What is an Orbit?

We often talk about things being in orbit. The Sun orbits the galaxy, the Earth orbits the Sun, the Moon orbits the Earth and so on. But what is an orbit?

The Moon orbiting the Earth taken by the Galileo space probe. Credit: NASA

In the 1600s, an English scientist named Isaac Newton came up with three laws of motion. The first of Newton’s laws state that an object in motion will stay in motion unless it is acted upon by another force. To visualise what this means, imagine someone on a skateboard going down a hill. All goes well until the skateboard hits as stone and stops. As a result of Newton’s first law, the person on the skateboard will continue to move forward and fall off the skateboard.

This law is essential to understanding an orbit, as is gravity. For the purpose of this, we can imagine gravity as a force attracting two objects. For example the Moon and Earth are attracted to each other by gravity (in reality this is more complex because the Sun is also attracted to the Moon and Earth as are all other Solar System objects).

Imagine a cannon on the surface of the Earth. According to Newton’s first law the cannonball will keep moving in a straight line unless some force acts on it when fired. The gravity of the Earth pulls the cannonball towards it. This bends the path of the ball towards the Earth. As we know the Earth is round and this means the surface of the Earth also curves downward. The problem is that the ball doesn’t go far enough before it hits the ground. The next time we fire the cannon we put some extra gun powder in it and the ball goes further but still hits the Earth. Eventually, if we put enough gunpowder in the cannon the ball will go all the way around without hitting the Earth. This is an orbit.

Cannonball fired with increasing speed.

This means that the the ball is always falling towards the Earth but missing it. All orbits are ellipses. An ellipse is very similar to a oval. Some orbits are very long narrow ellipses while others, such as the orbits of the planets are almost circular.

How Do Spacecraft Get Into Obit?

Orbital mechanics is the study of the motions of satellites. When it comes to spacecraft, orbits can be strange things. When a rocket is trying to get into orbit it doesn’t just fly straight up. If it did, it would just fall back to the Earth. This is called a ballistic trajectory and the first rockets were built like this as they were based on earlier missiles. To orbit, we need to go around the Earth fast enough that we keep missing the ground. For a satellite to stay in low earth orbit such as the ISS, you need to move at a speed of about 26,600 km/h. This motion needs to be around the Earth rather than straight up.

Ballistic Trajectory

For a rocket to get to orbit, the rocket flies straight up or close to it initially but slowly begins to bank and fly at an angle. Eventually the engine is turned off and the rocket continues using the momentum to fly to the top of the ballistic trajectory. Once at the very top, the rocket fires again, this time increasing the speed of the rocket sideways. Eventually the rocket will reach orbital velocity and is in orbit.

The rockets orbit isn’t a perfect circle and to make the orbit circular you need to wait on the rocket to reach the highest point in the orbit and fire the engines in the direction you want to go. This increase in speed will increase the height of the orbit on the other side. This is one of the strange things about orbits. If you want to increase the height of the orbit you fire the engine in the direction you are flying to speed up. This increases the height. If you want to lower the orbit the engines are fired in the opposite direction which slows the rocket down, reducing the height of the orbit.

The ISS orbits the Earth and needs occasional boosts to keep it in orbit Image: NASA

Relating this back to Newton’s first law of motion, when a rocket or satellite such as the ISS is in low Earth orbit there is another force acting upon it called friction. At the height satellites operate there is very little atmosphere, but there is still some molecules. These bang into the satellites and cause a small amount of friction. This adds up, and over time slows the craft down, lowering its orbit. This is why satellites such as the ISS need to occasionally boost their orbit by firing little rockets. Otherwise it would eventually slow down too much, get too low and enter the atmosphere where it would burn up!

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