The Vulcanoids are a postulated asteroid group that may orbit in a gravitationally stable region between 0.08 and 0.21 astronomical units (Sun-to-Earth lengths) from the Sun. By comparison, the planet Mercury orbits the Sun at a distance of between 0.3 and 0.46 astronomical units.
Confirming or disconfirming the existence of the Vulcanoids is very difficult because of the Sun's glare. Astronomers have been looking for the Vulcanoids since an eclipse in 1901, but have yet to have any luck. Vulcanoids are named after Vulcan, a hypothetical planet postulated to explain unusual motions in Mercury's orbit. These variations were later explained by Einstein's theory of general relativity, eliminating the need for a Vulcan.
There are various reasons why we should expect the Vulcanoid asteroids to exist. Other dynamically stable regions of the solar system, such as the asteroid belt and the Kuiper Belt, contain numerous asteroids, making little reason to expect the Vulcanoid belt to be empty. Mercury has numerous surface scars, also boosting the likelihood that Vulcanoids exist. But some scientists have argued that all Vulcanoids have already impacted Mercury or fallen into the Sun. Some models of the solar system show all Vulcanoids disappearing more than a billion years ago, while others predict that they still exist. The matter will not be settled until we increase our ability to observe the inner solar system, preferably by sending more robotic probes.
Just like the human eye, many of the world's best observatories contain delicate optical equipment that would be incinerated if exposed directly to the Sun's rays. This means that special telescopes and equipment must be used to observe the extremely luminous region around the Sun. Recent efforts have focused on cameras mounted on sub-orbital spaceplanes.
The inner solar system is relatively poorly explored. Manned trips to this region in the near future are out of the question due to the extreme heat and radiation in the inner reaches of the solar system. It will likely take a sophisticated probe, or much better telescopes, to confirm or disconfirm the existence of the Vulcanoids for good.