Acceleration of the rotation of asteroid 1862 Apollo by radiation torques


The anisotropic reflection and thermal re-emission of sunlight from an asteroid’s surface acts as a propulsion engine. The net propulsion force (Yarkovsky effect) changes the orbital dynamics of the body at a rate that depends on its physical properties; for irregularly shaped bodies, the propulsion causes a net torque (the Yarkovsky–O'Keefe–Radzievskii–Paddack or YORP effect) that can change the object’s rotation period and the direction of its rotation axis1,2. The Yarkovsky effect has been observed directly3, and there is also indirect evidence of its role in the orbital evolution of asteroids over long time intervals4,5,6. So far, however, only indirect evidence exists for the YORP effect through the clustering of the directions of rotation axes in asteroid families6,7,8. Here we report a change in the rotation rate of the asteroid 1862 Apollo, which is best explained by the YORP mechanism. The change is fairly large and clearly visible in photometric lightcurves, amounting to one extra rotation cycle in just 40 years even though Apollo’s size is well over one kilometre. This confirms the prediction that the YORP effect plays a significant part in the dynamical evolution of asteroids.

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Figure 1: YORP-induced phase offset in lightcurves.
Figure 2: Sample lightcurves from the 2005 apparition.
Figure 3: Phase offset evolution in time.


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We thank D. Vokrouhlický for assistance and discussions, P. Pravec for Apollo data, and W. Bottke and A. Harris for comments. The work of M.K. was supported by the Academy of Finland, that of J.Ď. by the Grant Agency of the Czech Republic, and that of Y.N.K. and N.M.G. by the Ministry of Education and Science of the Ukraine.

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Correspondence to Mikko Kaasalainen.

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Supplementary Information

This file contains Supplementary Table S1 (observations of Apollo) and Supplementary Figures S1-S4 illustrating Apollo lightcurves (S1), Apollo shape model (S2), simulated YORP behaviour of the model (S3), orbital evolution of Apollo( S4). (PDF 730 kb)

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Kaasalainen, M., Ďurech, J., Warner, B. et al. Acceleration of the rotation of asteroid 1862 Apollo by radiation torques. Nature 446, 420–422 (2007).

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