Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Catastrophic disruptions as the origin of bilobate comets


Several comets observed at close range have bilobate shapes1, including comet 67P/Churyumov–Gerasimenko (67P/C–G), which was imaged by the European Space Agency’s Rosetta mission2,3. Bilobate comets are thought to be primordial because they are rich in supervolatiles (for example, N2 and CO) and have a low bulk density, which implies that their formation requires a very low-speed accretion of two bodies. However, slow accretion does not only occur during the primordial phase of the Solar System; it can also occur at later epochs as part of the reaccumulation process resulting from the collisional disruption of a larger body4, so this cannot directly constrain the age of bilobate comets. Here, we show by numerical simulation that 67P/C–G and other elongated or bilobate comets can be formed in the wake of catastrophic collisional disruptions of larger bodies while maintaining their volatiles and low density throughout the process. Since this process can occur at any epoch of our Solar System’s history, from early on through to the present day5, there is no need for these objects to be formed primordially. These findings indicate that observed prominent geological features, such as pits and stratified surface layers4,5, may not be primordial.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Rent or buy this article

Prices vary by article type



Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Ratio of minor to major principal axes for each reaccumulated body with major axes over 1 km, when fit to a prolate spheroidal shape.
Fig. 2: Temperature increases resulting from catastrophic collisions at a range of impact speeds.
Fig. 3: Post-impact porosity distributions showing the amounts of mass at a given porosity or below for the catastrophic disruption simulations used in this study.


  1. Sunshine, J. M., Thomas, N., El-Maarry, M. R. & Farnham, T. L. Evidence for geologic processes on comets. J. Geophys. Res. Planets 121, 2194–2210 (2016).

    Article  ADS  Google Scholar 

  2. Sierks, H. et al. On the nucleus structure and activity of comet 67P/Churyumov–Gerasimenko. Science 347, aaa1044 (2015).

    Article  Google Scholar 

  3. Massironi, M. et al. Two independent and primitive envelopes of the bilobate nucleus of comet 67P. Nature 526, 402–405 (2015).

    Article  ADS  Google Scholar 

  4. Michel, P. & Richardson, D. C. Collision and gravitational reaccumulation: possible formation mechanism of the asteroid Itokawa. Astron. Astrophys. 554, L1 (2013).

    Article  ADS  Google Scholar 

  5. Rickman, H. et al. Comet 67P/Churyumov–Gerasimenko: constraints on its origin from OSIRIS observations. Astron. Astrophys. 583, A44 (2015).

    Article  Google Scholar 

  6. Jutzi, M., Benz, W. & Michel, P. Numerical simulations of impacts involving porous bodies. I. Implementing sub-resolution porosity in a 3D SPH hydrocode. Icarus 198, 242–255 (2008).

    Article  ADS  Google Scholar 

  7. Benz, W. & Asphaug, E. Impact simulations with fracture. I. Method and tests. Icarus 107, 98–116 (1994).

    Article  ADS  Google Scholar 

  8. Hirabayashi, M. et al. Fission and reconfiguration of bilobate comets as revealed by 67P/Churyumov–Gerasimenko. Nature 534, 352–355 (2016).

    Article  ADS  Google Scholar 

  9. Jutzi, M., Benz, W., Toliou, A., Morbidelli, A. & Brasser, R. How primordial is the structure of comet 67P/C–G? Combined collisional and dynamical models suggest a late formation. Astron. Astrophys. 597, A61 (2017).

    Article  ADS  Google Scholar 

  10. Stadel, J. G. Cosmological N-body Simulations and Their Analysis. PhD thesis, Univ. Washington (2001).

  11. Richardson, D. C., Quinn, T., Stadel, J. & Lake, G. Direct large-scale N-body simulations of planetesimal dynamics. Icarus 143, 45–59 (2000).

    Article  ADS  Google Scholar 

  12. Schwartz, S. R., Richardson, D. C. & Michel, P. An implementation of the soft-sphere discrete element method in a high-performance parallel gravity tree-code. Granul. Matter 14, 363–380 (2012).

    Article  Google Scholar 

  13. Schwartz, S. R., Yu, Y., Michel, P. & Jutzi, M. Small-body deflection techniques using spacecraft: techniques in simulating the fate of ejecta. Adv. Space Res. 57, 1832–1846 (2016).

    Article  ADS  Google Scholar 

  14. Zhang, Y. et al. Creep stability of the proposed AIDA mission target 65803 Didymos: I. Discrete cohesionless granular physics model. Icarus 294, 98–123 (2017).

    Article  ADS  Google Scholar 

  15. Groussin, O. et al. Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov–Gerasimenko from OSIRIS observations. Astron. Astrophys. 583, A32 (2015).

    Article  Google Scholar 

  16. Jutzi, M. & Benz, W. Formation of bi-lobed shapes by sub-catastrophic collisions. A late origin of comet 67P’s structure. Astron. Astrophys. 597, A62 (2017).

    Article  ADS  Google Scholar 

  17. Collins, G. S. Numerical simulations of impact crater formation with dilatancy. J. Geophys. Res. Planets 119, 2600–2619 (2014).

    Article  ADS  Google Scholar 

  18. Jutzi, M. & Asphaug, E. The shape and structure of cometary nuclei as a result of low-velocity accretion. Science 348, 1355–1358 (2015).

    Article  ADS  Google Scholar 

  19. Davidsson, B. J. R. et al. The primordial nucleus of comet 67P/Churyumov–Gerasimenko. Astron. Astrophys. 592, A63 (2016).

