Asteroids are
minor planets (
small Solar System bodies and
dwarf planets) that are not
comets, especially those of the
inner Solar System. They have also been called
planetoids, especially the larger ones. These terms have historically been applied to any astronomical object orbiting the
Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as small objects in the
outer Solar System were discovered, their
volatile-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids. Thus the term
asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System out to the orbit of
Jupiter. They are grouped with the outer bodies—
centaurs,
Neptune trojans, and
trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles. In this article the term "asteroid" refers to the minor planets of the inner Solar System.
There are millions of asteroids, many thought to be the shattered remnants of
planetesimals, bodies within the young Sun's
solar nebula that never grew large enough to become
planets. The large majority of known asteroids orbit in the
asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the
Jupiter Trojans). However, other orbital families exist with significant populations, including the
near-Earth asteroids. Individual asteroids are classified by their characteristic
spectra, with the majority falling into three main groups:
C-type,
S-type, and
M-type. These were named after and are generally identified with
carbon-rich,
stony, and
metallic compositions, respectively.
Only one asteroid,
4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be naked-eye visible for a short time.