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The Population of Binary and Triple Near-Earth Asteroids Observed with the Arecibo Planetary Radar System

Presentation #104.01 in the session Planetary Defense! Part One.

Published onOct 20, 2022
The Population of Binary and Triple Near-Earth Asteroids Observed with the Arecibo Planetary Radar System

It is estimated that binary and triple asteroid systems represent approximately 15% of the near-Earth asteroid (NEA) population. Multiple asteroid systems can help to understand the formation mechanism and evolution of small bodies in the solar system, which is especially important in Earth’s vicinity as we monitor how NEA orbits evolve with time. Example of missions to binary asteroid systems are the ongoing NASA’s Double Asteroid Redirection Test (DART) to the 65803 Didymos system, and the upcoming Janus mission, which will visit two binary NEAS, (175706) 1996 FG3 and (35107) 1991 VH. DART will be the first planetary defense mission demonstration ever performed, while Janus primary objectives are to identify and understand how binary asteroid systems form and evolve. Small body missions frequently select targets that can be observed with radar prior to the mission, including DART and Janus, as ground-based radar is a powerful tool for physical and dynamical characterization, helping to plan the mission. We will present the population of binary and triple NEAs observed with the Arecibo’s planetary radar system.

Arecibo’s S-band (2.38 GHz, 12.6 cm) planetary radar system helped to discover or confirm nearly 60 NEAs in multiple systems, 73% classified as potentially hazardous asteroids (PHAs). The vast majority of these observed systems are in a binary configuration, only three were clearly triple systems in delay-Doppler images (3122 Florence, 1994 CC, and 2001 SN263). Although the most common configuration is to have a primary component much larger than the secondary one (or “moonlet”), radar images were able to reveal four equal-mass binaries (69230 Hermes, 2005 UP156, 1994 CJ1, and 2017 YE5), where the barycenter lies near the geometric center of the system. The detection of a satellite in radar images can help to extract important information about the system, for example, by knowing the separation distance between two components and mutual orbital period, it is possible to calculate the mass of the system and estimate density. In terms of orbit classification, at least 71% of the multiple asteroid systems observed at the Arecibo Observatory were part of the Apollo group, followed by 17% Amor, 8% Aten, and only one belonging to the Atira group (163693 Atira). In addition, approximately 50% of the radar-observed NEA systems have heliocentric orbit eccentricity e ≥ 0.5, with the binary NEA 5143 Heracles having the most eccentric orbit (e = 0.772).

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