Science Questions

Can satellites collide?

Mon, 18th May 2015

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Harry asked:

I just wanted to know why satellites in orbit do not collide with one another? I hear there are hundreds of these flying objects in space. Do they have a predetermined orbit path before launch?


We put Harry's question to Space Boffin Richard Hollingham...

GPS satellite

Richard - There are actually hundreds of thousands of these objects in space. There are an estimated half a million pieces of what's called space debris, particularly in low Earth orbits - thatís where an awful lot of the satellites are. Not the communication satellites. So, not the satellites we use for broadcast or for pinging mobile phone calls around the world, but the sort of satellites that maybe observe the Earth, the sort of satellites, particularly scientific satellites and spacecraft with humans in them. So, the International Space Station for instance is in this particular orbit. Now, these half a million objects vary from flecks of paint - and these can cause severe damage. Imagine stuff thatís pinging around the Earth at almost 8 meters per second! So, you got all these stuff pinging around the Earth. The biggest stuff is about 20,000 pieces of space debris and this is tracked by the US. Itís tracked here in the UK by the RAF using radar. So, they know where all the stuff is. It sounds like a lot but the Earth is pretty big. Now obviously, you donít want to hit one of these when you're launching a satellite. So, they launch into orbits where there isnít the stuff. So, they're tracking the stuff continuously. They donít know where the flecks of paint are. Youíve got pretty good odds that you're not going to hit a fleck of paint. So, they do launch into orbit. They're very careful where they launch. They donít want to launch into the orbit of say MVSat which is a big redundant Ė it suddenly went dead a couple of years ago - European satellite, the size of a double-decker bus. You donít want to put your satellite in the same orbit as that.

Kat - So, when you're planning an orbit for satellite, itís basically about how fast and exactly the sort of the angle you shoot it up over the Earth at?

Richard - You work all that stuff out. What's also interesting Ė so, thatís into low Earth orbit - is if you are launching say, a telecommunication satellite. Now, these sit in what are called geostationary orbits. So essentially, they orbit the Earth as the same speed the Earth rotates. So, they sit above the same part of the Earth all the time. Sky TV for instance has one above Europe right now beaming down to peopleís satellite dishes on the sides of their house.

Kat - Other channels are available.

Richard - Other channels are available, but if you want, okay, sport on the BBC. If you watch that from around the world, thatíll come via satellite. They actually have to navigate their way through this space debris and not only the satellites that are orbiting. They actually plan and sometimes they have to divert their course. So, imagine crossing the road, they're having to weave between the traffic to get up to this high orbit. So, these are all things they take into account.

Kat - It sounds like a stellar version of Frogger!


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