DDA 2015 – Stochastic YORP On Real Asteroid Shapes

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Jay W. McMahon (UC Boulder)


Since its theoretical foundation and subsequent observational verification, the YORP effect has been understood to be a fundamental process that controls the evolution of small asteroids in the inner solar system. In particular, the coupling of the YORP and Yarkovsky effects are hypothesized to be largely responsible for the transport of asteroids from the main belt to the inner solar system populations. Furthermore, the YORP effect is thought to lead to rotational fission of small asteroids, which leads to the creation of multiple asteroid systems, contact binary asteroids, and asteroid pairs. However recent studies have called into question the ability of YORP to produce these results. In particular, the high sensitivity of the YORP coefficients to variations in the shape of an asteroid, combined with the possibility of a changing shape due to YORP accelerated spin rates can combine to create a stochastic YORP coefficient which can arrest or change the evolution of a small asteroid’s spin state. In this talk, initial results are presented from new simulations which comprehensively model the stochastic YORP process. Shape change is governed by the surface slopes on radar based asteroid shape models, where the highest slope regions change first. The investigation of the modification of YORP coefficients and subsequent spin state evolution as a result of this dynamically influenced shape change is presented and discussed.


  • Background
    • YORP controls small asteroid spin evolution
    • YORP highly sensitive to location of features on surface (Statler 2009)
    • “stochastic YORP” (Cotto-Figueroa 2013)
    • “stochastic YORP” $\rightarrow$ evolution of asteroid families (Bottke et al. 2015)
  • Motivation
    • Do shapes change as spin increases?
    • How does shape evolution map to YORP coefficients?
  • Shape evolution
    • Regolith will flow “downhill”
    • Body will reshape to relax to some slope limit (Scheeres 2015)
    • This study: use actual radar-derived asteroid shapes instead of idealized sphere/ellipsoid
    • Use (101955)Bennu
      • Apollo asteroid
      • OSIRIS-REx sample return target
  • Results
    • 5-m boulder (as spin limitis approached):
      • effect on obliquity very small
      • larger effects on spin rate
      • shape of boulder matters
    • Much future work to do

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