An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A → near-Earth asteroid (NEA) with → perihelion distances between 1.017 and 1.3 → astronomical units. The Amor asteroids approach the orbit of the Earth from beyond, but do not intersect it. Most Amors do cross the orbit of Mars. It is estimated that 32% of the total number of NEAs are Amors. One of the larger Amors is → Eros.

→ Amor; → asteroid.

A member of a class of → near-Earth asteroids (NEAs) that have orbital → semi-major axes greater than that of the Earth (> 1 → astronomical unit) but → perihelion distances less than the Earth's → aphelion distance (less than 1.017 AU); thus, they cross the Earth's orbit when near the perihelia of their orbits. They are named for the prototype Apollo.

→ Apollo; → asteroid.

1) A small rocky object orbiting the Sun. There are millions of asteroids moving in orbits in the main → asteroid belt, between → Mars and → Jupiter and in the → Kuiper belt. The largest and the first discovered, → Ceres, about 1,000 km in size, is now classified as → dwarf planet (2006 IAU General Assembly). The largest asteroid in the solar system (Ceres apart), is → Pallas, with a size of 582 × 556 × 500 km. On the other hand, the smallest asteroid ever studied is the 2 meters space rock 2015 TC25, which was observed when it made a close flyby of Earth in October 2015. See also → near-Earth asteroid; → binary asteroid.
2) Math.: A → hypocycloid with four → cusps in which the → radius of the rolling → circle is a → quarter of the radius of the fixed circle. It has the → parametric equations x = a cos³θ, y = a sin³θ, where a is the radius of the fixed circle.

Gk. asteroeides "star-like," from aster, → astro- + → -oid "like, resembling."

Sayyârak "small planet," from sayyâré, → planet, + -ak "diminutive suffix."
Axtarvâr, from axtar, → astro-, + -vâr, → -oid.

The region of the → solar system located between → Mars and → Jupiter where over a million objects bigger than 1 km across orbit the Sun. Another region populated by minor bodies lies beyond the orbit of → Neptune, the → Kuiper belt.

→ asteroid; → belt.

1) For an asteroid whose orbit is precisely known, a number and optionally a proper name, e.g. (7) Iris, (24101) Cassini, (99942) Apophis.
2) For an asteroid whose orbit is not known, a provisional designation composed of four elements: number.letter.letter.(optionally)number. The first number indicates the year of discovery. The first letter denotes the half-month of the discovery (A: first half of January, Y: second half of December; the letter "I" is excluded). The second letter and the following number indicate the order of discovery within the half-month. For example, the first asteroid discovered in the first half of May 1960 is: 1960 JA. Since more than 25 objects (without "I") might be detected within a half-month, the number following the second letter indicates the number of 25 discoveries. Hence, 2001 SD3 was discovered in the second half of September 2001 and was the (D =) 4 + (25 x 3) or the 79th object found during that period.

→ asteroid; → designation.

A group of asteroids that share the same or similar proper orbital elements (semi-major axis, eccentricity, inclination). In 1918, the Japanese astronomer K. Hirayama first recognized some non random concentrations of asteroid elements. He noticed that certain "groups" of asteroids had similar orbital elements, and hence he first introduced the concept of "asteroid families," and identified three of them: Koronos, Eos, and Themis. The names of these groups were chosen by the parent (brightest) asteroid that the smaller group asteroids follow. Some of the more common asteroid families include the Trojans, which are actually not an asteroid family, but a group of asteroids caught in the Sun-Jupiter gravitational equilibrium points known as L3 and L4 → Lagrangian points.

→ asteroid; → family.

Systematic observation of the sky in particular searching for → asteroids that may have a close approach to the Earth. → near-Earth object.

→ asteroid; → survey.

Bardid, → survey; sayyârakhâ plural of sayyârak, → asteroid.

A member of a class of → near-Earth asteroids with → perihelion distances between 0.983 and 1.0 → astronomical units. It is estimated that 6% of the total number of NEAs are Atens.

→ Aten; → asteroid.

A division of → C-type asteroids whose members have relatively low albedos (0.04 to 0.08) and the → ultraviolet absorption below 0.5 μm is small or absent. Examples include → 2 Pallas, 379 Huenna (diameter 62 km), and → 101955 Bennu.

→ type; → asteroid.

