The Amor asteroids
are a group of near-Earth asteroids
named after the asteroid 1221 Amor
. They approach the orbit of the Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars. The two moons of Mars, Deimos
, may be Amor asteroids that were captured by Mars' gravity.
The most famous member of this group is 433 Eros, which was the first asteroid to be orbited and then landed upon by a human probe (NEAR Shoemaker).
There are over 1200 Amor asteroids known today. Under 200 of them are numbered, and over 50 of them are named.
Subdivisions by semi-major axis
Amor asteroids can be partitioned into four sub-groups, 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.
Amor I asteroids that do cross the orbit of Mars (like 433 Eros), do so from the inside.
Amor I asteroids that have semi-major axes very close to Earth's (such as 1992 JD) can be considered Arjuna asteroids because they have very low eccentricities and thus Earth-like orbits.
The Amor II subgoup has a semi-major axis between that of Mars (1.52 au) and the main 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 main 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 the 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 main 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 a few 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 considered Amor IV asteroids. They are all Jupiter-crossers. Though they have very high eccentricities (0.65 to 0.75), they are not as eccentric as most Damocloids and comets, which tend to have eccentricities around 0.9. The only numbered and named Amor IV asteroid is 3552 Don Quixote
. 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 the Earth, 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 you use.
If you use 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 you use the more precise definition of an Amor, those outer Earth-grazers which never get closer to the Sun than the 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 you use 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 the 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 the 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 Dionyses
have permanent names.
Although, by definition, no Amor asteroid actually currently crosses the Earth's orbit (see Earth-crosser asteroid
), the definition of an Earth-crossing asteroid
(ECA) is broad enough so that many, if not most, Amor asteroids are also ECAs. An ECA has to be able to some day
cross the orbit of the Earth, not just today. If an Amor makes an approach close enough to the Earth, Mars, or Jupiter, it is possible that the gravitational effect of that encounter will alter the asteroid's orbit. Repeated close encounters may eventually cause the planet to cross the Earth's orbit. If astronomers determine that this can happen, the Amor asteroid is classified as an Earth-crossing asteroid. Of course, after its orbit has changed, it will no longer be an Amor asteroid, and will be reclassified as an Apollo asteroid and Earth-crosser asteroid. It can take many years of observation before an asteroid can be classified as an ECA.