Vulcanoid asteroid and Sun. Artwork of an asteroid that is circling the Sun within the orbit of the innermost planet Mercury. Known as vulcanoids, these are a hypothetical group of asteroids that would have a gravitationally stable orbit between 0.08 and 0.21 AU (astronomical units), equivalent to 12-30 million kilometres. Because such asteroids are so close to the Sun, observation to confirm the theory is difficult. If such asteroids do exist, the intense heat will have evaporated much of the material, leaving concentrated deposits of metals that could be used in future commercial space mining.