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Vulcanoids are a hypothetical group of 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 nineteenth-century astronomers fruitlessly searched for to explain the excess precession of Mercury's perihelion. Virtually all 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.

The hypothesis that there could be such a population of asteroids was put forward by Charles Dillon Perrine, one of the most active observational astronomers of that time. But despite more than a century of searching since the 1901 eclipse, no Vulcanoids have ever been found, despite ground-based searches and more recent searches by NASA using high-altitude F-18 aircraft and Black Brant suborbital rockets. Such searches are extremely difficult due to the glare of the Sun. Additionally, the space-based Solar and Heliospheric Observatory (SOHO) would have been able to see any bright objects near the Sun (for example, it has seen hundreds of small comets). If Vulcanoids exist, for the expected albedo they cannot be more than 60 km in diameter, since previous searches would have found anything larger. Vulcanoid asteroids, if they exist, would be a special subclass of Apohele asteroids.

Nevertheless, it is thought Vulcanoids could exist because the region of space being searched is gravitationally stable. Also, the heavily cratered surface of Mercury means a population of Vulcanoids probably existed in the very early days of the solar system.

However, one theory argues that, over long timescales, the orbits of Vulcanoids are not completely stable, due to the Yarkovsky effect. The dynamical lifetime of a Vulcanoid is measured in tens of millions of years, and according to David Vokrouhlicky, Paolo Farinella and William F. Bottke, Jr. the vulcanoid population would have been depleted within a billion years of the formation of Mercury. According to their 2000 paper, there should be no original Vulcanoids left, having either impacted Mercury or been drawn into the Sun. They also argue that the current data on bombardment indicate that Mercury, to the extent it is mapped, does not show a signature pattern of cratering which would indicate that Vulcanoids ever existed. However, they speculate that a small population could exist within available measurements, though better data of the Mercurian surface would be required to establish this.

Future searches for Vulcanoids will likely use small space-based telescopes, which can see very close to the Sun. SOHO is not the best instrument for the task, but suitable spacecraft have been proposed to look for near-Earth objects. Indirect evidence for Vulcanoids may come from the MESSENGER probe, which is scheduled to fly by Mercury three times and then insert into a planetary orbit.

The entire inner solar system is not well explored, with only one probe having visited Mercury: Mariner 10, which managed to photograph 45% of the planet in a series of fly-bys in 1973 and 1974.

• Britt, Robert R. (Jan. 26, 2004) Elusive Vulcanoids: Search Reaches New Heights
• David Vokrouhlicky, Paolo Farinella and William F. Bottke, Jr., The Depletion of the Putative Vulcanoid Population via the Yarkovsky Effect. Icarus 148 147-152 (2000)