Size, density, and structure of comet Shoemaker - Levy 9 inferred from the physics of tidal breakup

Erik Asphaug, Willy Benz

Research output: Contribution to journalArticlepeer-review

239 Scopus citations


Detailed consideration of possible fragmentation mechanisms shows that Comet Shoemaker - Levy 9 (SL9) had negligible effective strength, even in comparison with tide and selfgravity, by the time it attained perijove in 1992. This reduces the tidal physics to a computable basis: we model the elongation of this "rubble-pile," and the onset of the instability which created a chain of gravitationally bound clumps, using an N-body code with self-gravity and simple collisions. Gravitational clumping depends only on the density ρc of the progenitor (for a given orbit), and chain length then scales linearly with initial diameter dc. We thus constrain from chain morphology that ρc ≈ 0.6 g cm-3 and from chain length that dc ≈ 1.5 km. Our numerical results accurately match analytical derivations for the threshold of tidal breakup, and lead to general relations for erosion and disruption of strengthless or regolith-covered bodies. For a given random encounter, we show that a rubble-pile comet, or one mantled in deep regolith, is half as likely to be destroyed by tides as it is to impact an outer planet. Because of their similar density ratio, the same holds true for rubble-pile asteroids encountering terrestrial planets. Split comets such as SL9 near Saturn are unlikely: the required periapses intersect the planet.

Original languageEnglish (US)
Pages (from-to)225-248
Number of pages24
Issue number2
StatePublished - Jun 1996

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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