Arecibo Planetary Radar Observations of Near-Earth Asteroids: December 2017 – December 2019

We observed 191 near-Earth asteroids (NEAs) using the Arecibo Observatory’s S-band planetary radar system from December 2017 through December 2019. We present radar cross sections for 167 NEAs; circular-polarization ratios for 112 NEAs based on Doppler-echo-power spectra measurements; and radar albedos, constraints on size and spin periods, and surface-feature and shape evaluation for selected 37 NEAs, for which we obtained well-resolved delay-Doppler radar images with a range resolution of 75 m or finer. Figure 1 shows a representative sample of eight of these NEAs with the name or the designation and the vertical × the horizontal image resolution displayed under each panel. Out of 33 NEAs with an estimated effective diameter of at least 200 m and a sufficient image quality to give clues of the shape, at least four (~12%) are binary asteroids, including one equal-mass binary asteroid, 2017 YE5, and at least ten (~30%) are contact binary asteroids. This is roughly twice the previous estimate of contact-binary NEAs with diameters of 200 m or more, which suggests that they could be more common than what has been previously thought. For five out of 112 asteroids with reliable measurements in both circular polarizations, we measured circular-polarization ratios greater than 1.0, which could indicate that they are E-type asteroids, while the mean with the one-sigma standard deviation was 0.37 ± 0.23 for all 112 asteroids. Three of the five asteroids ((418900) 2009 BE2, 2019 QY1, and 2019 RC) had not been classified before, one (2015 JD1) had likely been misclassified, and one ((141593) 2002 HK12) was previously classified in the broader X-complex, but could be sub-classified as E-type also based on its visual geometric albedo that we we were able to refine using the radar images. Further, we find a mean opposite-sense circular-polarization radar albedo of 0.21 ± 0.11 for 41 asteroids (0.19 ± 0.06 for eleven S-complex asteroids). We identified two asteroids, 2011 WN15 and (505657) 2014 SR339, as possible metal-rich NEAs based on their unusually high radar albedos, and discuss possible evidence of water ice in 2017 YE5. We also discuss how the evaluation of radar scattering properties can support the taxonomic classification of asteroids in general.