These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet, but as minor planets in the outer Solar System were discovered, they were often distinguished from traditional asteroids. In this article the term “asteroid” refers to the minor planets of the inner Solar System.
- They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles.
- There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets.
- The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter Trojans).
- However, other orbital families exist with significant populations, including the near-Earth asteroids.
- On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt.
- The finding is unexpected because comets, not asteroids, are typically considered to “sprout jets and plumes”.
- A newly discovered asteroid is given a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month.
- In 1851, after the fifteenth asteroid (Eunomia) had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook).
- The circle was then numbered in order of discovery to indicate a specific asteroid (although he assigned ① to the fifth, Astraea, while continuing to designate the first four only with their existing iconic symbols).
- The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery.
- If so, the object receives a catalog number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.
- 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.
- Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.
- Vesta, too, has a differentiated interior, though it formed inside the Solar System’s frost line, and so is devoid of water; its composition is mainly of basaltic rock such as olivine.
- This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through accumulation of debris after collisions between asteroids.
- Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.
- There are at least 67 main-belt asteroids that are known to have satellites.
Main Belt Asteroids with Moons
Of the many asteroids that have satellites, few are given proper names. Most minor planet moons are classified by the year they were found. Some of the better known asteroid and moon pairs with names are
- Ida and its moon Dactyl
- Kalliope and its moon Linus
- Sylvia and its moons Remus and Romulus
- Kleopatra and its moons Alexhelios and Cleoselene
- Eugenia and its moons Petit-Prince and the less charmingly named S/2004 (45) 1
One main-belt asteroid, (90) Antiope, has a companion named S/2000 (90) 1 that is approximately the same size as Antiope. The pair are often considered to be a double asteroid. These two rubble piles appear to orbit each other instead of one around the other.
If Antiope and S/2000 (90) 1 are a double asteroid and not an asteroid-satellite pair, then the largest main-belt asteroid satellite is Linus at about 38 kilometers across. Its parent, Kalliope, is 166 kilometers across. Linus was discovered in 2001 and is named after the mythological son of Calliope.
Of the 502,195 currently known asteroids, there are bound to be more satellites than what has been so far discovered. Some satellites are believed to be created through capture of smaller objects being pulled in by larger objects’ gravity wells. Others may have once been a part of the parent asteroid. Kleopatra is one such parent.
Kleopatra was recently discovered to be not a solid object but a large pile of rubble, which is unexpected for an asteroid of its size (217 kilometers). Some of the other asteroids named here that are also large piles of debris are Kalliope, Antiope, and Sylvia. Scientists studying Kleopatra believe that an oblique collision with the asteroid caused its two moons to spin out from the pile and begin to orbit.
Although many asteroids are visible with amateur equipment, and at least one can be seen with the unaided eye, the size of the companion moons make them too small for amateur observers to spot.
The first asteroid moon was not discovered until 1993, and this was by the spacecraft Galileo that flew past the asteroid belt. It was not until images taken in 2004 that the asteroid Sylvia was discovered to have two moons, followed by Eugenia and its two moons. This discovery required the use of the telescope at the European Southern Observatory in Chile.
An asteroid with three or more moons has yet to be discovered.