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{{Short description|Hypothetical asteroids orbiting the Sun}}
{{For|similar sounding terms|Vulcanite (disambiguation)}}
[[File:vulcanoidorbits.svg|right|thumb|270px|The zone, represented by the orange region, in which vulcanoids may exist, compared with the orbits of [[Mercury (planet)|Mercury]], [[Venus]] and [[Earth]]]]
The '''vulcanoids''' are a [[List of hypothetical Solar System objects|hypothetical]] population of [[asteroid]]s that orbit the [[Sun]] in a dynamically stable zone inside the orbit of the planet [[Mercury (planet)|Mercury]]. They are named after the hypothetical planet [[Vulcan (hypothetical planet)|Vulcan]], which was proposed on the basis of irregularities in Mercury's orbit that were later found to be explained by [[general relativity]]. So far, no vulcanoids have been discovered, and it is not yet clear whether any exist.

If they do exist, the vulcanoids could easily evade detection because they would be very small and near the bright glare of the Sun. Due to their proximity to the Sun, searches from the ground can only be carried out during twilight or solar eclipses. Any vulcanoids must be between about {{convert|100|m|ft}} and {{convert|6|km|mi}} in diameter and are probably located in nearly circular orbits near the outer edge of the gravitationally stable zone between the Sun and Mercury. These should be distinguished from [[Atira asteroids]], which may have perihelia within the orbit of Mercury, but whose aphelia extends as far as the orbits of Venus or within Earth's orbital path. Because they cross the orbit of Mercury, these bodies are not classed as vulcanoids.

The vulcanoids, should they be found, may provide scientists with material from the first period of [[planet formation]], as well as insights into the conditions prevalent in the early [[Solar System]]. Although every other gravitationally stable region in the Solar System has been found to contain objects, non-gravitational forces (such as the [[Yarkovsky effect]]) or the influence of a [[planetary migration|migrating planet]] in the early stages of the Solar System's development may have depleted this area of any asteroids that may have been there.

==History and observation==

Celestial bodies interior to the orbit of Mercury have been hypothesized, and searched for, for centuries. The German astronomer [[Christoph Scheiner]] thought he had seen small bodies passing in front of the Sun in 1611, but these were later shown to be [[sunspot]]s.<ref>{{cite journal | last = Drobyshevskii | first = E. M. | title = Impact Avalanche Ejection of Silicates from Mercury and the Evolution of the Mercury / Venus System | journal = Soviet Astr | volume = 36 | issue = 4 | pages = 436–443 | date=1992| bibcode = 1992SvA....36..436D}}</ref> In the 1850s, [[Urbain Le Verrier]] made detailed calculations of Mercury's orbit and found a small discrepancy in the planet's [[perihelion precession]] from predicted values. He postulated that the gravitational influence of a small planet or ring of asteroids within the orbit of Mercury would explain the deviation. Shortly afterward, an amateur astronomer named [[Edmond Modeste Lescarbault|Edmond Lescarbault]] claimed to have seen Le Verrier's proposed planet [[Astronomical transit|transit]] the Sun. The new planet was quickly named [[Vulcan (hypothetical planet)|Vulcan]] but was never seen again, and the anomalous behaviour of Mercury's orbit was explained by [[Albert Einstein|Einstein]]'s [[general relativity|general theory of relativity]] in 1915. The vulcanoids take their name from this hypothetical planet.<ref>{{cite book  | last = Standage  | first = Tom  | author-link = Tom Standage  | title = The Neptune File  | publisher = Allen Lane, The Penguin Press  |date=2000| location = Harmondsworth, Middlesex, England  | pages = 144–149  | isbn = 0-7139-9472-X}}</ref> What Lescarbault saw was probably another sunspot.<ref>{{cite book | last = Miller | first = Ron | title = Extrasolar Planets | publisher = Twenty-First Century Books |date=2002| page = 14 | url = https://books.google.com/books?id=6AHX9kKEEw8C&pg=PA1 | isbn = 978-0-7613-2354-9 }}</ref>

[[File:Zatm lagan.jpg|left|thumb|270px|[[Total solar eclipse]]s provide an opportunity to search for vulcanoids from the ground.]]

Vulcanoids, should they exist, would be difficult to detect due to the strong glare of the nearby Sun,<ref name="PlanSoc">{{cite web | title = Vulcanoids | publisher = The Planetary Society | url = http://www.planetary.org/programs/projects/vulcanoids/ | access-date = 2008-12-25 | url-status = dead | archive-url = https://web.archive.org/web/20090108102412/http://planetary.org./programs/projects/vulcanoids/ | archive-date = 2009-01-08 }}</ref> and ground-based searches can only be carried out during twilight or during [[solar eclipse]]s.<ref name="Roach" /> Several searches during eclipses were conducted in the early 1900s,<ref name="Campbell">{{cite journal | last = Campbell | first = W.W. |author2=Trumpler, R. | title = Search for Intramercurial Objects | journal = Publications of the Astronomical Society of the Pacific | volume = 35 | issue = 206 | pages = 214 |date=1923 | bibcode = 1923PASP...35..214C | doi =  10.1086/123310| s2cid = 122872992 }}</ref> which did not reveal any vulcanoids, and observations during eclipses remain a common search method.<ref name="VulcFAQ" /> Conventional telescopes cannot be used to search for them because the nearby Sun could damage their optics.<ref name="Britt" />

