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Asteroids or planetoids are small solid bodies in planetary systems. Compared to planets and dwarf planets, they are smaller and often of irregular shape. They were created from remains of planetary matter which wasn’t merged with planets during forming of the system from proto-planetary disk or with latter dynamic evolution of comets or some other objects. They most often circle around parent star on their own trajectory or as satellites of some bigger planets. Some of them we find tied with gravitational forces to planets, in groups that orbit the path of planets, in front or behind. Although until recently it was thought otherwise, it was discovered that asteroids can have their own satellites. Most asteroids in solar system are located in the belt between Mars and Jupiter, and in Kuiper belt. Until now over 170.000 is discovered and over 11.000 was named, apart from their catalogue number. It is estimated that in our solar system there could be several million asteroids. Still there is no exact definition of an asteroid. Experts differ in opinion on how to properly categorize asteroids according to their mass, size, composition and position in the planetary system. There is a proposal to call bodies larger than 50 m in diameter as asteroids, smaller than planets and with metal or stone composition. Even smaller bodies that completely decompose and burn up upon entry in the atmosphere of Earth would fall in the category of meteoroids, while asteroids would be called those that can reach Earth’s surface. Asteroids close to Earth are a common name for all asteroids that in one part of their orbit approach the Earth’s orbit. NEA is divided into 3 groups, called by individual asteroid from the group. - First group is made by Amor asteroids. There are about 2500 such asteroids known (February 2009).
- Second group are Apollo asteroids. There is 3000 known asteroids with such orbits (February 2009), and it is estimated that there is about 500 to 1000 of ones that are larger than 1 km. 2101 Aodins, 2-kilometer asteroid, in 1937 passed only 2 million kilometers from Earth. 4179 Tuatusu, one of Apollo asteroids, in 1992, as the first asteroid had its size and shape determined by radar.
- Third group are made by Aton asteroids. There is about 500 of such asteroids (February 2009). In recent years several passes of asteroids close to Earth was recorded.
It is common that asteroids are grouped according to orbital characteristics and according photometric and spectroscopic properties, which point to differences in structure. According to orbital characteristics, asteroids are divided in groups and families. The group is usually named by the asteroid that was first discovered, or the biggest one. In the beginning asteroids were divided in three groups according to the composition of surface material, that is surface properties: color, reflection coefficient and spectral type. Number of groups in this division grows with the discoveries of new asteroids and there is currently 14 of them. Basic three groups are: - C-type: Carbon asteroids- 75% of known asteroids belong to this group.
- S-type: Silica asteroids- 17% of asteroids.
- M-type: Metal asteroids- most of the remaining asteroids.
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