Introduction: Small NEOs may 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 so-called minimoons also in the Earth-Moon system. Only one minimoon, 2006 RH120 has been discovered until recently (Kwiatkowski et al. 2009). Minimoons have been identified as viable targets for in situ exploration of metre-sized asteroids, as test cases for initial steps of asteroid resource utilisation (Granvik et al. 2013, Jedicke et al. 2018), and as constraints for the size-frequency distribution of metre-sized NEOs (Harris & D’Abramo 2015, Granvik et al. 2016, Tricarico 2017, Brown et al. 2002). So far, the observational evidence of the minimoon population has been lacking.
Observations: The object 2020 CD3 was discovered on February 15th 2020 at the Mt. Lemmon station of the Catalina Sky Survey, and was reported to be on a geocentric orbit the following night. We report the results of the astrometric and photometric observational campaign of 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 it is the second temporary natural satellite in the Earth-Moon system. We describe its discovery circumstances, physical characterisation, rotational period, orbital evolution, and possible origin.
Discussion: 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 assess the capture duration and rotation period of 2020 CD3 in context of simulation and similar objects.
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.
Bolin et al. (2014), Icarus 241, 280
Brown et al. (2002), Nature, 420, 294
Fedorets et al. (2017) Icarus, 285, 83
Fedorets et al. (2020), Icarus, 338, 113517
Granvik et al. (2012), Icarus, 218, 262
Granvik et al. (2013) in V. Badescu ed. Asteroids: Prospective Energy and Material Resources, 151
Granvik et al. (2016), Nature, 530, 303
Harris & D’Abramo (2015), Icarus, 257, 302
Jedicke et al. (2018) FrASS, 5, A13
Kwiatkowski et al. (2009), A&A, 495, 967
Tricarico et al. (2017), Icarus, 284, 416