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Binary Asteroid Systems’ Orbital Dynamics Evolve Differently

binary asteroid

Three radar images from Arecibo Observatory unambiguously reveal the binary nature of near-Earth asteroid 66391 (1999 KW4). The images have resolution of 30 meters per pixel in the vertical direction and 0.3 Hz in the horizontal direction and are sums of all data collected at this resolution on each day. The primary component, the larger and wider body at the center of each frame, is more than one kilometer in diameter, while its satellite is about one-third the size of the primary.

Credit: Arecibo/NASA/NSF 


Long-term observations of two binary near-Earth asteroid systems have allowed researchers to constrain the long-term mutual orbital evolution of their components, and to derive physical and dynamical properties of the binary systems. Scientists including PSI’s Eva Lilly obtained thorough photometric observations of two binary near-Earth asteroids 66391 (1999 KW4) and (88710) 2001 SL9 taken from 2000 to 2019. 

Lilly collected photometric observations for asteroid (88710) 2001 SL9 over three nights with the University of Hawaii 88-inch telescope and helped with the data reduction. 

The findings are described in a new Icarus paper “A satellite orbit drift in binary near-Earth asteroids (66391) 1999 KW4 and (88710) 2001 SL9 – Indication of the BYORP effect” ( PSI’s Juan Sanchez is a co-author. 

“Our results show the components of (88710) 2001 SL drift inwards (the semi-major axis decreases), while the components of (66391) 1999 KW4 drift outwards (semi-major axis is expanding). The only other well-known system studied before – (175706) 1996 FG3 has components in dynamical equilibrium. These three binary systems present examples of the three states of the mutual orbital evolution – equilibrium, expanding and contracting in the population of near-Earth binary asteroids, and provide observational confirmation that synchronous binary asteroids are in a state of stable equilibrium between binary YORP (BYORP) effect and mutual body tides. 

The Yarkovsky–O'Keefe–Radzievskii–Paddack  (YORP) effect is a result of sunlight being absorbed and re-radiated as heat on a small body, which creates a thermal torque capable of modifying the asteroid’s rotation rate and obliquity.  The increase in spin rate can be so significant it could change a body’s shape and eventually lead to asteroid’s break-up. Essentially, the BYORP effect is a version of the YORP effect affecting orbital properties of a binary asteroid. It can either separate the binary components or cause them to collide. 

Our understanding of the YORP and BYORP effects sheds light on the physical and dynamical evolution of small NEAs, and is very important for planning future space missions and for assessing the impact risk these bodies could pose to Earth.

Feb. 14, 2021
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