A Beginner’s Guide to Meteorites, Asteroids, Comets, & Meteors
What's the difference between all these rocks from space?
Here at the National Space Centre, we often get asked about the differences between meteorites, meteoroids, meteors, asteroids, and comets.
These are all essentially space rocks, but what we call them depends on how big they are, what they’re made of, and where they’re found.
At school I was always taught the rhyme:
“meteoroids avoid, and meteorites fall from a height”
However, this simple rhyme doesn’t include the other objects, so we’ve come up with this handy guide to help you.
Asteroids are basically balls of rock and metal, varying in size from 1 meter to over 100 kilometres.
Not all asteroids are made of the same material. The larger ones have a rocky or iron core, similar to small planets. They have a nearly spherical shape, and they can be considered as ‘failed planets’, with not quite enough mass to join the planet club.
The smallest asteroids are usually piles of space rubble held together by gravity. They are usually more irregular in shape and thought to be rocks that have broken off of larger objects in the Solar System. These small asteroids could also be the leftovers from the very formation of the planets 4.5 billion years ago.
There are thousands of asteroids in our Solar System, and many of them orbit between Mars and Jupiter, in something called the Asteroid Belt.
No human has been to an asteroid, so how do we know what they are made of?
There are many ways to find out what things in space are made of, without actually visiting them. One way is by measuring ‘albedo’. Albedo is a measurement of how much sunlight an object reflects. Different materials have different reflective properties – just compare your reflection in a mirror versus a piece of wood.
Another way of finding out what an object is made of is by measuring its density. The density is found from the volume and the mass of a body. Although we cannot actually put planetary bodies on a weighing scale, we can observe their interactions with other things in space and from this work out their mass. The easiest way of doing this is by circling the object with a spacecraft of known mass and observing how its orbit changes due to the object’s gravity, and therefore its mass.
But soon we will have an actual sample of an asteroid to study here on Earth, thanks to NASA’s Osirius-REx mission. This spacecraft arrives at the asteroid 101955 Bennu in August 2018 and will collect up to 2 kilograms of the asteroid for return to Earth by 2023.
Comets are often nicknamed ‘dirty snowballs’, which is a bit unfair on such grand, complex worlds. But there’s some truth in the name – they carry a lot of ice because they form in the outer regions of the Solar System, far from the Sun’s heat.
The main difference between asteroids and comets is that they are made of different materials. While asteroids are made of metal and rock, comets are made of dust, ice, and rock. Comets, like asteroids, range in size between 100 metres and 30 kilometres across, and we know of about 5,000 of them!
Comets, unlike asteroids, have tails. When comets approach the Sun in their orbit, the Sun’s light causes material from the comet to heat up and turn to vapour, creating two majestic tails streaming out behind the comet. One tail is made up of gas particles, and the other is made of dust particles.
Comets often have elliptical, or eccentric orbits, meaning that it can take them over a thousand years to orbit the Sun. Short period comets are those which take less than 200 years to orbit the Sun. For reference, Neptune – the furthest planet from the Sun – takes 164.8 years to complete a full orbit, and Pluto – the famous dwarf planet – takes around 248 years.
Comets and asteroids do occasionally collide with the Earth, especially so when the Earth was relatively young. Many scientists believe that these collisions could be the way that Earth first got water and other important ingredients for life.
Of course, it is also believed that such a collision is what sealed the fate of the dinosaurs 65 million years ago.
One of the most famous comets is Halley’s comet, which has been seen throughout history, and most recently in 1986. To find out more about how this comet may have shaped history, read our blog.
Today we know a lot more about comets thanks to the European Space Agency mission, Rosetta. In 2014, this spacecraft caught up with and landed on a comet, measuring its chemistry and activity as it got close to the Sun.
A meteor is what is often referred to as a shooting star, and comes from the Greek phrase for ‘high in the air’.
It is the fate of objects that come into Earth’s atmosphere. A meteor is a piece of rock, dust, or metal burning up as it hurtles through the atmosphere at speeds between 11-72 kilometres per second!
So a meteor could be a piece of a comet, asteroid, meteorite, or even a micrometeorite falling through the atmosphere.
Not all meteors land on Earth. In fact most them burn up entirely in the atmosphere – luckily for us!
Meteors are more common at certain times, and this is usually when the Earth’s orbit takes it through path of a comet. The tail of a comet leaves behind a trail of debris – dust and grains of rock – that the Earth can pass through regularly. When these pieces of dust enter the atmosphere, they cause the natural fireworks known as meteor showers.
For more information about meteors, check out our meteor infographic in our blogs about the major meteor showers that grace our skies each year.
Not all meteors come from nature. When astronauts use the toilet on the International Space Station, there must be somewhere for the solid waste to go. This is stored in sealed containers and when a container is full they release it from the Space Station to burn up in Earth’s atmosphere.
So, next time you see a ‘shooting star’ it may not be quite as romantic as it seems!
Meteoroids are small objects in space that range from 10 micrometres to 1 metre. Therefore, they are objects that are smaller than an asteroid. Objects smaller than 10 micrometres are called ‘micrometeoroids’, or simply, dust!
Most meteoroids contain nickel and iron and fall into three main categories: iron, stone, and stony-iron. Many meteoroids come from the Asteroid Belt, but they can also come from planetary bodies or other objects in the Solar System.
Once a meteoroid enters the Earth’s atmosphere, it becomes a meteor. If it survives entry through the atmosphere, then the name changes again to a meteorite.
Meteorites are chunks of rock or metal from space that have passed through Earth’s atmosphere and have landed on Earth’s surface (or on another world).
Meteorites usually have characteristic fingerprint-like marks on them from where they have melted in the atmosphere. They are also usually magnetic due to their typically high iron content. Sometimes meteorites are so large and hot that they cause fireballs across the sky, and can therefore by followed and found.
At other times the meteorites are small and are not necessarily visible when they fall. Meteorites that are smaller than 2 millimetres are classed as micrometeorites. To find out more about the meteorites in our collection, have a look at these blog posts:
You can touch a meteorite in person, and learn more about these 4.5-billion-year-old rocks, in our Solar System gallery at the National Space Centre.
About the author: Elspeth Lewis is a physics student at the University of Leicester and works as a Science Interpreter at the National Space Centre.