Eunomia family
Encyclopedia
The Eunomia family of asteroids
is a large grouping of S-type
asteroid
s named after the Greek goddess Eunomia
. It is the most prominent family in the intermediate main belt. About 5% of all main belt asteroids belong to this family.
, the largest of all the "stony" S-type
asteroids, It is about 300 km across along its longest axis, has a 250 km mean radius, and lies close to the barycenter
of the family. Eunomia has been estimated to contain about 70–75% of the matter from the original parent body. This had a mean diameter of about 280 km and was disrupted by the catastrophic impact that created the family. It is likely that the parent body was at least partly differentiated
, because the surface of Eunomia and spectra of the smaller family members show some variation. This notwithstanding, other studies have indicated that the body that was definitively shattered by the impact that created the family was probably already somewhat fragmented by previous smaller collisions. The impactor was likely a smaller, yet still very substantial asteroid of 50 km diameter (or so) that hit at a speed of about 22,000 km/h.
The other Eunomian asteroids are quite regularly distributed in orbital space around Eunomia. The next largest member identified by the analysis was 258 Tyche
of 65 km diameter. However, its orbit lies at the very margin of what can be considered the family region, and it may well be an interloper. The largest clear family members are about 30 km diameter, with several asteroids in this size range.
Spectroscopic studies have shown that the family members span a noticeable range of compositions, although all remain within the S spectral class
. As such they are of generally stony (rather than icy) surface composition that includes silicate
s and some nickel
-iron
, and are quite bright for their size.
The family contains relatively large numbers of small objects. Since most of these smaller objects are "eroded" away over time due to secondary collisions, gravitational perturbations, and the Yarkovsky effect
, this indicates that the Eunomia family was created relatively recently (on an astronomical timescale).
The Cassini-Huygens
spacecraft flew by 2685 Masursky
, a small family member, in 2000. However, the encounter distance
of about one million kilometers was too large for surface features to be resolved.
with Jupiter
, at relatively high inclinations.
A HCM numerical analysis by Zappalà et al. determined a large group of 'core' family members, whose proper orbital elements
lie in the approximate ranges
! !! ap
!! ep !! ip
>
min
2.54 AU
0.121
11.6°
max
2.72 AU
0.180
14.8°
At the present epoch
, the range of osculating
orbital elements
of these core members is
The Zappalà 1995 analysis found 439 core members, while a search of a recent proper element database for 96944 minor planet
s in 2005 yielded 4649 objects lying within the rectangular-shaped region defined by the first table above. This would give about 5% of all main belt asteroids.
members: 85 Io
, 141 Lumen
, 546 Herodias
, 657 Gunlöd
, 1094 Siberia
, and 1275 Cimbria
.
Asteroid family
An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions...
is a large grouping of S-type
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s named after the Greek goddess Eunomia
Eunomia (goddess)
Eunomia was a minor Greek goddess of law and legislation, and one of the daughters of Themis and Zeus.-Mythology:...
. It is the most prominent family in the intermediate main belt. About 5% of all main belt asteroids belong to this family.
Characteristics
By far the largest member is 15 Eunomia15 Eunomia
15 Eunomia is a very large asteroid in the inner asteroid belt. It is the largest of the stony asteroids, and somewhere between the 8th-to-12th-largest main-belt asteroid overall...
, the largest of all the "stony" S-type
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
asteroids, It is about 300 km across along its longest axis, has a 250 km mean radius, and lies close to the barycenter
Barycentric coordinates (astronomy)
In astronomy, barycentric coordinates are non-rotating coordinates with origin at the center of mass of two or more bodies.The barycenter is the point between two objects where they balance each other. For example, it is the center of mass where two or more celestial bodies orbit each other...
of the family. Eunomia has been estimated to contain about 70–75% of the matter from the original parent body. This had a mean diameter of about 280 km and was disrupted by the catastrophic impact that created the family. It is likely that the parent body was at least partly differentiated
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...
, because the surface of Eunomia and spectra of the smaller family members show some variation. This notwithstanding, other studies have indicated that the body that was definitively shattered by the impact that created the family was probably already somewhat fragmented by previous smaller collisions. The impactor was likely a smaller, yet still very substantial asteroid of 50 km diameter (or so) that hit at a speed of about 22,000 km/h.
The other Eunomian asteroids are quite regularly distributed in orbital space around Eunomia. The next largest member identified by the analysis was 258 Tyche
258 Tyche
258 Tyche is a relatively large main belt asteroid discovered in 1886. It is classified as an S-type. Tyche orbits very close to the Eunomia family of asteroids, and could be a member based on composition. However, it is larger than all family members apart from 15 Eunomia while lying at the very...
of 65 km diameter. However, its orbit lies at the very margin of what can be considered the family region, and it may well be an interloper. The largest clear family members are about 30 km diameter, with several asteroids in this size range.
