The science of space rocks

How we classify asteroids, and why we’re so keen to study them

An artist's impression of a large asteroid belt around Vega, the second brightest star in the northern night sky.
Credit: NASA/JPL-Caltech


Written by Jasmin Fox-Skelly


Asteroids do not have the most glamourous of reputations.

How can they, when they are commonly known as ‘space rocks’?

Rocks, boring and ordinary, that just so happen to be in space.

Cold, airless, barren lumps that orbit the Sun, left over from the formation of the Solar System.

How can they possibly compare to the allure of Mars, Saturn, or even demoted Pluto?

Although less dynamic than the planets, space rocks have a charm and a usefulness all of their own.

Because they are, in fact, the leftovers, they could hold the key to how the Solar System first formed and how life on Earth began. 

Not to mention that some are so rich in valuable metals that private companies are racing to be the first to mine them.

But – as Asteroid Day on 30 June reminds us – one could plough into planet Earth and wipe out civilisation as we know it, the sequel to the 10km-wide body that crashed into Mexico’s Yucatan Peninsula millions of years ago and almost certainly contributed to the extinction of the dinosaurs.  

Asteroids are surprisingly varied.

The smallest ever studied is 2015 TC25, a 2m-wide body that made a close flyby of Earth in 2015; many are smaller still, whilst the largest is 578km-wide Vesta.

Most are irregular in shape because they lack the gravity needed to make them spherical.

Some are solid rock, but others consist of loose collections of rubble bound together by gravity.

One asteroid between Saturn and Uranus has its own rings, whilst another has six comet-like tails.

Many are cratered – Vesta has one impact basin so wide it covers 95 per cent of the asteroid’s diameter.

Some have their own moons, and others orbit each other in pairs. 


Class wars

One simple way to classify asteroids is on the basis of where in the Solar System they reside.

The majority exist between Mars and Jupiter, in a disc known as the asteroid belt, but they can be found across the Solar System, including in the stable gravitational wells surrounding the major planets (asteroids of this type are known as Trojans) or on orbits that bring them much closer to Earth – if they come within 1.3 AU of the Sun, they are officially near-Earth asteroids. 

But even asteroids that coexist in the same region of space can be vastly different in terms of their composition, which scientists infer from their spectral profile, colour and albedo (reflectivity).

It’s a tricky business as there are multiple ‘taxonomies’ in use, none of which take precedence, and they can be quite confusing – one goes so far as to include 14 classes.

Very broadly speaking, asteroids can be considered in three major groupings: carbonaceous, or C type, containing large amounts of carbon; silicaceous, or S type, which are stony in composition; and metallic, or M type, which are often (but not always) dominated by iron-nickel.

Most (but not all) asteroids fall into one of these three groups.

There’s a very valuable reason why astronomers go to such lengths to study and characterise asteroids.

They are the parts of Solar System that did not become planets, time capsules to the past that have remained unchanged for billions of years.

They are a window to what the early Solar System was like.    

So far our knowledge of how planets formed comes almost entirely from studying meteorites that have fallen to Earth, 99.8 percent of which originated from asteroids.

Many of these have remained unchanged in composition since they first formed 4.6 billion years ago.

These meteorites, which are classed as ‘Primitive’ formed while the planets themselves were still forming, and so provide direct evidence of conditions at that time. 


Asteroid Ryugu as seen by the Japanese Hayabusa2 spacecraft from an altitude of 6km. The image was captured with the Optical Navigation Camera - Telescopic (ONC-T) on 20 July 2018.
Credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST.


Remnant revelations 

Models of star and planet formation predict that the planets originally formed from a disc of dust and gas surrounding the young Sun.

As the disc cooled, different materials began to condense, and then solidify at different distances from the.

Eventually these accumulated together to form planets.

Studying the composition of these oldest primitive meteorites revealed which materials condensed first – minerals rich in calcium, aluminium and titanium it turns out, as these were found in one of the oldest meteorites, known as ‘Allende’. 

Allende is an example of a carbonaceous chondrite, the most ancient of all space rocks.

These resemble the Sun in composition, although they contain less hydrogen, carbon, nitrogen, and noble gases, as these are too volatile to have condensed in the inner Solar System.

The slightly younger rocks reveal that the last to condense were the carbonaceous compounds and ices made from water, ammonia, and methane.

Other younger meteorites that have fallen to Earth shine a light on the processes that go on inside asteroids and planets as they form.

These ‘Processed’ or ‘differentiated’ meteorites resemble igneous rocks found on Earth.

They appear to have been part of a larger body that broke up at some intermediate stage in the history of the Solar System, after having gone through a stage of heating and volcanism.

Unlike the older ‘Primitive’ meteorites that have all their ingredients jumbled together, Processed space rocks contain concentrated elements like iron from a core, or volcanic rock from a crust or mantle. 

This shows that they became hot enough to melt and separate into distinct layers of rock and pure iron-nickel. 

Asteroids could also be the key to solving one of science’s biggest mysteries – how life on Earth first started.

It’s possible that meteors could have brought some of the key ingredients of life to Earth, such as water and amino acids. 

This is one of the reasons that astronomers are so keen to study samples acquired directly from asteroids.

Two promising missions are underway right now: JAXA’s Hayabusa 2 and NASA’s OSIRIS-REX.

The Japanese probe has arrived at asteroid 162173 Ryugu and is due to return to Earth in December 2020.

OSIRIS-REX’s target is 101955 Bennu; it will reach the asteroid in December 2018 and return a sample to us in 2023.


The basic classes of asteroids

Carbonaceous asteroids (C type)

Famous examples: Pallas, Hygiea, Davida

The most common space rocks, accounting for 76 per cent of all asteroids. They are coal-black in colour and are rich in carbon-based compounds, clay and silicate rocks. They contain a lot of water molecules but hardly any metals. 


Silicaceous asteroids (S type)

Famous examples: Gaspra, Ida

S types are made of rocky silicate minerals, as well as metals like nickel, iron and magnesium, but unlike C types they contain little water. They make up 16 per cent of known asteroids. S types are notably brighter than C types.


Metallic asteroids (M type)

Famous examples: Psyche, Lutetia

Five per cent of known asteroids are M types, making them the third most abundant. They tend to contain more metallic elements than other types – including rare metals such as platinum – but not always. Lutetia, for instance, has some stony characteristics. 

Jasmin Fox-Skelly is an astronomy and science writer



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