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Asteroids - Agreed Terms - April 2022

Asteroids comprise a very large proportion of more massive smaller Solar System objects. They vary from the size of dwarf planets to the smallest objects we want to recognise. Obviously there are immense smaller objects, down even to specks of dust which may be orbiting the Sun. To avoid problems of nomenclature (and there are many overlaps) one path is to discount objects beyond the orbit of Neptune, e.g., Trans Neptunian Objects (TNOs).

So, to start with, what are the largest asteroids?

First, let’s compare the largest, Ceres, with the sizes of Earth and Moon.


Note that the largest asteroid, Ceres (now a dwarf planet) is only a fraction the size of our Moon. Sometimes, because of its new status, you will find Ceres excluded from the list, with another asteroid named largest.

Ceres has a diameter of 946 km. Other asteroids range in size from Vesta – otherwise the largest, at about 329 miles (530 km) in diameter – to bodies that are less than 33 feet (10 meters) across.

The total mass of asteroids combined is less than that of Earth’s Moon.

Diameters, and other characteristics, of top 38 asteroids are given here:

List of exceptional asteroids - Wikipedia

Where are asteroids found?

We have excluded bodies of similar size outside the orbit of Neptune, so that most asteroids, then, orbit the Sun in the Asteroid Belt, between the orbits of Mars and Jupiter.


Some asteroids, generically called Trojans, orbit 60 degrees ahead or behind some planets, in the same orbit. These were first discovered around Jupiter. Other smaller groups will be mentioned in passing.

Classification > asteroids-comets-and-meteors

The three broad compositional classes of asteroids are C-, S-, and M-types.

The C-type (chondrite) asteroids are most common. They probably consist of clay and silicate rocks, and are dark in appearance . . .
The S-types (“stony”) are made up of silicate materials and nickel-iron.
The M-types are metallic (nickel-iron).

A few words about collisions might be appropriate here. Larger Solar System bodies become "differentiated". Heavier metallic components migrate to the core, followed by layers of mainly silicate, and finally a surface crust. There is an interface or intermediary layer between the core and the silicate layer. It has been suggested that, if these bodies are involved in collisions they might form smaller asteroids or smaller bodies deriving from such layers. Thus, M-types would result from metallic core fragments, C-types from silicate layers, and S-types from stony (otherwise known as stony-iron) interfacial layers

This process was probably repeated down the fragment chain.
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I am retired early and interested in everything from the Big Bang (and before) from Cosmology to Astronomy and Planetary Sciences to Geology including the 'geology' of exoplanets and miscellaneous objects including asteroids. That covers quite a...
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"There never was a good war, or a bad peace."
Continuing with the types of asteroid:

The C-type (chondrite) asteroids are most common. They probably consist of clay and silicate rocks, and are dark in appearance . . .
The S-types (“stony”) are made up of silicate materials and nickel-iron.
The M-types are metallic (nickel-iron).

Much of the information on asteroids is obtained from meteorites, since these are much more accessible on Earth.

Asteroids, Meteorites and Comets Linda T Elkins-Tanton Chelsea House 2006
This book, part of an excellent series, contains a wealth of greater detail.

"About 85% of meteorites now falling to Earth are chondrites (calculated from observed falls). Nevertheless, proportionately somewhat more iron and stony-iron are found, because they do not look so much like terrestrial rocks, and so are easier to recognise."

Chondrite - Wikipedia › wiki › Chondrite

"To guess which meteorite composition represents the most primitive material, scientists have compared the elements in the meteorite with the elements that make up the Sun. The Sun, since it contains more than 99% of the material in the Solar System, is probably a good measure of an average solar system composition." . . . . . . . . .
"Chondrites have compositions very similar to the Sun's, except for the very volatile elements like helium and hydrogen. If the abundance of elements in the Sun is plotted against the abundance of elements in chondrite meteorites, the plot forms almost a straight line. Other meteorites and especially planetary materials do not have comparable elemental abundances to the Sun."
"The chondrite meteorites (hence parent asteroids) are named after tiny round bodies that they contain, called chondrules. Chondrules are rounded heterogeneous bodies that contain both crystal and glass. Because they are rounded, they are thought to have cooled before being incorporated into the meteorites (or precursors) (otherwise their round, smooth outlines would have been shaped by the crystals around them). They are further thought to be droplets that condensed from a liquid. These droplets are thought to represent the earliest solar system material. . . . . . . Radioisotopic dating showed that chondrules formed at approximately 4.586 billion years ago - and that is the age of the solar system.”
“Chondrite meteorites generally consist of chondrules and the minerals olivine and pyroxene, with a fine grained matrix filling in the gaps.”

Carbonaceous chondrites
Carbonaceous chondrites . . . . . . are given their name because they contain because they contain hydrocarbons and other organic material. Some of this material can be attributed to contamination after landing on Earth, but much of it is primary material that the meteorite carried with it. The carbonaceous material contains hydrocarbons in rings and chains, and amino acids. . . . . . . The parent bodies for carbonaceous chondrites must have been smaller than for some other classes of chondrites because the hydrocarbons in these materials would not survive much processing."


