Cometary Science Newsletter

May 2021
Michael S. P. Kelley (

Refereed Articles

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

Systematics and Consequences of Comet Nucleus Outgassing Torques

  • Jewitt, D. 1
  1. UCLA

Anisotropic outgassing from comets exerts a torque sufficient to rapidly change the angular momentum of the nucleus, potentially leading to rotational instability. Here, we use empirical measures of spin changes in a sample of comets to characterize the torques and to compare them with expectations from a simple model. Both the data and the model show that the characteristic spin-up timescale, T, is a strong function of nucleus radius, r. Empirically, we find that the timescale for comets (most with perihelion 1 to 2 AU and eccentricity near 0.5) varies as T ~ 100 r2, where r is expressed in kilometers and T is in years. The fraction of the nucleus surface that is active varies as fA ~ 0.1 r-2. We find that the median value of the dimensionless moment arm of the torque is kT = 0.007 (i.e.~0.7% of the escaping momentum torques the nucleus), with weak (<3 sigma) evidence for a size dependence kT ~ 0.001 r2. Sub-kilometer nuclei have spin-up timescales comparable to their orbital periods, confirming that outgassing torques are quickly capable of driving small nuclei towards rotational disruption. Torque-induced rotational instability likely accounts for the paucity of sub-kilometer short-period cometary nuclei, and for the pre-perihelion destruction of sungrazing comets. Torques from sustained outgassing on small active asteroids can rival YORP torques, even for very small (<1 g/s) mass loss rates. Finally, we highlight the important role played by observational biases in the measured distributions of T, fA and kT.

The Astronomical Journal (In press)

arXiv: 2103.10577

Initial Characterization of Active Transitioning Centaur, P/2019 LD2 (ATLAS), Using Hubble, Spitzer, ZTF, Keck, Apache Point Observatory, and GROWTH Visible and Infrared Imaging and Spectroscopy

  • Bolin, B.T. 1, 2
  • Fernandez, Y.R. 3
  • Lisse, C.M. 4
  • Holt, T.R. 5,6
  • and 41 co-authors
  1. California Institute of Technology, USA
  2. IPAC, USA
  3. University of Central Florida, USA
  4. Johns Hopkins University Applied Physics Laboratory, USA
  5. University of Southern Queensland, Australia
  6. Southwest Research Institute, USA

We present visible and mid-infrared imagery and photometry of temporary Jovian co-orbital comet P/2019 LD2 taken with Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3), Spitzer Space Telescope/Infrared Array Camera (Spitzer/IRAC), and the GROWTH telescope network, visible spectroscopy from Keck/Low-Resolution Imaging Spectrometer (LRIS), and archival Zwicky Transient Facility observations taken between 2019 April and 2020 August. Our observations indicate that the nucleus of LD2 has a radius between 0.2 and 1.8 km assuming a 0.08 albedo and a coma dominated by ~100 μm-scale dust ejected at ~1 m s−1 speeds with a ~1'' jet pointing in the southwest direction. LD2 experienced a total dust mass loss of ~108 kg at a loss rate of ~6 kg s−1 with Afρ/cross section varying between ~85 cm/125 km2 and ~200 cm/310 km2 from 2019 April 9 to 2019 November 8. If the increase in Afρ/cross section remained constant, it implies LD2's activity began ~2018 November when within 4.8 au of the Sun, implying the onset of H2O sublimation. We measure CO/CO2 gas production of lesssim1027 mol s−1/lesssim1026 mol s−1 from our 4.5 μm Spitzer observations; g–r = 0.59 ± 0.03, r–i = 0.18 ± 0.05, and i–z = 0.01 ± 0.07 from GROWTH observations; and H2O gas production of lesssim80 kg s−1 scaling from our estimated C2 production of QC2 ~ 7.5 1024 mol s−1 from Keck/LRIS spectroscopy. We determine that the long-term orbit of LD2 is similar to Jupiter-family comets having close encounters with Jupiter within ~0.5 Hill radius in the last ~3 y and within 0.8 Hill radius in ~9 y. Additionally, 78.8% of our orbital clones are ejected from the solar system within 1 × 106 yr, having a dynamical half-life of 3.4 × 105 yr.