A member of a population of double objects in the main → Asteroid Belt or the → Kuiper Belt which are gravitationally bound together. So far about 200 such binary systems have been identified, while their number is increasing. 243 Ida was the first binary asteroid to be discovered during the Galileo spacecraft flyby in 1993. Other examples are → Antiope and Kalliope in the main belt and QG₂₉₈ in the Kuiper Belt. The importance of these objects resides in the fact that systems with well measured orbital parameters allow the total mass to be estimated. If the sizes of the components are known then their densities can be accurately calculated. Density is an important parameter since it yields information about composition and internal structure.

→ binary; → asteroid.

An → asteroid that belongs to the family of → carbonaceous asteroids. They are → depleted in → hydrogen and → helium, have chemical ratios akin to solar composition, and show low → albedo (0.03-0.09). C-type asteroids are the most common variety, forming around 75% of known asteroids. They inhabit → main belt's outer regions.

C stands for → carbonaceous; → type; → asteroid.

An → asteroid whose orbit around the Sun lies typically between the orbits of → Jupiter and → Neptune Neptune (5 to 30 → astronomical units). The first Centaur, called → Chiron, was discovered in 1977, but since then more than 100 roughly similar objects have been found. Three centaurs, Chiron, 60558 Echeclus, and 166P/NEAT 2001 T4, have been found to display → cometary  → comas. Chiron and 60558 Echeclus are now classified as both asteroids and → comets. Most of the Centaur asteroids are probably dormant comets from the → Kuiper belt which have been pulled in by the gravity of → outer planets.

→ Centaurus; → asteroid.

An asteroid having a → co-orbital motion.

→ co-orbiting; → asteroid.

A relatively uncommon → carbonaceous carbonaceous asteroid whose spectrum contains a strong → ultraviolet  → absorption feature below 0.5 μm (→ Tholen classification). In the → SMASS classification it corresponds to the Cg and Chg types , depending on the presence or absence (respectively) of the absorption feature at 0.7 μm. The most remarkable "asteroid" in this type is → Ceres (now classified as a → dwarf planet).

→ type; → asteroid.

The asteroids found on the outer edge of the main asteroid belt in a 2:3 orbital resonance with Jupiter. The group is not an asteroid family since the members are not physically related. The group consists of asteroids with semi-major axes between 3.70 AU and 4.20 AU, eccentricities less than 0.30, and inclinations less than 20°. It is dominated by D- and P-type asteroids.

Named for the prototype 153 Hilda, discovered by Johann Palisa (1848-1925) on November 2, 1875, and named Hilda after a daughter of his teacher, the astronomer Theodor von Oppolzer (1841-1886); → asteroid.

A type of → asteroid that consists of metallic → iron and displays high → albedo (0.10-0.18). M-type asteroids inhabit the middle part of the → asteroid belt.

M for → metallic; → type; → asteroid.

An → asteroid whose orbit lies partly between 0.983 and 1.3 → astronomical units from the Sun, so that it passes close to the Earth. Currently thousands of near-Earth asteroids are known, ranging in size up to about 30 km. Among them, there are between 500 and 1,000 such asteroids larger than one km in diameter. They are divided into three subclasses: → Amor asteroids, → Apollo asteroids, and → Aten asteroids. See also → near-Earth object.

→ near; → earth; → asteroid.

An asteroid that could make a threatening close approach to the Earth. In technical terms a PHA is defined as having an → absolute magnitude of 22 or brighter and an → Earth Minimum Orbit Intersection Distance (MOID) of less than 0.05 → astronomical unit or 7.5 million km.

→ potentially; → hazardous; → asteroid.

A moderately bright type of asteroids (albedo 0.10 to 0.22) consisting mainly of iron- and magnesium-silicates such as olivine and pyroxene. They are dominant in the inner main belt within 2.2 AU, common in the central belt within about 3 AU, but become rare further out. The largest is 15 Eunomia (about 330 km in its largest dimension).

S stands for silicaceous, → silicate; → asteroid.

A type of → asteroid containing → pyroxene and → olivine silicates, probably mixed with metallic iron, similar to → stony meteorites. S-type asteroids show high albedo of 0.10-0.22. They include about 17% of known asteroids and occupy the inner → asteroid belt.

S for → stone; → type; → asteroid.