Editing Yarkovsky effect (section)

==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. The thermal properties of the surface cause a lag between the absorption of radiation from the Sun and the emission of radiation as heat, so the surface is warmest not when the Sun is at its peak but slightly later. 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 [[direct motion|prograde]] rotator, then 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 motion|retrograde]] rotator spirals inward. The diurnal effect is the dominant component for bodies with diameter greater than about 100 m.<ref name=Bottke06review>{{cite journal |last=Bottke, Jr. |first=William F. |url=https://astro.troja.mff.cuni.cz/davok/papers/ann_rev_06.pdf |title=The Yarkovsky and YORP Effects: Implications for Asteroid Dynamics |journal=[[Annual Review of Earth and Planetary Sciences|Annu. Rev. Earth Planet. Sci.]] |volume=34 |pages=157–191 |year=2006 |doi=10.1146/annurev.earth.34.031405.125154 |bibcode=2006AREPS..34..157B |s2cid=11115100 |display-authors=etal |access-date=2021-08-12 |archive-date=2021-08-12 |archive-url=https://web.archive.org/web/20210812022037/https://astro.troja.mff.cuni.cz/davok/papers/ann_rev_06.pdf |url-status=live }}</ref>
* '''Seasonal''' effect: This is easiest to understand for the idealised case of a non-rotating body orbiting the Sun, for which each "year" consists of exactly one "day". As it travels around its orbit, the "dusk" hemisphere which has been heated over a long preceding time period is invariably in the direction of orbital motion. The excess of thermal radiation in this direction causes a braking force that always causes spiraling inward toward the Sun. In practice, for rotating bodies, this seasonal effect increases along with the [[axial tilt]]. It dominates only if the diurnal effect is small enough. This may occur because of very rapid rotation (no time to cool off on the night side, hence an almost-uniform [[longitude|longitudinal]] temperature distribution), small size (the whole body is heated throughout), or an axial tilt close to 90°. The seasonal effect is more important for smaller asteroid fragments (from a few metres up to about 100 m), provided their surfaces are not covered by an insulating [[regolith]] layer and they do not have exceedingly slow rotations. Additionally, on very long timescales over which the spin axis of the body may be repeatedly changed by collisions (and hence also the direction of the diurnal effect changes), the seasonal effect will also tend to dominate.<ref name=Bottke06review />

In general, the effect is size-dependent, and will affect the semi-major axis of smaller asteroids, while leaving large asteroids practically unaffected. For kilometre-sized asteroids, the Yarkovsky effect is minuscule over short periods: the force on asteroid [[6489 Golevka]] has been estimated at 0.25 [[newton (unit)|newtons]], for a net acceleration of 10<sup>&minus;12</sup>&nbsp;m/s<sup>2</sup>. But it is steady; over millions of years an asteroid's orbit can be perturbed enough to transport it from the [[asteroid belt]] to the inner Solar System.

The mechanism is more complicated for bodies in strongly [[eccentricity (orbit)|eccentric]] orbits.