Keeping satellites in orbit

Dr Allan McInnes tells us about forces acting on satellites once they are in orbit, how to keep a satellite in orbit and the purpose of a graveyard orbit.

Points of interest:

• The US Federal Communications Commission requires all geostationary orbits to commit to moving to a graveyard orbit.

• About one-third of satellites succeed in moving to one.

Transcript

DR ALLAN MCINNES
The Earth is what we call an oblate spheroid so it’s kind of squished at the poles, and then it’s also lumpy. It has continents and other bumps in it so the gravitational field around the Earth is not perfectly regular, and you get tugged in different ways as you orbit around the Earth. Over time, that tends to change the shape of your orbit. The other problem we have is that space is not empty. For starters, a lot of satellites – even though they’re above most of what we think of as the atmosphere of the Earth – are still in some of the gas that’s around the Earth, which means that, as they fly around, they experience drag as the atmosphere pushes on them. It’s like sticking your hand out a car window and feeling air blowing it around.

With satellites, we want them to stay in a particular orbit, and if they’re being shoved around in space by all these forces, what we need to do is apply other forces that will cause them to shift back into the orbit we want them to be in. So most satellites have a bunch of small rockets – what we call thrusters – that they can use to just fire a little burst every now and then that nudges them one way or another.

Do satellites stay in orbit forever? Well, mostly not – it depends on which orbit we’re talking about. Low-orbiting satellites below a few thousand kilometres are low enough that drag from the air has a big effect, and over time, the orbit will decay. So the drag from the air slows the satellite down, the satellite loses energy and the size of the orbit gets smaller and smaller until it gets into a part of the air where friction builds up so much that the satellite just burns up. In higher orbits particularly out towards sort of 36 000 kilometres – what we’d call a geostationary orbit – in principle, they could stay up there forever. The orbit will tend to shift over time but it will stay orbiting the Earth in the same way that the Moon still orbits the Earth after millions of years. But usually we don’t want them to stay in a particular orbit forever.

A satellite has a useful lifetime of between 5 and 15 years depending on the satellite. It’s hard to design them to last much longer than that, either because the solar arrays stop working or because they run out of fuel to allow them to maintain the orbit that they’re supposed to be in. So when they hit the end of their useful life, we want to move them out of the way so they’re not interfering with things, and we’ll move them into what’s known as a graveyard orbit. So that’s one that’s not an orbit that’s usually used for functioning satellites, it’s just there to act as a sort of, I guess a dump, where we can put things that aren’t being used.

Acknowledgements:
Andrew Z. Colvin
Oblate spheroid background plate courtesy of AugPi
http://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License_1.2
http://creativecommons.org/licenses/by-sa/3.0/deed.en
Phil Nolan
UARS orbital path and re-entry video animation courtesy of Analytical Graphics, Inc