Small Body

Comet Halley nucleus

small body, also called small solar system body (SSSB), 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.