Chasing Tails: Active Asteroid, Centaur, and Quasi-Hilda Discovery with Astroinformatics and Citizen Science

Abstract

The discovery of activity emanating from asteroid (4015) Wilson-Harrington in 1950 (Harris, 1950) prompted astronomers to realize that comet-like activity, such as comae and tails, is not limited to comets. Fewer than 30 of these “active asteroids” have been discovered (Chandler et al., 2018) in the last 70 years, yet they promise to hold clues about fundamental physical and chemical processes at play in our solar system (Jewitt, 2012; Hsieh et al., 2015a). Activity is attributed to sublimation for roughly half of these objects, highlighting asteroids as a “volatile reservoir.” In this context a volatile reservoir is any dynamical group of bodies in the solar system that is known to harbor volatiles. Understanding the past and present volatile distribution in the solar system has broad implications ranging from informing future space exploration pro- grams to helping us understand how planetary systems form with volatiles prerequisite to life as we know it, especially water. Notably, the origin of Earth’s water is essentially unknown, although it is now believed that asteroids account for at least some of the terrestrial volatile budget (Alexander, 2017).

A second volatile reservoir came to light following the 1977 discovery of Centaur (2060) Chiron (Kowal & Gehrels, 1977). Centaurs, found between the orbits of Jupiter and Neptune, are thought to be icy objects originating from the Kuiper Belt, a circumstellar region between the orbit of Neptune (30 au) and about 50 au from the Sun (Jewitt, 2009). The Kuiper Belt is roughly 200 times more massive than the Asteroid Belt. Nevertheless, active Centaurs are also rare, with fewer than 20 discovered to date (Chandler et al., 2020).

We set out to increase the number of known active objects in order to (1) enable the study of these active objects as populations, and (2) search for new volatile reservoirs. I proposed to the NSF Graduate Research Fellowship Program (GRFP) to create a Citizen Science project designed to carry out an outreach program while searching through millions of images of known asteroids in order to find previously unknown active objects. My proposal was selected for funding, and on 31 August 2022 we successfully launched Active Asteroids (http://activeasteroids.net), a NASA Partner, and discoveries have been abundant ever since.

In this dissertation I present (1) Hunting for Activity in Repositories with Vetting-Enhanced Search Techniques (HARVEST), a pipeline that extracts images of known solar system objects for presentation to Citizen Scientists, (2) our proof-of-concept demonstrating Dark Energy Camera (DECam) images are well-suited for activity detection (Chandler et al., 2018), (3) how we discovered a potential new recurrent activity mechanism (Chandler et al., 2019), (4) a Centaur activity discovery plus a novel technique for estimating which species are sublimating (Chandler et al., 2020), (5) how our discovery of an additional activity epoch for an active asteroid enabled us to classify the object as a member of the Main-belt Comet (MBC) (Chandler et al., 2021b), a rare (< 10) active asteroid subset that orbits in the Asteroid Belt that is known for sublimation-driven activity, (6) a dynamical pathway that can explain the presence of some of the active asteroids, and (7) the Citizen Science project Active Asteroids, including initial results.