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Arecibon planetaarisen tutkan Maan lähiasteroidihavainnot: 2017 joulukuu - 2019 joulukuu
We successfully observed 191 near-Earth asteroids using the Arecibo Observatory's S-band planetary radar system from 2017 December through 2019 December. We present radar cross sections for 167 asteroids; circular-polarization ratios for 112 asteroids based on Doppler-echo-power spectra measurements; and radar albedos, constraints on size and spin periods, and surface-feature and shape evaluation for 37 selected asteroids using delay-Doppler radar images with a range resolution of 75 m or finer. Out of 33 asteroids with an estimated effective diameter of at least 200 m and sufficient image quality to give clues of the shape, at least 4 (∼12%) are binary asteroids, including 1 equal-mass binary asteroid, 2017 YE5, and at least 10 (∼30%) are contact-binary asteroids. For 5 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 and the 1σ standard deviation were 0.37 ± 0.23. Further, we find a mean opposite-sense circular-polarization radar albedo of 0.21 ± 0.11 for 41 asteroids (0.19 ± 0.06 for 11 S-complex asteroids). We identified two asteroids, 2011 WN15 and (505657) 2014 SR339, as possible metal-rich objects based on their unusually high radar albedos, and discuss possible evidence of water ice in 2017 YE5.Peer reviewe
An Aphelion Analysis of Comet 2P/Encke
The rotational and secular light curve of the comet 2P/Encke has been a matter of debate for some decades. Understanding of the rotational properties is fundamental to deriving information on the interior structure of comets, which in turn has implications for solar system formation and evolution. Encke is one of the best observed comets throughout history and yet these basic properties have been highly elusive. While the basic rotational period is broadly known to be approximately 11 hours, it seems to change by almost 1 percent per orbit, and the cause of the unusual single-peaked rotational light curve morphology, which could be attributed to the shape of the nucleus, complex rotation, variable albedo across the surface or a combination of those factors, has been similarly difficult to pin down. Here we present a comparison between optical and mid-Infrared light curves of Encke obtained in 2005, ruling out albedo variations and arguing for a period of 11.03 hours. Furthermore, radial profile analyses indicate that the unexpected brightness near aphelion cannot be attributed to more than 20 percent extended emission and the nucleus does truly resemble a point source