Establishing Earth's Minimoon Population through Characterization of Asteroid 2020 CD$_3$

TL;DR

This paper characterizes Earth's second known minimoon, asteroid 2020 CD3, confirming its natural origin, comparing its properties with models, and discussing implications for understanding minimoon populations and dynamics.

Contribution

It provides the first detailed characterization of asteroid 2020 CD3, validating theoretical models of minimoon physical and dynamical properties and highlighting discrepancies in rotation period predictions.

Findings

2020 CD3 is a silicate asteroid with a diameter of about 1.2 meters.

Its capture duration of at least 2.7 years aligns with models involving lunar encounters.

Its rotation period is significantly longer than theoretical expectations.

Abstract

We report on our detailed characterization of Earth's second known temporary natural satellite, or minimoon, asteroid 2020 CD3. An artificial origin can be ruled out based on its area-to-mass ratio and broadband photometry, which suggest that it is a silicate asteroid belonging to the S or V complex in asteroid taxonomy. The discovery of 2020 CD3 allows for the first time a comparison between known minimoons and theoretical models of their expected physical and dynamical properties. The estimated diameter of 1.2+0.4-0.2 m and geocentric capture approximately a decade after the first known minimoon, 2006 RH120, are in agreement with theoretical predictions. The capture duration of 2020 CD3 of at least 2.7 yr is unexpectedly long compared to the simulation average, but it is in agreement with simulated minimoons that have close lunar encounters, providing additional support for the orbital models. 2020 CD3's atypical rotation period, significantly longer than theoretical predictions, suggests that our understanding of meter-scale asteroids needs revision. More discoveries and a detailed characterization of the population can be expected with the forthcoming Vera C. Rubin Observatory Legacy Survey of Space and Time.