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 hypothetical population of asteroids that orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the 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 and 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 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 sunspots. 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 Lescarbault claimed to have seen Le Verrier's proposed planet transit the Sun. The new planet was quickly named Vulcan but was never seen again, and the anomalous behaviour of Mercury's orbit was explained by Einstein's general theory of relativity in 1915. The vulcanoids take their name from this hypothetical planet. What Lescarbault saw was probably another sunspot.
left|thumb|270px|[[Total solar eclipses 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, and ground-based searches can only be carried out during twilight or during solar eclipses. which did not reveal any vulcanoids, and observations during eclipses remain a common search method. 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 and made observations with the Southwest Universal Imaging System—Airborne (SWUIS-A). This does not include objects like sungrazing comets, which, although they have perihelia inside the orbit of Mercury, have far greater semi-major axes. All other similarly stable regions in the Solar System have been found to contain objects, although non-gravitational forces such as radiation pressure, Poynting–Robertson drag may have depleted the vulcanoid area of its original contents. There may be no more than 300–900 vulcanoids larger than in radius remaining, if any. 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. 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. The inner edge is not sharply defined: objects closer than 0.06 AU are particularly susceptible to Poynting–Robertson drag and the Yarkovsky effect,
The maximum possible volume of the vulcanoid zone is very small compared to that of the asteroid belt. Vulcanoids are unlikely to have inclinations of more than about 10° to the ecliptic.
Physical characteristics
Any vulcanoids that exist must be relatively small. Previous searches, particularly from the STEREO spacecraft, rule out asteroids larger than in diameter. They would be almost hot enough to glow red hot.
There is evidence that Mercury was struck by a large object relatively late in its development, and explaining the thinness of Mercury's mantle compared to the mantles of the other terrestrial planets. If such an impact occurred, much of the resulting debris might still be orbiting the Sun in the vulcanoid zone.
Significance
Vulcanoids, being an entirely new class of celestial bodies, would be interesting in their own right,