Orbit
A vulcanoid is an asteroid in a stable orbit with a semi-major axis less than that of Mercury (i.e. 0.387 AU). This does not include objects like sungrazing comets which, although they have a perihelion inside the orbit of Mercury, have a far greater semi-major axis.
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. 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 and the Yarkovsky effect 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. 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 outer edge of the vulcanoid zone is approximately 0.21 AU from the Sun. More distant objects are unstable due to the gravitational influence of Mercury and would be perturbed into Mercury-crossing orbits on timescales of the order of 100 million years. The inner edge is not sharply defined: objects closer than 0.06 AU are highly susceptible to Poynting–Robertson drag and the Yarkovsky effect, and even out to 0.09 AU vulcanoids would have temperatures of 1,000 K or more, which is hot enough for evaporation of rocks to be the limiting factor in their lifetime.
The volume of the vulcanoid zone is very small compared to that of the asteroid belt. 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. Vulcanoids are unlikely to have inclinations of more than about 10° to the ecliptic. Mercury trojans, asteroids trapped in Mercury's Lagrange points, are also possible.
Read more about this topic: Vulcanoid Asteroid
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