In 1998, astronomers analysed data from the [[Solar and Heliospheric Observatory|SOHO]] spacecraft's [[Large Angle and Spectrometric Coronagraph|LASCO]] instrument, which is a set of three [[coronagraph]]s. The data taken between January and May of that year did not show any vulcanoids brighter than [[Apparent magnitude|magnitude]] 7. This corresponds to a diameter of about {{convert|60|km|mi}}, assuming the asteroids have an [[albedo]] similar to that of Mercury. In particular, a large planetoid at a distance of 0.18 AU, predicted by the theory of [[scale relativity]], was ruled out.<ref name="Schumacher" />

Later attempts to detect the vulcanoids involved taking astronomical equipment above the interference of [[Atmosphere of Earth|Earth's atmosphere]], to heights where the twilight sky is darker and clearer than on the ground.<ref name="Whitehouse" /> In 2000, planetary scientist [[Alan Stern]] performed surveys of the vulcanoid zone using a [[Lockheed U-2]] spy plane. The flights were conducted at a height of {{convert|21300|m|ft}} during twilight.<ref name="David">{{cite news|last=David |first=Leonard |title=Astronomers Eye 'Twilight Zone' Search for Vulcanoids |work=Space.com |year=2000 |url=http://www.space.com/news/chasing_asteroids_000124.html |access-date=2008-12-25 |url-status=dead |archive-url=https://web.archive.org/web/20080724132657/http://www.space.com/news/chasing_asteroids_000124.html |archive-date=July 24, 2008 }}</ref> In 2002, he and [[Dan Durda]] performed similar observations on an [[F-18]] fighter jet. They made three flights over the [[Mojave Desert]] at an altitude of {{convert|15000|m|ft}} and made observations with the Southwest Universal Imaging System&mdash;Airborne (SWUIS-A).<ref name="Dryden" />

Even at these heights the atmosphere is still present and can interfere with searches for vulcanoids. In 2004, a [[sub-orbital spaceflight]] was attempted in order to get a camera above Earth's atmosphere. A [[Black Brant (rocket)|Black Brant]] rocket was launched from [[White Sands, New Mexico]], on January 16, carrying a powerful camera named VulCam,<ref name="Alexander" /> on a ten-minute flight.<ref name="PlanSoc" /> This flight reached an altitude of {{convert|274000|m|ft}}<ref name="Alexander" /> and took over 50,000 images. None of the images revealed any vulcanoids, but there were technical problems.<ref name="PlanSoc" />

Searches of NASA's two [[STEREO]] spacecraft data have failed to detect any vulcanoid asteroids.<ref name="Steffl2013" /> It is doubtful that there are any vulcanoids larger than {{convert|5.7|km}} in diameter.<ref name="Steffl2013" />

The ''[[MESSENGER]]'' [[space probe]] took a few images of the outer regions of the vulcanoid zone; however, its opportunities were limited because its instruments had to be pointed away from the Sun at all times to avoid damage.<ref name="Choi">{{cite news | last = Choi | first = Charles Q. | title = The Enduring Mysteries of Mercury| work = Space.com|date=2008| url = http://www.space.com/scienceastronomy/080114-mm-mystery-mercury.html| access-date = 2008-12-25}}</ref><ref name="Chapman" /> Before its demise in 2015, however, the craft failed to produce substantial evidence on vulcanoids.

==Orbit==

A vulcanoid is an asteroid in a stable orbit with a [[semi-major axis]] less than that of Mercury (i.e. 0.387&nbsp;[[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&ndash;0.21&nbsp;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&ndash;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&nbsp;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&nbsp;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&nbsp;AU are particularly susceptible to Poynting–Robertson drag and the Yarkovsky effect,<ref name="Evans" /> and even out to 0.09&nbsp;AU vulcanoids would have temperatures of 1,000&nbsp;[[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>

==Physical characteristics==

Any vulcanoids that exist must be relatively small. Previous searches, particularly from the [[STEREO]] spacecraft, rule out asteroids larger than {{convert|6|km|mi}} in diameter.<ref name=Steffl2013/> The minimum size is about {{convert|100|m|ft}};<ref name=Evans/> particles smaller than 0.2&nbsp;[[Micrometre|μm]] are strongly repulsed by radiation pressure, and objects smaller than 70&nbsp;m would be drawn into the Sun by [[Poynting–Robertson effect|Poynting–Robertson drag]].<ref name=Schumacher/> Between these upper and lower limits, a population of asteroids between {{convert|1|km|mi}} and {{convert|6|km|mi}} in diameter is thought to be possible.<ref name=Whitehouse>{{cite news | last = Whitehouse | first = David | title = Vulcan in the Twilight Zone | work = BBC News | url = http://news.bbc.co.uk/2/hi/science/nature/2063200.stm | access-date = 2008-12-25 | date=2002-06-27}}</ref> They would be almost hot enough to glow red hot.<ref name=Noll/>
 
