Radar Astrometry of Near-Earth Asteroids from the Arecibo Observatory: 2018-2019

Arecibo Observatory has the world's most powerful planetary radar system, which provides ground-based observations whose quality could only be exceeded with a spacecraft flyby. It allows one to characterize near-Earth objects (NEOs) in terms of size, shape, spin, and surface properties, and to discover natural satellites that form binary and triple asteroid systems. In addition, radar astrometry is a valuable tool for orbit refinement, providing precise measurements that can significantly improve the accuracy of orbit determination.

We present an overview of radar astrometric observations obtained using the Arecibo planetary radar system since January 2018. Especially for newly discovered objects, that usually have large orbit uncertainties, Doppler and/or range measurements can prevent the object from being lost and requiring re-discovery in the future. We'll present a list of targets that had their orbits secured after radar observations, and an evaluation of how much radar helped to reduce their orbital uncertainties.

Every year about 40-60 recently discovered NEOs are observed at Arecibo. From January 1, 2018 to March 31, 2019, 90 NEOs were observed at Arecibo, including a comet. A total of 44 were newly discovered objects, almost half of the observed targets, and 33 are designated as potentially hazardous asteroids (PHAs). At least one Doppler measurement was obtained for each target, plus at least one range measurement for 55 of them.

Radar offers the great advantage of controlling the properties of the transmitted signal. The changes in the received echo compared to the transmitted signal provides clues about the characteristics of the object. The time delay gives information about the distance to a target with precision as fine as meters, and the Doppler-shifted frequency of the echo provides the radial velocity with precision as fine as mm/s, making it possible to detect even small changes in the orbit due to perturbations, such as the nongravitational acceleration generated by the Yarkovsky effect. Radar astrometry can also quickly eliminate impact false alarms with the improvement of estimates of an asteroid's orbital elements. Therefore, radar astrometry is crucial for planetary defense.