Nobody has been to Jupiter. How do we know what it looks like?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain how telescopes, satellites and space probes help us learn
Name ways satellites help people, such as weather and phone signals
People have walked on only one other body, the Moon. No person has been to another planet. Yet we know what Jupiter's clouds look like. Three kinds of tool do the visiting for us.
The first tool is the telescope. A telescope collects more light than your eye can. So it shows things that are faint or far away.
Draw it first
Draw how a telescope collects light
1 Draw a far star and six arrows of light coming from it. The star is so far away that the arrows run side by side.
2 Draw your eye as a small opening. It catches only two of the six arrows.
3 Draw a telescope as a wide opening. It catches all six arrows. A round opening 3 times as wide catches 9 times as much light.
A telescope has a big lens or mirror to catch the light. The wider it is, the more light it catches. More light means a brighter picture, so fainter and farther things can be seen.
Try it
Catch rice like a telescope catches light
You need: a small cup about 5 cm wide, a wide bowl about 15 cm wide, a tray or a cloth to work on, a handful of dry rice or small beans, paper and a pencil.
Be safe: Do not eat the rice. Sweep up any that falls on the floor, so that nobody slips.
Put the cup and the bowl side by side on the tray.
Hold a pinch of rice high above them. Let it fall like rain, evenly over both.
Do this until the cup has caught a few grains. Stop.
Count the grains in the cup and the grains in the bowl. Write both numbers down.
Divide the bowl's number by the cup's number. How many times as much did the bowl catch?
What you will see: The bowl is 3 times as wide as the cup. But it catches about 9 times as much rice. A wide mirror catches far more light, in the same way.
If you cannot try it, this is what happens.
The second tool is the satellite. A satellite is a body that goes round a planet. The Moon is a natural satellite of the Earth. People have also built satellites and sent them up. They orbit the Earth, held by gravity.
A satellite goes round the Earth on its orbit, held by gravity.
Satellites help us in many ways. They send down pictures of clouds, so we get weather forecasts. They carry phone and television signals across the world. They help a phone find where you are. They look at forests, farms, ice and the sea.
Four jobs for satellites that orbit the Earth.
The third tool is the space probe. A space probe is a machine sent to travel to another planet or moon. It has cameras and other instruments. It sends pictures and measurements back to the Earth by radio.
Some probes fly past a planet. Some go into orbit round it. Some land on it. No person has to go.
See it three ways
As a picture
In words
A telescope stays on the ground, or in orbit, and collects light. A satellite goes round the Earth. A space probe leaves the Earth and goes to another planet.
The probe's messages travel as radio waves. Radio waves go about 300 000 km every second. That is very fast, but space is very big. A signal needs time to cross it.
Radio and light go 300 000 km every second. Sunlight takes about 8 minutes to reach us.
Time = distance ÷ 300 000 km each second. A message from the Moon takes about 1.3 seconds. A message from a probe far out in the solar system can take hours.
A wrong idea many people have
We know about the planets because people have visited them.
Why it feels right: Pictures of planets look like photographs taken by a traveller.
The test that breaks it: Ask who took the picture. It was taken by a camera on a probe or on a satellite. Or it came from a telescope. No person was there.
What is true: We learn about the planets from light collected by telescopes. We also learn from probes and satellites that go there for us.
Key words
telescope
a tool that collects more light than your eye, to show faint and far things.
satellite
a body that goes round a planet, such as a built one that orbits the Earth.
space probe
a machine sent to travel to another planet or moon.
Back to your guess
The answer is B: Telescopes, satellites and space probes collect light and send us pictures and measurements.
Telescopes collect light from far away. Satellites orbit the Earth and look down. Space probes travel to other planets and send pictures and measurements home.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What does a telescope do?
See the answer
It collects more light, so faint and far things can be seen.
A telescope has a wide lens or mirror that catches more light than an eye. So it shows things that are faint or far.
If you chose A: Nothing moves. A telescope only collects more light and makes the picture bigger and brighter. Go to S3.12.8.
If you chose B: Never look at the Sun, not even through a telescope. It can blind you in a moment. Go to S3.12.8.
What is a satellite?
See the answer
A body that goes round a planet, such as a machine that orbits the Earth
A satellite goes round a bigger body. The Moon is a natural satellite. Built satellites orbit the Earth.
If you chose B: That is a space probe. A satellite stays in orbit round a planet or a moon. Go to S3.12.8.
If you chose C: Stars are far-off suns. A satellite is a body that goes round a planet, held by its gravity. Go to S3.12.8.
Which machine travels to another planet and sends pictures home?
See the answer
A space probe
A space probe is sent to travel to another planet or moon. It sends its pictures back by radio.
If you chose A: A weather satellite stays in orbit round the Earth. It does not go to another planet. Go to S3.12.8.
If you chose C: A telescope on the ground stays on the Earth. It collects light but does not travel. Go to S3.12.8.
Where do the cloud pictures in a weather report come from?
See the answer
Satellites that orbit high above the Earth
Weather satellites orbit the Earth. They look down on the clouds and send pictures to the forecasters.
If you chose A: A telescope on the ground looks up at space. Cloud pictures come from machines that look down from above. Go to S3.12.8.
If you chose B: A space probe goes far away. Weather pictures come from satellites that stay in orbit close to the Earth. Go to S3.12.8.
A radio signal travels 300 000 km every second. A probe is 2 700 000 km away. How long does a signal take to reach it?
See the answer
9 seconds
Time = distance ÷ speed = 2 700 000 ÷ 300 000 = 9 s.
If you chose B: You counted one zero too many. 300 000 × 90 is far more than 2 700 000. Go to S3.12.8.
If you chose C: You divided the speed by the distance, upside down. Time = distance ÷ speed. Go to S3.12.8.
Which of these is a way that satellites help people?
See the answer
They carry phone and television signals across the world.
Satellites pass signals over long distances, so a call or a television picture can cross the world. They also send weather pictures and help a phone find its position.
If you chose A: Satellites only watch the weather from above. They do not make it. Go to S3.12.8.
If you chose C: A satellite is far too small to light the ground. You may see one as a moving dot of reflected sunlight. Go to S3.12.8.
From an earlier chapter
A comet is near 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 from the Sun. So the tail points away from it.
If you chose A: The Sun's light and wind push the tail away from the Sun, not towards it. Go to S3.12.7.
If you chose B: The tail is not smoke left behind. It always points away from the Sun, even as the comet goes away. Go to S3.12.7.
Keep it
Copy this into your exercise book.
Telescopes collect light to show far things. Satellites orbit the Earth for weather, signals and positions. Space probes travel to other planets and send messages by radio.
Draw: The Earth with a telescope, a satellite on a ring and a probe on an arrow.
