Binary asteroids

Planetary scientists Seth Jacobson and Daniel Scheeres, both from University of Colorado, have developed a theory that explains the formation of synchronous binary asteroids, as well as all other dynamical classes of Near Earth Asteroids (NEAs). Their model follows the dynamical evolution of rotationally fissioned asteroids – asteroids whose rotation has increased to break-up speed.

Contrary to what you might think, astronomers believe that most asteroids are not solid monoliths of rock orbiting the Sun, but are better described as “rubble piles” – a collection of smaller boulders bound together by their mutual gravitational force. It is thought that low-speed collisions amongst asteroids can fracture them, but as long as the relative velocity amongst the shattered fragments of an asteroid is less than their mutual escape velocity, the fragments can remain gravitationally bound as a rubble pile.

Evidence that asteroids may indeed be rubble piles include their relatively low rotation rates (which needs to be low so as not to fling the small fragments away from each other), low bulk density (regardless of their surface composition, a rubble pile would have high porosity and hence low bulk density), and the fact that binary asteroids – or asteroids with satellites – are relatively common.  The first asteroid satellite, the Ita-Dactyl system, was discovered in 1993 by the Galileo spacecraft on its way to Jupiter. Since then many more have been discovered, and about 2% of Main Belt asteroids and 15% of NEAs are binaries.

Animation of the orbit of the binary Near-Earth Asteroid (66391) 1999 KW4 (Credit: NASA/JPL)

When sunlight hits the day-side of an asteroid, some of the light will be scattered while some will be absorbed and then re-emitted as heat or thermal radiation. For a rotating, irregularly shaped asteroid, this results in asymmetric re-emission of thermal radiation. This causes a small torque that changes the rotation rate and orientation of the spin axis of the asteroid.  This is known as the YORP (or Yarkovsky–O’Keefe–Radzievskii–Paddack) effect, and while the torque is very small, over long periods of time the asteroid’s spin rate can increase.  The smaller the asteroid, the larger the effect and small asteroids (generally less than 10 km) can be rotationally fissioned by the YORP effect, effectively turning one asteroid into two asteroids.

If the original asteroid was a rubble pile, then Jacobson and Scheeres argue that both asteroids in the binary must be rubble piles. They suggest that the smaller of the pair can undergo a ‘secondary fission’ – not due to the YORP effect this time, but due to a coupling of the spin and orbital states for the binary system.  The smaller secondary asteroid can “steal” rotational energy from the larger primary asteroid, increasing its rotation rate (while decreasing the rotation of the larger asteroid and changing the shape of their mutual orbit). The smaller asteroid can rotationally fission, producing a chaotic ternary (3-body) system. Energy can swap between the spins and orbits of these three bodies, as well as be damped by tidal effects, which can result in a wide range of outcomes. Impacts between members of the ternary or the escape of one member will help stabilise the resulting binary system.

They conclude that rotational fission can explain all NEA systems, including synchronous binaries (where the spin rate of the secondary equals the rotation rate of the binary pair), high eccentricity binaries (which are asynchronous and have highly oval orbits), ternary systems (3-body systems thought to form after a binary asteroid  undergoes a secondary rotational fission), and contact binaries (which results when gravity pulls the two asteroids together until they touch, forming a single irregularly shaped asteroid).

“NEAs are actively evolving systems driven by these processes and the observed asteroid classes are stages in this evolution”, the authors conclude. This work sheds light on the  mechanism that contributes to the formation and potential disintegration of small asteroids.

For more information, see

[Sheridan Lacey & Sarah Maddison]

** Note: the 2014 arXiv paper by Jacobson & Scheeres (arxiv:1404.0801) was in fact published in Icarus in 2011 (Icarus, 2011, 214, 161) !

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