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==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 = 436β443 | 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 = 144β149 | 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—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.
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