Thevulcanoids are ahypothetical population ofasteroids that orbit theSun in a dynamically stable zone inside the orbit of the planetMercury. They are named after the hypothetical planetVulcan, which was proposed on the basis of irregularities in Mercury's orbit that were later found to be explained bygeneral 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 100 metres (330 ft) and 6 kilometres (3.7 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 fromAtira 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 ofplanet formation, as well as insights into the conditions prevalent in the earlySolar System. Although every other gravitationally stable region in the Solar System has been found to contain objects, non-gravitational forces (such as theYarkovsky effect) or the influence of amigrating planet in the early stages of the Solar System's development may have depleted this area of any asteroids that may have been there.
Celestial bodies interior to the orbit of Mercury have been hypothesized, and searched for, for centuries. The German astronomerChristoph Scheiner thought he had seen small bodies passing in front of the Sun in 1611, but these were later shown to besunspots.[1] In the 1850s,Urbain Le Verrier made detailed calculations of Mercury's orbit and found a small discrepancy in the planet'sperihelion 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 namedEdmond Lescarbault claimed to have seen Le Verrier's proposed planettransit the Sun. The new planet was quickly namedVulcan but was never seen again, and the anomalous behaviour of Mercury's orbit was explained byEinstein'sgeneral theory of relativity in 1915. The vulcanoids take their name from this hypothetical planet.[2] What Lescarbault saw was probably another sunspot.[3]
Vulcanoids, should they exist, would be difficult to detect due to the strong glare of the nearby Sun,[4] and ground-based searches can only be carried out during twilight or duringsolar eclipses.[5] Several searches during eclipses were conducted in the early 1900s,[6] which did not reveal any vulcanoids, and observations during eclipses remain a common search method.[7] Conventional telescopes cannot be used to search for them because the nearby Sun could damage their optics.[8]
In 1998, astronomers analysed data from theSOHO spacecraft'sLASCO instrument, which is a set of threecoronagraphs. The data taken between January and May of that year did not show any vulcanoids brighter thanmagnitude 7. This corresponds to a diameter of about 60 kilometres (37 mi), assuming the asteroids have analbedo similar to that of Mercury. In particular, a large planetoid at a distance of 0.18 AU, predicted by the theory ofscale relativity, was ruled out.[9]
Later attempts to detect the vulcanoids involved taking astronomical equipment above the interference ofEarth's atmosphere, to heights where the twilight sky is darker and clearer than on the ground.[10] In 2000, planetary scientistAlan Stern performed surveys of the vulcanoid zone using aLockheed U-2 spy plane. The flights were conducted at a height of 21,300 metres (69,900 ft) during twilight.[11] In 2002, he andDan Durda performed similar observations on anF-18 fighter jet. They made three flights over theMojave Desert at an altitude of 15,000 metres (49,000 ft) and made observations with the Southwest Universal Imaging System—Airborne (SWUIS-A).[12]
Even at these heights the atmosphere is still present and can interfere with searches for vulcanoids. In 2004, asub-orbital spaceflight was attempted in order to get a camera above Earth's atmosphere. ABlack Brant rocket was launched fromWhite Sands, New Mexico, on January 16, carrying a powerful camera named VulCam,[13] on a ten-minute flight.[4] This flight reached an altitude of 274,000 metres (899,000 ft)[13] and took over 50,000 images. None of the images revealed any vulcanoids, but there were technical problems.[4]
Searches of NASA's twoSTEREO spacecraft data have failed to detect any vulcanoid asteroids.[14] It is doubtful that there are any vulcanoids larger than 5.7 kilometres (3.5 mi) in diameter.[14]
TheMESSENGERspace 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.[15][16] Before its demise in 2015, however, the craft failed to produce substantial evidence on vulcanoids.
A vulcanoid is an asteroid in a stable orbit with asemi-major axis less than that of Mercury (i.e. 0.387 AU).[7][17] This does not include objects likesungrazing comets, which, although they haveperihelia inside the orbit of Mercury, have far greater semi-major axes.[7]
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 theSun.[18] All other similarly stable regions in theSolar System have been found to contain objects,[8] although non-gravitational forces such asradiation pressure,[9]Poynting–Robertson drag[18] and theYarkovsky effect[5] may have depleted the vulcanoid area of its original contents. There may be no more than 300–900 vulcanoids larger than 1 kilometre (0.62 mi) in radius remaining, if any.[19] A 2020 study found that theYarkovsky–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.[20] 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 theKuiper belt.[18]
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 intoMercury-crossing orbits on timescales of the order of 100 million years.[18] (Some definitions would nonetheless include such unstable objects as vulcanoids as long as their current orbits lie completely interior to that of Mercury.)[21] The inner edge is not sharply defined: objects closer than 0.06 AU are particularly susceptible to Poynting–Robertson drag and the Yarkovsky effect,[18] and even out to 0.09 AU vulcanoids would have temperatures of 1,000 K or more, which is hot enough for evaporation ofrocks to become the limiting factor in their lifetime.[22]
The maximum possible volume of the vulcanoid zone is very small compared to that of theasteroid belt.[22] 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.[23] Vulcanoids are unlikely to haveinclinations of more than about 10° to theecliptic.[7][18] Mercurytrojans, asteroids trapped in Mercury'sLagrange points, are also possible.[24]
Any vulcanoids that exist must be relatively small. Previous searches, particularly from theSTEREO spacecraft, rule out asteroids larger than 6 kilometres (3.7 mi) in diameter.[14] The minimum size is about 100 metres (330 ft);[18] particles smaller than 0.2 μm are strongly repulsed by radiation pressure, and objects smaller than 70 m would be drawn into the Sun byPoynting–Robertson drag.[9] Between these upper and lower limits, a population of asteroids between 1 kilometre (0.62 mi) and 6 kilometres (3.7 mi) in diameter is thought to be possible.[10] They would be almost hot enough to glow red hot.[17]
It is thought that the vulcanoids would be very rich inelements with a highmelting point, such asiron andnickel. They are unlikely to possess aregolith because such fragmented material heats and cools more rapidly, and is affected more strongly by theYarkovsky effect, than solid rock.[5] Vulcanoids are probably similar to Mercury in colour and albedo,[7] and may contain material left over from the earliest stages of the Solar System's formation.[12]
There is evidence that Mercury was struck by a large object relatively late in its development,[5] a collision which stripped away much of Mercury's crust and mantle,[16] and explaining the thinness of Mercury'smantle compared to the mantles of the otherterrestrial planets. If such an impact occurred, much of the resulting debris might still be orbiting the Sun in the vulcanoid zone.[13]
Vulcanoids, being an entirely new class of celestial bodies, would be interesting in their own right,[24] but discovering whether or not they exist would yield insights into theformation and evolution of the Solar System. If they exist they might contain material left over from the earliest period of planet formation,[12] and help determine the conditions under which theterrestrial planets, particularly Mercury, formed.[24] 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,[16] making that planet's surface appear older than it actually is.[24] If vulcanoids are found not to exist, this would place different constraints on planet formation[24] and suggest that other processes have been at work in the inner Solar System, such asplanetary migration clearing out the area.[18]
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.
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