There’s a reason they call it space: there’s a lot of it. From the distance between planets, to the tiny relative size of satellites compared to the volume of orbital space around the Earth, there seems to be a lot of ‘clear air’ (for want of a better term) available for space flight.
But things aren’t exactly what they seem when it comes to the latter case of the orbital space around the Earth. And this is because of an effect that’s become known as the ‘Kessler Syndrome’, given that name based on a seminal paper co-authored in 1978 by astrophysicist and NASA scientist, Donald Kessler.
In “Collision Frequency of Artificial Satellites: The Creation of a Debris Belt”, Kessler and co-author Burton Cour-Palais pointed out that, as the number of artificial satellites in earth orbit increases, the probability of collisions between satellites also increases. And furthermore, that any initial satellite collision would produce orbiting fragments – a cloud of unintentional missiles traveling at tens of kilometres per second – each of which would increase the probability of further collisions, leading to what is known as “collisional cascading”.
If you’ve seen Alfonso Cuarón’s 2013 movie Gravity, you know exactly how this works:
Once the exponential chain of collisions begins, it will keep going for decades, until there is a ‘shell’ of debris surrounding the Earth, orbiting at high speed, capable of destroying any other object that enters that zone of destruction.
And this isn’t a hypothetical: according to Donald Kessler, it has already begun. “The cascade is happening right now,” Kessler told the Guardian, after being started by a collision in 2009 between two communications satellites, the Iridium 33 and the derelict Russian military satellite Kosmos-2251. That one event is said to have created around a thousand pieces of debris that will likely orbit the Earth for the next 10,000 years.
If Kessler’s scenario plays out in coming decades, it will have grave implications for modern society – cascading collisions of satellites and a resulting ‘dead zone’ that limits our ability to place satellites in orbit could play havoc with telecommunications, weather forecasting and other systems that depend on this technology.
But there is another possible implication: could an orbital shell of debris grow to a size and density where it is virtually impervious to spacecraft attempting to leave the planet?
At the moment transiting the debris field seems less of a problem than that of the issue with satellites, which have to sit within this zone – as Kessler says, “you can cross the street a lot safer than living in the middle of it.” But taken to the extreme, is it possible that residents of a planet might become ‘prisoners’ on it, within a century of their civilisation learning how to escape their gravity well and become a space-faring civilisation?
In short: could the answer to the Fermi Paradox – ‘if aliens exist, where is everybody?’ – be that they’re all isolated on their own planets, locked in by an orbital debris field created by their own technological success?
One would hope that any advanced civilisation would eventually be able to figure out a way to clear a path so they can leave their own planet. But all the same, perhaps we should add the ‘Prison Planet’ possibility to the list of proposed answers to the Fermi Paradox, like the Zoo hypothesis and the ‘Great Filter‘ theory.