A New Method for Estimating the Absolute Magnitude Frequency Distribution of Near Earth Asteroids (NEAs)

TL;DR

This paper introduces a simple, assumption-based method to estimate the absolute magnitude distribution of near-Earth asteroids using known data, revealing a consistent power-law slope and estimating around 100 million NEAs larger than 10 meters.

Contribution

It provides a novel, model-free approach to derive the NEA magnitude distribution directly from observational data without complex population modeling.

Findings

Power-law slope of 0.50 ± 0.03 for H < 27

Estimated ~10^8 NEAs with H < 27

No evidence of additional structure in the distribution

Abstract

The distribution of solar system absolute magnitudes ($H$) for the near-Earth asteroids (NEAs) observable near opposition -- i.e. Amors, Apollos, and Atens ($A^3$) -- is derived from the set of ALL currently known NEAs. The result is based only on common sense assumptions of uniformly random distributions and that the orbital phase space and $H$-magnitude distribution of known NEAs is representative of the total population. There is no population or other modeling and no assumption on albedo except in interpreting the result as a size-frequency distribution (SFD). The analysis is based on the 18355 $A^3$ NEAs cataloged by the MPC as of June 2018. The observations from 9 of the top programs (in terms of number of distinct NEAs observed) and the smaller but deeper DECam NEO Survey are used, comprising 74696 measurements of 13466 NEAs observed within 30 deg of opposition. The only parameter in the analysis is an estimate of the detection magnitude limits for each program. A single power-law slope for the cumulative distribution, $\log(N<H)=0.50\pm0.03H$, for $H < 27$ is found with no evidence for additional structure. A turn-over fainter than 27th magnitude may occur, but the population of known NEAs is dropping off rapidly because they are difficult to detect and so possibly is a completeness effect. Connecting to the nearly complete census of the brightest/biggest NEAs (diameter $> {\sim}2$Km) provides a normalization that estimates ${\sim}10^8 A^3$ NEAs with $H < {\sim}27$ corresponding to NEAs greater than ${\sim}10$m in diameter for reasonable typical albedos. Restricting the analysis to Earth crossing asteroids (10839 known, 7336 selected, 36541 observed) produces the same power-law slope.