It is thought that the vulcanoids would be very rich in [[Chemical element|elements]] with a high [[melting point]], such as [[iron]] and [[nickel]]. They are unlikely to possess a [[regolith]] because such fragmented material heats and cools more rapidly, and is affected more strongly by the [[Yarkovsky effect]], than solid rock.<ref name=Roach/> Vulcanoids are probably similar to Mercury in colour and albedo,<ref name=VulcFAQ/> and may contain material left over from the earliest stages of the Solar System's formation.<ref name=Dryden>{{cite news | title = NASA Dryden, Southwest Research Institute Search for Vulcanoids | publisher = NASA | date = 2002 | url = http://www.nasa.gov/centers/dryden/news/NewsReleases/2002/02-20.html | access-date = 2008-12-25 | archive-date = 2019-05-03 | archive-url = https://web.archive.org/web/20190503104116/https://www.nasa.gov/centers/dryden/news/NewsReleases/2002/02-20.html | url-status = dead }}</ref>

There is evidence that Mercury was struck by a large object relatively late in its development,<ref name=Roach/> a collision which stripped away much of Mercury's crust and mantle,<ref name=Chapman>{{citation | first = C.R.  | last =  Chapman |author2=Merline, W.J. |author3=Solomon, S.C. |author4=Head, J.W. III |author5=Strom, R.G.   | title = First ''MESSENGER'' Insights Concerning the Early Cratering History of Mercury | publisher = Lunar and Planetary Institute |date=2008| url = http://www.lpi.usra.edu/meetings/bombardment2008/pdf/3014.pdf | access-date= 2008-12-26}}</ref> and explaining the thinness of Mercury's [[mantle (geology)|mantle]] compared to the mantles of the other [[terrestrial planet]]s. If such an impact occurred, much of the resulting debris might still be orbiting the Sun in the vulcanoid zone.<ref name=Alexander>{{cite news| last = Alexander| first = Amir| title = Small, Faint, and Elusive: The Search for Vulcanoids| publisher = The Planetary Society| date = 2004| url = http://www.planetary.org/news/2004/0202_Small_Faint_and_Elusive_The_Search.html| access-date = 2008-12-25| url-status = dead| archive-url = https://web.archive.org/web/20081011231437/http://www.planetary.org/news/2004/0202_Small_Faint_and_Elusive_The_Search.html| archive-date = 2008-10-11}}</ref>

==Significance==

Vulcanoids, being an entirely new class of celestial bodies, would be interesting in their own right,<ref name=Campins/> but discovering whether or not they exist would yield insights into the [[formation and evolution of the Solar System]]. If they exist they might contain material left over from the earliest period of planet formation,<ref name=Dryden/> and help determine the conditions under which the [[terrestrial planets]], particularly Mercury, formed.<ref name=Campins/> In particular, if vulcanoids exist or did exist in the past, they would represent an additional population of impactors that have affected no other planet but Mercury,<ref name=Chapman/> making that planet's surface appear older than it actually is.<ref name=Campins/> If vulcanoids are found not to exist, this would place different constraints on planet formation<ref name=Campins/> and suggest that other processes have been at work in the inner Solar System, such as [[planetary migration]] clearing out the area.<ref name=Evans/>

==See also==
*[[594913 ꞌAylóꞌchaxnim]] (the only known asteroid always within Venus' orbit)
*{{mpl|2021 PH|27}}, an asteroid in the Atira group with the smallest semi-major axis among asteroids
*[[Atira asteroid]] (asteroids always within Earth's orbit)
*[[Groups of minor planets]]
*[[Kreutz sungrazer]]
*[[List of hypothetical Solar System objects]]
*[[List of Mercury-crossing minor planets]]
*[[Vulcan (hypothetical planet)]]

==References==
{{reflist|colwidth=30em
| refs =

<ref name=Steffl2013>{{cite journal
  |last=Steffl |first=A. J.
  |author2=Cunningham, N. J. |author3=Shinn, A. B. |author4= Stern, S. A. 
  |title=A Search for Vulcanoids with the STEREO Heliospheric Imager
  |journal=Icarus
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 }}</ref>
}}

{{Small Solar System bodies}}
{{Solar System}}
{{Asteroids}}

{{good article}}

[[Category:Hypothetical bodies of the Solar System]]
[[Category:Atira asteroids|*]]
[[Category:Mercury (planet)]]
[[Category:Solar System]]
[[Category:Hypothetical astronomical objects]]