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The vulcanoids are a hypothetical population of asteroids that orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, whose existence was disproven in 1915. So far, no vulcanoids have been discovered, and it is not yet clear if any exist. If they do exist, the vulcanoids could easily evade detection because they would be very small and near the bright glare of the Sun. Due to their proximity to the Sun, searches from the ground can only be carried out during twilight or solar eclipses. Any vulcanoids must be between about and in diameter and are probably located in nearly circular orbits near the outer edge of the gravitationally stable zone. The vulcanoids, should they be found, may provide scientists with material from the first period of planet formation, as well as insights into the conditions prevalent in the early Solar System. Although every other gravitationally stable region in the Solar System has been found to contain objects, non-gravitational forces (such as the Yarkovsky effect) or the influence of a migrating planet in the early stages of the Solar System's development may have depleted this area of any asteroids that may have been there. ==History and observation== Celestial bodies interior to the orbit of Mercury have been hypothesized, and searched for, for centuries. The German astronomer Christoph Scheiner thought 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,〔(【引用サイトリンク】 publisher = The Planetary Society )〕 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 , assuming the asteroids have an albedo similar to that of Mercury. In particular, a large planetoid at a distance of 0.18 AU, 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 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 and made observations with the Southwest Universal Imaging System—Airborne (SWUIS-A).〔 Even at these heights the atmosphere is still present and can interfere with searches for vulcanoids. 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 〔 and took over 50,000 images. None of the images revealed any vulcanoids, but there were technical problems.〔 Searches of NASA's two STEREO spacecraft data have failed to detect any vulcanoid asteroids.〔 It is doubtful that there are any vulcanoids larger than in diameter.〔 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vulcanoid」の詳細全文を読む スポンサード リンク
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