Cometary Science Newsletter

January 2020
Michael S. P. Kelley (

Refereed Articles

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

New Insights into Interstellar Object 1I/2017 U1 (ʻOumuamua) from SOHO/STEREO Nondetections

  • Hui, Man-To 1,2
  • Knight, Matthew M. 3
  1. Institute for Astronomy, University of Hawaii, USA
  2. Department of Earth, Planetary and Space Sciences, UCLA, USA
  3. Department of Astronomy, University of Maryland, USA

Object 1I/2017 U1 (ʻOumuamua) is the first interstellar small body ever discovered in the solar system. By the time of discovery, it had already passed perihelion. To investigate the behavior of ʻOumuamua around perihelion, we searched for it in Solar and Heliospheric Observatory (SOHO) and Solar TErrestrial RElations Observatory (STEREO) images from early 2017 September (preperihelion), but did not detect it. The nondetection of ʻOumuamua by STEREO renders more stringent constraints on its physical properties thanks to the extreme forward-scattering observing geometry. Assuming a geometric albedo of pV = 0.1, the effective scattering cross-section of any dust coma was ≲(2.1± 0.2) × 104 m2. Assuming that it behaved like a typical solar system comet, this would correspond to a total mass of ≲20 ± 2 kg and a water production rate of ≲(6.1 ± 0.5) × 1025 s−1 at a heliocentric distance of rH = 0.375 au. If scaled to post-discovery rH, the water production rate would be smaller than any of the previously reported upper limits by at least an order of magnitude. To exhibit the reported nongravitational motion with our default assumptions requires a nucleus bulk density ≲40 kg m−3; higher bulk densities are possible for other assumptions. Alternatively, we show that thermal fracturing could have plausibly removed an inert surface layer between these observations and discovery, thus initiating activity after ʻOumuamua left the field of view of STEREO.

The Astronomical Journal (Published)

DOI: 10.3847/1538-3881/ab50b8 NASA ADS: 2019arXiv191010303H arXiv: 1910.10303

The Nucleus of Interstellar Comet 2I/Borisov

  • Jewitt, D. 1
  • Hui, M. 2
  • Kim, Y. 3
  • Mutchler, M. 4
  • Weaver, H. 5
  • Agarwal, J. 3
  1. UCLA
  2. University of Hawaii
  3. Max Planck Institute for Solar System Science
  4. Space Telescope Science Institute
  5. Johns Hopkins Applied Physics Laboratory

We present high resolution imaging observations of interstellar comet 2I/Borisov (formerly C/2019 Q4) obtained using the Hubble Space Telescope. Scattering from the comet is dominated by a coma of large particles (characteristic size 0.1 mm) ejected anisotropically. Convolution modeling of the coma surface brightness profile sets a robust limit to the spherical-equivalent nucleus radius r_n < 0.5 km (geometric albedo 0.04 assumed). We obtain an independent constraint based on the non-gravitational acceleration of the nucleus, finding r_n > 0.2 km (nucleus density 500 kg/m3 assumed). The profile and the non-gravitational constraints cannot be simultaneously satisfied if density < 25 kg/m3; the nucleus of comet Borisov cannot be a low density fractal assemblage of the type proposed elsewhere for the nucleus of 1I/ʻOumuamua. We show that the spin-up timescale to outgassing torques, even at the measured low production rates, is comparable to or shorter than the residence time in the Sun's water sublimation zone. The spin angular momentum of the nucleus should be changed significantly during the current solar fly-by. Lastly, we find that the differential interstellar size distribution in the 0.5 mm to 100 m size range can be represented by power laws with indices < 4 and that interstellar bodies of 100 m size scale strike Earth every one to two hundred million years.

The Astrophysical Journal Letters (In press)

arXiv: 1912.05422

Pre-discovery Activity of New Interstellar Comet 2I/Borisov Beyond 5 AU

  • Ye, Q.-Z. 1
  • Kelley, M. S. P. 1
  • and 16 coauthors
  1. University of Maryland

Comet 2I/Borisov, the first unambiguous interstellar comet ever found, was discovered in August 2019 at ~3 au from the Sun on its inbound leg. No pre-discovery detection beyond 3 au has yet been reported, mostly due to the comet's proximity to the Sun as seen from the Earth. Here we present a search for pre-discovery detections of comet Borisov using images taken by the Catalina Sky Survey (CSS), Pan-STARRS and Zwicky Transient Facility (ZTF), with a further comprehensive follow-up campaign being presented in Bolin et al. (2019). We identified comet Borisov in ZTF images taken in May 2019 and use these data to update its orbit. This allowed us to identify the comet in images acquired as far back as December 2018, when it was 7.8 au from the Sun. The comet was not detected in November 2018 when it was 8.6 au from the Sun, possibly implying an onset of activity around this time. This suggests that the activity of the comet is either driven by a more volatile species other than H2O, such as CO or CO2, or by exothermic crystallization of amorphous ice. We derive the radius of the nucleus to be <7 km using the non-detection in November 2018, and estimate an area of ~0.5–10 km2 has been active between December 2018 and September 2019, though this number is model-dependent and is highly uncertain. The behavior of comet Borisov during its inbound leg is observationally consistent with dynamically new comets observed in our solar system, suggesting some similarities between the two.

The Astronomical Journal (In press)

arXiv: 1911.05902