From the films Armageddon and Melancholia to hypotheses about the demise of the dinosaurs, asteroids are regularly thought of as deadly objects on a collision course with planet Earth. But asteroids, and especially their terrestrial counterparts meteorites, are more than mere lumps of space rock. They allow scientists to handle directly pieces of material from other planets and study the evolution of our Solar System. And, as the world found out early last year when a meteor travelling at over 50 times the speed of sound exploded over the city of Chelyabinsk in Russia, they are closer to us than we think.
The Space Rock Glossary
Asteroids are small bodies in the inner solar system, mostly present in a large belt between the orbits of Mars and Jupiter. They are the remnants of what scientists call planetesimals – aggregates of rock and dust in the early Solar System that did not grow large enough to become full-sized planets. A small asteroid is called a meteoroid.
When a meteoroid enters the Earth’s atmosphere, friction and interactions with gases cause it to heat up, leaving a streak of bright visible debris called a meteor, or shooting star. Most such meteoroids are no larger than a pebble.
When a piece of asteroid or meteoroid survives its journey through the atmosphere and its collision with the Earth’s surface, it is called a meteorite. These are what planetary scientists go hunting for in some of the most remote places on Earth. They collect them and take them back to the lab to study what goes on beyond our atmosphere.
Unravelling the story of the Solar System
Meteorites are generally classified into three broad categories. Stony meteorites are rocks. They are mainly composed of silicon-based minerals such as those we find on Earth. Iron meteorites are metallic and largely composed of iron and nickel. Stony-iron meteorites are – you’ve guessed it – meteorites that contain both rock and metal.
Now why are these different meteorites interesting to scientists?
Because they were formed at different times during the evolution of the planets in the Solar System.
The Earth formed over 4.5 billion years ago, within a hot spinning disk of cosmic dust and gas around the Sun. Small grains within this planetary disk started to collide with one another, slowly accumulating into larger bodies, which eventually became our planets. This is a process scientists call planetary accretion.
As the Earth grew, denser elements such as iron and nickel sunk to its interior – a process known as planetary differentiation. This is the reason the Earth is made of a dense central metallic core and a less dense surrounding mantle. The outermost layer that we live on, and that makes up our continents and ocean floors, is called the crust. The crust further differentiated from the mantle later in our planet’s history.
The larger asteroids in the Solar System would have undergone similar initial differentiation processes, and the different classes of meteorites reflect material from different stages of this differentiation.
By far the most common stony meteorites are called chondrites. They are fascinating because they represent the very early material of the Solar System, before the onset of planetary differentiation. Scientists study what elements and minerals are found in chondrites, and some even contain small amounts of organic matter. These give important clues into the formation of the planets and processes such as the synthesis of organic compounds.
Iron meteorites are generally thought to have originated from the dense metallic core of shattered asteroids, i.e. after these had already undergone differentiation. They can tell us about the nature and timing of this planetary differentiation process.
Stony-iron meteorites are differentiated and contain roughly equal parts of rock and metal. They can also provide important information on the timing of this differentiation process.
Hunting for meteorites
Until the start of the 20th century, most collected meteorites were iron or stony-iron samples, because they look different to typical Earth rocks and were more easily recognisable. It wasn’t until the 1930s and 40s, when a man called Harvey Nininger went off looking for space rocks in the great US plains, that people realised that meteorites were much more common than originally thought.
Most meteorites are found in deserts, rather pristine environments that have not been altered by human activities. This includes Australia, the Sahara, Arabia, and the largest desert on Earth: Antarctica. Over 23,000 meteorites from Antarctica have already been classified, and thousands more are waiting to be tended to.
The Chelyabinsk event
On 15 February 2013, the sky exploded over the region of Chelyabinsk in the southern Urals in Russia. An undetected asteroid travelling at over 18 km per second and carrying roughly 500 thousand tons of kinetic energy – several tens of times more than the Hiroshima bomb – suddenly entered the Earth’s atmosphere. Its incredible speed and shallow travel angle caused the meteor to explode roughly 23 km above the ground, bursting into many small pieces and creating a giant shock wave. The shock wave swept the countryside and over 7000 buildings were damaged and hundreds of injured residents had to seek medical attention.
The Chelyabinsk meteor is the largest space object to have entered the Earth’s atmosphere since the Tunguska meteor caused widespread destruction in remote Siberia in 1908. It is the most damaging event recorded in terms of human injuries.
Many meteorites have been recovered from this event. Although scientists have been unable to pinpoint the meteor’s parent asteroid, the meteorites’ composition matches that of a particular type of chondrite with low concentrations of iron and metal.
If you want to know more about the Chelyabinsk meteor, you can listen to this interview with geologist and planetary scientist Dr Matt Genge from the Department of Earth Science and Engineering at Imperial by clicking on the player below. And if you want to see some real meteorites and learn about what they can tell us of our cosmic past, you can always head down to the Natural History Museum, where hundreds of specimens are being studied every year.
IMAGE: Asteroid 243 Ida, NASA.