Yarkovsky effect
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In physics, the Yarkovsky effect is a force felt by a body caused by the momentum carried away by the thermal photons that it emits. It was discovered by the Russian civil engineer Ivan Osipovich Yarkovsky (1844–1902), who worked on scientific problems in his spare time. Writing in a pamphlet around the year 1900, Yarkovsky noted that the diurnal heating of a rotating object in space would cause it to experience a force that, while tiny, could lead to large secular effects in the orbits of small bodies, especially meteoroids and small asteroids. Yarkovsky’s remarkable insight would have been consigned to oblivion had it not been for the brilliant Estonian astronomer Ernst J. Öpik (1893–1985), who read Yarkovsky’s pamphlet sometime around 1909. Decades later Öpik, recalling the pamphlet from memory, discussed the possible importance of the Yarkovsky effect for moving meteoroids about the solar system (E. J. Öpik, Collision probabilities with the planets and the distribution of interplanetary matter, Proceedings of the Royal Irish Academy, 54A, pp. 165–199 (1951)).
In a nutshell, the Yarkovsky effect is a consequence of the warming of the asteroid's surface as it rotates under the Sun. Suppose an asteroid rotates like the Earth does, albeit maybe a little faster; the face exposed to the Sun warms up, and then rotates to the night side where it cools off. The "early night" side will be warmer than the "late night" side and therefore will radiate a little more (black body radiation scales as the fourth power of temperature). The radiation pressure from that side will be a little stronger than that from the other side, and the asteroid will thus be subjected to a net sideways thrust. Depending on how the axis of rotation is oriented, this net force may accelerate or decelerate the asteroid on its orbit. As a result, the orbit's semi-major axis will increase or decrease slightly but steadily.
The description above is for the diurnal Yarkovsky effect. There is another form, called the seasonal effect, which dominates if the object spins fast enough that the day/night (longitudinal) temperature difference tends to become negligible. In that case, the object's obliquity comes into play: in the same way as the Earth's, an asteroid's poles will spend half the orbital period in sunlight and the other half in shadow, so a large temperature difference may occur latitudinally. The seasonal effect is more important for smaller asteroidal fragments (from a few metres to ~100 m), provided their surfaces are not covered by an insulating regolith layer and they do not have exceedingly slow rotations.
The Yarkovsky effect is minuscule: 6489 Golevka is estimated to be subjected to a force of about 0.25 newton, for a net acceleration of 10−10 m/s². But it is steady; over millions of years it can perturb an asteroid's orbit enough to transport it from the main belt to the inner solar system.
It is very hard to predict the exact impact that the Yarkovsky effect will have on a specific asteroid's orbit, however, due to the fact that it is affected by a great many variables that are hard to account for with limited observational information. The Yarkovsky effect depends upon the shape of the asteroid, its orientation, its rotation rate, and its albedo; these factors are further complicated by the effects of shadowing and thermal "reillumination", which are not relevant for bodies with convex shapes, and the effect of directly reflected sunlight (radiation pressure), which is usually neglected for spherical bodies with uniform albedo.
In general, the effect is size dependent, and will affect the semi-major axis of smaller asteroids. Large asteroids will be mostly unaffected.
Even for the simple case of the pure seasonal Yarkovsky effect on a spherical body in a circular orbit with 90° obliquity, semi-major axis changes could differ by as much as a factor of two between cases with uniform albedo and cases with a strong north/south albedo asymmetry. For higher eccentricity, greater differences are possible. Depending on the orbit and spin axis, the Yarkovsky semi-major axis change may be reversed simply by changing from a spherical to a non-spherical shape.
The effect was first measured in 1991-2003 on the asteroid 6489 Golevka. The asteroid drifted 15 km from its predicted position over 12 years (the orbit was established with great precision by a series or radar observations in 1991, 1995 and 1999).
Reference
- Chesley, Steven R.; Ostro, Steven J.; Vokrouhlický, David; Čapek, David; Giorgini, Jon D.; Nolan, Michael C.; Margot, Jean-Luc; Hine, Alice A.; Benner, Lance A. M.; Chamberlin, Alan B.; Direct Detection of the Yarkovsky Effect via Radar Ranging to Asteroid 6489 Golevka, Science 302, 1739-1742 (2003)