A shooting star flashes across the sky. Is it a star that is falling?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Describe moons, dwarf planets, asteroids and comets
Explain what a shooting star really is
The solar system holds more than the Sun and the eight planets. It holds moons, dwarf planets, asteroids and comets. Each is a different kind of body.
A moon is a body that goes round a planet or a dwarf planet. The Earth has one Moon. Mercury and Venus have none. Mars has two.
The giant planets have many moons. New ones are still being found, so we say about.
Moons of each planet. Saturn has the most, about 270.
A dwarf planet is round, like a planet. But it shares its path round the Sun with many other bodies. Pluto and Ceres are dwarf planets.
Dwarf planets are small. Pluto is smaller than our Moon. The table gives the width of some bodies.
Pluto and Ceres are dwarf planets. Both are smaller than the Moon.
An asteroid is a lump of rock that goes round the Sun. Most asteroids lie in a belt between Mars and Jupiter. There are millions of them, and most are small.
The asteroid belt lies between Mars and Jupiter. Not to scale.
A comet is a ball of ice and dust on a long, thin orbit round the Sun. Far from the Sun it is a frozen lump with no tail.
Near the Sun the comet warms. Gas and dust stream off it. The Sun's light and wind push them away from the Sun, so the comet gets a tail. The tail always points away from the Sun.
Draw it first
Draw a comet on its long orbit
1 Draw the Sun and a long, thin orbit. Draw the comet far away with no tail, and a little nearer with a short tail.
2 Draw the comet near the Sun, too. Near the Sun its tail is longest.
3 Draw each tail pointing straight away from the Sun. Name the parts.
A comet's tail points away from the Sun even when the comet is moving away. The tail is not smoke blown out behind it. It is pushed by the Sun.
Now, the shooting star. Tiny bits of rock and dust fly through space. Many are smaller than a pea. Sometimes one meets the Earth and flies into our air.
The bit moves at tens of kilometres every second. It squeezes the air in front of it. The air gets very hot. The bit glows and burns up, high above the ground, about 100 km up.
See it three ways
As a picture
In words
A small bit of rock or dust burns up in the air. The glowing streak is called a meteor. People call it a shooting star, but it is not a star. A real star is a far-off sun.
We call the streak of light a meteor. Most bits burn up completely, so nothing reaches the ground. If a piece does reach the ground, it is a meteorite.
Try it
Watch for shooting stars
You need: an adult to come with you, a warm blanket or mat to lie on, a safe, open place with a clear dark sky, paper and a pencil.
Be safe: Go with an adult, to a safe place away from roads. Never go alone at night. Stay on the mat and do not wander in the dark.
Go out at night with an adult. Choose a safe place, away from roads, water and steep edges.
Lie on the mat and look at the sky. Wait for ten minutes, so your eyes get used to the dark.
Watch for 30 minutes. Every time you see a bright streak, make a mark on your paper.
After each 10 minutes, count the marks and write the number down.
What you will see: You may see a few streaks, or none. Some nights are better than others. Each streak is a bit of rock or dust burning up, high in the air.
If you cannot try it, this is what happens.
A wrong idea many people have
A shooting star is a star that falls to the Earth.
Why it feels right: The streak looks like a star that has come loose and dropped across the sky.
The test that breaks it: The stars are still in their places after a streak has gone. A real star is a sun, very far away. It does not drop across the sky.
What is true: A shooting star is a meteor. It is a small bit of rock or dust that burns up high in the air.
Key words
moon
a body that goes round a planet or a dwarf planet.
dwarf planet
a small, round body that shares its path with many others.
asteroid
a lump of rock that goes round the Sun, mostly in a belt between Mars and Jupiter.
comet
a ball of ice and dust on a long orbit, with a tail near the Sun.
meteor
the streak of light when a bit of rock or dust burns up in the air.
meteorite
a piece of space rock that reaches the ground.
Back to your guess
The answer is A: It is a small bit of rock or dust burning up high in the air.
A shooting star is a small bit of rock or dust from space. It burns up in the air high above us. A real star is a huge, far-off sun, and it does not fall.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What is a shooting star?
See the answer
A small bit of rock or dust burning up high in the air
The bit flies into the air at great speed. The air heats it and it glows. We see the streak and call it a meteor.
If you chose A: Stars are huge, far-off suns. They do not fall. The streak is a tiny bit of rock or dust close to the Earth. Go to S3.12.7.
If you chose C: A planet is a big ball that goes round a star. A shooting star is a tiny bit of rock or dust that burns up. Go to S3.12.7.
A comet is close to the Sun. Which way does its tail point?
See the answer
Away from the Sun
The Sun's light and wind push the comet's gas and dust away. So the tail always points away from the Sun.
If you chose A: It is not smoke trailing behind. The Sun's light and wind push the tail away from the Sun, even as the comet goes away. Go to S3.12.7.
If you chose B: The Sun pushes the gas and dust away. So the tail points away from the Sun, not towards it. Go to S3.12.7.
Where is the main belt of asteroids?
See the answer
Between Mars and Jupiter
Most asteroids go round the Sun in a belt between the orbits of Mars and Jupiter.
If you chose B: Mars is the next planet after the Earth, and there is no belt between them. The belt is farther out. Go to S3.12.7.
If you chose C: Beyond Neptune lie icy bodies and dwarf planets such as Pluto. The main belt of asteroids is nearer the Sun. Go to S3.12.7.
Why is Pluto called a dwarf planet?
See the answer
It is round, but it shares its path with many other bodies.
A planet clears its path of other bodies. A dwarf planet is round but shares its path with many others. Pluto is small, too: smaller than our Moon.
If you chose A: Pluto is round. A dwarf planet is round, like a planet. It is the shared path that makes it a dwarf. Go to S3.12.7.
If you chose C: Pluto does have moons. Having a moon does not decide whether a body is a planet. Go to S3.12.7.
Which planet has the most moons?
See the answer
Saturn
The table shows about 270 moons for Saturn, more than any other planet. New moons are still being found, so we say about.
If you chose A: The Earth has only one Moon. The giants have far more. Go to S3.12.7.
If you chose B: Jupiter has about 95 moons, but Saturn has about 270. Look at the table of moons. Go to S3.12.7.
From an earlier chapter
What keeps a moon going round its planet?
See the answer
The planet's gravity pulls on it.
The planet pulls on its moon all the time. The moon moves sideways, so its path curves into an orbit.
If you chose B: The Sun does pull on a moon. But the planet's gravity bends the moon's path into an orbit round the planet. Go to S3.12.3.
If you chose C: With no pull, a moon would fly off in a straight line, like your ball. Go to S3.12.3.
Kofi reads that the Moon is 3 474 km wide and Pluto is 2 377 km wide. How much wider is the Moon than Pluto?
See the answer
1 097 km
3 474 − 2 377 = 1 097 km.
Keep it
Copy this into your exercise book.
Moons orbit planets. Asteroids are rocks, mostly between Mars and Jupiter. Comets are icy and their tails point away from the Sun. A shooting star is a meteor, burning up.
Draw: The Sun and a long thin orbit. Draw a comet near the Sun, its tail pointing away.
