Characterization of Temporarily Captured Minimoon 2020 CD₃ by Keck Time-resolved Spectrophotometry

Abstract We present time-resolved visible spectrophotometry of 2020 CD 3 , the second known minimoon. The spectrophotometry was taken with the Keck I/Low Resolution Imaging Spectrometer between wavelengths 434 and 912 nm in the B , g , V , R , I , and RG850 filters as it was leaving the Earth–Moon system on 2020 March 23 UTC. The spectrum of 2020 CD 3 resembles V-type asteroids and some lunar rock samples with a 434–761 nm reddish slope of ∼18%/100 nm ( g – r = 0.62 ± 0.08 and r – i = 0.21 ± 0.06) with an absorption band at ∼900 nm corresponding to i – z = −0.54 ± 0.10. Combining our measured H of 31.9 ± 0.1 with an albedo of 0.35 typical for V-type asteroids, we determine 2020 CD 3 's diameter to be ∼0.9 ± 0.1 m, making it the first minimoon and one of the smallest asteroids to be spectrally studied. We use our time-series photometry to detect significant periodic light-curve variations with a period of ∼573 s and amplitude of ∼1. In addition, we extend the observational arc of 2020 CD 3 to 37 days, to 2020 March 23 UTC. From the improved orbital solution for 2020 CD 3 , we estimate the likely duration of its capture to be ∼2 yr and the nongravitational perturbation on its orbit due to radiation pressure with an area-to-mass ratio of (6.9 ± 2.4) × 10 −4 m 2 kg −1 implying a density of 2.3 ± 0.8 g cm −3 , broadly compatible with other meter-scale asteroids and lunar rock. We searched for prediscovery detections of 2020 CD 3 in the Zwicky Transient Facility archive as far back as 2018 October but were unable to locate any positive detections.