# 02Orbits and satellites

Es02TA of the Earth in Space topic
• ## 01 Going around in circlesEs02TAnugget01 Activity

### Demonstrating the need for a centripetal force

What the activity is for

This activity provides a memorable demonstration of motion in a circle and demonstrates the need for an inward centripetal force for circular motion.

What to prepare

• a long length of cord with a large rubber bung on the end
• a bucket half filled with water
• a length of rope

Safety note: The teacher should try these demonstrations in advance. Check that the bucket and the rope will exert the force required. Make sure pupils stay out of the firing line.

What happens during this activity

This is a demonstration for the school yard or playing field. Line the class up, shoulder to shoulder, facing you in a line. Take the rubber bung and start swinging it around in a circular path on the end of the cord (cowboy style above your head). Draw attention to the way in which this particular satellite is being pulled along its circular path by the tension in the cord.

Now warn the class that you are going to release the cord so that the satellite can escape from its orbit! Tell them not to worry. You'll release the satellite as it passes directly in front of the class.

Some pupils may become apprehensive. But, of course, if released at this point the satellite continues along its tangential path, parallel to the line of pupils.

The demonstration can be extended by offering to use a bucket of water as a satellite. This does not introduce any new ideas, but the class will be fascinated by the possibility of you becoming soaked!

With the bucket half full of water, start swinging it around on a short length of rope and gradually pay out the rope to increase the radius of orbit of the satellite.

What force is acting to keep the water in the bucket moving along a circular path?

The question is certainly worth asking, but only after you have got the satellite back to Earth. (Answer: the force is provided by the sides and bottom of the bucket acting on the water.)

• ## 02 Satellites in the newsEs02TAnugget02 Activity

### Uses of satellites

What the activity is for

To explore the range and variety of the uses of satellites.

What to prepare

• a collection of web site addresses (the ESA and NASA sites will lead to many others) and clippings to get started
• a large chart for recording the results of the pupils' researches

What happens during this activity

Find examples of how satellites are used now in communications, weather forecasting, business, agriculture, resource management, transport, and science by clipping relevant articles in magazines and newspapers.

Start a satellite wall display. You might like to prepare a grid, using the headings given above for the rows, and an example per column. Different groups of pupils can be given different areas to research, to achieve some kind of balance.

We suggest using this populated grid to discuss the advantages and disadvantages of using satellites rather than old technologies, such as communications satellites compared to telephone wires.

Amongst other things that you can discuss, satellites stop working for various reasons. Depending on the orbit of the satellite one of two things occurs. Satellites in low Earth orbit will eventually burn up in the Earth's atmosphere. Satellites in geostationary orbit can be moved farther out into a less desirable orbit.

Discuss why satellites are not brought back to Earth (cost). Consider the analogy to the world's oceans that were once considered a limitless area to discard waste. Do you think the same thing will happen to outer space? How can that be avoided? What consequences does this have for our planning for using satellites?

• ## 03 A stable orbit for a satelliteEs02TAnugget03 Activity

### Modelling a stable orbit

What the activity is for

Here you compare the distance the satellite is carried sideways away from the Earth by its own motion and the distance that it falls towards the Earth due to the pull of gravity over the same time interval. You then show how balancing these can give a path that keeps the satellite a constant height above the planet, so in an orbit. The satellite falls away from a straight line.

What to prepare

• the modelling program VnR running on a computer connected to a large display.

What happens during this activity

Use the modelling tool to give an account of this diagram.

You might build the model shown in clip 1 or a more complex one as shown in clip 2.

View clip

In both cases it is probably best to build the model up with the class, so some practice beforehand will lead to a more relaxed performance.

You can also take some pressure off by having a pupil do the keyboard work, or by preparing a half completed model, perhaps just listing the variables.

You can also, if needed, go so far as accounting for geostationary satellites, by a simple extension to this last model – the radius affects both the orbital speed and the gravitational force. This model shows the two distances that must be in balance.

• ## 04 Passive sensing with satellitesEs02TAnugget04 Activity

### Passive sensing

What the activity is for

Here you look at the performance of a satellite camera.

What to prepare

• a webcam connected to a computer, able to run both video capture and still image capture programs (low resolution is an advantage – you should easily be able to get as low as 640 by 480, but older cameras go as low as 160 by 160)
• Duplo or other large children's building blocks
• Lego or other small children's building blocks
• A3 paper and felt tip
• a high street catalogue detailing the current cost of digital cameras

What happens during this activity

Make up a landscape from both kinds of block. Fly the camera over them at a carefully chosen height, capturing an image or images as you go, so that you can easily distinguish each of the large blocks, but not the small. The images will show this because of their pixellation. You should be able to increase the magnification of the image, by zooming in, to see the pixels. Emphasise that the camera just cannot see things that are smaller than one pixel.

