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Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies
in orbit around the Sun, especially in the inner Solar System; they are smaller than planets but larger than meteoroids. The
term "asteroid" has historically been applied primarily to minor planets of the inner Solar System, as the outer
Solar System was poorly known when it came into common usage. The distinction between asteroids and comets is made on visual
appearance: Comets show a perceptible coma while asteroids do not.
Traditionally, small bodies orbiting the Sun were
classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid. The
term "asteroid" is ill-defined. It never had a formal definition, with the broader term minor planet being preferred
by the International Astronomical Union until 2006, when the term "small Solar System body" (SSSB) was introduced
to cover both minor planets and comets. The 2006 definition of SSSB says that they "include most of the Solar System
asteroids, most trans-Neptunian objects (TNOs), comets, and other small bodies". Other languages prefer "planetoid"
(Greek for "planet-like"), and this term is occasionally used in English for the larger asteroids. The word "planetesimal"
has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System
was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets,
but is not in common use.
When found, asteroids were seen as a class of objects distinct from comets, and there
was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between
an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects
have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary
activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids.
A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids"
with notably eccentric orbits are probably dormant or extinct comets. For
almost two centuries, from the discovery of the first asteroid, Ceres, in 1801 until the discovery of the first centaur, 2060
Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944
Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently
resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids, though there was
debate over whether they should be classified as asteroids or as a new type of object. Then, when the first trans-Neptunian
object, 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms
were invented to sidestep the issue: Kuiper belt object (KBO), trans-Neptunian object (TNO), scattered-disc object (SDO),
and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not
expected to exhibit much cometary activity; if centaurs or TNOs were to venture close to the Sun, their volatile ices would
sublimate, and traditional approaches would classify them as comets and not asteroids.
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The innermost of these are the Kuiper belt objects (KBOs), called "objects"
partly to avoid the need to classify them as asteroids or comets. KBOs are believed to be predominantly comet-like in composition,
though some may be more akin to asteroids. Furthermore, most do not have the highly eccentric orbits associated with comets,
and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized
to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected
by the Stardust probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum
between asteroids and comets" rather than a sharp dividing line. The
minor planets beyond Jupiter's orbit are rarely called "asteroids", but all are commonly lumped together under the
term "asteroid" in popular presentations. We include Trojans (bodies
captured in Jupiter's 4th and 5th Lagrange points), Centaurs (bodies in orbit between Jupiter and Neptune), and trans-Neptunian
objects (orbiting beyond Neptune) in our definition of "asteroid" as used on this site, even though they may more
correctly be called "minor planets" instead of asteroids.
Because it is becoming increasingly common for the term "asteroid" to be restricted
to minor planets of the inner Solar System, this article will restrict itself for the most part to the classical asteroids:
objects of the main asteroid belt, Jupiter trojans, and near-Earth objects. When
the IAU introduced the class small solar system bodies in 2006 to include most objects previously classified as minor planets
and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have
become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally
the term 'small solar system body' will be preferred." Currently only the largest object in the asteroid belt, Ceres,
at about 950 km (590 mi) across, has been placed in the dwarf planet category, although there are several large asteroids
(Vesta, Pallas, and Hygiea) that may be classified as dwarf planets when their shapes are better known.
Asteroid (Click on photo's
to enlarge)
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