Arecibo Observatory radar observation of potentially hazardous asteroid 2020 NK1

We report Arecibo S-band (2380 MHz, 12.6 cm) radar observation of the recently discovered potentially hazardous asteroid (PHA) 2020 NK1. Delay-Doppler astrometry and imaging data were obtained on July 31, 2020, when it passed at a distance of 0.055 au (21 lunar distances). Additional observations planned for the prior day had been canceled due to a tropical storm. Based on 88 optical measurements from 12 observatories, this object had been flagged by the Sentry hazard warning system as a Torino 1 case (the only non-zero Torino value on the current list), and the third highest threat in terms of cumulative Palermo rating (-1.73), having seven virtual impact scenarios in the years spanning 2086-2101. Including two new Doppler measurements (± 0.1 and ± 0.2 Hz) and a round-trip delay measurement (± 4 μs) in the orbit solution reduced the uncertainty parameter (U) from 8 to 4, securing orbit knowledge and eliminating the potential impact risk later in the century from the Sentry hazard list. The next Earth encounter less than 0.1 au was determined to be in 2043, at less than half the distance, 0.023 au (9 lunar distances). The signal-to-noise ratio (SNR) was high enough to obtain delay-Doppler images with delay resolutions of 0.5 μs (75 m), 0.2 μs (30 m), and 0.05 μs (7.5 m), revealing an elongated shape with the longest axis diameter measuring approximately 1 km. The narrow appearance in the images suggests a slow rotation period of tens of hours.