Vulcanoid Asteroid - History and Observation

History and Observation

Celestial bodies interior to the orbit of Mercury have been hypothesized, and searched for, for centuries. The German astronomer Christoph Scheiner believed he had seen small bodies passing in front of the Sun in 1611, but these were later shown to be sunspots. In the 1850s, Urbain Le Verrier made detailed calculations of Mercury's orbit and found a small discrepancy in the planet's perihelion precession from predicted values. He postulated that the gravitational influence of a small planet or ring of asteroids within the orbit of Mercury would explain the deviation. Shortly afterward, an amateur astronomer named Edmond Lescarbault claimed to have seen Le Verrier's proposed planet transit the Sun. The new planet was quickly named Vulcan but was never seen again, and the anomalous behaviour of Mercury's orbit was explained by Einstein's General theory of relativity in 1915. The vulcanoids take their name from this hypothetical planet. What Lescarbault saw was probably another sunspot.

Vulcanoids, should they exist, would be difficult to detect due to the strong glare of the nearby Sun, and ground-based searches can only be carried out during twilight or during solar eclipses. Several searches during eclipses were conducted in the early 1900s, which did not reveal any vulcanoids, and observations during eclipses remain a common search method. Conventional telescopes cannot be used to search for them because the nearby Sun could damage their optics.

In 1998, astronomers analysed data from the SOHO spacecraft's LASCO instrument, which is a set of three coronagraphs. The data taken between January and May of that year did not show any vulcanoids brighter than magnitude 7. This corresponds to a diameter of about 60 kilometres (37 mi), assuming the asteroids have an albedo similar to that of Mercury. In particular a large planetoid at a distance of 0.18AU, predicted by the theory of Scale relativity, was ruled out.

Later attempts to detect the vulcanoids involved taking astronomical equipment above the interference of Earth's atmosphere, to heights where the twilight sky is darker and clearer than on the ground. In 2000, planetary scientist Alan Stern performed surveys of the vulcanoid zone using a Lockheed U-2 spy plane. The flights were conducted at a height of 21,300 metres (69,900 ft) during twilight. In 2002, he and Dan Durda performed similar observations on an F-18 fighter jet. They made three flights over the Mojave desert at an altitude of 15,000 metres (49,000 ft) and made observations with the Southwest Universal Imaging System—Airborne (SWUIS-A).

Even at these heights the atmosphere is still present and able to interfere with vulcanoid searches. In 2004, a sub-orbital spaceflight was attempted in order to get a camera above Earth's atmosphere. A Black Brant rocket was launched from White Sands, New Mexico, on January 16, carrying a powerful camera named VulCam, on a ten-minute flight. This flight reached an altitude of 274,000 metres (899,000 ft) and took over 50,000 images. Due to technical problems, none of the images were able to reveal any vulcanoids.

The MESSENGER space probe may provide evidence regarding vulcanoids. Its opportunities will be limited because its instruments need to be pointed away from the Sun at all times to avoid damage. The spacecraft has already taken a few of a planned series of images of the outer regions of the vulcanoid zone.