small body

small body, any natural solar system object other than the Sun and the major planets and dwarf planets and their satellites (moons). The small bodies populate the solar system in vast numbers and include the mostly rocky asteroids, or minor planets, the predominantly icy comets, and the fragments of such bodies—commonly called meteoroids—over a continuum of sizes down to microscopic grains known as interplanetary dust particles or micrometeoroids. The term small body is sometimes restricted to objects that can be observed telescopically while still outside Earth’s atmosphere. In practice, that results in a lower limit of a few metres on the diameters of small bodies.

The use of meteoroid has expanded to describe any small chunk of matter in interplanetary space, especially one less than a few tens of metres in size. The term formerly was limited to small bodies that were near enough to Earth to eventually enter its atmosphere. In that narrower meaning, meteoroid is usually linked with the terms meteor, meteorite, or both. As a meteoroid travels through the atmosphere, friction heats it up, creating a glowing trail of hot ionized gases (plasma) called a meteor. If the object survives both its passage through the atmosphere and its subsequent impact on the ground, it is called a meteorite. Over 99 percent of meteorites are fragments of asteroids. A small group is known to be of lunar origin, and a second group is generally acknowledged to have come from Mars (see Mars: Meteorites from Mars). There is also reason to believe that some are fragments of the rocky remnants of comets, although that remains to be firmly established.

Populations

The largest known small bodies, in the conventional sense, are several icy Kuiper belt objects found orbiting the Sun beyond the orbit of Neptune. Ceres—which is the largest main-belt asteroid and is now considered a dwarf planet—is roughly 950 km (590 miles) in diameter. The distant Kuiper belt object Eris, roughly the same size as Pluto, which has a diameter of about 2,350 km (1,460 miles), is also designated a dwarf planet rather than a small body, as are Haumea and Makemake, both of which have diameters of about 1,450 km (900 miles). These four dwarf planets are known as plutoids. At least four other Kuiper belt objects are also believed to be larger than Ceres. In the 1990s astronomers recognized that Pluto, which has a diameter of about 2,300 km (1,400 miles), is a giant icy leftover from the time when the planets formed. Pluto had been classified as a planet from its discovery in 1930 but was reclassified as a dwarf planet in 2006. Primarily for historical reasons, the moons of the planets are not considered small bodies. Nevertheless, many moons are asteroid-size or smaller, and some are probably captured asteroids or comet nuclei.

Although small bodies are found throughout the solar system, most of those known are concentrated in several regions. Those regions in which they travel in fairly stable orbits include (1) the main asteroid belt, between the orbits of Mars and Jupiter, (2) the gravitationally stable points (Lagrangian points) along the orbital paths of Jupiter, Mars, Uranus, Neptune, and Earth where objects called Trojan asteroids are clustered (see celestial mechanics: The restricted three-body problem), (3) the Kuiper belt, a disk-shaped zone of icy bodies whose members are also called trans-Neptunian objects because they all have orbits beyond that of Neptune, and (4) the spherical Oort cloud of icy bodies inferred to be orbiting the Sun at distances typically more than 1,000 times that of Neptune or Pluto. In none of the above categories is Sedna, an object discovered in 2003 in the outer reaches of the solar system that is perhaps as large as 40 percent the size of Pluto. Sedna has a highly elliptical orbit in which its closest approach to the Sun is more than twice that of Neptune—i.e., billions of kilometres beyond the recognized extent of the Kuiper belt; it may be a member of a population of objects lying between the Kuiper belt and the Oort cloud.

Other small bodies travel in unstable orbits that cross the paths of one or more of the planets. Those include (1) most observed comets, (2) near-Earth asteroids, most with orbits that cross either Earth’s orbit or Mars’s orbit and some with orbits that lie mostly or entirely inside Earth’s orbit and cross the orbits of Venus or of both Venus and Mercury while closely approaching Earth’s, (3) Centaur objects, icy bodies that are thought to have been gravitationally perturbed out of the Kuiper belt and now travel mainly between the orbits of Jupiter and Neptune, and (4) individual idiosyncratic planet-crossing objects such as the asteroid Hidalgo, which travels between the inner edge of the asteroid belt and a point just beyond the orbit of Saturn. All objects in planet-crossing orbits eventually collide with the Sun or a planet or are permanently ejected from the solar system, although some survive for long periods (up to hundreds of millions of years) because of stabilizing resonances.

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