Space is big. Really, really big. But the bit of it around planet Earth is rapidly filling up.

On Wednesday night this week, two defunct, retired satellites came within a hair’s breadth of colliding, 900km above the US city of Pittsburgh. The IRAS satellite, launched in 1983, passed within metres of the GGSE-4 satellite, up there since 1967, at a relative speed of nine miles per second.

Had they collided, they would have spread debris across space in what is known as low Earth orbit. This cloud of debris could have spread, causing further damage to other satellites, conjuring up images of the catastrophic domino effect of collisions that did for George Clooney in 2013’s Gravity (sorry, spoiler alert…).

As far-fetched as such cinematic disasters may be, collisions like the one we narrowly avoided on Wednesday keep space scientists awake at night. It’s not a matter of if a collision will happen, but when.

In early 2009, two communications satellites – Iridium 33 and Kosmos-2251 – collided in low Earth orbit, creating a cloud of thousands of large pieces of debris and many more smaller pieces.

Over time, many of these fragments have de-orbited and burned up harmlessly in the Earth’s upper atmosphere. After the collision, the world’s space agencies had to calculate the risk to the thousands of satellites that orbit the Earth, and connect us globally.

Glasgow Times:

The main risk is to the astronauts and cosmonauts that live and work in space aboard the International Space Station (ISS). A collision with even a small piece of debris could be life-threatening, if it was travelling quickly enough to puncture the pressurised hull of the ISS.

In 2012, all six ISS crew members were instructed to board the Soyuz escape modules in the event of a collision, as a piece of debris from the Iridium-Kosmos collision passed within 120m. Luckily it was a near miss.

Not all space debris comes from accidental collisions. In 2007, the Chinese Space Agency decided to test a ground-based anti-satellite missile system by targeting a defunct satellite in low Earth orbit. This militarisation of space is nothing new – governments have flown spy satellites and many other top-secret space systems for decades – but it seems to be a growing trend.

Six weeks ago President Donald Trump created the United States Space Force (USSF), an arm of the US armed forces. The new USSF logo was unveiled this week to widespread amusement, as it looks uncannily similar to the Starfleet logo from Star Trek.

Despite this, space exploration is still an overwhelmingly peaceful endeavour, with nations co-operating in orbit to build incredible feats of engineering like the International Space Station. And the number of satellites is growing to meet the demands of us earthlings.

Elon Musk’s SpaceX recently launched more than 200 satellites called Starlink. The aim is to launch around 12,000 into space by 2025 to provide global broadband internet.

They haven’t been unanimously popular. Astronomers around the world complain that too many satellites buzzing around the Earth will make it harder to see the stars. The skies above towns and cities are already full of artificial reflected streetlight, but they will also soon be full of thousands of satellite trails, making it even harder to see the stars.

Regardless of these issues, we will continue to put more and more satellites into space, driven by our demand for more connectivity, more science, more data.

Improving the design of satellites will help keep space tidy. Indeed, Scotland is leading the way in the production of small, efficient satellites, with more satellites manufactured in Glasgow than in any city in the world outside of California.

One of Scotland’s thriving satellite companies is Clydespace, based just across the Clyde from Glasgow Science Centre. Clydespace builds cubesats, tiny micro satellites that can piggy-back on rockets commissioned to launch larger satellites, thereby significantly reducing the cost and the number of distinct objects in space. This week we welcomed the CEO, Craig Clark, on to our board of trustees.

As space gets busier we’ll have to come up with new and innovative ways to keep it safe and clean. Modern satellites, unlike the two lumps of space junk that whizzed past one another above Pittsburgh two nights ago, are built to

de-orbit at the end of their lives.

And space agencies and private companies are exploring technologies that will one day allow us to clear out low Earth orbit of all the junk we’ve left up there over the past 63 years.