Spectroscopic studies have shown that the family members span a noticeable range of compositions, although all remain within the S spectral class
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
. As such they are of generally stony (rather than icy) surface composition that includes silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s and some nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, and are quite bright for their size.
The family contains relatively large numbers of small objects. Since most of these smaller objects are "eroded" away over time due to secondary collisions, gravitational perturbations, and the Yarkovsky effect
Yarkovsky effect
The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum...
, this indicates that the Eunomia family was created relatively recently (on an astronomical timescale).
The Cassini-Huygens
Cassini-Huygens
Cassini–Huygens is a joint NASA/ESA/ASI spacecraft mission studying the planet Saturn and its many natural satellites since 2004. Launched in 1997 after nearly two decades of gestation, it includes a Saturn orbiter and an atmospheric probe/lander for the moon Titan, although it has also returned...
spacecraft flew by 2685 Masursky
2685 Masursky
The asteroid 2685 Masursky is a main-belt asteroid. It was discovered by Edward Bowell in 1981. It was named after Harold Masursky , a planetary geologist at the U.S. Geological Survey, Flagstaff, who worked on numerous space missions....
, a small family member, in 2000. However, the encounter distance
of about one million kilometers was too large for surface features to be resolved.
Location and size
The Eunomia family is located between the 3:1 and 8:3 resonancesOrbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
with Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, at relatively high inclinations.
A HCM numerical analysis by Zappalà et al. determined a large group of 'core' family members, whose proper orbital elements
Proper orbital elements
The proper orbital elements of an orbit are constants of motion of an object in space that remain practically unchanged over an astronomically long timescale...
lie in the approximate ranges
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
!! ep !! ip
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
>
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
At the present epoch
Epoch (astronomy)
In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as celestial coordinates, or elliptical orbital elements of a celestial body, where these are subject to perturbations and vary with time...
, the range of osculating
Osculating orbit
In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space is the gravitational Kepler orbit In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space (at a given moment of time) is the gravitational Kepler orbit In astronomy,...
orbital elements
Orbital elements
Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are generally considered in classical two-body systems, where a Kepler orbit is used...
of these core members is
| a Semi-major axis The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape... | e | i Inclination Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit... |
|
|---|---|---|---|
| min | 2.53 AU Astronomical unit An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance.... |
0.078 | 11.1° |
| max | 2.72 AU | 0.218 | 15.8° |
The Zappalà 1995 analysis found 439 core members, while a search of a recent proper element database for 96944 minor planet
Minor planet
An asteroid group or minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid...
s in 2005 yielded 4649 objects lying within the rectangular-shaped region defined by the first table above. This would give about 5% of all main belt asteroids.
Interlopers
A number of interlopers have been identified, which share the same orbital elements as the true family members, but can not have come from the same breakup because of spectral (hence, compositional) differences. The following have been identified in a spectral survey, and also by inspection of the PDS asteroid taxonomy data set for non-S-typeS-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
members: 85 Io
85 Io
85 Io is a large, dark main-belt asteroid of the C spectral class. It is probably a primitive body composed of carbonates. Like 70 Panopaea it orbits within the Eunomia asteroid family but it is not related to the shattered parent body....
, 141 Lumen
141 Lumen
141 Lumen is a dark , large rocky asteroid 130 km in diameter orbiting in the main belt near the Eunomia family of asteroids. It is not, however, physically related to the group, being of the wrong spectral class...
, 546 Herodias
546 Herodias
546 Herodias is a C-type asteroid orbiting in the Main belt near the Eunomia family. However, it is not a family member but an un-related interloper in the region because its composition is inconsistent with membership. Its diameter is about 66 km, its albedo around 0.053, and its rotation...
, 657 Gunlöd
657 Gunlöd
657 Gunlöd is a C-type asteroid orbiting in the Main belt near the Eunomia family. However, it is not a family member but an un-related interloper in the region because its composition is inconsistent with membership. Its diameter is about 43 km, its albedo around 0.042, and its rotation...
, 1094 Siberia
1094 Siberia
1094 Siberia is a minor planet orbiting the Sun. Initially it received the designation 1926 CB. It is now named after Siberia. The numerical designation indicates this was the 1094th asteroid discovered.-1094 Siberia in fiction:...
, and 1275 Cimbria
1275 Cimbria
1275 Cimbria is a main-belt asteroid discovered on November 30, 1932 by Reinmuth, K. at Heidelberg.- External links :*...
.