So why not make all satellite cameras very high resolution?

1. Cost – you have to make 6 million picture elements all perfect on a chip to get a 6 megapixel camera. These are much more expensive than 2 megapixel cameras (a high street catalogue might come in useful here).
2. The file sizes are also huge (each picture has to be transmitted as a series of 1s and 0s) and as you do not want to bring the satellite down again (why not?) all that information will need to be transmitted back to Earth. Have you ever had to wait for a very big picture to download from the Internet? Have one bookmarked here for demonstration if possible, but make sure it is not cached (so that it has to be downloaded).

If you can video with the camera, set it to a very low frame rate, then repeat the fly-by, but this time capturing a continuous series of frames. This allows you to develop a discussion about the need for a good choice of frame rate, to capture images often enough, particularly if you want to see movement. Get children to use these results to design landscapes where some features will be visible, and some not. Try out a selection of their landscapes, keeping the same camera settings and fly-by height, to see if they have understood the issues of resolution.

• ## 05 Active sensing with satellitesEs02TAnugget05 Activity

### Active sensing

What the activity is for

Some satellites send out pulses, and then time how long these pulses take to return, in order to see what lies beneath them. Here you'll model that process.

What to prepare

• Plasticine, or other modelling clay
• straws
• graph paper and pencil
• an ultrasonic ruler if possible (available from many DIY stores)
• prepared electrical ducting

What happens during this activity

Introduce the concept of working out how far away a reflector is by timing an echo. Either use the ultrasonic ruler or clap and listen. Draw out the importance of the round trip time, from which you can calculate the distance.

Now show the prepared channels, explaining how the top surface represents the path of the satellite, and hidden below this is the landscape. Make some landscape and place it inside the ducting. Ask how to find out what distance below the satellite the landscape is using the straw.

Point out that from repeated readings you can build up a map.

This is how radar maps of planets are built up, as are maps of the sea floor.

Now put each pair to work making a landscape, then sealing it into the ducting. Each group passes their landscape on to the next group, who use the straw to gather data, before plotting a map on the graph paper.

One subtlety is that the straw gives a map of depth below the satellite, but often people want a map of heights above some agreed datum. Subtracting the depth from a constant gives the required map. Faster classes might be left to puzzle this out. For classes needing more support you might ask for a map of depth, or tell them how to process the data to give a map of height.

After the maps are made a lively discussion can ensue. Only those cheating can find the finer detail of a landscape; this type of probing can only reconstruct the landscape on a scale determined by the spatial resolution of the sampling.

• ## 06 How the global positioning system worksEs02TAnugget06 Activity

### Building a GPS

What the activity is for

This is a class activity, making a physical model of the GPS that uses three measurements to plots positions.

What to prepare

• 3 lengths of rope, red, green and blue, knotted at 0.5 metre intervals
• a space approximately 3 metre square in the middle of the classroom
• drawing compasses, pencil and paper

What happens during this activity

Explain that you can find your place in two dimensions by knowing how long three signals take to reach you. Place three pupils around the perimeter of the open area, with firm instructions not to move. They will be the red, green and blue satellites. Their job is to pay out the red, or blue, or green rope, keeping it taut. Appoint one pupil as the explorer, he or she enters the open square holding one end of each of the knotted ropes. The other end is held by the appropriate satellite.

Instruct the explorer to take a fix every so often (3 to 7 fixes might be appropriate, depending on the patience and ability of the class), as they move around the space. To do this they need only count the number of knots of rope between them and the satellite. This represents the trip time for a signal from the satellite. A recorder writes down these values.

Then the class can reconstruct the journey using the fixed locations of the three satellites and a pair of compasses. You might like to prepare a sheet containing the fixed locations, with a scale for the knots on the bottom of the sheet, to allow different attempts to be overlaid for easy comparison.

The brave will note that this whole experiment can be scaled up to work outside.

Further questions to explore with the class:

• Does it matter where the satellites are, so long as we know where they are? [no]
• What happens if the satellites move? [model it with one – all readings from that satellite drift]
• What happens if the clocks in the satellites do not send out their signals at the same time? [model it with one – all readings from that satellite gain or lose an extra metre of rope]
• What do we need to know in order to fix the explorer's position more accurately? [more knots in the rope, to be able to work out the times more accurately]
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