    Article  Google Scholar 

  20. Morbidelli, A. & Rickman, H. Comets as collisional fragments of a primordial planetesimal disk. Astron. Astrophys. 583, A43 (2015).

    Article  ADS  Google Scholar 

  21. Singer, K. N. et al. Impact craters on Pluto and Charon indicate a deficit of small Kuiper belt objects. Am. Astron. Soc. Div. Planetary Sci. Conf. 48, 213.12 (2016).

  22. El-Maarry, M. R. et al. Regional surface morphology of comet 67P/Churyumov–Gerasimenko from Rosetta/OSIRIS images. Astron. Astrophys. 583, A26 (2015).

    Article  Google Scholar 

  23. Vincent, J.-B. et al. Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse. Nature 523, 63–66 (2015).

    Article  ADS  Google Scholar 

  24. Libourel, G. et al. Search for primitive matter in the Solar System. Icarus 282, 375–379 (2017).

    Article  ADS  Google Scholar 

  25. Michel, P., Benz, W., Tanga, P. & Richardson, D. C. Collisions and gravitational reaccumulation: forming asteroid families and satellites. Science 294, 1696–1700 (2001).

    Article  ADS  Google Scholar 

  26. Richardson, D. C., Michel, P., Walsh, K. J. & Flynn, K. W. Numerical simulations of asteroids modelled as gravitational aggregates with cohesion. Planet. Space Sci. 57, 183–192 (2009).

    Article  ADS  Google Scholar 

  27. Jutzi, M. SPH calculations of asteroid disruptions: the role of pressure dependent failure models. Planet. Space Sci. 107, 3–9 (2015).

    Article  ADS  Google Scholar 

  28. Cundall, P. A. & Strack, O. D. L. A discrete numerical model for granular assemblies. Geotechnique 29, 47–65 (1979).

    Article  Google Scholar 

  29. Rickman, H. The nucleus of comet Halley: surface structure, mean density, gas and dust production. Adv. Space Res. 9, 59–71 (1989).

    Article  ADS  Google Scholar 

  30. A’Hearn, M. F. et al. Deep impact: excavating comet Tempel 1. Science 310, 258–264 (2005).

    Article  ADS  Google Scholar 

  31. A’Hearn, M. F. et al. EPOXI at comet Hartley 2. Science 332, 1396–1400 (2011).

    Article  ADS  Google Scholar 

  32. Duxbury, T. C., Newburn, R. L. & Brownlee, D. E. Comet 81P/Wild 2 size, shape, and orientation. J. Geophys. Res. 109, E12S02 (2004).

    ADS  Google Scholar 

  33. Lamy, P. L., Toth, I. & Weaver, H. A. Hubble Space Telescope observations of the nucleus and inner coma of comet 19P/1904 Y2 (Borrelly). Astron. Astrophys. 337, 945–954 (1998).

    ADS  Google Scholar 

  34. Soderblom, L. A. et al. Observations of comet 19P/Borrelly by the Miniature Integrated Camera and Spectrometer aboard Deep Space 1. Science 296, 1087–1091 (2002).

    Article  ADS  Google Scholar 

Download references


S.R.S. and P.M. acknowledge support from the Centre National d’Études Spatiales, as well as the Academies of Excellence on Complex Systems and Space, Environment, Risk and Resilience of the Initiative d’EXcellence ‘Joint, Excellent, and Dynamic Initiative’ (IDEX JEDI) of the Université Côte d’Azur. M.J. acknowledges support from the Swiss National Centre of Competence in Research PlanetS, and S.M. acknowledges support from the Jet Propulsion Laboratory. Computation was performed using the YORP computing cluster run by the Center for Theory and Computation at the University of Maryland’s Department of Astronomy and the Mésocentre de Calcul Intensif ‘Simulations Intensives en Géophysique, Astronomie, Mécanique, et Mathématiques’ hosted by the Côte d’Azur Observatory in Nice. For data visualization, the authors made use of the freeware, multi-platform, ray-tracing package, Persistence of Vision Raytracer.

Author information

Authors and Affiliations



S.R.S. performed the soft-sphere/N-body numerical simulations, analysed the numerical results and led the research. M.J. performed the SPH numerical simulations and contributed to the analyses. P.M. initiated the collaboration between the institutions to investigate catastrophic impacts as an origin story, and helped steer the study to address questions in the context of Solar System origins. S.M. provided essential and detailed context for the Rosetta findings and outstanding questions regarding comet 67P/G–C. Y.Z. analysed the friction angles and implemented the rolling friction subroutine used. D.C.R. guided the development of all N-body-related code. All authors contributed to interpretation of the results and preparation of the manuscript.

Corresponding author

Correspondence to Stephen R. Schwartz.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figures 1–4, captions for Supplementary Videos 1–4.


Supplementary Video 1

Animation showing the formation of the largest remnant after a 150 m s–1 catastrophic impact (18° friction angle).

Supplementary Video 2

Animation showing the formation of the largest remnant after a 150 m s–1 catastrophic impact (29° friction angle).

Supplementary Video 3

One example of gravitational reaccumulation of marginally bound impact fragments.

Supplementary Video 4

One example of the formation of a bilobate body from a catastrophic disruption simulation.

Rights and permissions

Reprints and Permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Schwartz, S.R., Michel, P., Jutzi, M. et al. Catastrophic disruptions as the origin of bilobate comets. Nat Astron 2, 379–382 (2018).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing