[Paper Review] NEOMOD: A New Orbital Distribution Model for Near Earth Objects

David Nesvorný, Rogerio Deienno+11 more|arXiv (Cornell University)|Jun 15, 2023

Astro and Planetary Science4 citations

TL;DR

NEOMOD presents a new orbital distribution model for Near Earth Objects (NEOs) by simulating main-belt asteroid dynamics and calibrating results against Catalina Sky Survey observations. It reveals size-dependent sampling of main-belt sources—particularly ν₆ and 3:1 resonances—where small NEOs (H ≈ 25) are dominated by ν₆, while large NEOs (H ≈ 15) originate from the 3:1 resonance, requiring maximal Yarkovsky drift and specific asteroid obliquities (0° or 180°) near resonances to sustain flux.

ABSTRACT

Near Earth Objects (NEOs) are a transient population of small bodies with orbits near or in the terrestrial planet region. They represent a mid-stage in the dynamical cycle of asteroids and comets, which starts with their removal from the respective source regions -- the main belt and trans-Neptunian scattered disk -- and ends as bodies impact planets, disintegrate near the Sun, or are ejected from the Solar System. Here we develop a new orbital model of NEOs by numerically integrating asteroid orbits from main belt sources and calibrating the results on observations of the Catalina Sky Survey. The results imply a size-dependent sampling of the main belt with the $ν_6$ and 3:1 resonances producing $\simeq 30$\% of NEOs with absolute magnitudes $H = 15$ and $\simeq 80$\% of NEOs with $H = 25$. Hence, the large and small NEOs have different orbital distributions. The inferred flux of $H<18$ bodies into the 3:1 resonance can be sustained only if the main-belt asteroids near the resonance drift toward the resonance at the maximal Yarkovsky rate ($\simeq 2 imes 10^{-4}$ au Myr$^{-1}$ for diameter $D=1$ km and semimajor axis $a=2.5$~au). This implies obliquities $θ\simeq 0^\circ$ for $a<2.5$~au and $θ\simeq 180^\circ$ for $a>2.5$~au, both in the immediate neighborhood of the resonance (the same applies to other resonances as well). We confirm the size-dependent disruption of asteroids near the Sun found in previous studies. An interested researcher can use the publicly available NEOMOD Simulator to generate user-defined samples of NEOs from our model.

Motivation & Objective

• To develop a physically consistent, observationally calibrated model of the orbital and absolute magnitude distribution of Near Earth Objects (NEOs) for improved impact risk assessment and survey planning.
• To resolve discrepancies in NEO flux estimates by incorporating size-dependent dynamics, disruption near perihelion, and Yarkovsky drift effects.
• To test the consistency of observed NEO fluxes with theoretical asteroid source models, particularly the 3:1 and ν₆ resonances.
• To provide a flexible, updatable framework for future integration with data from upcoming surveys (e.g., Vera C. Rubin Observatory, NEO Surveyor).

Proposed method

• Numerical N-body integrations of 10⁵ main-belt asteroids from source resonances (e.g., 3:1, ν₆) to simulate NEO injection into the inner solar system.
• Calibration of simulated NEO detections against actual Catalina Sky Survey (CSS) observations using Bayesian inference via the MultiNest algorithm.
• Use of cubic splines to model the absolute magnitude distribution of NEOs, enabling flexible and accurate representation across H = 15 to H = 28.
• Incorporation of size-dependent disruption of NEOs at small perihelion distances, based on Granvik et al. (2016), to account for observational incompleteness.
• Model optimization via MultiNest to infer source contributions, Yarkovsky drift rates, and obliquity constraints from observational data.
• Development of the publicly available NEOMOD Simulator to generate user-defined NEO samples from the calibrated model.

Experimental results

Research questions

• RQ1 What is the relative contribution of different main-belt resonances (e.g., 3:1, ν₆) to the NEO population across different absolute magnitudes?
• RQ2 Can the observed flux of H < 18 NEOs into the 3:1 resonance be sustained by Yarkovsky drift, and what does this imply about asteroid obliquity?
• RQ3 How do size-dependent disruption effects near perihelion influence the orbital distribution of small versus large NEOs?
• RQ4 What is the contribution of inactive comets (JFCs) to the current NEO population, based on CSS detection statistics?
• RQ5 How do the model-predicted impact fluxes on Earth, Mars, and Moon compare with previous estimates and crater chronology data?

Key findings

• The ν₆ and 3:1 resonances produce approximately 30% and 80% of NEOs with absolute magnitudes H = 15 and H = 25, respectively, indicating strong size-dependent source contributions.
• The flux of H < 18 NEOs into the 3:1 resonance requires main-belt asteroids near the resonance to drift at the maximum Yarkovsky rate (~2×10⁻⁴ au Myr⁻¹ for D = 1 km at a = 2.5 au), implying obliquities of ~0° for a < 2.5 au and ~180° for a > 2.5 au.
• The average time between terrestrial impacts of 10 m bolides is estimated at ~30 years, ~3 times longer than previous estimates, suggesting possible undercounting in CSS for H ≈ 25–28.
• The PM excess for meteorite falls from ν₆ and 3:1 sources is 0.47±0.02 and 0.50±0.05, respectively, aligning better with Morbidelli & Gladman (1998) than with Wisdom (2017, 2020), but lower than the observed 0.63±0.02.
• The contribution of inactive comets (JFCs) to the NEO population is constrained to α_JFC < 0.017 (68.3% credible interval), with Bayes factor slightly favoring a model with no cometary contribution.
• The Mars-to-Moon impact flux ratio (Ma/Mo) is 7.1 for H = 15 and 4.7 for H = 25, yielding R_b = 2.0 and 1.2, respectively—significantly lower than the previously used R_b ≈ 2.6 in crater chronology studies.