Grade by Grade
E. Earth and Space Systems · Space

Moons, Asteroids, Comets and Meteoroids

E2.1 A1.2 A1.4About 45 minutes
Learning goals
  • I can describe natural satellites (moons), including Earth's Moon.
  • I can tell the difference between asteroids, comets and meteoroids.
  • I can explain the difference between a meteoroid, a meteor and a meteorite.
  • I can do a fair test to see how drop height changes the size of a crater.

Words to know

natural satellite
A natural object that orbits a planet or another body. Moons are natural satellites.
asteroid
A rocky or metal object, smaller than a planet, that orbits the Sun. Most are in the asteroid belt.
comet
A ball of ice, dust and rock that orbits the Sun. Near the Sun it grows a glowing cloud and tails.
meteoroid
A small piece of rock or metal travelling through space, much smaller than an asteroid.
meteor
The streak of light we see when a meteoroid enters Earth's atmosphere and burns up. Also called a shooting star.
meteorite
A piece of a meteoroid that survives the trip through the atmosphere and lands on the ground.
crater
A bowl-shaped hole made when an object from space hits a surface.

A shooting star that isn't a star

Have you ever seen a quick streak of light zip across the night sky? People call it a shooting star. But it is not a star at all! It is a tiny bit of space rock, often smaller than a grain of sand, burning up as it rushes into Earth's air. In this lesson you will meet the smaller members of the solar system.

A bright streak of light crossing a starry night sky above a lake and pine trees.
A "shooting star" is really a meteor: a tiny piece of space rock burning up as it rushes into Earth's air. AI-generated image

Natural satellites: moons

A natural satellite is a natural object that orbits a planet. We usually call them moons.

Earth's Moon:

  • It is about 3475 km across, a bit more than one quarter of Earth's width.
  • It is about 384 400 km from Earth.
  • It is rocky and covered with craters, mountains and dark, flat areas made by ancient lava.
  • It has almost no air, so there is no weather, no wind, and no sound can travel.
  • It takes about 27 days to orbit Earth.

Other moons: Mercury and Venus have no moons. Mars has two small moons. The giant planets have many: NASA's 2025 count was 95 for Jupiter and 274 for Saturn, and new ones are still being found. Jupiter's moon Ganymede is the biggest moon in the solar system. It is even wider than the planet Mercury.

ObjectWhat it is made ofWhere it is foundWhat it looks like
Asteroidrock and metalmostly in the asteroid belt between Mars and Jupiterlumpy rocks, from a few metres to hundreds of kilometres wide
Cometice, dust and rock (a "dirty snowball")comes from the cold, far edges of the solar system and travels in long, stretched orbitsnear the Sun: a fuzzy glowing head and long tails
Meteoroidsmall bits of rock or metalall over the solar system; many are crumbs from comets and asteroidsfar too small to see in space
The small bodies of the solar system.

Comets grow tails

Far from the Sun, a comet is just a frozen chunk of ice, dust and rock. As it gets close to the Sun, it heats up. Some of the ice turns straight into gas, and dust is released. This makes a fuzzy cloud around the comet, called the coma, and long tails.

The tails always point away from the Sun, because sunlight and the stream of particles from the Sun push the gas and dust outward.

Sunfar away:frozen, no tailgetting closer:a small tailnear the Sun:a long tail pointingAWAY from the Sunmoving away: the tail still points awaynot to scale
A comet's long, stretched orbit. Far from the Sun it has no tail; near the Sun it grows a tail that always points away from the Sun, even when the comet is heading back out.

Meteoroid, meteor, meteorite: three names for one journey

  1. In space, a small rock is a meteoroid.
  2. When it rushes into Earth's atmosphere, it heats up and glows. The streak of light is a meteor.
  3. If a piece survives and lands on the ground, it is a meteorite.

Most meteoroids are so small that they burn up completely. When Earth passes through a trail of dust left by a comet, we see many meteors in one night. This is a meteor shower. The Perseids, which happen every August, are a well-known one.

Memory trick: a meteorite has hit the ground.

1. meteoroida small rockin space2. meteora streak of lightin Earth's air3. meteoritea piece thatreached the groundair
Three names for one journey: a meteoroid in space becomes a meteor when it glows in our air, and a meteorite if a piece lands.
Worked example

Let's sort these space objects

Sort each one: moon, asteroid, comet, meteoroid, meteor or meteorite?

