The population of natural Earth satellites

TL;DR

This study models the population and characteristics of Earth's irregular natural satellites, revealing that at any time there is typically at least one 1-meter object temporarily orbiting Earth, with detailed capture dynamics and frequency.

Contribution

First comprehensive calculation of Earth's irregular natural satellite population and their capture dynamics based on NEO data and solar system influences.

Findings

At least one 1-meter diameter NES is present at any time.

Average capture duration is approximately 286 days.

Capture events show seasonal preference in January and July.

Abstract

We have for the first time calculated the population characteristics of the Earth's irregular natural satellites (NES) that are temporarily captured from the near-Earth-object (NEO) population. The steady-state NES size-frequency and residence-time distributions were determined under the dynamical influence of all the massive bodies in the solar system (but mainly the Sun, Earth, and Moon) for NEOs of negligible mass. To this end, we compute the NES capture probability from the NEO population as a function of the latter's heliocentric orbital elements and combine those results with the current best estimates for the NEO size-frequency and orbital distribution. At any given time there should be at least one NES of 1-meter diameter orbiting the Earth. The average temporarily-captured orbiter (TCO; an object that makes at least one revolution around the Earth in a co-rotating coordinate system) completes $(2.88\pm0.82)\rev$ around the Earth during a capture event that lasts $(286\pm18)\days$. We find a small preference for capture events starting in either January or July. Our results are consistent with the single known natural TCO, 2006 RH$_{120}$, a few-meter diameter object that was captured for about a year starting in June 2006. We estimate that about 0.1% of all meteors impacting the Earth were TCOs.