We humans are a messy lot.

We leave junk behind us, everywhere.

Space is no different.

We may find that clouds of orbiting space junk, travelling at hypersonic speeds, could seriously interfere with our future space operations.

What is space junk?

Space junk is the total collection of dead and non-functioning artificial objects that we humans have left in space.

The oldest piece of space junk is the Vanguard One satellite, dating back to 1958.

Space junk includes old satellites, spent rocket stages that were used to launch something, dust from solid rocket motors, and even coolant from obsolete Russian nuclear-powered satellites — which are still orbiting above us.

More personally, Michael Collins lost a camera on the Gemini 10 mission, and the astronaut Ed White even lost a glove.

There is a lot of junk up there orbiting our planet. After all, we have had more than 5,000 space launches since the beginning of the Space Age in 1957.

Each launch often delivered several objects into orbit, so these 5,000 launches have given us over 30,000 large space objects.

How much junk is out there?

In 2016, the best estimates were that there were billions of pieces of space junk smaller than one millimetre, hundreds of millions of objects between one and 10mm in size, and about half a million objects in the range 1 to 10cm.

There are about 100,000 bits of space junk bigger than 5cm.

All told, estimates suggest there are about 5,000 tonnes of space junk orbiting the Earth.

In 2017, the US Space Command's Joint Space Operations Centre was regularly tracking more than 22,000 objects orbiting the Earth.

But fewer than 5 per cent of these were functioning spacecraft — over 95 per cent is junk.

They track them so that they can manoeuvre active spacecraft out of the way of an impact.

There are various orbits that are useful for commercial, military and scientific programs — and space junk is in all of them.

For example, the geostationary orbit is about 36,000km up. It's called "geostationary" because satellites up there take 24 hours to orbit the Earth, which by a nice coincidence, is how long the Earth takes to spin on its own axis.

So an object in geostationary orbit will appear to hover over one fixed position above the Earth, even though it is actually travelling at some 3km per second.

We have many communications and weather satellites in the geostationary orbit.

A rocket blasts off from the European space centre at Kourou, French Guiana. ( Getty Images: Jerome Vallette )

A little closer to home is the low Earth orbit, which ranges from 200 to 2,000km above the surface.

The Hubble Space Telescope is up there at around 540 km of altitude, while the International Space Station is around 350km.

There is about 1,900 tonnes of space junk in low Earth orbit, but almost all of it — about 98 per cent — is accounted for by just 1,500 objects, each over 100kg.

Objects in low Earth orbit circle the Earth some 15 times each day.

Depending on their orbit, they can zip past each other, or smash into each other, at speeds up to 17km per second.

Making more, on purpose

Both the USA and the Soviet Union carried out multiple series of anti-satellite weapons tests in the 1960s and 1970s.

In 1985, a single US test destroyed a one-tonne satellite at 525km of altitude, creating thousands of pieces of space junk bigger than 1cm.

In 2007, the Chinese tested their anti-satellite weapons systems by destroying one of their own weather satellites — the Fengyun IC.

This was the largest single space debris event in history.

It created more than 2,300 pieces bigger than 3cm across, 35,000 pieces bigger than 1cm, and 1 million pieces bigger than 1mm.

Mostly though, the collisions happen by accident.

On 10 February 2009, a defunct Kosmos spacecraft weighing 800kg smashed into the functioning Iridium 33 satellite weighing about 700kg.

This happened at a closing speed of 11.7km per second — over 42,000kph.

That's seriously big stuff running into big stuff.

On a much smaller scale, in 2016, the British astronaut Tim Peake photographed a 7mm crack in the window of the Cupola module of the International Space Station.

It was caused by running into a hypersonic microscopic piece of space debris.

Looking at a different spacecraft, in the Space Shuttle a window had to be replaced every second flight — due to running into this invisible space junk.

So the lesson is, we need to clean up our act, and our space junk.