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

Abstract

Context. Near-Earth asteroid (25143) Itokawa was visited by the Hayabusa spacecraft in 2005, resulting in a highly detailed shape and surface topography model. This model has led to several predictions for the expected radiative torques on this asteroid, suggesting that its spin rate should be decelerating. Aims. To detect changes in rotation rate that may be due to YORP-induced radiative torques, which in turn may be used to investigate the interior structure of the asteroid. Methods. Through an observational survey spanning 2001 to 2013 we obtained rotational lightcurve data at various times over the last five close Earth-approaches of the asteroid. We applied a polyhedron-shape-modelling technique to assess the spin-state of the asteroid and its long term evolution. We also applied a detailed thermophysical analysis to the shape model determined from the Hayabusa spacecraft. Results. We have successfully measured an acceleration in Itokawa's spin rate of dω/dt = (3.54 ± 0.38) × 10 −8 rad day −2 , equivalent to a decrease of its rotation period of ∼45 ms year −1 . From the thermophysical analysis we find that the centre-of-mass for Itokawa must be shifted by ∼21 m along the long-axis of the asteroid to reconcile the observed YORP strength with theory. Conclusions. This can be explained if Itokawa is composed of two separate bodies with very different bulk densities of 1750 ± 110 kg m −3 and 2850 ± 500 kg m −3 , and was formed from the merger of two separate bodies, either in the aftermath of a catastrophic disruption of a larger differentiated body, or from the collapse of a binary system. We therefore demonstrate that an observational measurement of radiative torques, when combined with a detailed shape model, can provide insight into the interior structure of an asteroid. Futhermore, this is the first measurement of density inhomogeneity within an asteroidal body, that reveals significant internal structure variation. A specialised spacecraft is normally required for this.

Physical characterisation of near-Earth asteroid (1620) Geographos

“…Especially when previous studies have shown that the YORP effect can be highly sensitive to unresolved shape features and surface roughness (Statler 2009;Rozitis & Green 2012), the shape model resolution (Breiter et al 2009), and internal bulk density distribution (Scheeres & Gaskell 2008;Lowry et al 2014). However, Geographos has a relatively high YORPcoefficient of 0.01 (see Rossi et al 2009or Rozitis & Green 2013b for a definition), and Rozitis & Green (2013a) showed that asteroids with high values are less sensitive to the inclusion of concavities in their global shape model.…”

Photometric survey, modelling, and scaling of long-period and low-amplitude asteroids

“…However, so far only the spin-up of the rotation period has been directly detected (e.g. Lowry et al 2007Lowry et al , 2014; Kaasalainen et al 2007;Ďurech et al 2008) The spatial distribution of asteroid spin axes suggests that the largest bodies generally preserved their primordial, prograde spin, while smaller ones, with diameters less than 30 km, seem to be strongly affected by the YORP effect that pushes these axes towards extreme values of obliquities (Hanuš et al 2013). The spins of prograde rotators under the YORP effect influence can be captured into spin-orbit resonances, sometimes even forming spin clusters (Slivan 2002;Kryszczyńska et al 2012).…”

Looking into the evolution of granular asteroids in the Solar System

“…Asteroid 1999 KW4 could have a strong core, and asteroid Itokawa could possibly have a weak interior [6]. As encouraging as all these results and comparisons are, we still have not observed evidence that the formation mechanism here proposed has actually happened.…”

“…The angular velocity of this rotation, in the context of NEOs, can be accelerated by the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect which results from an interplay between solar radiation pressure and the shape of an asteroid [5,6]. Very succinctly, when the photons that come from the Sun ime-mail: diego.sanchez-lana@colorado.edu e-mail: dscheeres@colorado.edu e-mail: thirabayashi@purdue.edu e-mail: simon.tardivel@colorado.edu pact on the surface of an asteroid, they will be reflected, absorbed and re-emitted from it anisotropically due to its irregular shape.…”