Perhaps the first question to think about is how they get into orbit in the first place. 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.
Exactly the same combination of falling and moving sideways works for anything in orbit. The planet, moon, or satellite falls towards the things that it's orbiting. That's the effect of gravity. But it also travels forwards, at just the right speed, so that it the sideways movement compensates for the movement caused by the falling. So the orbiting thing stays the same distance away from what it's orbiting around.
Thinks about trying to achieve this balance artificially, which is what we do every time we launch an artificial satellite. You can imagine this in two steps: one, use a rocket to get a satellite to the planned height; two, fire some thrusters to set it going it sideways.
If the satellite is
thrown sideways too slowly it will fall to Earth because the pull of gravity is too great. If the satellite is thrown sideways 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.
You have to set the horizontal speed of the satellite so that the gravitational pull of the Earth tugs it round on its orbital path.
When talking about planets and moons with children it is quite likely that someone will pose the (very good) question:
Abi: Miss, what keeps the moon 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.
There are several ideas about how natural satellites in the universe (these could be moons, planets, stars, or even galaxies) got to start moving sideways as well as falling. These rely on matter that's already swirling coming together to make up the orbiting systems. And once stuff is swirling, or spinning, it's rather hard to stop, as you'll know if you have ever tried ice skating. IN any case, it's as well that there are orbits, with the fine balance between falling and moving sideways, otherwise we would not have a home, as Earth would not be.
Before Newton, people used to think that circular motion was
natural and needed no explanation. Newton's great insight was to realise that motion in a straight line is the natural form of unforced motion and that it is circular motion that requires an explanation.
This led Newton to ponder on the nature of the force that keeps the Moon in orbit around the Earth. The incident that is supposed to have suggested the answer to him was the apple falling on his head. If gravity can act between the Earth and the apple, why should it not also act between the Earth and the Moon? Newton's subsequent calculations led him to the idea that gravity is a universal force acting between all objects.