The YORP effect on 25 143 Itokawa

Abstract

Context. The asteroid 25143 Itokawa is one of the candidates for the detection of the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect in the rotation period. Previous studies were carried out up to the 196 608 facets triangulation model and were not able to provide a good theoretical estimate of this effect, raising questions about the influence of the mesh resolution and the centre of mass location on the evolution the rotation period. Aims. The YORP effect on Itokawa is computed for different topography models up to the highest resolution Gaskell mesh of 3 145 728 triangular faces in an attempt to find the best possible YORP estimate. Other, lower resolution models are also studied and the question of the dependence of the rotation period drift on the density distribution inhomogeneities is reexamined. A comparison is made with 433 Eros models possessing a similar resolution. Methods. The Rubincam approximation (zero conductivity) is assumed in the numerical simulation of the YORP effect in rotation period. The mean thermal radiation torques are summed over triangular facets assuming Keplerian heliocentric motion and uniform rotation around a body-fixed axis.Results. There is no evidence of YORP convergence in Gaskell model family. Differently simplified meshes may converge quickly to their parent models, but this does not prove the quality of YORP computed from the latter. We confirm the high sensitivity of the YORP effect to the fine details of the surface for 25 143 Itokawa and 433 Eros. The sensitivity of the Itokawa YORP to the centre of mass shift is weaker than in earlier works, but instead the results prove to be sensitive to the spin axis orientation in the body frame. Conclusions. Either the sensitivity of the YORP effect is a physical phenomenon and all present predictions are questionable, or the present thermal models are too simplified.

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

“…YORP spin-down). In particular, studies based on the Hayabusa-derived shape models predicted rotational accelerations (Scheeres et al 2007;Breiter, et al 2009) of (-5.5 to -2.0) × 10 −7 rad day −2 , which differ significantly from our observed value of 3.54 × 10 −8 rad day −2 . These predictions were also inconsistent with an upper limit of |ν| < 1.5 × 10 −7 rad day −2 that was derived from existing light-curve observations in 2008 (Ďurech et al 2008b).…”

“…Assuming a moment of inertia, I Z , of 7.77 × 10 14 kg m 2 (Breiter et al 2009) along with a moderately rough surface at cm scales (Ostro et al 2004;Müller et al 2005) with a uniform spatial distribution, the ATPM predicts a rotational acceleration of -1.80 × 10 −7 rad day −2 , consistent with previous determinations. A COM shift can reconcile our YORP model with the observed value, which we can determine by combining the ATPM with the methodology used for calculating such COM offsets (Scheeres & Gaskell 2008).…”

“…To explain the inconsistency, it was suggested that a non-uniform internal mass distribution that shifted the centre-of-mass (COM) away from the "centre-of-figure" towards the "head" of Itokawa could be a possible cause (Scheeres & Gaskell 2008). Other theoretical work indicated that the YORP effect can be extremely sensitive to unresolved shape features (Statler 2009), the shape model resolution (Breiter, et al 2009), and surface roughness (Rozitis & Green 2012), such that the error in any prediction could be very large.…”

“…The ATPM calculates the YORP rotational acceleration line with shadowing and global self-heating effects included. For comparison purposes, the zero YORP rotational acceleration lines with none of these effects included (Scheeres & Gaskell 2008), and with only shadowing included (Breiter et al 2009), are plotted as the dotted and dashed lines respectively. As demonstrated in Rozitis & Green (2013), if global self-heating effects are neglected then YORP predictions are generally more accurate if shadowing is also not included.…”

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.…”