Asteroid
0Asteroid, by Wikipedia https://en.wikipedia.org/wiki?curid=791 / CC BY SA 3.0
#Solar_System
#Asteroids
#Minor_planets
253 Mathilde, a C-type asteroid measuring about 50 km （30 mi） across, covered in craters half that size.
Photograph taken in 1997 by the NEAR Shoemaker probe.
Diagram of the Solar System's asteroid belt 2014 JO25 imaged by radar during its 2017 Earth flyby An asteroid is a minor planet of the inner Solar System.
Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope 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 that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main asteroid belt.
Thus the term "asteroid" now generally refers to the minor planets of the inner Solar System, including those co-orbital with Jupiter.
Larger asteroids are often called planetoids.
Millions of asteroids exist: many are shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets.
The vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter （the Jupiter trojans）.
However, other orbital families exist with significant populations, including the near-Earth objects.
Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type.
These were named after and are generally identified with carbon-rich, metallic, and silicate （stony） compositions, respectively.
The sizes of asteroids varies greatly; the largest, Ceres, is almost 1,000 km （600 mi） across and massive enough to qualify as a dwarf planet.
Asteroids are somewhat arbitrarily differentiated from comets and meteoroids.
In the case of comets, t...
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