Carbonaceous chondrite - Wikipedia › wiki › Carbonaceous_chondrite

Enstatite chondrites
"Enstatite chondrites . . . contain tiny sulphide minerals that show that the meteorite material cooled in a matter of days. This rapid cooling rate requires that the original material was in small pieces and therefore was broken by violent impacts from a larger hotter body."

"Achondrites are igneous rocks, that is, meteorites which have crystallized from a silicate melt. They do not contain chondrules or the other markers of early, undifferentiated solar system materials. These meteorites are the remnants of larger bodies that formed early in the solar system and at least began the process of differentiation and evolution." . . . . . .
"The achondrite classes howardite, eucrite, and diogenite are now generally accepted to have come from a parent body that was broken by impacts, leaving behind the large asteroid 4 Vesta to orbit the Sun in the Asteroid Belt. Other achondrites may have originated in parent bodies that no longer exist or may not have been identified yet.


Stony (stony-irons)
"Stony-iron meteorites consist of roughly equal mixtures of silicate material and metal phases The metal in these meteorites closely resembles the metal of iron meteorites, and the silicate portions resemble achondrites. Stony-irons are therefore thought to represent portions of differentiated bodies that incorporated both mantle and core material. They are divided into two main classes - the pallasites and the mesosiderites."
"Pallasites are among the most beautiful rocks in the solar system . . . . . . They are thought to have originated as rocks along the boundary of the core and silicate mantle of the early planetesimals that also yielded the iron meteorites from their cores and the achondrites from their mantles and surfaces. Pallasites usually consist of a mixture of iron metal and Widmannstatten patterns and near translucent yellow to green olivine crystals up to an inch in diameter." . . . . . . . . .
"Mesosiderites, on the other hand, consist of silicate minerals with iron that appears never to have melted completely and recrystallized into core-like materials. They appear to exist as metal mixed with basalt or gabbro, both igneous rocks, or minerals settled from igneous rocks."

Stony-iron meteorite - Wikipedia

Iron meteorites

"Iron meteorites are believed to represent the cores of planetesimals that differentiated and were then fractured into pieces. These meteorites usually contain almost no silicate material and are therefore direct and unmixed samples of the cores of large, early bodies in the solar system. As such, they are windows into core-forming processes, which remain a matter of much hypothesizing and little data, since scientists cannot sample the cores of the terrestrial planets directly."
"Iron meteorites consist of iron with 5-20% nickel and traces of gallium, germanium, carbon, sulphur and iridium. These elements are arranged mainly into the minerals kamacite, and tetrataenite, with various other minerals in small amounts (these so-called accessory minerals include martensite, awaruite, troilite, shriebersite, and graphite. . . . . . . "
"Isotopic studies of the meteorites by a variety of researchers indicate that the cores of the parent bodies all formed within about 5 million years of the start of the solar system and that the last of the parent bodies had fully solidified no later than 4.6 Ga. While meteorites are travelling in space, their surfaces are bombarded by cosmic rays. The amount of damage to minerals in the meteorite can be measured, and thus the length of time that the meteorite has been exposed to cosmic rays can be calculated. Cosmic ray exposure ages from the surfaces of iron meteorites indicate that the meteorite broke up from their parent bodies and have been travelling through space for between 200 million years and one billion years before breaking into the pieces that eventually fell to Earth. These exposure ages imply that larger iron meteorite bodies survived intact for between 3.5 and 4.4 billion years."
"Though iron meteorites make up only about 5% of observed meteorite falls, and are therefore thought to make up about 5% of meteorites in near-Earth orbit, they make up the largest % by mass of any meteorite group in collections. . . . . . . The largest iron meteorite yet found is a meteorite in Namibia, named Hoba, which weighs 123,000 pounds (55,000 kg)



Work in progress. Last edited 11th April 17.20 BST.
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"There never was a good war, or a bad peace."
Now something a lot of us (should be) interested in:

Near Earth Asteroids

There is every spectrum of interest from:
There should be maximum search/find/destroy
No interest; I can't do anything about it
. . . . . . and everything in between.

This is very helpful:

NEO Basics (

Atirasa<1.0 au
Q<0.983 au
NEAs whose orbits are contained entirely with the orbit of the Earth (named after asteroid 163693 Atira).
Atensa<1.0 au
Q>0.983 au
Earth-crossing NEAs with semi-major axes smaller than Earth's (named after asteroid 2062 Aten).
Apollosa>1.0 au
q<1.017 au
Earth-crossing NEAs with semi-major axes larger than Earth's (named after asteroid 1862 Apollo).
Amorsa>1.0 au
1.017<q<1.3 au
Earth-approaching NEAs with orbits exterior to Earth's but interior to Mars' (named after asteroid 1221 Amor).
PHAsMOID<=0.05 au
Potentially Hazardous Asteroids: NEAs whose Minimum Orbit Intersection Distance (MOID) with the Earth is 0.05 au or less and whose absolute magnitude (H) is 22.0 or brighter.


Apollo asteroid - Wikipedia › wiki › Apollo_asteroid

The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, discovered by German astronomer Karl Reinmuth in the 1930s.

