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Vulcanoid
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==Orbit== A vulcanoid is an asteroid in a stable orbit with a [[semi-major axis]] less than that of Mercury (i.e. 0.387 [[Astronomical unit|AU]]).<ref name="VulcFAQ">{{cite web|title=FAQ: Vulcanoid Asteroids |publisher=vulcanoid.org |date=2005 |url=http://www.vulcanoid.org/faq |access-date=2008-12-27 |url-status=dead |archive-url=https://web.archive.org/web/20080724162509/http://www.vulcanoid.org/faq |archive-date=July 24, 2008 }}</ref><ref name="Noll">{{cite web|last = Noll | first = Landon Curt | title = Vulcanoid Search during a Solar eclipse|date=2007|url = http://www.paulnoll.com/China/Eclipse/eclipse-vulcanoid-scenes.html | access-date = 2008-12-24}}</ref> This does not include objects like [[sungrazing comet]]s, which, although they have [[perihelion|perihelia]] inside the orbit of Mercury, have far greater semi-major axes.<ref name="VulcFAQ" /> The vulcanoids are thought to exist in a gravitationally stable band inside the orbit of Mercury, at distances of 0.06–0.21 AU from the [[Sun]].<ref name="Evans">{{cite journal|author=Evans, N. Wyn|author2=Tabachnik, Serge |title=Possible Long-Lived Asteroid Belts in the Inner Solar System|journal=Nature|volume=399|issue=6731|pages=41–43|date=1999|arxiv=astro-ph/9905067|doi=10.1038/19919|bibcode = 1999Natur.399...41E |s2cid=4418335 }}</ref> All other similarly stable regions in the [[Solar System]] have been found to contain objects,<ref name="Britt">{{cite news | last = Britt | first = Robert Roy | title = Vulcanoid search reaches new heights | work = Space.com| date=2004| url = http://www.nbcnews.com/id/4066544 | archive-url = https://web.archive.org/web/20151019112317/http://www.nbcnews.com/id/4066544/ | url-status = dead | archive-date = October 19, 2015 | access-date = 2008-12-25}}</ref> although non-gravitational forces such as [[radiation pressure]],<ref name="Schumacher">{{Cite journal| last1 = Schumacher| first1 = G.| last2 = Gay | first2 = J.| title = An Attempt to detect Vulcanoids with SOHO/LASCO images | journal = Astronomy & Astrophysics | volume = 368| issue = 3 | pages = 1108–1114 | date = 2001 | doi = 10.1051/0004-6361:20000356 | bibcode=2001A&A...368.1108S| doi-access = free}}</ref> [[Poynting–Robertson effect|Poynting–Robertson drag]]<ref name="Evans" /> and the [[Yarkovsky effect]]<ref name="Roach">{{cite news | last = Roach| first = John| title = Fighter Jet Hunts for "Vulcanoid" Asteroids| publisher = [[National Geographic Society|National Geographic]] News| date=2002 | url = http://news.nationalgeographic.com/news/2002/04/0430_020506_vulcanoids.html | archive-url = https://web.archive.org/web/20020508112241/http://news.nationalgeographic.com/news/2002/04/0430_020506_vulcanoids.html | url-status = dead | archive-date = May 8, 2002 | access-date = 2008-12-24}}</ref> may have depleted the vulcanoid area of its original contents. There may be no more than 300–900 vulcanoids larger than {{convert|1|km|mi}} in radius remaining, if any.<ref name="Vokro">{{Cite journal | last1 = Vokrouhlický | first1 = David | last2 = Farinella | first2 = Paolo | last3 = Bottke| first3 = William F. Jr. | title = The Depletion of the Putative Vulcanoid Population via the Yarkovsky Effect| journal = Icarus| volume = 148| issue = 1| pages = 147–152| date = 2000| doi = 10.1006/icar.2000.6468 | bibcode=2000Icar..148..147V| s2cid = 55356387 }}</ref> A 2020 study found that the [[Yarkovsky–O'Keefe–Radzievskii–Paddack effect]] is strong enough to destroy hypothetical vulcanoids as large as 100 km in radius on timescales far smaller than the age of the Solar System; would-be vulcanoid asteroids were found to be steadily spun up by the YORP effect until they rotationally fission into smaller bodies, which occurs repeatedly until the debris is small enough to be pushed out of the vulcanoid region by the Yarkovsky effect; this would explain why no vulcanoids have been observed.