At any given moment, thousands of planes are in the sky.

More people are flying than ever before, and on popular routes, such as Melbourne to Denpasar, or London to New York, multiple planes will fly the same route at once.

Pilots will make the most of jetstreams where they can, which are high-wind passages than can shave hours off flight times.

However, since most of them are only 5km high and 160km wide, it means aircraft become concentrated into what's really a tiny portion of the sky.

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And while that's all well and good when you can see the other planes on radar, there are huge black spots over the world's largest oceans.

So how do they avoid colliding mid-air?

Mid-air traffic routes ensure planes don't collide.

Basically, it all comes down to air traffic control.

Over the Atlantic Ocean, for example, planners in Canada and Scotland agree on about 10 mid-sky highways each day, which are programmed into each plane's autopilot.

Captains request the track they want, and when they drop off radar they use a set of coordinates to track their progress, reporting their location by radio.

They have to maintain a separation of 10 minutes from other planes travelling the same direction, or 15 minutes from planes that cross paths.

If you're travelling at 1000km/h, that's means they're about 160km apart at any time.