Rights: University of Waikato Published 27 March 2013 Download

Dr Allan McInnes explains why satellites need large rockets to launch them into orbit and why many satellites are launched from near the equator.

Point of interest:
A satellite in an equatorial orbit provides repeated observations of the same area. An example of this is a weather satellite.


If we want to orbit at 200 kilometres, then we need to get to that height. But that’s not enough because if we were at 200 kilometres and not moving at all, we’d just fall right back down to Earth – that would be the gravity pulling us back down again. So we also need to have a certain amount of speed going sideways that’s going to cause us to stay in that orbit in the same way that if you’re swinging a tennis ball on the end of a string, you need to give it some speed so it actually swings instead of just falling down. So the way we get a satellite up to both the altitude it needs to be at and going at the speed we need is to put it on a rocket.

We need to use a rocket because we’re trying to go above the atmosphere, so planes that use the atmosphere to fly aren’t going to work. If you’re flying a plane, you have oxygen in the atmosphere that will burn the fuel. Once you’re above the atmosphere, you need something else, and the way a rocket works is that it takes fuel with it and also it takes something to burn that fuel. We actually carry oxygen or some other kind of oxidiser. So that’s part of the reason rockets look so big actually is because they’ve got to carry not just fuel but also oxidiser – you’ve got to carry a lot of stuff just to get a small satellite into orbit.

A lot of satellites need to be sent into orbits that are what we call equatorial, so ones that circle the equator, and the easiest way to do that is to start from near the equator. There’s a very small advantage just from the fact that you get a little bit of extra energy from the Earth because it’s spinning. But the main advantage is just that because you’re near the equator, you can go into pretty much an equatorial orbit straight away. If you start from a higher latitude, then you have to do a manoeuvre that changes the angle of the orbit. So you have to flip from being something that might be at 30 or 40° angle to the equator to something that’s going around the equator. And that manoeuvre – which is called a plane change manoeuvre – can be quite expensive. It takes a lot of rocket fuel to do it, which means you either need a much, much bigger rocket or you can launch a smaller satellite.


Cryosat II footage courtesy of European Space Agency/ESA.