Characterisation of 2020 CD3, Earth’s second minimoon

Introduction

Small solar system objects may occasionally become captured temporarily by planets. Theoretical models (Granvik et al. 2012, Fedorets et al. 2017) predict the existence of a steady-state population of these objects, also known as minimoons, also in the Earth-Moon system. Only one minimoon, 2006 RH120 has been discovered until recently (Kwiatkowski et al. 2009). Since minimoons spend a significant amount of time in Earth’s vicinity, they have been identified as outstanding targets for in situ exploration, or test cases for initial steps of asteroid resource utilisation (Granvik et al. 2013, Chyba et al. 2014, Brelsford et al. 2016, Jedicke et al. 2018). Moreover, not only are minimoons outstanding targets to constrain the size-frequency distribution of metre-sized asteroids (Harris & D’Abramo 2015, Granvik et al. 2016, Tricarico 2017, Brown et al. 2002), but also for studying the structure of the smallest asteroids. However, until now, the observational evidence of the minimoon population has been lacking.

Observations

The object 2020 CD3 was discovered on February 15^th 2020 at the Mt. Lemmon station of the Catalina Sky Survey, and was noticed to be on a geocentric orbit the following night. We report the results of the astrometric and photometric observational campaign to characterise 2020 CD3 performed by Gemini North, LDT, NOT, CFHT, CSS, and other telescopes during spring 2020. By investigating the solar radiation pressure signature on the astrometry of 2020 CD3, and broad-band photometry, we present evidence that 2020 CD3 is indeed the second temporary natural satellite in the Earth-Moon system. We describe its discovery circumstances, physical characterisation, rotational period and orbital evolution.

Discussion

Using 2020 CD3 as an example case, we discuss the challenges of discovering minimoons with contemporary surveys. For the first time, we are able to compare the observational evidence of minimoons with the theoretical models. We also assess the capture duration and rotation period of 2020 CD3 in context of simulation and similar objects. Finally, we compare the origin of minimoons as captured objects from the NEO population against their origin as lunar ejecta, and show why the first mechanism is dominant.

Prospects

The discovery of 2020 CD3, and the comparison to discovery predictions with other surveys (Bolin et al. 2014), assures that the expectation of discovery of tens of minimoons with LSST is realistic (Fedorets et al. 2020). With the anticipated growth of the population of minimoons, the path for further exploration of minimoons is foreseen.

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