Vulcanoid asteroid
Vulcanoids are hypothetical asteroids that may orbit in a dynamically stable zone between 0.08 and 0.21 astronomical units from the Sun, well within the orbit of Mercury. They take their name from the hypothetical planet Vulcan, which eighteenth-century astronomers fruitlessly searched for to explain the excess precession of Mercury's perihelion. The anomaly in Mercury's orbit later turned out to be an effect explained by general relativity, removing the need to postulate the existence of Vulcan. No Vulcanoids have ever been found, despite ground-based searches and more recent searches by NASA using high-altitude F-18 aircraft. Such searches are extremely difficult due to the glare of the Sun. If Vulcanoids exist, for the expected albedo they may be no more than 60 km in diameter, since previous searches would have found anything larger. Nevertheless, it is thought Vulcanoids could exist because the region of space being searched is gravitationally stable, and all similarly stable regions of the solar system have been found to contain objects. Also, the heavily cratered surface of Mercury means a population of Vulcanoids probably existed in the very early days of the solar system. Future searches for Vulcanoids may use Tier One's SpaceShipOne, the winner of the X-Prize competition. Vulcanoid asteroids, if they exist, would be a special subclass of Apohele asteroids.
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