Editing YORP effect

{{Short description|Second-order variation on the Yarkovsky effect that changes rotation rates of small bodies}}
{{distinguish|Yarkovsky effect}}The '''Yarkovsky–O'Keefe–Radzievskii–Paddack effect''', or '''YORP effect''' for short, changes the rotation state of a small [[astronomical body]] – that is, the body's [[rotation period|spin rate]] and the [[Axial tilt|obliquity]] of its [[Poles of astronomical bodies|pole]](s) – due to the [[scattering]] of [[solar radiation]] off its surface and the [[Emission (electromagnetic radiation)|emission]] of its own [[thermal radiation]].

The YORP effect is typically considered for [[asteroid]]s with their [[heliocentric orbit]] in the [[Solar System]]. The effect is responsible for the creation of [[Binary asteroid|binary]] and [[tumbling asteroids]] as well as for changing an asteroid's pole towards 0[[Degree (angle)|°]], 90°, or 180° relative to the [[ecliptic plane]] and so modifying its heliocentric radial drift rate due to the [[Yarkovsky effect]].

== Term ==
The term was coined by [[David P. Rubincam]] in 2000<ref>{{Cite journal |last=Rubincam |first=D |title=Radiative Spin-up and Spin-down of Small Asteroids |url=https://zenodo.org/record/1229840 |journal=Icarus |language=en |volume=148 |issue=1 |pages=2–11 |doi=10.1006/icar.2000.6485 |bibcode=2000Icar..148....2R |year=2000 |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> to honor four important contributors to the concepts behind the so-named YORP effect. In the 19th century, [[Ivan Osipovich Yarkovsky|Ivan Yarkovsky]] realized that the [[thermal radiation]] escaping from a body warmed by the Sun carries off [[momentum]] as well as [[heat]]. Translated into modern physics, each emitted [[photon]] possesses a momentum ''p'' = ''E/c'' where ''E'' is its [[energy]] and ''c'' is the [[speed of light]]. Vladimir Radzievskii applied the idea to rotation based on changes in [[astronomical albedo|albedo]]<ref>Radzievskii (1954)</ref> and Stephen Paddack realized that shape was a much more effective means of altering a body's spin rate.<ref>{{Cite journal|last=Paddack|first=S. J.|date=1969-01-01|title=Rotational bursting of small celestial bodies: Effects of radiation pressure.|journal=Journal of Geophysical Research|volume=74|issue=17|pages=4379–4381|doi=10.1029/JB074i017p04379|issn=0148-0227|bibcode=1969JGR....74.4379P}}</ref> Stephen Paddack and [[John A. O'Keefe (astronomer)|John O'Keefe]] suggested that the YORP effect leads to rotational bursting and by repeatedly undergoing this process, small asymmetric bodies are eventually reduced to dust.<ref>S. J. Paddack, J. W. Rhee, ''Geophys. Res. Lett'' '''2''', 365 (1975)</ref><ref>{{Cite journal|last=Okeefe|first=J. A.|date=1975-04-01|title=Tektites and their origin|journal=NASA STI/Recon Technical Report N|volume=75|pages=23444|bibcode=1975STIN...7523444O}}</ref>

== Physical mechanism ==
In principle, [[electromagnetic radiation]] interacts with the surface of an asteroid in three significant ways: radiation from the [[Sun]] is (1) [[Absorption (electromagnetic radiation)|absorbed]] and (2) [[Diffuse reflection|diffusively reflected]] by the surface of the body and the body's internal energy is (3) [[Emission (electromagnetic radiation)|emitted]] as [[thermal radiation]]. Since [[photon]]s possess [[momentum]], each of these interactions leads to changes in the [[angular momentum]] of the body relative to its [[center of mass]]. If considered for only a short period of time, these changes are very small, but over longer periods of time, these changes may [[Integration (mathematics)|integrate]] to significant changes in the angular momentum of the body. For bodies in a [[heliocentric orbit]], the relevant long period of time is the [[orbital period]] (i.e. year), since most asteroids have [[rotation period]]s (i.e. days) shorter than their orbital periods. Thus, for most asteroids, the YORP effect is the secular change in the rotation state of the asteroid after averaging the [[Solar Radiation|solar radiation]] torques over first the rotational period and then the orbital period.

