Asteroids: The ultimate guide to killer space rocks

Asteroids are small solar system bodies (SSSBs) that fall between the size of a planet and the size of a meteoroid.

Ranging from just a few hundred centimetres to a few hundred kilometres in diameter, asteroids are made of rocks and metals. Most are carbon-based, while others are silicate or metal-based. They are left over from the formation of the solar system more than 4 billion years ago.

Millions of asteroids orbit the Sun in the Main Asteroid Belt, a massive ring between 300 and 600 million kilometres in diameter between the orbits of Mars and Jupiter. Most asteroids have elliptical orbits and take a few years to orbit the Sun. Some astronomers think the Asteroid Belt comprises remnants from a planet that was destroyed during a collision, while others believe that they are material left over from a planet that never formed due to Jupiter’s strong gravitational pull.

asteroid Vesta
The giant asteroid Vesta photographed by NASA’s Dawn spacecraft

How do asteroids form?

There are three types of asteroid: carbonaceous (C-type), siliceous (S-type) and metallic (M-type) variants, each corresponding to the composition of an asteroid, be that stony, stony-iron or iron. The composition of an asteroid – be that shape or material – is dependent on when and what it was formed from, as well as if it has undergone reconstruction post collision.

Initially, at the dawn of the solar system, most asteroids were much larger than now commonly found by astronomers, with sizes more consistent with a planet such as Mars and shapes varying wildly. However, the radioactive decay of elements within the asteroid rock melted these larger bodies, and during their fluid stage, gravity pulled them into spherical shapes before they cooled. At this point, though, many smaller asteroids – which cooled more efficiently than their larger brethren – did not reach melting point and retained their uniform rocky-metallic composition and their initial irregular shape.

This process of asteroid formation can be seen vividly when contrasting many of the asteroids that modern scientists and astronomers are currently studying. Take the asteroid Ceres (Ceres was the first asteroid to be discovered and is now considered by some astronomers as a dwarf planet) for example – this is a large asteroid (it has an equatorial radius of 487km) and, in turn, is both spherical in structure and carbonaceous composition (C-class), as it was pulled apart easily and cooled slowly. However, if you compare Ceres to Ida for example, which is a small asteroid (it has a mean radius of 15.7km), you find the latter is both irregular in shape (funnily, it looks like a potato) and heavily composed of iron and magnesium-silicates (S-class).

Where are asteroids found?

Asteroid orbits
The orbits of over 1,000 known Potentially Hazardous Asteroids (PHAs)

The majority of asteroids in our solar system are found in a concentration known as the main belt, which lies between Mars and Jupiter. This belt contains thousands of asteroids and takes roughly four and a half years to orbit the Sun on a slightly elliptical course and low inclination. Despite the fact that they all orbit in the same direction, collisions do occur at low velocities (for such large objects) and these cause the asteroids to be continuously broken up into smaller variants. Of this main belt, certain groups have been captured into peculiar orbits, such as the Trojan group of asteroids that follow Jupiter’s orbit, or the Amor or Apollo groups, which cross the paths of Earth and Mars respectively and the Aten group, which sits inside Earth’s own orbit.

What’s the difference between an asteroid, a meteoroid and a comet?

Despite common misconceptions, these space phenomena are different from each other. An asteroid is a small solar system body in orbit around the Sun, which are sometimes referred to or classed as minor planets.

A meteoroid, however, is a sand-to-boulder- sized particle of debris drifting through space in orbit around the Sun or other bodies. They are smaller than asteroids and tend to travel at higher speeds; their composition ranging from iron to ice. Crucially, though, meteoroids differ from meteors and meteorites, although they are all part of the same body. A meteor is the visible streak of light that occurs when a meteoroid enters the Earth’s atmosphere, while a meteorite is the remaining part of the meteoroid that impacts Earth.

Finally, comets are ice-based small solar system bodies that when close to the Sun display a visible coma (a nebulous temporary atmosphere) and tail of ice, dust and rock particles. Comets, unlike asteroids, generally origin in the outer solar system.

What happens when asteroids collide with each other and with Earth?

Asteroid crater
The crater in Northern Arizona, USA was created form the impact of a large asteroid approximately 50,000 years ago

When asteroids collide with each other there are three main outcomes, each of which depends on the size of the impacting asteroid. If the incoming asteroid is 1/50,000th the size of the larger body then it will merely create a large crater, sending small fragments out into space. If the impactor is roughly 1/50,000th the size of the impacted, then the latter will fracture before breaking into rock and dust, before being pulled back together into a ball of rubble by gravity. Finally, if the incoming asteroid is larger than 1/50,000th the size of the other, larger asteroid, then it will immediately shatter into pieces and form a mini belt of smaller asteroids.

Very rarely, asteroids collide with the Earth, the most notable of which in the past 100 million years was the instigator of the Cretaceous-Tertiary extinction event that wiped out the majority of the dinosaurs 65.5 million years ago. However, there is evidence across the world of many other lesser- sized asteroids impacting the Earth, with their craters remaining a testament to their size. Importantly, their size is not directly represented by the size of the crater, which is roughly ten times the size of the impacting body. These impacts are postulated to have occurred infrequently over the Earth’s 4 billion year life span.

Is an asteroid going to hit Earth soon?

Our friends at All About Space magazine asked asteroid researcher Professor Alan Fitzsimmonds this and many other questions about the potential threat of asteroids. See what he had to say here.

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Plus take a look at:

Top 5 facts about asteroids

Do all planets and asteroids in our Solar System orbit in the same direction? 

Why does the asteroid belt in our Solar System stay in place?