Thread by @ProfHughLewis

Here's a thread containing the slides and thoughts I shared at today's @seradata space conference. I wasn't able to invest much time to prepare the talk, so some of the slides will look familiar to those attending April's ESA #SpaceDebris conference. Some are new [1/n]

[alt text: talk title "The Space Debris Environment - Current Status and Evolution of the Risk"]

I put this slide together using data from celestrak.com. It shows the historical evolution of the orbital object population (as recorded in the public catalogue). The highlight statistic is that active spacecraft make up 20% of the current catalogue population [2/n]

The trends are interesting! Three key things to take away: (1) the rate at which objects have been added to the catalogue has always exceeded the rate at which objects have decayed, hence the growth of the "on-orbit" population [3/n]

(2) Since the end of 2006, the rate at which objects have been added to the catalogue has increased by about 50% while the rate at which objects have decayed has reduced by about 10% [4/n]

(3) The growth in the catalogued and on-orbit populations is, I think, exponential and quite possibly super-exponential (although that cannot continue, of course) [5/n]

The recent conjunction data from #SOCRATES reveals about 7500-8000 conjunctions within 5 km and about 400 conjunctions within 1 km per day (as of end of May 2021). Many of these involve #Starlink [6/n]

Again, the trend is interesting (and also quite alarming) with substantial increases in the conjunction rate just in the last two years. The model used for the fit is 'super'-exponential [7/n]

[alt text: an increase of 92% since May 2019 for conjunctions within 5 km and of 159% since May 2019 for conjunctions within 1 km]

Exponential growth occurs when the rate at which the population grows is proportional to the number of orbital objects in the population [8/n]

We see exponential growth in many natural populations, although the growth is often limited by some factor (e.g. amount of resources available) but can we really expect to see the same thing in a technological system? [9/n]

The answer is yes! Here, the simulated evolution of the #SpaceDebris population into the future is shown for two very different scenarios, one where the sector adopts mitigation measures widely & one where it doesn't. Both show (super-)exponential population growth [10/n]

This has implications for #SpaceSustainability. The definition adopted by the UN includes the need to preserve the space environment for future generations. Exponential #SpaceDebris growth may prevent this & more stringent measures (e.g. ADR) may be needed [11/n]

I ended with this prototype #SpaceSustainability dashboard, with metrics calculated from simulations. On the left are statistics showing the best outcome we might be able to achieve with debris mitigation. On the right, what our path looks like now [12/n]

Think of the annual growth rate as the interest you might pay on a debt (we have 'borrowed' more of the space environment than we can afford). The interest we owe is currently accumulating at about 5% per year [13/n]

Think of the 'R' value like this: for every object added to the on-orbit catalogue, the environment adds a little extra. On our current pathway, the environment is adding one extra trackable object for every 20 we do (and we are adding objects at a rate of 1.6 per day) [14/n]

I think the previous-but-one tweet really sums up the #SpaceDebris & #SpaceSustainability problems. We have borrowed more of the environment than we/it can afford. The interest we are paying on this debt, in terms of collision risk, is accumulating rapidly [15/n]

There are things we can (and definitely should) do to reduce the interest rate we are paying, but it's only through more robust measures such as Active Debris Removal that we'll be able to reduce the debt. [16/16, end]