== Observations ==
In 2007 there was direct observational confirmation of the YORP effect on the small asteroids [[54509 YORP]] (then designated {{mpl|2000 PH|5}})<ref name="LowryFitzsimmons2007">{{cite journal|last1=Lowry|first1=S. C.|last2=Fitzsimmons|first2=A.|last3=Pravec|first3=P.|last4=Vokrouhlicky|first4=D.|last5=Boehnhardt|first5=H.|last6=Taylor|first6=P. A.|last7=Margot|first7=J.-L.|last8=Galad|first8=A.|last9=Irwin|first9=M.|last10=Irwin|first10=J.|last11=Kusnirak|first11=P.|title=Direct Detection of the Asteroidal YORP Effect|journal=Science|volume=316|issue=5822|year=2007|pages=272–274|issn=0036-8075|doi=10.1126/science.1139040|bibcode=2007Sci...316..272L|pmid=17347414|s2cid=26687221 |url=https://pure.qub.ac.uk/portal/files/5430838/Science_2007_Lowry_272_4.pdf|access-date=2019-09-23|archive-date=2019-09-23|archive-url=https://web.archive.org/web/20190923110706/https://pure.qub.ac.uk/portal/files/5430838/Science_2007_Lowry_272_4.pdf|url-status=live}}</ref><ref name="TaylorMargot2007">{{cite journal|last1=Taylor|first1=P. A.|last2=Margot|first2=J.-L.|last3=Vokrouhlicky|first3=D.|last4=Scheeres|first4=D. J.|last5=Pravec|first5=P.|last6=Lowry|first6=S. C.|last7=Fitzsimmons|first7=A.|last8=Nolan|first8=M. C.|last9=Ostro|first9=S. J.|last10=Benner|first10=L. A. M.|last11=Giorgini|first11=J. D.|last12=Magri|first12=C.|title=Spin Rate of Asteroid (54509) 2000 PH5 Increasing Due to the YORP Effect|journal=Science|volume=316|issue=5822|year=2007|pages=274–277|issn=0036-8075|doi=10.1126/science.1139038|bibcode=2007Sci...316..274T|pmid=17347415|s2cid=29191700 |doi-access=free}}</ref> and [[1862 Apollo]].<ref>{{cite journal |author=Kaasalainen, Mikko |author2=Ďurech, Josef |author3=Warner, Brian D. |author4=Krugly, Yurij N. |author4-link=Yurij N. Krugly |author5=Gaftonyuk, Ninel M. | date = 2007 | title = Acceleration of the rotation of asteroid 1862 Apollo by radiation torques | journal = Nature | volume = 446 | issue = 7134 | pages = 420–422 | doi = 10.1038/nature05614 | bibcode=2007Natur.446..420K | pmid=17344861|s2cid=4420270 }}</ref> The spin rate of 54509 YORP will double in just 600,000 years, and the YORP effect can also alter the axial tilt and [[precession]] rate, so that the entire suite of YORP phenomena can send asteroids into interesting resonant spin states, and helps explain the existence of [[binary asteroid]]s.<ref>{{cite journal | last1 = Rubincam | first1 = D. P. | last2 = Paddack | first2 = S. J. | date = 2007 | title = As Tiny Worlds Turn | journal = Science | volume = 316 | issue = 5822 | pages = 211–212 | doi = 10.1126/science.1141930 | pmid = 17431161 | citeseerx = 10.1.1.205.5777 | s2cid = 118802966 }}</ref>

Observations show that asteroids larger than 125&nbsp;km in diameter have rotation rates that follow a [[Maxwell–Boltzmann distribution|Maxwellian frequency distribution]], while smaller asteroids (in the 50 to 125&nbsp;km size range) show a small excess of fast rotators. The smallest asteroids (size less than 50&nbsp;km) show a clear excess of very fast and slow rotators, and this becomes even more pronounced as smaller-sized populations are measured. These results suggest that one or more size-dependent mechanisms are depopulating the centre of the spin rate distribution in favour of the extremes. The YORP effect is a prime candidate. It is not capable of significantly modifying the spin rates of large asteroids by itself, so a different explanation must be sought for objects such as [[253 Mathilde]].

