Vol. 562
In section 10. Planets and planetary systems

The internal structure of asteroid (25143) Itokawa as revealed by detection of YORP spin-up

by S. C. Lowry, P. R. Weissman, S. R. Duddy, et al., A&A 562, A48


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The internal structure of asteroids can tell us a lot about how they were formed and thus about collision processes in our solar system and the processes that are responsible for planet formation. Asteroid (25143) Itokawa is interesting in this respect because of its distinctive bean shape filled with boulders, as revealed by spacecraft Hayabusa in 2005. Using photometric data spanning 2001 to 2013, Lowry et al. show that the asteroid's spin rate has been decreasing by a mere 45 msec/year. This is due to an exchange of angular momentum between absorbed and reflected solar photons and thermal emission from the asteroid through the so-called YORP effect. Using detailed topographic models of the asteroids, the authors are able for the first time to provide evidence of density inhomogeneities within an asteroid. They show that Itokawa is formed of two separate bodies with very different densities, i.e. 1.75 and 2.85 g/cm3, respectively. Various explanations for the origin of this internal structure exist (catastrophic disruption of a larger differentiated body or the collapse of a binary system) but will have to be put to the test of these new constraints.