Comets, Asteroids and Meteors

Space is not empty

When we think about the Solar System, we usually picture the Sun and its eight planets. But in between and all around them, space is sprinkled with smaller objects — chunks of rock, lumps of metal and balls of dirty ice. They are the leftovers from when the Solar System formed, about 4.6 billion years ago.

Three of these objects often get muddled up: comets, asteroids and meteors. They sound similar, but they are quite different. Once you know what each one is, you will be able to explain shooting stars, spot the difference between a comet and an asteroid, and even watch some of them with your own eyes.

Asteroids: the rocky leftovers

Asteroids are lumps of rock and metal that orbit the Sun, just like planets do — only they are far smaller. Some are the size of a car. The largest, called Ceres, is about 940 kilometres across, big enough to be round, but most asteroids are lumpy, potato-shaped rocks.

Most asteroids live together in a huge ring called the asteroid belt, which sits between the planets Mars and Jupiter. There are millions of them there. You might imagine a crowded, crashing field of rocks like in films, but the belt is actually mostly empty space — the asteroids are spread so far apart that spacecraft fly straight through without trouble.

Why is there a belt instead of a planet? Long ago, the giant gravity of nearby Jupiter kept stirring up this region, so the rocks could never clump together into a single world. The asteroid belt is, in a way, the planet that never finished forming. Scientists love asteroids because they are like fossils of the early Solar System, frozen just as they were billions of years ago.

Comets: dirty snowballs with tails

Comets are also leftovers from the Solar System's birth, but they are made of different stuff. A comet is a mix of ice, dust and rock frozen together, which is why scientists nickname them "dirty snowballs". Most comets spend their lives in the cold, dark, outer edges of the Solar System, far beyond Neptune.

Now and then, a comet's long orbit brings it swooping in towards the Sun — and that is when the magic happens. As the comet warms up, its ice turns straight into gas, releasing dust. The Sun's light and a stream of particles called the solar wind push this gas and dust away from the comet, forming a glowing tail that can stretch for millions of kilometres. A comet's tail always points away from the Sun, no matter which way the comet is travelling.

One of the most famous is Halley's Comet, which loops past Earth roughly every 76 years. It last appeared in 1986 and will return around 2061. People have recorded it for over 2,000 years — it even appears stitched into a thousand-year-old tapestry from medieval Europe.

Meteors: the shooting stars

Have you ever seen a quick streak of light dart across the night sky? People call it a shooting star, but it is not a star at all. The proper name is a meteor.

Here is what really happens. Space is full of tiny rocks and grains of dust, often no bigger than a pebble or even a grain of sand. These are called meteoroids. When one of them crashes into Earth's air, it is travelling incredibly fast — tens of thousands of kilometres per hour. The rubbing against the air heats it up so much that it glows white-hot and burns away in a bright streak. That glowing streak is the meteor.

Most meteoroids are so small that they burn up completely, high in the sky, and never reach the ground. But if a piece is big enough to survive the fiery fall and land, that piece is called a meteorite. Museums display real meteorites you can sometimes touch — actual rocks from space.

So the three words describe the same object at three different moments:

Stage	Where it is	Name
Floating in space	Out in the Solar System	Meteoroid
Burning in the sky	In Earth's atmosphere	Meteor (shooting star)
Landed on the ground	On Earth's surface	Meteorite

Why these space rocks matter

Comets and asteroids are not just exciting — they help us understand where we came from. Many scientists think comets and asteroids delivered some of the water and the building blocks of life to the early Earth when they crashed into it billions of years ago. So studying them is a way of studying our own origins.

They have also shaped life's story. About 66 million years ago, a huge asteroid (or comet) struck Earth and helped wipe out the dinosaurs. You can read more about that in Dinosaurs and Fossils. Today, space agencies keep a careful watch on large asteroids and have even tested gently nudging one to change its path — just in case.

Try it yourself: watch a meteor shower

Several times a year, Earth passes through the dusty trail left behind by a comet. When that happens, lots of meteoroids hit our air at once, and we get a meteor shower — dozens of shooting stars in a single night. The best part: you need no equipment at all, just your eyes.

1. Find out when the next big shower is. Reliable ones include the Perseids (around mid-August) and the Geminids (around mid-December). A quick check with a grown-up online will give the date.
2. Pick a clear, dark night, away from bright streetlights if you can. A garden, balcony or park is fine.
3. Let your eyes adjust to the dark for about 20 minutes — avoid looking at phone screens, which spoil your night vision.
4. Lie back, get cosy and watch a wide patch of sky. Be patient. Count how many shooting stars you see in an hour.

Each streak you spot is a tiny crumb of a comet, burning up just for you. Wrap up warm, bring a grown-up, and enjoy the show.

Want to explore further? See Planets of the Solar System to meet the worlds these rocks travel between, and The Sun, the Moon and the Stars for a gentle first look at the night sky.