Asteroid Day (also known as International Asteroid Day) is an annual global event which is held on the anniversary of the Siberian Tunguska event that took place on June 30, 1908, the most harmful known asteroid-related event on Earth in recent history. The United Nations has proclaimed it be observed globally on June 30 every year in its resolution with an aim to raise awareness about asteroids and what can be done to protect the Earth, its families, communities, and future generations from a catastrophic event.
Asteroids are minor planets, small pieces of rocks, especially of the inner Solar System. Larger asteroids have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered they were typically found to have volatile-rich surfaces similar to comets. As a result, they were often distinguished from objects found in the main asteroid belt.
The first asteroid, Ceres, was discovered accidentally in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. Later that year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, which was found to be between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.
It is thought that planetesimals in the asteroid belt evolved much like the rest of the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.
Generally, there are three main types of asteroids:
Dark C (carbonaceous) asteroids, which make up most asteroids and are in the outer belt. They includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles.
Bright S (silicaceous) asteroids and are in the inner belt, closer to Mars. They are relatively bright (albedo -10.22) and are believed to consist of metallic nickel-iron mixed with iron- and magnesium-silicates.
Bright M (metallic) asteroids. They sit in the middle of the asteroid belt and are mostly made up of metallic iron and nickel and have a bright (albedo -10.18).
There are also D type, known as the Trojan asteroids of Jupiter and are dark and carbonaceous in nature, and V type that are distant asteroids between the orbits of Jupiter and Uranus, and they may have originated in the Kuiper Belt. While these have not been studied extensively, it has been suggested that they have a composition of organic-rich silicates, carbon and anhydrous silicates, possibly with water ice in their interiors.
Asteroids are also categorized by their position in the solar system:
Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group).
Near-Earth Asteroids (NEAs): ones that closely approach the Earth.
Atens: semi-major axes less than 1.0 AU and aphelion distances greater than 0.983 AU;
Apollos: semi-major axes greater than 1.0 AU and perihelion distances less than 1.017 AU;
Amors: perihelion distances between 1.017 and 1.3 AU;
Trojans: located near Jupiter's Langrange points (60 degrees ahead and behind Jupiter in its orbit). Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether. Curiously, there are many more in the leading Lagrange point (L4) than in the trailing one (L5).
(There may also be a few small asteroids in the Lagrange points of Venus and Earth that are also sometimes known as Trojans; 5261 Eureka is a "Mars Trojan".)
International Asteroid Search Campaign (IASC) organise annual global asteroid hunting campaign where students analyse CCD images using Astrometrica and hunt for asteroids.
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