Microgravity: living on the International Space Station

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# 4 How the ISS stays up there

The ISS is just a satellite in orbit. It wasn’t launched in one go. It is a combination of many smaller satellites launched over several years and joined together. It is obviously a massive satellite when compared with the others. It does occupy a very low orbit though, mainly to avoid all of the other satellites and especially space debris. This was shown graphically in the film Gravity (2013)!

However, because of this relatively low orbit on the edge of the Earth’s atmosphere, the ISS must be reboosted occasionally. The Earth’s atmosphere slows it down, resulting in the ISS falling slowly back to Earth.

You can now test your knowledge in the next activity.

## Activity 6 A test on the ISS

Timing: Allow approximately 15 minutes

1. How high is the ISS from Earth? Choose the correct answer from the options below.

a.

400 km

b.

10 km

c.

1000 km

d.

Near the orbit of the Moon

e.

100 000 km

2. Using the correct answer from Question 1, what is the approximate percentage of the distance from the ISS to the Earth’s surface compared with the radius of the Earth? (Hint: divide the distance in Question 1 by the radius of the Earth (about 6000 km) and then multiply by 100. This is your answer in %.)

a.

7%

b.

70%

c.

0.07%

3. The ISS’s speed is approximately:

a.

28 000 km/h

b.

100 km/h

c.

1000 km/h

The ISS’s speed is approximately 28 000 km/h.

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But what keeps the ISS in orbit around the Earth? Your first (and correct) answer is probably ‘gravity’. This ‘force’ of gravity’ gives the feeling of weight. But people often mix up scientific terms in everyday speech. For example, how often do you hear people saying that they need to lose weight? Mass and weight are not the same. Mass is a measure of the amount of ‘stuff’ you have and is measured in kg, whereas weight is a force, due to gravity, which is measured in N.

This force of gravity helps the ISS to orbit the Earth in a similar way to the Moon orbiting the Earth. The speed of the ISS and the Moon around the Earth are just enough to keep them orbiting.

You know that the Moon is a natural satellite of the Earth (that is, not manufactured), but what types of satellite are launched by humans into orbit? Two particularly distinct orbit trajectories are:

• geostationary – staying above the same part of the Earth (for example, Sky TV)
• polar orbiting (for example, monitoring weather systems).

These are shown in Figure 12.

Figure 12 Geostationary and polar orbiting satellites.

Various global positioning and communication satellites are placed in orbits that are somewhere between geostationary and polar.

But how else other than gravity does the ISS stay in motion around the Earth? The answer to this is circular motion. Watch Video 3 which demonstrates circular motion with water in a bucket. Pay attention to what happens to the water when the swinging motion is stopped.

Skip transcript: Video 3 Circular motion of water.

#### Transcript: Video 3 Circular motion of water.

TOM:
OK, the first thing we do is take the bucket. We have the water in the bucket, and we start to emulate circular motion by gradually building up speed, and then confidently swinging it around in a motion where the water stays in the bucket. If you slow it down very carefully, the water doesn't fall out of the bucket.
The trick now is to show you circular motion as we're going around, what happens when the force stops. The water falls out of the bucket. At the wrong time, the water will fall out and continue towards the centre. As proof, there's very little water left in the bucket.
End transcript: Video 3 Circular motion of water.
Video 3 Circular motion of water.
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In the next video, circular motion is demonstrated using a bicycle. Watch this video before moving onto the first practical experiment in this course.

Skip transcript: Video 4 Circular motion of a bicycle.

#### Transcript: Video 4 Circular motion of a bicycle.

[SIGHING]
HELEN:
So I may not be the best bicyclist in the whole world, but ever since I learned to ride, and probably when you learned to ride as well, we learnt that when we go around the corner on our bicycles, we have to lean into the corner so that we don't fall over. And that's all about balancing forces, something we do very naturally when riding our bikes. And this next practical is all about motion in a circle and learning about how we balance those forces and how those forces help us to maintain that motion in a circle.
So Tom and I are now back inside in the warm in the lab. And we've gathered together all the things we need to do the experiment about motion in a circle. First of all, we've got a jar of lentils, pasta, rice. Anything like that will do. Second, we've got some plastic cups. This one is a clear one. This one is a paper one. Either one is just as good. And what we need to do is we need to take the cup, and we need to make a small hole on either side of the cup.
Now, I happen to be using the point of a compass, but you could use scissors or actually a pencil. Either one will do. And what you need to do is thread some string through each side of the cup to make something like this, which Tom made earlier-- a piece of string with two knots on it that can hang equally from either side of the cup. And now what I'm going to do is I'm going to ask Tom to fill the cup as bravely as he feels like with some lentils from the jar. Can you do that, Tom?
TOM:
Thank you, Helen. Well, I'm going to take the jar of lentils. I'm going to place some of these into the cup. I think that should be enough.
HELEN:
OK, Tom. Why don't you go and give that cup a whirl around in a circle?
TOM:
I shall certainly try, Helen. Thank you.
HELEN:
Now, the important thing for Tom to do is not to spill any of the lentils. Let's see if it's possible.
[LAUGHTER]
Well done, Tom.
TOM:
Thank you, Helen.
[CLAPPING]
There we go, and not a single lentil lost.
HELEN:
Wow, Tom. That was amazing. But I'm really confused. At some points, the cup was clearly upside down. How did the lentils not all just fall out?
TOM:
And it's amazing when you look at that. The fact that the lentils don't fall out, the reason behind that is the forces are balanced. So as it's going round in circular motion, the forces are balanced so that the lentils stay in as it's going round in a circle, very similar to what happens in the International Space Station. Now, you need to look at the text now as we move again to talking about circular motion and the International Space Station, how it stays in orbit around the Earth.
End transcript: Video 4 Circular motion of a bicycle.
Video 4 Circular motion of a bicycle.
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