Space Rocks: Whats What

Meteor, asteroid, comet, fireball: what’s the difference? Know your meteorite from your meteoroid with OGN's simple handy guide.


Mankind has observed space rocks for millennia. We now watch TV at night. Once upon a time we watched the sky for our nightly entertainment, lying beside the warm glow of a crackling fire.


Iron meteorites have been prized throughout history: such as Tutankhamun's meteoric iron dagger, or the Buddha carved from a meteorite that fell 15,000 years ago? Halley's Comet, along with a famous arrow striking King Harold, was immortalised in the Bayeux Tapestry, made sometime in the 11th Century.


But what's the difference between an asteroid and a comet? Or a meteorite and a meteoroid? Answers to these questions, as well as a simple round-up of the different types of space rock, are below.


Shooting star streaking across the night sky

Asteroids: Small, rocky objects, often irregularly shaped, leftover from the formation of the Solar System. Asteroid Bennu made the news two years ago when a NASA mission successfully landed on the asteroid's surface and collected over 400g worth of samples. But astronomers must be patient for a little while longer as the spacecraft is still on its way back home, due to swing by in September 2023 to deliver the precious cargo.


Comets: Comets are the snowballs of the cosmos. Large, icy bodies of frozen gases, dust and rock, with a frozen nucleus. Arguably the most famous comet is Halley's Comet, which is due to return to our skies in July 2061. Comets are so varied in size, orbit and composition that this has resulted in numerous sub-classifications over the years but, fundamentally, there are three main parts to a comet: Nucleus (the solid core), Coma (the gases expelled by the nucleus) and Tail (the stream of gas and dust left in the comet's wake).


Meteoroid: Fragments and debris from asteroids and comets. When a meteoroid enters the Earth’s atmosphere and burns up, it leaves a glowing trail in the sky and is known as a meteor – or more popularly, a shooting star. Meteoroids can be as small as a grain of sand.


Meteor shower: Multiple meteoroids burning up in the Earth's atmosphere. As comets or asteroids travel around the Sun, they leave behind a trail of debris in their wake. When Earth’s orbit intersects with this debris, the result is hundreds (or thousands) of bright trails, appearing to radiate from one point in the night sky. This is why we often see meteor showers at the same time every year, and meteors are more numerous on certain nights.


So, a meteor could more accurately be described as an event, rather than an object: when that tiny particle (called a meteoroid) enters the upper parts of the atmosphere and heats the air around it to incandescence. This is the glow we see as a meteor.


Fireball: An exceptionally bright meteor that can be seen over a wide area.


Meteorite: Meteoroids that survive the journey through the Earth's atmosphere and fall to the surface. So far, more than 69,000 meteorites have been found (and named) on Earth.

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