There is a long list of Apollo asteroids here - perhaps the most interesting being the largest known Apollo asteroid is 1866 Sisyphus, with a diameter of about 8.5 km. This is of a size similar to the one thought to have destroyed the dinosaurs. (10km).
Asteroids larger than approximately 35 meters across can pose a threat to a town or city.

Potentially Hazardous Asteroids

Work in progress. Last edited 23rd April 22.50 BST.
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Nov 10, 2020
One technical point to add here is that as they are currently defined asteroid and dwarf planet are not mutually exclusive categories. As such Ceres is both an asteroid and a dwarf planet.

Composition wise the origin of various asteroids are still somewhat unresolved as we don't yet have a well established time frame within planet formation and how asteroids played a role in this exactly. C type asteroids are particularly interesting as samples from cometary material brought back to Earth indicate that in terms of isotopic ratios comets appear to chemically be C type suggesting this class of asteroids represent the nuclei of extinct comets that have become trapped in the inner solar system largely thanks to the gravitational influence of Jupiter.
Also supporting this picture is the largest C type body within the asteroid belt Ceres which appears to have ammonium salts suggesting it may have formed out beyond the nitrogen frost line before it got dragged into the inner solar system during Jupiter's inward migration.
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"There never was a good war, or a bad peace."
Yes, it is rather complex, especially when you take into account the relationship with asteroids.

Here is one interesting reference:

How are asteroid compositions and classifications determined? (Intermediate) - Curious About Astronomy? Ask an Astronomer (

“How are asteroid compositions and classifications determined? (Intermediate)
How do you know what an asteroid is made out of, and what is the classification system for asteroids?
There are many ways to tell what asteroids are made of. One way is to send a spacecraft there, for example NASA's NEAR spacecraft that orbited Eros for a year. While in orbit, the spacecraft used an infrared camera and spectrometer and an x-ray/gamma ray spectrometer to look at the composition of the asteroid.

However, we can't send spacecraft to every asteroid, and most asteroid compositions are determined using infrared spectroscopy from ground based telescopes. In the infrared, different minerals absorb different wavelengths of light. By looking at the infrared spectral absorptions, and comparing them to spectra of minerals measured on Earth, it is possible to identify the composition. This is still a difficult process, though, because asteroids are faint, and so it can be difficult to get a good enough detection to be sure about the spectrum. Usually you need to observe the object for a significant fraction of its rotation period which means that you get one spectrum for the entire object; you can't see compositional differences between different regions on the asteroid. Also, asteroids are combinations of many minerals, and so astronomers argue over what combination of minerals can form a particular asteroid spectrum. Sometimes it looks like several mineral combinations could give you similar spectra, so it can be hard to tell which one is correct.

Another way to determine the composition is to use radar. With planetary radar you send out a radio signal to the asteroid and look at what is reflected back. The radio waves react differently to different materials, for example metals look quite different from rock. This is a relatively new technique, since only in the past couple decades have we had the technology to look at lots of small and faraway objects with radar. Therefore, there aren't as many asteroids that have been classified based on radar observations.

Asteroids are classified using a lettering system which, in my opinion, is one of the more non-intuitive classification systems in astronomy. There are usually 14 classifications (A,B,C,D,E,F,G,M,P,Q,R,S,T,and V), but some scientists don't believe that some of the classifications should be distinct and some of the classifications seem to contain many more types. The asteroids are placed into a letter group based on their spectral characteristics, not based on their "real-life" characteristics, but some of the letters correspond to familiar things. For example, S type asteroids are "stony" and M type are probably metallic. Just as an example, the description for the "A" classification might be "extremely reddish shortward of 0.7 microns; strong absorbtion feature longward of 0.7 microns..." (quoted from Tholen and Barucci, 1989).

To add to the confusion, meteorites, which are pieces of planets or asteroids that have fallen to Earth, are classified in a similar but separate manner. This is because we can measure the meteorites in a lab, so it's much easier to tell what they are made of. Right now, people are trying to connect the meteorite classes with the asteroid classes, but it's hard because we don't get the same information from telescopic spectroscopy that we get from a lab on Earth. So it's possible that one type of meteorite could come from asteroids in multiple classes, or several types of meteorite could come from a single asteroid class.

Cat :)
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"There never was a good war, or a bad peace."
Interesting information on Trojan Asteroids has emerged in:
[2204.08617] Size Distribution of Small Jupiter Trojans in the L5 Swarm (

Trojan asteroids were originally named after the groups of asteroids orbiting 60° ahead and behind the orbit of Jupiter, but it seems to be applied now to any asteroids sharing the orbit of a planet.

The reference indicated estimates that the number of Trojans orbiting Jupiter at 260,000 larger than 1 km, as well as innumerable smaller objects going down to dust size. The figure quoted puts Jupiter's Trojans larger than 1 km at roughly 10% those of corresponding size in the Asteroid Belt. It is also noted that the L4 stream (ahead of Jupiter) has 40% more asteroids larger than 2 km than there are in the L5 stream (behind Jupiter in its orbit). This suggests that the Trojans were captured by Jupiter when forming further out in the Solar System before migrating inward.

Cat :)
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