Main-belt in the context of "10 Hygiea"

In planetary astronomy, a centaur is a small Solar System body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Centaurs generally have unstable orbits because of this; almost all their orbits have dynamic lifetimes of only a few million years, but there is one known centaur, 514107 Kaʻepaokaʻāwela, which may be in a stable (though retrograde) orbit. Centaurs typically exhibit the characteristics of both asteroids and comets. They are named after the mythological centaurs that were a mixture of horse and human. Observational bias toward large objects makes determination of the total centaur population difficult. Estimates for the number of centaurs in the Solar System more than 1 km in diameter range from as low as 44,000 to more than 10,000,000.

The first centaur to be discovered, under the definition of the Jet Propulsion Laboratory and the one used here, was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed centaur is 10199 Chariklo, which at 250 kilometers in diameter is as big as a mid-sized main-belt asteroid, and is known to have a system of rings. It was discovered in 1997.

A Mars-crossing asteroid (MCA, also Mars-crosser, MC) is an asteroid whose orbit crosses that of Mars. Some Mars-crossers numbered below 100000 are listed here. They include the two numbered Mars trojans 5261 Eureka and (101429) 1998 VF31.

Many databases, for instance the JPL Small-Body Database (JPL SBDB), only list asteroids with a perihelion greater than 1.3 AU as Mars-crossers. An asteroid with a perihelion less than this is classed as a near-Earth object even though it is crossing the orbit of Mars as well as crossing (or coming near to) that of Earth. Nevertheless, these objects are listed on this page. A grazer is an object with a perihelion below the aphelion of Mars (1.67 AU) but above the Martian perihelion (1.38 AU). The JPL SBDB lists 13,500 Mars-crossing asteroids. Only 18 MCAs are brighter than absolute magnitude (H) 12.5, which typically makes these asteroids with H<12.5 more than 13 km in diameter depending on the albedo. The smallest known MCAs have an absolute magnitude (H) of around 24 and are typically less than 100 meters in diameter. There are over 21,600 known Mars-crossers of which only 5751 have received a MPC number.

A distant minor planet, or distant object, is any minor planet found beyond Jupiter in the outer Solar System that is not commonly thought of as an "asteroid". The umbrella term is used by IAU's Minor Planet Center (MPC), which is responsible for the identification, designation and orbit computation of these objects. As of January 2025, the MPC maintains 6101 distant objects in its data base.

Most distant minor planets are trans-Neptunian objects and centaurs, while relatively few are damocloids, Neptune trojans or Uranus trojans. All distant objects have a semi-major axis (average distance from the Sun) greater than 6 AU. This threshold, which is just beyond the orbit of Jupiter (5.2 AU), ensures that the vast majority of "true asteroids" – such as the near-Earth, Mars-crosser, main-belt and Jupiter trojan populations – are excluded from the distant minor planets.

A Kirkwood gap is a gap or dip in the distribution of the semi-major axes (or equivalently of the orbital periods) of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter. The gaps were first noticed in 1866 by Daniel Kirkwood, who also correctly explained their origin in the orbital resonances with Jupiter while a professor at Jefferson College in Canonsburg, Pennsylvania.

For example, there are very few asteroids with semimajor axis near 2.50 AU, period 3.95 years, which would make three orbits for each orbit of Jupiter (hence, called the 3:1 orbital resonance). Other orbital resonances correspond to orbital periods whose lengths are simple fractions of Jupiter's. The weaker resonances lead only to a depletion of asteroids, while spikes in the histogram are often due to the presence of a prominent asteroid family (see List of asteroid families).

130 Elektra is a large outer main-belt asteroid and quadruple system with three minor-planet moons. It was discovered on 17 February 1873, by astronomer Christian Peters at Litchfield Observatory, New York, and named after Electra, an avenger in Greek mythology.