Vulcanoid asteroid

From Free net encyclopedia

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 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.

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.

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.

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.

Vulcanoid asteroids, if they exist, would be a special subclass of Apohele asteroids.

[edit]