<ref>{{Cite journal|url=https://ui.adsabs.harvard.edu/abs/2020AAS...23527701C/abstract|bibcode = 2020AAS...23527701C|title = The YORP Effect Can Efficiently Destroy 100 Kilometer Planetesimals at the Inner Edge of the Solar System|last1 = Collins|first1 = M. D.|journal = American Astronomical Society Meeting Abstracts #235|year = 2020|volume = 235|pages = 277.01}}</ref> The gravitational stability of the vulcanoid zone is due in part to the fact that there is only one neighbouring planet. In that respect it can be compared to the [[Kuiper belt]].<ref name="Evans" /> The outer edge of the vulcanoid zone is approximately 0.21 AU from the Sun. Objects more distant than this are unstable due to interactions with Mercury and would be perturbed into [[List of Mercury-crossing minor planets|Mercury-crossing]] orbits on timescales of the order of 100 million years.<ref name="Evans" /> (Some definitions would nonetheless include such unstable objects as vulcanoids as long as their orbits lie completely interior to that of Mercury.)<ref name="Greenstreet2012">{{cite journal |title = The orbital distribution of Near-Earth Objects inside Earth's orbit |url = https://static1.squarespace.com/static/5743c691d51cd42eed1e15ea/t/57451dc89f72665be88257b5/1464147402569/Greenstreetetal2012_NEOSSat1model.pdf |first1 = Sarah |last1 = Greenstreet |first2 = Henry |last2 = Ngo |first3 = Brett |last3 = Gladman |date = January 2012 |journal = Icarus |volume = 217 |issue = 1 |pages = 355–366 |doi = 10.1016/j.icarus.2011.11.010 |bibcode = 2012Icar..217..355G |hdl = 2429/37251 |quote = The existence of a non-negligible population of Venus-decoupled Vatiras thus begs the question as to whether any objects reach orbits entirely interior to that of Mercury. Accepted convention would likely to be to call such an object a Vulcanoid, although the term is usually intended to mean an object which has been resident inside Mercury for the entire lifetime of the Solar System.|hdl-access= free }}</ref> The inner edge is not sharply defined: objects closer than 0.06 AU are particularly susceptible to Poynting–Robertson drag and the Yarkovsky effect,<ref name="Evans" /> and even out to 0.09 AU vulcanoids would have temperatures of 1,000 [[Kelvin|K]] or more, which is hot enough for evaporation of [[rock (geology)|rock]]s to become the limiting factor in their lifetime.<ref name="Lewis">{{cite book | last = Lewis | first = John S. | title = Physics and Chemistry of the Solar System | publisher = Academic Press| date= 2004| page = 409| url = https://books.google.com/books?id=uY79k7Nx-egC&pg=PA409| isbn = 978-0-12-446744-6}}</ref> The maximum possible volume of the vulcanoid zone is very small compared to that of the [[asteroid belt]].<ref name=Lewis/> Collisions between objects in the vulcanoid zone would be frequent and highly energetic, tending to lead to the destruction of the objects. The most favourable location for vulcanoids is probably in circular orbits near the outer edge of the vulcanoid zone.<ref name=DurdaStern>{{cite journal | author = Stern, S.A. |author2=Durda, D.D. | title = Collisional Evolution in the Vulcanoid Region: Implications for Present-Day Population Constraints | journal = Icarus |volume=143 |issue=2 |page=360 |date=2000 |doi=10.1006/icar.1999.6263 | bibcode=2000Icar..143..360S|arxiv = astro-ph/9911249 |s2cid=11176435 }}</ref> Vulcanoids are unlikely to have [[inclination]]s of more than about 10° to the [[ecliptic]].<ref name=VulcFAQ/><ref name=Evans/> Mercury [[Trojan (astronomy)|trojans]], asteroids trapped in Mercury's [[Lagrange point]]s, are also possible.<ref name=Campins>{{cite journal|last1=Campins|first1=H.|last2=Davis|first2=D. R.|last3=Weidenschilling|first3=S. J.|last4=Magee|first4=M.|title=Searching for Vulcanoids|journal=Completing the Inventory of the Solar System, Astronomical Society of the Pacific Conference Proceedings |volume=107| pages=85–96| date=1996| bibcode=1996ASPC..107...85C}}</ref>
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