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The asteroid and comet belts orbit the Sun from the inner rocky planets into outer parts of the Solar System, interstellar space. [16] [17] [18] An astronomical unit, or AU, is the distance from Earth to the Sun, which is approximately 150 billion meters (93 million miles). [19]
Over 200 asteroids are known to be larger than 100 km, [63] and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. [64] The number of asteroids in the main belt steadily increases with decreasing size.
Over 200 asteroids are known to be larger than 100 km, [45] and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. [46] The absolute magnitudes of most of the known asteroids are between 11 and 19, with the median at about 16. [47]
The Kuiper belt (/ ˈ k aɪ p ər / KY-pər) [1] is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. [2] It is similar to the asteroid belt, but is far larger—20 times as wide and 20–200 times as massive.
The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice. [195] It extends between 30 and 50 AU from the Sun. It is composed mainly of small Solar System bodies, although the largest few are probably large enough to be dwarf planets. [ 196 ]
Compared to the entire mass of the asteroid belt, the mass loss necessary to sustain the NEA population is relatively small; totalling less than 6% over the past 3.5 billion years. [27] The composition of near-Earth asteroids is comparable to that of asteroids from the asteroid belt, reflecting a variety of asteroid spectral types. [123]
[4] [5] However, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called Manx comets.
In the current Solar System, these small bodies are usually also classified by dynamics and composition, and may have subsequently evolved [10] [11] [12] to become comets, Kuiper belt objects or trojan asteroids, for example.