Earth and Space | Forces and Motion

How do satellites stay in orbit?

Physics Narrative for 11-14 Supporting Physics Teaching

Getting into orbit

Perhaps the first question to think about is how they get into orbit in the first place. Let's try a thought experiment that was first suggested by Sir Isaac Newton himself.

Imagine a mountain on the Earth's surface that is so big that its summit sticks out above the Earth's atmosphere (it would need to be about ten times as high as Mount Everest). Supposing you climb to the top of this mountain and throw a cricket ball horizontally outwards. The ball is pulled by gravity so that it falls to the ground along a curved path.

Let's assume you now try a lot harder and the ball travels much farther outwards before it hits the ground.

You now summon up all of your strength and manage to throw the ball so fast that it flies outwards and as it falls, its path follows the curvature of the Earth. The ball follows this falling path right round the Earth. In fact, you need to duck as it comes by after completing one orbit! You have managed to throw the ball into orbit around the Earth so that it is now an Earth satellite.

Putting satellites into orbit

Putting satellites into orbit involves the same kinds of actions and ideas. First of all the satellite is placed on top of a huge rocket to carry it away from the Earth and up through the atmosphere. Once it is at the required height, sideways rocket thrusts of just the right strength are applied to send the satellite into orbit at the correct speed.

If the satellite is thrown out too slowly it will fall to Earth because the centripetal pull of gravity is too great. If the satellite is thrown out too fast it will escape from the Earth's orbit because the gravitational pull is not sufficient to provide the required centripetal force. With the correct launch speed the satellite continues in its falling orbit around the Earth.

It is just a matter of setting the horizontal speed of the satellite such that the gravitational pull of the Earth (at the given height) tugs it round on its orbital path.

When talking about satellites with pupils it is quite likely that someone will pose the (very good) question:

Cas: Miss, what keeps the satellite going?

The short answer to the question is:

Teacher: Nothing keeps it going, it keeps going itself.

As the satellite is launched from the carrier rocket, a rocket thrust acts to throw it out in the desired direction at the prescribed speed. The crucial point to understand here is that the satellite speeds up only for as long as the rocket thrust is acting. Once the rocket motor is switched off the satellite continues at the final speed achieved, neither speeding up nor slowing down, and the gravitational pull of the Earth continuously tugs the satellite in and along its orbital path. In this sense, the satellite just keeps going itself.

If the satellite was moving through empty space it would stay in its orbit forever, there being no forces acting to speed it up or to slow it down. In reality low orbit Earth satellites are not travelling through empty space and so experience a resistive force or drag due to the thin atmosphere which they encounter. In such circumstances, occasional rocket thrusts are needed to maintain the motion of the satellite, otherwise it will fall to Earth.

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