Editing YORP effect (section)

== Examples ==
Assume a rotating spherical asteroid has two wedge-shaped fins attached to its equator, irradiated by parallel rays of sunlight. The [[Reaction (physics)|reaction]] force from photons departing from any given surface element of the spherical core will be normal to the surface, such that no [[torque]] is produced (the force vectors all pass through the centre of mass).[[File:YORP effect - wedged sphere.svg|thumb|A spherical asteroid with two wedge-shaped projections. Re-radiated light from the "B" fin has the same magnitude as the "A" fin, but is not parallel to the incoming light. This produces a torque on the object.]]

Thermally-emitted photons [[wiktionary:reradiate|reradiated]] from the sides of the wedges, however, can produce a torque, as the normal vectors do not pass through the centre of mass. Both fins present the same cross section to the incoming light (they have the same height and width), and so absorb and reflect the same amount of energy each and produce an equal force. Due to the fin surfaces being oblique, however, the normal forces from the reradiated photons do not cancel out. In the diagram, fin A's outgoing radiation produces an equatorial force parallel to the incoming light and no vertical force, but fin B's force has a smaller equatorial component and a vertical component. The unbalanced forces on the two fins lead to torque and the object spins. The torque from the outgoing light does not average out, even over a full rotation, so the spin accelerates over time.<ref name="Rubincam 2000 pp. 2–11">{{cite journal | last=Rubincam | first=D | title=Radiative Spin-up and Spin-down of Small Asteroids | journal=Icarus | publisher=Elsevier BV | volume=148 | issue=1 | year=2000 | pages=2–11 | doi=10.1006/icar.2000.6485 | bibcode=2000Icar..148....2R | url=https://zenodo.org/record/1229840 | access-date=2019-12-11 | archive-date=2020-02-26 | archive-url=https://web.archive.org/web/20200226134341/https://zenodo.org/record/1229840 | url-status=live }}</ref>

An object with some "windmill" asymmetry can therefore be subjected to minuscule torque forces that will tend to spin it up or down as well as make its axis of rotation [[precession|precess]]. The YORP effect is zero for a rotating [[ellipsoid]] ''if'' there are no irregularities in surface temperature or [[albedo]].

In the long term, the object's changing [[obliquity]] and rotation rate may wander randomly, chaotically or regularly, depending on several factors. For example, assuming the [[Sun]] remains on its [[equator]], asteroid [[951 Gaspra]], with a radius of 6&nbsp;km and a [[semi-major axis]] of 2.21 [[astronomical unit|AU]], would in 240 Ma (240 million years) go from a rotation period of 12 h to 6 h and vice versa. If [[243 Ida]] were given the same radius and orbit values as Gaspra, it would spin up or down twice as fast, while a body with [[Phobos (moon)|Phobos']] shape would take several [[1000000000 (number)|billion]] years to change its spin by the same amount.

Size as well as shape affects the amount of the effect. Smaller objects will spin up or down much more quickly. If Gaspra were smaller by a factor of 10 (to a radius of 500 m), its spin will halve or double in just a few million years. Similarly, the YORP effect intensifies for objects closer to the Sun. At 1 AU, Gaspra would double/halve its spin rate in a mere 100,000 years. After one million years, its period may shrink to ~2 h, at which point it could start to break apart.{{citation needed|date=February 2020}} According to a 2019 model, the YORP effect is likely to cause "widespread fragmentation of asteroids" as the Sun expands into a luminous [[red giant]], and may explain the dust disks and apparent infalling matter observed at many [[white dwarf]]s.<ref>{{cite journal |last1=Veras |first1=Dimitri |last2=Scheeres |first2=Daniel J |title=Post-main-sequence debris from rotation-induced YORP break-up of small bodies – II. Multiple fissions, internal strengths, and binary production |journal=Monthly Notices of the Royal Astronomical Society |date=February 2020 |volume=492 |issue=2 |pages=2437–2445 |doi=10.1093/mnras/stz3565 | arxiv=2001.00949|doi-access=free }}</ref><ref>{{cite news |last1=Timmer |first1=John |title=When the Sun expands, it will trash all the asteroids |url=https://arstechnica.com/science/2020/02/when-the-sun-expands-it-will-trash-all-the-asteroids/ |access-date=20 February 2020 |work=Ars Technica |date=18 February 2020 |language=en-us |archive-date=20 February 2020 |archive-url=https://web.archive.org/web/20200220055707/https://arstechnica.com/science/2020/02/when-the-sun-expands-it-will-trash-all-the-asteroids/ |url-status=live }}</ref>

This is one mechanism through which [[binary asteroid]]s may form, and it may be more common than collisions and planetary near-encounter tidal disruption as the primary means of binary formation.

Asteroid {{mp|2000 PH|5}} was later named [[54509 YORP]] to honor its part in the confirmation of this phenomenon.