(a) A rock 500 km wide orbiting the Sun between Mars and Jupiter. (b) A streak of light across the sky. (c) An icy body with a long tail pointing away from the Sun. (d) A rock that orbits Mars. (e) A space rock found in a farmer's field.

  1. (a) Big rock, orbits the Sun, in the asteroid belt → asteroid.
  2. (b) A streak of light in our atmosphere → meteor.
  3. (c) Icy, with a tail pointing away from the Sun → comet.
  4. (d) It orbits a planet, not the Sun → moon (natural satellite).
  5. (e) It reached the ground → meteorite.

(a) asteroid, (b) meteor, (c) comet, (d) moon, (e) meteorite. Ask yourself: what does it orbit, what is it made of, and where is it now?

Just for fun

Why did the asteroid skip the big party in the asteroid belt?

It was feeling a little rocky. 🪨

Hands-on activity

Make craters: a fair test

You need: a baking pan or shallow box, flour (enough for a layer about 3 cm deep), a spoonful of cocoa powder or cinnamon, 3 round objects of different sizes such as a marble, a golf ball and a small rubber ball, a ruler or metre stick, newspaper or an old sheet, and a grown-up helper.

  1. Put the pan on newspaper on the floor. Fill it with flour and smooth the top. Sprinkle a thin layer of cocoa on top so craters show up.
  2. Question (A1.2): does dropping a "meteorite" from higher up make a bigger crater?
  3. Predict what will happen.
  4. Drop (do not throw) the marble from 30 cm. Measure the crater's width. Smooth the flour and do it again 2 more times.
  5. Repeat from 60 cm and 90 cm. Record your results in a table.
  6. Fair test: use the same marble, the same flour and the same way of letting go. Change only the height.
  7. Extra: try the different-sized balls from the same height.

What to look for: higher drops usually make wider craters, because the object is moving faster and has more energy when it hits. You may also see "rays" of cocoa and flour thrown out around the crater, like the bright rays around some craters on the Moon.

Safety (A1.4): marbles are a choking hazard, so keep them away from young children. Drop, never throw. Do not breathe in flour dust, and clean up spilled flour so no one slips.

Your turn

1

What is a natural satellite? Give an example.

2

What is the difference between an asteroid and a comet?

3

Which way do a comet's tails point?

4

What is a meteor?

5

What do we call a space rock that lands on Earth?

6

Which planets have no moons?

7

Why does the Moon have so many craters?

More practice: warm-up

1

Where are most asteroids found?

2

What is the biggest moon in the solar system?

3

What is another name for a meteor?

4

What is a comet made of?

5

How many moons does Mars have?

More practice: on your own

1

Put in order of the journey: meteorite, meteoroid, meteor.

2

Why does a comet only grow a tail when it gets near the Sun?

3

A rock is orbiting Saturn. Is it an asteroid or a moon? Explain.

4

In the crater test, Maya dropped the marble from 30 cm once, but from 90 cm she pushed it down hard. Is this a fair test? Explain.

5

Why do most meteoroids never reach the ground?

Challenge

1

Earth's diameter is 12 756 km and the Moon's is 3475 km. About how many Moons would fit across Earth? Show your thinking.

2

Earth has been hit by many space objects too, but we see far fewer craters on Earth than on the Moon. Give two reasons.

3

Design your own fair test: does the size of the ball change the crater size? What would you change, keep the same, and measure?

Check your understanding

Quiz

1.

What is a natural satellite?

2.

A ball of ice, dust and rock with tails pointing away from the Sun is...

3.

What is a meteor?

4.

A piece of space rock that lands on the ground is a...

5.

Where are most asteroids?

6.

Which is the biggest moon in the solar system?

For parents and teachers

Big idea (E2.1): this lesson finishes the components of the solar system: natural satellites, asteroids, comets and meteoroids. The crater investigation practises the scientific experimentation process (A1.2) and safety (A1.4).

How to help: do the crater test outdoors or on newspaper. Watch for a meteor shower together (the Perseids happen every August). Go somewhere dark with an adult, let your eyes adjust for about 20 minutes, and be patient.

Common difficulties:

  • Mixing up meteoroid, meteor and meteorite. Use the journey: in space, in the air, on the ground.
  • Thinking all moons are like ours. Moons vary hugely; Ganymede is wider than Mercury.
  • Moon counts. The number of known moons of the giant planets keeps changing as astronomers find small ones, so exact counts in books may differ.

Safety: marbles and small balls are choking hazards for young children.

Sources: NASA Planetary Fact Sheet (Moon size and distance, moon counts as of 2025).