Binary asteroid in the context of "Minor-planet moon"

65489 Ceto, as a binary also (65489) Ceto–Phorcys (provisional designation 2003 FX₁₂₈), is a binary trans-Neptunian object (TNO) discovered on March 22, 2003, by Chad A. Trujillo and Michael Brown at Palomar. It is named after the sea goddess Ceto from Greek mythology. It came to perihelion in 1989.

In astronomy, the barycenter (or barycentre; from Ancient Greek βαρύς (barús) 'heavy' and κέντρον (kéntron) 'center') is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important concept in fields such as astronomy and astrophysics. The distance from a body's center of mass to the barycenter can be calculated as a two-body problem.

If one of the two orbiting bodies is much more massive than the other and the bodies are relatively close to one another, the barycenter will typically be located within the more massive object. In this case, rather than the two bodies appearing to orbit a point between them, the less massive body will appear to orbit about the more massive body, while the more massive body might be observed to wobble slightly. This is the case for the Earth–Moon system, whose barycenter is located on average 4,671 km (2,902 mi) from Earth's center, which is 74% of Earth's radius of 6,378 km (3,963 mi). When the two bodies are of similar masses, the barycenter will generally be located between them and both bodies will orbit around it. This is the case for Pluto and Charon, one of Pluto's natural satellites, as well as for many binary asteroids and binary stars. When the less massive object is far away, the barycenter can be located outside the more massive object. This is the case for Jupiter and the Sun; despite the Sun being a thousandfold more massive than Jupiter, their barycenter is slightly outside the Sun due to the relatively large distance between them.

A binary system is a system of two astronomical bodies of the same kind that are comparable in size. Definitions vary, but typically require the center of mass to be located outside of either object. (See animated examples.)

The most common kinds of binary system are binary stars and binary asteroids, but brown dwarfs, planets, neutron stars, black holes and galaxies can also form binaries.

79360 Sila–Nunam (provisional designation 1997 CS₂₉) is a cold classical Kuiper belt object (cubewano) and binary system made up of components of almost equal size, called Sila and Nunam, orbiting beyond Neptune in the Solar System. The name of the system is the combined names of the two bodies, Sila and Nunam.

148780 Altjira (provisional designation 2001 UQ₁₈) appears to be a triple or contact binary double classical Kuiper belt object. The secondary is large compared to the primary, approximately 246 kilometres (153 mi) vs. 221 kilometres (137 mi). The lightcurve is quite flat (Δmag<0.10), which is indicative of a "quasi-spherical body with a homogeneous surface".The system mass is 4 × 10 kg.

Its companion was discovered on 6 August 2006, from images taken by the Hubble Space Telescope. The secondary's orbit has the following parameters: semi-major-axis, 9904 ± 56 km; period, 139.561 ± 0.047 days; eccentricity, 0.3445 ± 0.0045; and inclination, 35.19 ± 0.19° (retrograde). There is indirect evidence that Altjira may be an unresolved hierarchical triple system.

A contact binary is a small Solar System body, such as a minor planet or comet, that is composed of two bodies that have gravitated toward each other until they touch, resulting in a bilobated, peanut-like overall shape. Contact binaries are distinct from true binary systems such as binary asteroids where both components are separated. The term is also used for stellar contact binaries.

An example of a contact binary is the Kuiper belt object 486958 Arrokoth, which was imaged by the New Horizons spacecraft during its flyby in January 2019.

Dimorphos (formal designation (65803) Didymos I; provisional designation S/2003 (65803) 1) is a natural satellite or moon of the near-Earth asteroid 65803 Didymos, with which it forms a binary system. The moon was discovered on 20 November 2003 by Petr Pravec in collaboration with other astronomers worldwide. Dimorphos has a diameter of 177 meters (581 ft) across its longest extent.

Dimorphos is the smallest asteroid to be photographed and visited by a spacecraft. It was the target of the Double Asteroid Redirection Test (DART), a NASA space mission that deliberately collided a spacecraft with the moon on 26 September 2022 to alter its orbit around Didymos. Before the impact by DART, Dimorphos had a shape of an oblate spheroid with a surface covered in boulders but virtually no craters. The moon is thought to have formed when Didymos shed its mass due to its rapid rotation, which formed an orbiting ring of debris that conglomerated into a low-density rubble pile that became Dimorphos today.

Actaea, formal designation (120347) Salacia I, is the only known moon of the classical Kuiper belt object 120347 Salacia. Its diameter is estimated to be 393 km (244 mi), which is approximately one-half the diameter of Salacia; thus, Salacia and Actaea are viewed by William Grundy et al. to be a binary system. Assuming that the following size estimates are correct, Actaea is about the fifth-biggest known moon of a trans-Neptunian object, after Charon (1212 km), Dysnomia (615 km), Vanth (443 km), and Ilmarë (403 km).