The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km in diameter), as its influence is most significant for these bodies. History of discovery The effect was discovered by the Polish-Russian civil engineer Ivan Osipovich Yarkovsky (1844–1902), who worked in Russia on scientific problems in his spare time. Writing in a pamphlet around the year 1900, Yarkovsky noted that the daily heating of a rotating object in space would cause it to experience a force that, while tiny, could lead to large long-term effects in the orbits of small bodies, especially meteoroids and small asteroids. Yarkovsky's insight would have been forgotten had it not been for the 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 on movement of meteoroids about the Solar System. Mechanism The Yarkovsky effect is a consequence of the fact that change in the temperature of an object warmed by radiation (and therefore the intensity of thermal radiation from the object) lags behind changes in the incoming radiation. That is, the surface of the object takes time to become warm when first illuminated, and takes time to cool down when illumination stops. In general there are two components to the effect: Diurnal effect: On a rotating body illuminated by the Sun (e.g. an asteroid or the Earth), the surface is warmed by solar radiation during the day, and cools at night. Due to the thermal properties of the surface, there is a lag between the absorption of radiation from the Sun, and the emission of radiation as heat, so the warmest point on a rotating body occurs around the "2 PM" site on the surface, or slightly after noon. This results in a difference between the directions of absorption and re-emission of radiation, which yields a net force along the direction of motion of the orbit. If the object is a prograde rotator, the force is in the direction of motion of the orbit, and causes the semi-major axis of the orbit to increase steadily; the object spirals away from the Sun. A retrograde rotator spirals inward. The diurnal effect is the dominant component for