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The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars. The two moons of Mars, Deimos and Phobos, may be Amor asteroids that were captured by Mars's gravity.
There are three general criteria which an asteroid must meet to be considered a member of the Amor asteroid class:
- The asteroid's orbit must periodically bring it near Earth. To be considered "near", the asteroid must come closer to Earth than to any other major planet. The closest planet to Earth is Venus, which can come as close as 0.27 AU; this value is rounded up to 0.30 AU for the purpose of this definition. Therefore, an Amor asteroid must come within 0.30 AU of Earth's orbit.
- The asteroid's orbit must be outside the orbit of Earth (like the majority of asteroids). Asteroids that come close to Earth whose orbits are inside Earth's orbit are considered Apohele asteroids (although not all Apoheles necessarily get close to Earth).
- The asteroid's orbit must not cross Earth's orbit. The most commonly used definition of this (which Wikipedia uses) is that it never orbits closer to the Sun than Earth's average distance from the Sun (1.000 AU). A more strict definition is that "at any point along the asteroid's orbit, Earth's orbit is closer to the Sun". This takes into consideration the fact that Earth's orbit ranges between 0.983 and 1.016 AU from the Sun. It is more difficult to sort out the Amor asteroids from the non-Amor asteroids using this definition, however.
These three criteria boil down to a single test for membership: If an asteroid has a perihelion between 1.000 AU and 1.300 AU, it is an Amor asteroid. Any asteroid with this trait is considered an Amor-class asteroid, regardless of its semi-major axis, eccentricity, aphelion, inclination, physical properties, orbital stability, or place of origin.
An asteroid belongs to the Amor group if:
- Its orbital period is greater than one year. This is equivalent to saying that its semi-major axis (a) is greater than 1.0 AU. Mathematically, a > 1.0 AU.
- Its orbit does not cross Earth's orbit. That is, its lowest point (perihelion distance q) is higher than Earth's highest point (aphelion = 1.017 AU). Mathematically, q > 1.017 AU.
- It is a near-Earth object (NEO), that is, its perihelion distance q < 1.3 AU.
In summary, a > 1.0 AU and 1.017 AU < q < 1.3 AU.
While the above is the definition given in the source, the "a > 1.0 AU" constraint is not necessary because if the perihelion distance q is greater than 1.017 AU, then the semi-major axis a must also be greater than 1.017 AU (and greater than 1.0 AU).
Subdivisions by semi-major axis
Amor asteroids can be partitioned into four subgroups, depending on their average distance from the Sun.
The Amor I subgroup consists of Amor asteroids whose semi-major axes are in between Earth and Mars. That is, they have a semi-major axis between 1.000 and 1.523 AU. Less than one fifth of Amor asteroids belong to this subgroup. Amor I asteroids have lower eccentricities than the other subgroups of Amors.
Some Amor I asteroids, such as 15817 Lucianotesi, do not cross the orbit of Mars. They can be considered a part of an Earth–Mars belt. However, not all asteroids located entirely between the orbits of Earth and Mars are Amors.
The Amor II subgroup has a semi-major axis between that of Mars (1.52 AU) and the asteroid belt (2.12 AU). About a third of Amors, including 1221 Amor, belong to this group. They have moderate eccentricities (from 0.17 to 0.52), and all cross the orbit of Mars from the outside. Their orbits usually take them out into the asteroid belt.
Almost half of all Amor asteroids lie within the asteroid belt, and thus have semi-major axes between 2.12 and 3.57 AU. These can be considered main-belt objects with high enough eccentricities to come near Earth, usually 0.4 to 0.6.
Because their eccentricities are very large, about a third of Amor III asteroids have orbits that stretch beyond the asteroid belt and come within 1 AU of Jupiter. 719 Albert and 1036 Ganymed are two such asteroids. The most extreme Amor III asteroids (such as 5370 Taranis) are actually Jupiter crossers.
Because they lie within the asteroid belt, several Amor III asteroids also belong to subgroups of the asteroid belt. For instance, the first Alinda asteroid (in 1:3 resonance with Jupiter and close to a 4:1 resonance with Earth) discovered was 887 Alinda.
There are only 14 known Amor asteroids whose average distance from the Sun is beyond the asteroid belt. Their semi-major axes are greater than 3.57 AU and they are called Amor IV asteroids. All are Jupiter crossers. Though they have very high eccentricities (0.65 to 0.75), their orbits are not as eccentric as those of most Damocloids and comets, which tend to have eccentricities around 0.9. The only named Amor IV asteroid is 3552 Don Quixote and the only other numbered Amor IV asteroid is (85490) 1997 SE5. So far, no Amor asteroid has been discovered that crosses the orbit of Saturn.
Outer Earth-grazer asteroids
An outer Earth-grazer asteroid is an asteroid which is normally beyond Earth's orbit, but which can get closer to the Sun than Earth's aphelion (1.0167 AU), but not closer than Earth's perihelion (0.9833 AU). In other words, the asteroid's perihelion is between Earth's perihelion and aphelion. Outer Earth-grazer asteroids are split between Amor and Apollo asteroids, depending on the definition used.
If one uses the simple definition of an Amor (1.3000 AU > perihelion > 1.0000 AU), then asteroids whose perihelion is between 1.0000 AU (Earth's semi-major axis) and 1.0167 AU (Earth's aphelion) are Amor outer Earth-grazer asteroids, while those between 0.9833 AU (Earth's perihelion) and 1.0000 AU are considered Apollo outer Earth-grazer asteroids.
If one uses the more precise definition of an Amor, those outer Earth grazers which never get closer to the Sun than Earth does (at any angle along its orbit) are Amors, and those that do are Apollos. Some "simple" Amor asteroids are also "precise" Apollos, while some "precise" Amors are also "simple" Apollos. Which definition is used is only relevant to outer Earth grazers.
Potentially hazardous asteroids
Most potentially hazardous asteroids (PHAs) are either Aten asteroids or Apollo asteroids, and therefore cross the orbit of Earth. However, one tenth of PHAs are Amor asteroids. In order to be considered a PHA, its orbit has to get within 0.05 AU from Earth's orbit and the object has to be "big enough" to be a threat. An Amor asteroid therefore has to have a perihelion of less than 1.05 AU to be considered a PHA. About a fifth of Amors come this close to the Sun, and about a fifth of these are actually PHAs. Of the fifty known Amor PHAs, 2061 Anza, 3908 Nyx and 3671 Dionysus have permanent names.
|3908 Nyx||1980||Hans-Emil Schuster|
|1221 Amor||1932||Eugène Joseph Delporte|
|1036 Ganymed||1924||Walter Baade|
|887 Alinda||1918||Max Wolf|
|719 Albert||1911||Johann Palisa|
|433 Eros||1898||Gustav Witt|
- "NEO Groups". NASA/JPL Near-Earth Object Program Office. Retrieved 2012-06-04.
- List of Amor asteroids generated by the JPL Small-Body Database Search Engine Retrieved 2013-06-01
- List of numbered Amor asteroids generated by the JPL Small-Body Database Search Engine Retrieved 2012-07-19
- List of Amor asteroids with a>3.57 AU generated by the JPL Small-Body Database Search Engine Retrieved 2011-09-14