Asteroid Belt

The majority of asteroids are located in the “asteroid belt”, a flat dough-nut shaped feature lying between Mars and Jupiter in roughly the same orbital plane. The mean orbital distance of Mars from the sun is 228 million kilometres, and the mean orbital distance of Jupiter is 778 million kilometres. The belt is about 150 million kilometres wide—between 329,12 million and 478,71 million kilometres from the sun. This is termed the ‘main belt’ to distinguish it from near-Earth asteroids, the Trojan asteroids and the like.

The Inner Solar System
Nasa Jet Propulsion Laboratory. Image credit: Nasa/Caltech

Asteroids orbit in the same direction as the planets. All asteroids have craters, but some have been eroded and smoothed down by repeated minor collisions. There are three main compositional structures of asteroids: C, S and M-type. The vast majority are either carbonaceous (C-Type) or siliceous (S-Type). C-class dominate the middle and outer regions; S-class dominate the inner edge of the asteroid belt. The third most abundant is the metallic class (M-Type). You might also come across them referred to as: chondrite, stony and nickel-iron respectively. In general, most asteroids are made of rock and various metals, especially nickel and iron. Some are composed of clay and silicate. They also contain vast amounts of carbon. Analysis of various meteorites (asteroids that have made landfall on Earth) and samples returned to Earth‡ have also revealed precious metals such as platinum and gold; a wide variety of minerals including olivine and pyroxene; and even water or ice in their interiors.

‡ Japan’s Hayabusa landed on Itokawa (of the Apollo group) and returned an asteroid sample to Earth in 2010.

The main belt

The main belt is thought to be the chief source of meteoroids, the remains of shattered asteroids and some comets. The band has two distinct features: the outer-belt asteroids and the inner-belt asteroids.

There are over a billion asteroids in the asteroid belt. And yet their entire mass equates to just 4% of our very own moon. Jupiter’s massive gravitational influence prevents the asteroids from coalescing. And the bits are basically leftovers from failed planetary formations. The average asteroid is 1.5 million kilometres from its nearest neighbour—a sort of Heavenly social distancing. In other words, the asteroid belt is mostly empty. There is little chance of any of this debris managing to form a planet! Even less chance of a spacecraft colliding with anything should it traverse the asteroid belt.

The belt contains only 100 asteroids that are larger than 200 kilometres across. But there are 100 000 asteroids greater than about 20 kilometres across and a staggering billion that are over two kilometres along their longest axis. Some of them belong to “collisional families” where each group is the remnant of a collision of two bodies. There are up to 30 such families in the asteroid belt, and astronomers are sure they will find others.

‘Asteroid’ means star-like. They typically take four to five years to orbit the sun. Their orbits are slightly elliptical and of low inclination.

The largest asteroids live in the outer belt. The largest and first to be discovered is Ceres with a diameter of 952 kilometres. It was discovered in 1801 and is now classified as a dwarf planet and contains about 25% of the mass of all asteroids combined. Vesta, with a diameter of 525 kilometres, Pallas, slightly smaller at 510 kilometres across, and Hygiea with a diameter of 434 kilometres are the next largest objects in the asteroid belt, accounting for 9%, 7% and 2.9% respectively of the mass of the asteroid belt. In total, just these four bodies account for no less than 44% of the entire mass of the main asteroid belt.

The main belt describes the core region where about 93% of all discovered and numbered minor planets reside within the solar system. The Small-Body Database, managed by Nasa’s Jet Propulsion Laboratory (JPL) lists over 700 000 known main belt asteroids, as well as many comets. Newly discovered objects and their orbits are added on a daily basis. Search for JPL in your browser then type “Database” in their search option to find out more.

Outer-belt asteroids

Some think it is a good idea to divide the asteroid belt into inner and outer belts based on the so-called “Kirkwood Gap”. Personally, I don’t see much point in this but, for what it’s worth, I’ll explain it a bit. The inner belt is formed by asteroids orbiting nearer to mars at about 370 million kilometres mean distance from the sun. The outer belt ranges from about 490 million kilometres distant from the sun and outwards towards the orbit of Jupiter.

The Kirkwood Gap is a gap between sub-bands of asteroid clumps, assembled, one might say, under the influence of Jupiter’s gravity. Technically these are based on “orbital resonances” with Jupiter. For example, the group of asteroids in the 3:1 Kirkwood Gap will orbit the sun three times for every single orbit of Jupiter, thus passing the gravitational influence of that planet three times every 11.88 years.

There are four known asteroid groups collectively called outer belt asteroids. By definition they have orbital periods that range from more than one-half that of Jupiter to approximately Jupiter’s period. Three of the outer-belt groups are each headed by Cybele, Hilda, and Thule respectively. The fourth group is called the Trojan asteroids.

Cybele was discovered in 1861 and has a diameter of 237 kilometres. It skirts the outer edge of the asteroid belt. The Cybele asteroids have mean orbital distances from the sun of between 489 million – 554 million kilometres. The Cybele group consists of nearly 2 000 objects and a few collision families.