The Astronomical Journal (Published)

DOI: 10.3847/1538-3881/abd94b NASA ADS: 2021AJ....161..116B arXiv: 2011.03782

Time-series and Phase-curve Photometry of the Episodically Active Asteroid (6478) Gault in a Quiescent State Using APO, GROWTH, P200, and ZTF

  • Purdum, J. N. 1
  • Lin, Z. Y. 2
  • Bolin, B. T. 3
  • Sharma, K. 4
  • Choi, P. I. 5
  • Bhalerao, V. 6
  • Hanus, J. 7
  • Kumar, H. 6
  • Quimby, R. 1,8
  • van Roestel, J. C. 9
  • Zhai, C. 10
  • Fernandez, Y. R. 11
  • Lisse, C. M. 12
  • Bodewits, D. 13
  • Fremlin, C. 9
  • Golovich, N. R. 14
  • Hsu, C. Y. 15
  • Ip, W. H. 16
  • Ngeow, C. C. 15
  • Saini, N. S. 10
  • Shao, M. 10
  • Yao, Y. 9
  • Ahumada, T. 17
  • Anand, S. 9
  • Andreoni, I. 9
  • Burdge, K. B. 9
  • Burruss, R. 18
  • Chang, C. K. 15
  • Copperwheat, C. M. 19
  • Coughlin, M. 20
  • De, K. 9
  • Dekany, R. 18
  • Delacroix, A. 18
  • Drake, A. 9
  • Duev, D. 9
  • Graham, M. 9
  • Hale, D. 18
  • Kool, E. C. 21,22
  • Kasliwal, M. M. 9
  • Kostadinova, I. S. 9
  • Kulkarni, S. R. 9
  • Laher, R. R. 3
  • Mahabal, A. 9,23
  • Masci, F. J. 3
  • Mroz, P. J. 9
  • Neill, J. D. 9
  • Riddle, R. 18
  • Rodriguez, H. 18
  • Smith, R. M. 18
  • Walters, R. 9
  • Yan, L. 18
  • Zolkower, J. 18
  1. Department of Astronomy, San Diego State University, San Diego, CA
  2. Institute of Astronomy, National Central University, Taoyuan, Taiwan
  3. IPAC, Caltech, Pasadena, CA
  4. Department of Mechanical Engineering, Indian Institute of Technology Bombay, Powai, India
  5. Physics and Astronomy Department, Pomona College, Claremont, CA
  6. Department of Physics, Indian Institute of Technology Bombay, Powai, India
  7. Institute of Astronomy, Charles University, Prague, Czech Republic
  8. Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, Kashiwa, Japan
  9. Division of Physics, Mathematics and Astronomy, Caltech, Pasadena, CA
  10. Jet Propulsion Laboratory, Caltech, Pasadena, CA
  11. Department of Physics and Florida Space Inst., University of Central Florida, Orlando FL
  12. Johns Hopkins University Applied Physics Laboratory, Laurel, MD
  13. Physics Department, Auburn University, Auburn, AL
  14. Lawrence Livermore National Laboratory, Livermore, CA
  15. Graduate Institute of Astronomy, National Central University, Taiwan
  16. Institute of Astronomy, National Central University, Taiwan
  17. Department of Astronomy, University of Maryland, College Park, MD
  18. Caltech Optical Observatories, Caltech, Pasadena, CA
  19. Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK
  20. School of Physics and Astronomy, University of Minnesota, Minneapolis, MN
  21. The Oskar Klein Centre, Department of Astronomy, Stockholm University, Stockholm, Sweden
  22. Department of Physics and Astronomy, Macquarie University, Sydney, Australia
  23. Center for Data Driven Discovery, Caltech, Pasadena, CA

We observed the episodically active asteroid (6478) Gault in 2020 with multiple telescopes in Asia and North America and found that it is no longer active after its recent outbursts at the end of 2018 and the start of 2019. The inactivity during this apparition allowed us to measure the absolute magnitude of Gault of Hr = 14.63 ± 0.02, Gr = 0.21 ± 0.02 from our secular phase-curve observations. In addition, we were able to constrain Gault's rotation period using time-series photometric lightcurves taken over 17 hr on multiple days in 2020 August, September, and October. The photometric lightcurves have a repeating < 0.05 mag feature suggesting that (6478) Gault has a rotation period of ~2.5 hr and may have a semispherical or top-like shape, much like the near-Earth asteroids Ryugu and Bennu. The rotation period of ~2.5 hr is near the expected critical rotation period for an asteroid with the physical properties of (6478) Gault, suggesting that its activity observed over multiple epochs is due to surface mass shedding from its fast rotation spin-up by the Yarkovsky–O'Keefe–Radzievskii–Paddack effect.

The Astrophysical Journal Letters (Published)

DOI: 10.3847/2041-8213/abf2ca NASA ADS: 2021arXiv210213017P arXiv: 2102.13017