Arecibo Science Highlights of Observatory Planetary Radar Observations: 2019-2020

We present a summary of the radar experiments performed with the Arecibo Observatory Planetary Radar system during 2019-2020.  Located in Puerto Rico (18° 20' 36.6" N, 66° 45' 11.1" W) the Arecibo Observatory S-band (2380 MHz) radar system is capable of transmitting up to 1MW of power and uses the William E Gordon Telescope antenna of 305 m. The planetary radar science group focuses on performing follow-up (post discovery) observations of  known small bodies as well as recently discovered ones. Priority is given to objects on the CNEOS Sentry impact risk list, those classified as Potentially Hazardous (PHA’s) and those that are potential spacecraft mission targets (NHATS). Although currently operating at 35% power capacity, Arecibo has observed 92 objects since September 2019 to abstract submission date, distributed as: 61  recently discovered objects, 28 PHA’s, 2 planets and 1 comet. We present here some science highlights of  this year's observations of near-Earth objects (NEOs), including radar delay-Doppler images of 2020BX12, 2011WN15, 481394 (2006 SF6) and 162082 (1998HL1).

Introduction
The Arecibo Observatory is the largest and most powerful planetary radar system in the world, successfully observing  up to 130 asteroids a year. Funded by the NASA-NEO Observations program, the ground-based observations done using the S-band (2380 MHz, 12.6 cm) radar systems are a highly cost effective and rapid tool to constrain physical and dynamical properties of the targets in comparison to space missions. This Instrument has the capability of transmitting a signal with or without  phase modulation, providing extremely accurate astrometry measurements (range and radial velocity) on newly discovered objects, and track changes in the orbit of previously observed ones, such as those due to non-gravitational perturbations. Besides orbital characterization, radar data provides constraints on the object's size and rotation rate, is responsible for the discovery of satellites [1,2]  and for some cases can identify the shape and near-surface (meter-scale) structures up to a few wavelengths deep. 

Methods
The S-band system transmits a circularly polarized wave, and receives both the same-sense circular (SC) and opposite-sense circular polarization (OC) as transmitted. Radar observations usually start by a continuous-wave measurement to obtain the Doppler frequency spectrum of the echo. The measured Doppler spectrum bandwidth provides initial limits for rotation period and the object’s apparent diameter. From the measured received backscattered power in these two orthogonal states of polarization, it's possible to calculate the target's circular polarization ratio. Defined as the ratio of the SC and OC echo and commonly used as an indicator of the surface reflection properties. For targets with a relatively high signal-to-noise-ratio (SNR) we use phase modulation to produce delay-Doppler images, with range resolution as fine as 7.5 m per pixel in some cases. These images aid in the estimation of objects' diameter and provide an idea of the body's shape.

Results
Some highlights of our observations include: 162082 (1998 HL1) observed on October 25-28, 2019 with a delay-Doppler resolution of 75 m/px, its apparent diameter is estimated at 270 m, and its rotation period at approximately 11 hrs. Contact binary 481394 (2006SF6) was observed on November 11-15, 2019, with a delay-Doppler resolution of 7.5 m/px showing a maximum visible extent of 240 m. The rotation period is estimated to be 11.3 hrs and  it was observed at various orientations. 2011 WN15 was observed on December 12-13, 2019, with a delay-Doppler resolution of 7.5 m/px providing an estimate on diameter of 900 m, and a rotation period of up to 4 hours. 2020 BX12 observation on February 4-5, 2020, led to the discovery of a secondary body, images with delay-Doppler resolution of 7.5 m/px, showed a diameter of 165 m for the primary and no more than 70 m for the secondary. The apparent rotation period for the primary is about 2.8 hrs and 49 hrs or less for the secondary.

Acknowledgements:
The Arecibo Planetary Radar Program is fully supported by NASA’s Near-Earth Object Observations Program in NASA’s Planetary Defense Coordination Office through grant no. 80NSSC19K0523 awarded to University of Central Florida (UCF). UCF manages the National Science Foundation facility under a cooperative agreement with Yang Enterprises, Inc. and Universidad Ana G. Méndez.

References
[1] Benner, L.A., Nolan, M.C., Margot, J., Brozovic, M., Ostro, S.J., Shepard, M.K., Magri, C., Giorgini, J.D. and Busch, M.W., 2008, September. Arecibo and Goldstone radar imaging of contact binary near-Earth asteroids. In DPS (pp. 25-03).
[2] Rivera-Valentin, E.G., Taylor, P.A., Virkki, A. and Aponte-Hernandez, B., 2017. (163693) Atira. CBET, 4347, p.1.