How Many Hydrated NEOs Are There?

TL;DR

This study estimates the number of hydrated near-Earth objects (NEOs) using spectral proxies and orbital data, finding a significant population of accessible hydrated asteroids that could impact lunar surface composition.

Contribution

The paper introduces a novel estimation method for hydrated NEOs based on spectral classification and orbital dynamics, providing the first population-level estimates of their numbers and accessibility.

Findings

Approximately 53 ± 27 Ch-type hydrated NEOs larger than 1 km.

About 17 ± 9 of these are more accessible than the Moon's surface.

Potentially 700 ± 350 hydrated objects meet accessibility criteria regardless of size.

Abstract

Hydrated minerals are tracers of early solar system history, and have been proposed as a possible focus for economic activity in space. Near-Earth objects (NEOs) are important to both of these, especially the most accessible members of that community. Because there are very few identified hydrated NEOs, we use the Ch spectral class of asteroids as a proxy for hydrated asteroids, and use published work about NEO delivery, main-belt taxonomic distributions, NEO taxonomic distributions, and observed orbital distributions to estimate the number of hydrated asteroids with different threshold sizes and at different levels of accessibility. We expect 53 $\pm$ 27 Ch asteroids to be present in the known population of NEOs $>$ 1 km diameter, and using two different approaches to estimate accessibility we expect 17 $\pm$ 9 of them to be more accessible on a round trip than the surface of the Moon. If there is no need to define a minimum size, we expect 700 $\pm$ 350 hydrated objects that meet that accessibility criterion. While there are few unknown NEOs larger than 1 km, the population of smaller NEOs yet to be discovered could also be expected to contain proportionally-many hydrated objects. Finally, we estimate that hydrated NEOs are unlikely to bring enough water to account for the ice found at the lunar poles, though it is possible that asteroid-delivered hydrated minerals could be found near their impact sites across the lunar surface.