Thread by @ProfHughLewis

Here is a thread-based version of my talk at #orbitaldebris2019 yesterday. I haven't been able to include everything, but I hope it gives you a sense of what I talked about (Image Credit: 'Gravity' Warner Bros. Pictures 2013) #spacedebris

Only 10% of future traffic in LEO goes to altitudes >1000 km. In absolute terms, that is only 17 objects per year added to the population above 1000 km. With respect to the existing population, the peak spatial density (at 800 km) is 4 times higher than anywhere else (data: ESA)

Repeating the 2010-2017 launch traffic cycle with 90% PMD compliance & no explosions leads to what appears to be a fairly 'stable' population. The total no. objects increases at only 5 objects per year. Collision fragments are deposited at ever-slower rates. All looks good!

The projection of the debris population is not a future prediction. 'Blade Runner' was (arguably) about what it means to be human; the future setting allows that theme to be explored. The principle is the same for the simulation (Image Credit: 'Blade Runner, Warner Bros. 1982)

The simulations allow us to understand the fundamental behaviour of the 'system'. A bath is a great example of a simple system. The water level is what we are interested in, and it is controlled by water inflow (via a tap) and outflow (via a drain).

The water level rises if the tap is 'on' and the drain is 'closed'. The water level falls if the tap is 'off' and the drain is 'open'. The water level (can) remain constant if the water inflow is equal to the water outflow, i.e. dynamic equilibrium.

Human population dynamics are similar to space debris population dynamics. If the birth rate is higher than the death rate, we have exponential growth in the population. If the death rate is higher than the birth rate, we have exponential decay.

If the birth rate is the same as the death rate then we have dynamic equilibrium – the population (size) stays the same. These are the fundamental responses of the human population system and they are the same for space debris population system

Previously it looked like the space debris population system was near to 'dynamic equilibrium' but this is what happens if we just allow the simulation to continue - exponential growth. This means that collisions must be adding debris faster than it is removed by the atmosphere

Collisions occur throughout LEO (in the simulation) but the debris accumulates at an increasing rate only above 1000 km - the region that receives hardly any new launch traffic and did not have a high spatial density to begin with

The level of collision activity is still substantial at altitudes < 1000 km (75% of all the collisions occur here)

We can see that the interval between collisions decreases for altitudes > 1000 km. At the start of the simulation, collisions occur once every 50 years. By the end of the simulation it is once every 5 or 6 years.

The 'Kessler Syndrome' scenario happens in the simulation at a much slower rate than was portrayed in the movie 'Gravity', but the interval between collisions will continue to decrease if the simulation continues and perhaps it is not difficult to imagine such pace after all

A large proportion of the collisions in the simulation involved spacecraft and upper stages that had already manoeuvred to comply with the '25-year rule' (these are the points circled in the graph). As a 'countermeasure', the 25-year rule doesn't appear to be working

So, we do have a problem and it will take some effort to find and implement a solution (Image Credit: 'Apollo 13', Universal Pictures 1995)

But, "The future has not been written. There is no fate but what we make for ourselves." This is a call to everyone who can help us tackle the #spacedebris problem (Image Credit: 'Terminator 2: Judgment Day', TriStar Pictures 1991)