outbursts may have occurred a few days before perihelion and at ∼2.2 and 7.4 au postperihelion. The Afρ values derived from the final magnitudes are consistent with an r-1.5 dependence on heliocentric distance and are within a factor of 2-4 of those derived from spectroscopy and narrowband photometry. We present correlation equations for visual magnitudes and CO and H2O production rates that are consistent with the preperihelion brightness increasing due to CO outgassing until about 2.6-3.0 au from the Sun and then are strongly correlated with H2O production rates. We also present two generalized correlation equations that may be useful for observation planning and data analysis with the James Webb Space Telescope and other observatories.

The Planetary Science Journal (Published)

DOI: 10.3847/PSJ/abd32c NASA ADS: 2021PSJ.....2...17W

Narrowband Observations of Comet 46P/Wirtanen During its Exceptional Apparition of 2018/19 II: Photometry, Jet Morphology, and Modeling Results

  • Knight, M.M. 1,2
  • Schleicher, D.G. 3
  • Farnham, T.L. 2
  1. U.S. Naval Academy, USA
  2. University of Maryland, USA
  3. Lowell Observatory, USA

We report on our extensive photometry and imaging of Comet 46P/Wirtanen during its 2018/19 apparition and use these data to constrain modeling of Wirtanen's activity. Narrowband photometry was obtained on nine epochs from 2018 October through 2019 March as well as 10 epochs during the 1991, 1997, and 2008 apparitions. The ensemble photometry reveals a typical composition and a secular decrease in activity since 1991. Production rates were roughly symmetric around perihelion for the carbon-bearing species (CN, C3, and C2), but steeper for OH and NH outbound. Our imaging program emphasized CN, whose coma morphology and lightcurve yielded rotation periods reported in a companion paper (Farnham et al., PSJ, 2, 7). Here, we compare the gas and dust morphology on the 18 nights for which observations of additional species were obtained. The carbon-bearing species exhibited similar morphology that varied with rotation. OH and NH had broad, hemispheric brightness enhancements in the tailward direction that did not change significantly with rotation, which we attribute to their originating from a substantial icy grain component. We constructed a Monte Carlo model that replicates the shape, motion, and brightness distribution of the CN coma throughout the apparition with a single, self-consistent solution in principal axis rotation. Our model yields a pole having (R.A., Decl.) = 319 deg, −5 deg (pole obliquity of 70 deg) and two large sources (radii of 50 deg and 40 deg) centered at near-equatorial latitudes and separated in longitude by ∼160 deg. Applications of the model to explain observed behaviors are discussed.

Planetary Science Journal (In press)

NASA ADS: 2021arXiv210313486K arXiv: 2103.13486

Quantifying the Hypervolatile Abundances in Jupiter-Family Comet 46P/Wirtanen

  • McKay, A.J.1,2
  • DiSanti, M.A.1
  • Cochran, A.L.3
  • Bonev, B.2
  • Dello Russo, N.4
  • Vervack, R.J.4
  • Gibb, E.5
  • Roth, N.X.1,6
  • Saki, M.5
  • Khan, Y.5
  • Kawakita, H.6
  1. NASA Goddard Space Flight Center, Greenbelt, MD, USA
  2. American University, Washington, D.C., USA
  3. University of Texas at Austin/McDonald Observatory, Austin, TX, USA
  4. Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA
  5. University of Missouri-St. Louis, St. Louis, MO, USA
  6. Kyoto Sangyo University, Kyoto, Japan

We present analysis of IR and optical spectroscopy of Jupiter-family comet (JFC) 46P/Wirtanen obtained in 2019 January, when the comet had sufficient geocentric velocity to enable studies of the hypervolatiles CO and CH4, as well as [O I] emission. These species could not be studied near closest approach in mid-December because there was insufficient Doppler shift to separate the cometary emission from their corresponding telluric absorption lines. We employed the [O I] observations as a proxy for the CO2/H2O ratio, as CO2 cannot be observed directly from the ground, and space-based assets sensitive to CO2 were not able to observe 46P during this apparition. We focused our analysis on H2O, CO, CH4, C2H6, CH3OH, and CO2 (via [O I] emission). We detected strong emissions from H2O, C2H6, and CH3OH. Over the 3 nights, we found evidence for changing mixing ratios, mostly due to a variable H2O production rate. In 46P, C2H6 and CH3OH are enriched compared to cometary averages, with mixing ratios relative to H2O of ∼1% and ∼3%, respectively. Measurements of CH4 and CO have been especially rare in JFCs. We report significant 3σ upper limits on CH4/H2O < 0.97% and CO/H2O < 0.54%. They place CH4 being near-average or depleted, and CO being strongly depleted in 46P compared with Oort cloud comets. 46P has comparable CO/H2O to the few other measurements in JFCs, but enriched in C2H6 and CH3OH. Our inferred CO2/H2O mixing ratio is ∼15%, though accounting for systematic uncertainties from the lack of knowledge of [O I] photochemistry means a value between 10% and 20% is likely. The compositional profile of 46P is similar to another small, hyperactive comet: 103P/Hartley 2. The mechanism of CO2-driven water-rich ice grain production proposed for 103P/Hartley 2 may be operating on 46P as well.

The Planetary Science Journal (Published)

DOI: 10.3847/PSJ/abd71d NASA ADS: 2021PSJ.....2...21M