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Small-body dynamics

Cometary ↔ asteroidal reservoir mixing

Most “comet-like” fireballs aren't comets at all. Long-term integrations show the near-Earth “Jupiter-family” population is dominated by asteroidal interlopers.

Eccentricity versus semi-major axis, with asteroids and comets separated by Tisserand parameter lines
Asteroids (blue) and comets (red) in eccentricity–semi-major-axis space, split by the Tisserand parameter (values of 2 and 3). The “comet-like” region is heavily contaminated by asteroidal interlopers. Figure from Shober et al. (2024).

The mixing problem

How does volatile-rich material from the outer reservoirs blend with more refractory, inner-solar-system material in near-Earth space? That mixing controls the composition gradient of the young solar system and the delivery of water and organics to the terrestrial planets. In the classical picture, Jupiter-family comets (JFCs) arrive from the Kuiper belt and scattered disk on chaotic, low-inclination orbits with a Tisserand parameter between 2 and 3, while near-Earth asteroids are fed from the main belt through resonances and evolve more regularly.

What the fireballs actually show

During my PhD I used observed close encounters to quantify this mixing, first analysing the longest well-observed grazing fireball on record, in which an Apollo-type asteroidal meteoroid was scattered by Earth onto a JFC-type orbit. That motivated an observationally-anchored steady-state model of Earth close encounters: a small but persistent flux (~2.5 × 10⁸ objects yr⁻¹, ~0.1 % of the population in this size range) is transferred from asteroidal orbits (Tisserand parameter above 3) onto JFC-type orbits.

In my thesis’s final paper, long-term integrations of 50 sporadic DFN fireballs on JFC-type orbits found that only ~4 % show recent Jupiter encounters and genuinely cometary evolution, while 96 % behave as stable asteroidal bodies. During my Marie Curie postdoc I scaled this up, comparing 646 well-observed “JFC-like” fireballs from French, Australian, Czech and American sensors against 661 catalogued JFCs, and confirmed it: only about 1–5 % of cm–m bodies on JFC-type orbits are dynamically cometary, while roughly 22 % of NEOs classified as JFCs actually follow surprisingly stable, asteroid-like orbits.

Simple proxies like the Tisserand parameter are not enough at these sizes: dynamical stability diagnostics are essential to separate the reservoirs. The genuinely cometary material is mostly ground to dust, feeding the zodiacal cloud. I’m now finalising a study of >2,200 “asteroids on cometary orbits” (in preparation), combining long-term integrations, unsupervised clustering and physical properties (albedo, taxonomy) to map the mixing interface more finely.