In late 2013 asteroid [[P/2013 R3]] was observed breaking apart, likely because of a high rotation speed from the YORP effect.<ref>{{cite web
|url=http://www.spacetelescope.org/news/heic1405/
|title=Hubble witnesses an asteroid mysteriously disintegrating
|access-date=2014-03-06
|archive-date=2014-03-12
|archive-url=https://web.archive.org/web/20140312045223/http://www.spacetelescope.org/news/heic1405/
|url-status=live
}}</ref>

== 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.

== See also ==
* {{annotated link|101955 Bennu}}
* [[25143 Itokawa]]—Smallest asteroid to be visited by a spacecraft
* {{annotated link|54509 YORP}}
* {{annotated link|Radiation pressure}}
* {{annotated link|Radiometer}}
* {{annotated link|Yarkovsky effect}}

== Citations ==
{{Reflist}}

== General and cited references ==
* {{Cite book | last=O'Keefe | first=John A. | author-link=John A. O'Keefe (astronomer) | year = 1976 | title = Tektites and Their Origin |url=https://archive.org/details/tektitestheirori0000okee/mode/2up |url-access=registration | publisher = Elsevier}} [https://archive.org/details/nasa_techdoc_19750015372 Draft manuscript/report].
* {{cite journal | last1 = Paddack | first1 = Stephen J | year = 1969 | title = Rotational bursting of small celestial bodies: Effects of radiation pressure | journal = J. Geophys. Res. | volume = 74 | issue = 17| pages = 4379–4381 | doi=10.1029/jb074i017p04379 | bibcode=1969JGR....74.4379P}}
* {{cite journal| last=Radzievskii | first=V. V. | year=1954 | title=A mechanism for the disintegration of asteroids and meteorites | journal=[[Doklady Akademii Nauk SSSR]] | volume=97 | pages=49–52}}
* {{cite journal | last1 = Rubincam | first1 = David P | year = 2000 | title = Radiative spin-up and spin-down of small asteroids | url = https://zenodo.org/record/1229840| journal = Icarus | volume = 148 | issue = 1 | pages = 2–11 | doi=10.1006/icar.2000.6485 | bibcode=2000Icar..148....2R}}

== Further reading ==
* {{Cite journal | title = Extreme Sensitivity of the YORP Effect to Small-Scale Topography | last = Statler | first = Thomas S. | date = 2009-03-05 |arxiv = 0903.1119 | doi=10.1016/j.icarus.2009.03.003 | bibcode=2009Icar..202..502S | volume=202 | issue = 2 | journal=Icarus | pages=502–513| s2cid = 18935544 }}
* {{cite journal | last1 = Vokrouhlicky | first1 = David | author-link2 = William F. Bottke | last2 = Bottke | first2 = William F. | year = 2012| title = Yarkovsky and YORP effects| journal = [[Scholarpedia]] | volume = 7 | issue = 5| page = 10599 | doi = 10.2458/azu_uapress_9780816532131-ch027 |bibcode = 2012SchpJ...710599B | arxiv = 1502.01249 | doi-access = free }}

== External links ==
* {{Cite news | title = Asteroid Spin Changed by Sunlight | first = Irene | last = Klotz | date = 2007-03-07 | publisher = [[Discovery Communications|Discovery Communications, LLC.]] | url = http://dsc.discovery.com/news/2007/03/07/asteroidspin_spa.html?category=space&guid=20070307130000&dcitc=w19-502-ak-0000 | archive-url = https://web.archive.org/web/20080426041544/http://dsc.discovery.com/news/2007/03/07/asteroidspin_spa.html?category=space&guid=20070307130000&dcitc=w19-502-ak-0000 | url-status = dead | archive-date = 2008-04-26}}
* [http://www.sunflower-astronomy.com/KCKCC_Docs/NewsArticles/Asteroid_Rotation_Discovery_Reported.pdf Asteroid rotation discovery reported]

[[Category:Asteroids]]
[[Category:Orbital perturbations]]
[[Category:Radiation effects]]
[[Category:Rotation]]