Asteroids are broadly classified according to two criteria; one is their orbit (dynamical classification), the other is their surface composition (spectral classification). Today we’ll take a look at the dynamical classification system.
Asteroids can be found just about everywhere in the Solar System, which is why they are thought to be the leftovers from when the planets were formed. Unlike the planets, whose orbits are close to circular and all lie pretty much within the plane of the ecliptic (an imaginary plane that contains the orbit of the Earth around the Sun), asteroids tend to have more eccentric orbits (less circular) which are often at a significant angle to the ecliptic plane.
Based on which area of the Solar System an asteroid is found it can be classified into one of many groups and subgroups.
Near Earth Asteroids
Also called NEOs (Near Earth Objects, which would encompass comets too), NEAs are simply asteroids that come close to the Earth at some point in their orbits. How close? A general consensus is closer than 0.3 AU (about 45 million km). Depending on the size of their orbit, and how they approach the Earth (from inside or outside its orbit) these asteroids are classified into three groups.
Named after 2062 Aten, these asteroids with a semi-major axis less than 1 AU approach Earth’s orbit at aphelion (having an aphelion distance greater than 0.983 AU) and some of them cross Earth’s orbit. Over 640 are known. Click for a current list of Atens. There is a subset of Aten asteroids that never cross the Earth’s orbit, they are referred to as Apohele asteroids, or also Atira asteroids (after the first confirmed member of this group, 163693 Atira).
Named after 1862 Apollo, these asteroids with a semi-major axis greater than 1 AU approach Earth’s orbit at perihelion (having a perihelion distance less than 1.017 AU) and some of them cross Earth’s orbit. Over 3,800 are known. Click for a current list of Apollos.
Named after 1221 Amor, these asteroids with a semi-major axis greater than 1 AU, but less than 1.524 AU, can cross Mars’s orbit. Their closest approach to Earth’s orbit is at perihelion, but they never cross it (their perihelion distance is between 1.017 and 1.3 AU). Over 3,200 are known. Click for a current list of Amors.
Asteroid distribution between Mars and Jupiter.
Near Mars Asteroids
Named after 434 Hungaria, this family of asteroids orbits between Mars and Jupiter in a region of dual resonances: 9:2 with Jupiter and 3:2 with Mars, or distances of 1.78—2 AU. The eccentricity of their orbits is less than 0.18 with inclinations of 16—34°.
Named after 25 Phocaea, this family of asteroids reside at a distance from the Sun of 2.25—2.5 AU; the eccentricity of their orbits is greater than 0.1 with inclinations of 18—32°.
These asteroids cross the orbit of Mars, hence their name. Some Amor asteroids can also be classified as Mars-crossers seeing as their orbits take them out to Mars’s orbit. An external list of Mars-crossers can be found here.
Mid Solar System asteroids. The Main Belt is shown in white, Hildas are orange and the Jupiter Trojans are green. Note this diagram does not include all known asteroids.
Mid Solar System
Moving further away from Mars we enter the Mid Solar System, home of the Main Belt, popularly known as the Asteroid Belt. Despite its name it is not a uniformly populated region, and in fact there are families of asteroids that live outside the Main Belt. The proximity of giant planet Jupiter, with its strong gravitational influence, has greatly influenced the distribution of asteroids within the Mars-Jupiter region by creating zones of asteroidal abundance (“concentrations”) and scarcity (“gaps”) around orbitally stable and unstable resonances, respectively.
Stretching from 2.1 AU to 3.3 AU, the Main Belt houses the majority of asteroids in the Solar System (over 90% of all numbered minor planets). Over half of its mass is concentrated in four objects: 1 Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. It is estimated that there are several million asteroids in the main belt, ranging in size from the 975m diameter of Ceres down to millimeters. Their orbits have eccentricities below 0.4 and inclinations less than 30°, though the majority have eccentricities around 0.07 and inclinations less than 4°, thus living in rather circular orbits close to the ecliptic.
Named after 153 Hilda, this family of asteroids is maybe the most interesting from an orbital point of view. Their orbits are in a 3:2 resonance with Jupiter with eccentricities less than 0.3 and inclinations below 20°. While each asteroid moves along its own elliptical path, as a group they form a triangular shape (see diagram right, click for larger version) with corners located roughly at the 3 Lagrangian points L3, L4 and L5. This configuration has been shown to be quite stable, with a lifetime as long as the Solar System’s.
These asteroids orbit around the Jupiter Lagrangian points L4 and L5, essentially preceding and following Jupiter in its orbit, respectively. Those around the L4 point are called the Greeks, while those around the L5 are the Trojans. Each individual asteroid is named after a character from the Trojan War. Unlike the Hildas, which have small inclinations, the Trojans’ orbits can have inclinations up to 40°. It’s believed there are more Jupiter Trojans than asteroids in the Main Belt. Here is a list of currently known Jupiter Trojans.
Asteroids beyond the orbit of Saturn. The vertical axis shows their inclination in degrees, the horizontal axis is distance from the Sun in AU, with the corresponding orbital period in years. The relative size of the objects is indicated approximately by the size of the circles.
Outer Solar System
Centaurs
The asteroids of this family orbit between Jupiter and Neptune and cross one or more of the giant planets’ orbits. A more rigorous definition requires that they have a perihelion larger than Jupiter’s orbit and a semi-major axis less than Neptune’s. The Centaurs are dynamically unstable and are expected to move on from their current orbits within 1—10 million years. It is thought Centaurs are objects transitioning from the Kuiper Belt towards cometary orbits with aphelia in the region of Jupiter (to become so-called Jupiter comets).
Transneptunian Objects inhabit the space beyond Neptune; a current list of know TNOs can be found here. They can be divided into various groups according to their orbits:
The name comes from the ex-planet (now dwarf planet) Pluto, plus the addition of the diminutive suffix -ino, and denotes a family of objects in a 2:3 orbital resonance with Neptune. Despite having a longer orbital period than Neptune, the eccentricities of some Plutinos’ orbits bring them inside Neptune’s orbit at perihelion (Pluto being one such example), although the majority of Plutinos have small eccentricities and inclinations.
The Kuiper Belt refers to a collection of objects found in a region around 30—50 AU from the Sun, in near circular orbits that are non-resonant, unaffected by Neptune. To differentiate them from Plutinos and other Neptune-resonant objects, the are often call Cubewanos (Classical Kuiper Belt Objects).
Beyond the Kuiper Belt lie the objects in the Scattered Disk, generally in highly eccentric (up to 0.8) and inclined (up to 40°)orbits. The extent of the Scattered Disk is unknown, but is estimated to reach out to a few hundred AU, despite which, and because of these extreme orbits, many objects come as close as 35 AU to the Sun during perihelion. We believe long period comets originate in the Scattered Disk, thrown Sunward by a close encounter with Neptune.
NEO orbit diagrams courtesy of Andrew Buck, used through Creatives Commons license.