Keck Time-resolved Spectrophotometry of Temporarily-Captured Minimoon 2020 CD3 (original) (raw)

Abstract

&lt;div&gt; &lt;p&gt;Keck Time-resolved Spectrophotometry of Temporarily-Captured Minimoon 2020 CD&lt;sub&gt;3&lt;/sub&gt;&lt;/p&gt; &lt;p&gt;&lt;strong&gt;Bryce T. Bolin (1,2), &lt;/strong&gt;&lt;strong&gt;Christoffer Fremling (1), Timothy R. Holt (3,4), Matthew J. Hankins (1), Shreya Anand (1), Kishalay De (1), Kunal Deshmukh (5), Mansi M. Kasliwal (1), Alessandro Morbidelli (6), Josiah Purdum(7), Robert Quimby (7,8)&lt;/strong&gt;&lt;/p&gt; &lt;p&gt;(1) Division of Physics, Mathematics and Astronomy, Caltech, Pasadena, CA 91125, USA (bbolin@caltech.edu)&lt;/p&gt; &lt;p&gt;(2) IPAC, Mail Code 100-22, Caltech, 1200 E. California Blvd., Pasadena, CA 91125, USA&lt;/p&gt; &lt;p&gt;(3) Centre for Astrophysics, University of Southern Queensland, Queensland, Australia&lt;/p&gt; &lt;p&gt;(4) Southwest Research Institute, Department of Space Studies, Boulder, CO. USA.&lt;/p&gt; &lt;p&gt;(5) Dept. of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India&lt;/p&gt; &lt;p&gt;(6) Universit&amp;#233; C&amp;#244;te d&amp;#8217;Azur, Observatoire de la C&amp;#244;te d&amp;#8217;Azur, CNRS, Laboratoire Lagrange, Boulevard de l&amp;#8217;Observatoire, CS 34229, F-06304 Nice cedex 4, France&lt;/p&gt; &lt;p&gt;(7) Dept. of Astronomy, San Diego State University, 5500 Campanile Dr, San Diego, CA 92182, U.S.A.&lt;/p&gt; &lt;p&gt;(8) Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan&lt;/p&gt; &lt;p&gt;We report on rotationally-averaged visible spectrophotometry of minimoon 2020 CD&lt;sub&gt;3&lt;/sub&gt;, the second known asteroid to be temporarily gravitationally captured by the Earth-Moon system, taken with Keck I/LRIS&amp;#160; between 434 nm and 912 nm in B, g, V, R, I and RG850 bands as it was leaving the Earth-Moon system on 2020 March 23 UTC. We find that the broad-band spectrum of 2020 CD&lt;sub&gt;3&lt;/sub&gt; most closely resembles a V-type asteroid and some lunar rock samples with a reddish surface with a slope of 18 +/- 3 % between 434 nm and 761 nm corresponding to colors of g-r = 0.62 +/-0.08, r-i = 0.21 +/- 0.06 and an absorption band at ~900 nm corresponding to i-z = -0.54 +/- 0.10 (Isaacson et al. 2011, DeMeo &amp; Carry 2013) as seen in the left panel of Fig. 1. Assuming typical albedos of V-type asteroids, we constrain 2020 CD&lt;sub&gt;3&lt;/sub&gt;'s size to be ~1.0 +/- 0.1 m in diameter when combined with our measured H magnitude of 31.74 +/- 0.14.&amp;#160; In our time-series data, we detect periodic lightcurve variations corresponding to a rotation period of ~573 s and a Lomb-Scargle false alarm probability of &lt;10&lt;sup&gt;-4 &lt;/sup&gt;(Fig. 2, left panel) with a lightcurve amplitude of ~1 magnitude implying a b/a axial ratio of ~2.5 (Fig. 2, right panel), though this is atypical for meter-scale asteroids which are thought to have rotation periods ~60 s (Bolin et al. 2014). In addition, we refine the orbit of 2020 CD&lt;sub&gt;3&lt;/sub&gt; with our observations extending the observational arc to 37 days between 2020 February 15 and 2020 March 23 constraining its duration of capture of 2020 CD&lt;sub&gt;3&lt;/sub&gt; to be ~1.2 years a typical capture lifetime for minimoons (Granvik et al. 2012) as seen in the right panel of Fig. 1. We also constrain the effect of radiation pressure on its orbit with an estimated area-to-mass ratio of 6.9 +/- 2.1 10&lt;sup&gt;-4&lt;/sup&gt; m&lt;sup&gt;2&lt;/sup&gt;/kg&lt;sup&gt;*&lt;/sup&gt;. Using our diameter and area-to-mass ratio estimates, we calculate a density of 1.6-3.3 g/cm&lt;sup&gt;3&lt;/sup&gt;, broadly compatible with the densities of other small asteroids with densities constrained by radiation pressure measurements (Micheli et al. 2012) and lunar rock (~2.4 g/cm&lt;sup&gt;3&lt;/sup&gt;, Kiefer et al. 2012) somewhat less dense than the typical ~3.5g/cm&lt;sup&gt;3&lt;/sup&gt; density of V-type asteroids (Carry 2012). We searched for pre-discovery observations of 2020 CD&lt;sub&gt;3&lt;/sub&gt; in the ZTF archive (Masci et al. 2019) but were unable to locate any positive detections.&lt;/p&gt; &lt;/div&gt; &lt;p&gt;&lt;strong&gt;References: &amp;#160;&lt;/strong&gt;Bolin et al. 2014, Icarus, 241, 280-297, Carry 2012, PSS, 73, 98-118, DeMeo &amp; Carry 2013, Icarus, 226, 723-741, Kiefer et al. 2012, GRL, 39, 7, Isaacson et al. 2011, M&amp;PS,…