Do spacecraft ever collide?

The oldest spacecraft still in orbit was launched in 1958. It stopped working in 1964. But, when Vanguard-1 stopped working it didn’t just disappear. It stayed in orbit, its motion continually being perturbed.

For Vanguard-1 the main perturbing force is due to the Earth’s atmosphere, causing it to slowly lose altitude. Eventually it will burn up in the Earth’s atmosphere. But, Vanguard-1 went to quite a high orbit, so this really is a slow process. Vanguard-1 will likely remain in-orbit for at least 200 years after it stopped working.

And, it’s not just the spacecraft. The upper stage of its launch vehicle, slightly confusingly called the Vanguard rocket, is also still in orbit.

In the decades since, and with the thousands of spacecraft launched since, space debris has proliferated, from dead spacecraft and spent rockets, to fragments of them, ranging from remnants of explosive bolts used to hold-down, and then release objects, to flecks of paint, and propellant slag, or unburned fuel.

And, rather unhelpfully, spacecraft and rocket stages have a tendency to explode long after they have stopped working, creating clouds of debris.

We also have a bad habit of testing anti-satellite weapons (ASATs), some of which create only short lived debris fields when the target is in a low orbit, but some others have created long lasting debris fields that have grown and spread out over time.

Some of the more unusual debris comes from things astronauts have dropped, including a glove on the first US space-walk (EVA), various tools, several cameras, a mirror, and even a toothbrush. And, the cosmonauts on Mir used to throw bags of rubbish overboard as they couldn’t store them on-board.

By the late 1970’s engineers were starting to grasp the risk of the ever-growing space debris population, recognising the risk of a collision in space resulting in more debris, and so more collisions. This hypothesised a cascade effect creating a runaway scenario that was termed Kessler syndrome, and could result in certain regions of space becoming unusable, perhaps for centuries.

Today we have best-practise guidance in place to help avoid the creation of more debris. This includes guidelines on removing spacecraft from protected orbit regions. From geostationary orbit we tend to move spacecraft to a so-called graveyard orbit, above geostationary orbit.

In low-Earth orbit we tend to destroy spacecraft in the Earth’s atmosphere. However, not all spacecraft will completely burn up. For them we target a re-entry over the Southern Pacific, far away from people. But, sometimes that isn’t possible either, and so operators will simply take the risk and hope nobody is hurt.

It is also best-practise to make a spacecraft passive before turning it off in orbit. This helps avoid them breaking up or exploding by doing things like venting any unburned fuel and physically disconnecting batteries. Wherever possible, launch vehicles are also de-orbited rather than leaving them in space.

Large space objects are tracked from both the ground and space, but as objects get smaller than around 5-10cm, or roughly the size of a baseball, it becomes challenging to track them. Meaning there are a lot of objects big enough to do some serious damage that we simply have no idea where they are.

When there is a risk that a piece of space debris might hit a spacecraft, the operators will attempt to move it out of the way of the debris. This has a cost for the operator, and will shorten the spacecraft’s life. But, even the International Space Station has to perform avoidance manoeuvres.

A problem however is that we don’t know the precise location of the debris. So, sometimes we can’t be sure that manoeuvring won’t make the situation worse. Or, sometime we might not realise a collision is possible until it's too late. For the space station this means the crew will “shelter-in-place” by moving to inside one of the docked Soyuz vehicles so they can make a quick escape, if needed.

Sometimes that works. It’s always worked so far on the space station. But, sometimes it doesn’t. Which is what happened with the first major collision between two spacecraft, in February 2009, when a dead Russian spacecraft collided with an operational US commercial communications spacecraft 800km above Siberia. Both spacecraft were destroyed, creating thousands of fragments; much of which will stay in orbit for decades to come.

In March 2012 a fragment of that Russian spacecraft passed within 120m of the International Space Station whilst the crew sheltered-in-place inside the two docked Soyuz spacecraft.

Space debris, much like pollution on Earth is a negative externality. It often saves the spacecraft operator cost and any pollution doesn’t incur a cost, but has a negative effect on those not involved in that spacecraft.

Today there is no meaningful space debris legislation, nor enforcement in place. Rather, we rely on guidelines and best-practice. We rely on so-called gentlemanly conduct in a rapidly professionalising, congested, and contested environment. So, much like the climate crisis on Earth, without concerted international action and agreement we risk it becoming too late to save another natural environment from irresponsible human actions.