Hilda was discovered in 1875 and has a diameter of 175 kilometres. The orbits of the Hilda group ranges from 554 million — 628 million kilometres from the sun. The Hilda asteroid group consists of over 4 000 objects.

Thule was discovered in 1888. It has a diameter of 127 kilometres. The Thule group comprises just three objects, which orbit the sun at a distance of between 633 million and 667 million kilometres.


Members of the fourth group are called Trojan asteroids comprising something like 6 179 bodies. The Trojans are actually in two groups that, by definition, travel in the same orbit as Jupiter, one 60% in front of, and the other 60% behind, the giant planet. Their orbital period, of course, will be the same as that of Jupiter: 11.88 years.

 Inner-belt asteroids

There is only one known concentration of asteroids in the inner belt. It is called the Hungaria group, which occupies the region between Mars and the inner edge of the main asteroid belt. Hungaria asteroids have nearly circular (low eccentricity) orbits but large orbital inclinations to Earth’s orbital plane of 22.7°.

There are about 100 Hungaria asteroids. And, with an orbital period of about 2.5 years they orbit the sun roughly four times for every orbit completed by Jupiter. They are the innermost dense concentration of asteroids in the solar system.

The mean distance of the group at perihelion is 266.3 million kilometres from the sun, and 299.2 million kilometres at aphelion, so their orbits lie outside the orbit of Mars and cannot pass close to the planet.

Near-Earth asteroids

There are four near-Earth asteroid (NEA) groups, in each case named after their main representative: Atira, Amor, Apollo and Aten. The Atira asteroids are by far the smallest group of NEAs. The Amor group crosses the orbit of Mars; and the Apollo group crosses the orbit of Earth; while the Aten group follows a very small orbit, spending most of its time inside the orbit of Earth. Not surprisingly they are classified as “potentially hazardous asteroids”. About 500 fragments of asteroids (called meteors) crash into Earth every year. The latest numbers of asteroids in the Atira, Aten, Apollo and Amor asteroid groups are: 15, 45, 570 and 270 respectively, and that have diameters of larger than one kilometre. The Amor group has an orbital period of 5.3 years; the Apollo group an orbital period of 1.81 years.

Notable near-Earth asteroids include Eros, a member of the Amor group. It was discovered in 1898. It is 31 kilometres long and has an orbital period of 1.76 years. At its last close approach to Earth in 1975 Eros came within 22 million kilometres. The orbit is unstable, and astronomers believe is has a one in ten chance of hitting Earth or Mars within the next million years. Toutatis is much smaller, about 4.26 kilometres in length. It sweeps past Earth every four years. 2004 BL86 is an odd tumbling asteroid which came within 1 199 600 kilometres of Earth in January 2015. It is about 325 metres long and even has a little moon!

Cruithne (strange name—and a very strange orbit) is sometimes erroneously called Earth’s second moon. But it certainly has a weird relationship with our planet. It is a member of the Aten group. Cruithne flies near Earth once about every 385 years and is due again around 2300CE. It has a rather strange orbital path. It orbits the sun roughly once a year (Earth time), but the path also rotates like a hula hoop, as if around the sun’s waist. The pattern is repeated about once every 800 years. It passes close to Earth from behind every 385 years, and then passes in front 385 years later, and so on.

Apophis, just 370 metres in width, is a member of the Aten group. It is expected to reach Earth on Friday 13th April 2029 and pass as close as a mere 30 000 kilometres. At a precise distance of 30 405 kilometres, calculations in 2004 indicated a six-hundred-metre window—a so-called “gravitational keyhole”—offering a high probability of impact with Earth. More recent research has set this risk aside. A future impact on 13th April 2036 has not been ruled out, so the asteroid remains on the watch list. Incidentally, the 2029 encounter will alter the object’s residence from the Aten to the Apollo group.

Extreme orbits

There are some bodies with extremely eccentric orbits around our sun. Are they asteroids? Hidalgo may not be. Indeed, it is thought to be a defunct comet, having exhausted its gases when passing close to the sun. Elsewhere we have mentioned this body as a centaur. Hidalgo has a diameter of 52 kilometres. It was discovered in 1920. Its orbit ranges from 1 448 million kilometres at aphelion (just beyond Saturn) to just 302 million at perihelion.

One of the most extreme orbits is followed by Phaethon, the first asteroid to be discovered by a spacecraft (the Infrared Astronomical Satellite in 1983). Phaethon approaches to within 20.9 million kilometres of the Sun, well within the perihelion distance of Mercury, for example. By contrast, Phaethon’s aphelion is 359 million kilometres, taking it through the main asteroid belt. Because the parent bodies of all other meteor streams identified to date are comets, Phaethon is also considered by some to be a defunct comet—one that has lost its volatile materials and no longer displays the classic cometary features of a nebulous coma and a tail.

Solar System categories - average distance from the sun
Minor Planets58 million - 228 million
Asteroid Belt329 million - 479 million
Major Planets778 million - 4.5 billion
Kuiper Belt4.5 billion - 15 billion
Scattered Disc4.8 billion - 150 billion
Oort Cloud299 billion - 14.9 trillion

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By Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last updated: Wednesday, 1st April 2020