Amor asteroid

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The Amor asteroid group compared to the orbits of the terrestrial planets of the Solar System.
  Mars (M)
  Mars trojans
  Venus (V)
  Mercury (H)
  Amor asteroids
  Earth (E)

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 cross the orbit of Mars. The two moons of Mars, Deimos and Phobos, may be Amor asteroids that were captured by Mars's 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).

Definition 1[edit]

Amor asteroid Eros visited by NEAR Shoemaker in 2000

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.

Definition 2[edit]

An asteroid belongs to the Amor group if:[1]

  • 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,[1] 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).


There are 3729 Amor asteroids currently known.[2] 580 of them are numbered, and 75 of them are named.[3]

Subdivisions by semi-major axis[edit]

Amor asteroids can be partitioned into four subgroups, depending on their average distance from the Sun.

Amor I[edit]

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.[citation needed]

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.

Amor II[edit]

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.[citation needed] Their orbits usually take them out into the asteroid belt.

Amor III[edit]

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.

Amor IV[edit]

There are only 14 known Amor asteroids whose average distance from the Sun is beyond the asteroid belt.[4] 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.[4] So far, no Amor asteroid has been discovered that crosses the orbit of Saturn.


Outer Earth-grazer asteroids[edit]

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[edit]

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.

Well-known Amors[edit]

Name Year Discoverer
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


For a list articles about Amor asteroids, see Category:Amor asteroids.

This is a list of named Amor asteroids (see § top).[5]

Designation Prov. designation
433 Eros 1898 DQ
719 Albert 1911 MT
887 Alinda 1918 DB
1036 Ganymed 1924 TD
1221 Amor 1932 EA1
1580 Betulia 1950 KA
1627 Ivar 1929 SH
1915 Quetzalcoatl 1953 EA
1916 Boreas 1953 RA
1917 Cuyo 1968 AA
1943 Anteros 1973 EC
1980 Tezcatlipoca 1950 LA
2059 Baboquivari 1963 UA
2061 Anza 1960 UA
2202 Pele 1972 RA
2368 Beltrovata 1977 RA
2608 Seneca 1978 DA
3102 Krok 1981 QA
3122 Florence 1981 ET3
3199 Nefertiti 1982 RA
3271 Ul 1982 RB
3288 Seleucus 1982 DV
3352 McAuliffe 1981 CW
3551 Verenia 1983 RD
3552 Don Quixote 1983 SA
3553 Mera 1985 JA
3671 Dionysus 1984 KD
3691 Bede 1982 FT
3757 Anagolay 1982 XB
3908 Nyx 1980 PA
3988 Huma 1986 LA
4055 Magellan 1985 DO2
4401 Aditi 1985 TB
4487 Pocahontas 1987 UA
4503 Cleobulus 1989 WM
4587 Rees 3239 T-2
4947 Ninkasi 1988 TJ1
4954 Eric 1990 SQ
Designation Prov. designation
4957 Brucemurray 1990 XJ
5324 Lyapunov 1987 SL
5332 Davidaguilar 1990 DA
5370 Taranis 1986 RA
5620 Jasonwheeler 1990 OA
5653 Camarillo 1992 WD5
5751 Zao 1992 AC
5797 Bivoj 1980 AA
5863 Tara 1983 RB
5869 Tanith 1988 VN4
5879 Almeria 1992 CH1
6050 Miwablock 1992 AE
6456 Golombek 1992 OM
6569 Ondaatje 1993 MO
7088 Ishtar 1992 AA
7336 Saunders 1989 RS1
7358 Oze 1995 YA3
7480 Norwan 1994 PC
8013 Gordonmoore 1990 KA
8034 Akka 1992 LR
8709 Kadlu 1994 JF1
9172 Abhramu 1989 OB
9950 ESA 1990 VB
11284 Belenus 1990 BA
13553 Masaakikoyama 1992 JE
15745 Yuliya 1991 PM5
15817 Lucianotesi 1994 QC
16064 Davidharvey 1999 RH27
16912 Rhiannon 1998 EP8
18106 Blume 2000 NX3
20460 Robwhiteley 1999 LO28
21088 Chelyabinsk 1992 BL2
65803 Didymos 1996 GT
96189 Pygmalion 1991 NT3
154991 Vinciguerra 2005 BX26
162011 Konnohmaru 1994 AB1
164215 Doloreshill 2004 MF6
189011 Ogmios 1997 NJ6

See also[edit]


External links[edit]