Thread by @ProfHughLewis

I am seeing some ill-informed takes on today's near-miss in orbit so would like to offer some trajectory corrections if I may. Firstly, the chance that a single collision would trigger a catastrophic 'chain reaction' that would sweep through LEO is tiny.

For every close pass involving catalogued objects in orbit we can estimate a collision probability, or Pc. The Pc is between 0 and 1. If it is 1 we can say that a collision is certain. If it is 0 then we can say that a miss is certain.

The event today may have had a Pc between 0.02 & 0.2. In any case, the Pc was relatively small (compared to a Pc of 1) so a miss was the most likely outcome. For a chain reaction to occur a long & sustained sequence of collisions would need to take place.

For each event in that chain the most likely outcome would be a miss. The probability that collision 1 occurs *and* triggers collision 2 in the chain is even smaller than the original Pc. So the chance that the events in the chain continue will get smaller and smaller.

That's not to say that some version of a chain cannot happen. In fact we sometimes see these chains in our computer simulations: a fragment from an earlier collision hitting another object and creating more fragments that go on to hit other objects.

But these chains do not continue because the probabilities decrease to extremely small values after just a few events. The longest chain I have seen in one of our simulations is 7 events (we found that one amongst 25,000 Monte Carlo runs)

In a paper I am presenting at the European Conference on #SpaceDebris (starting April 20th btw) we simulated the simultaneous collisional breakup of the top 50 statistically most concerning derelict objects in LEO to see what might happen.

Spoiler: no catastrophic collision 'chain reaction' occurred.

It's not all good news though. The space debris population is growing in the manner predicted by Kessler but just not in the way represented in the movie 'Gravity'. We still have a lot of work to do to solve this problem we have created.

Secondly, large constellations and particularly #Starlink seem like easy targets for criticism when referring to the so-called #KesslerSyndrome or collision chain reactions. But the reality is somewhat different thanks to the atmosphere.

At Starlink altitudes the atmospheric drag experienced by the satellites would cause them to decay & re-enter within a relatively short period of time (a few years) even if they were to fail. This is a highly effective debris mitigation measure.

Again, we have simulated this & found that even if 90% of all Starlink satellites were to fail, the long-term impact on the environment is virtually negligible because the atmosphere provides an effective intervention.

Of course collisions could (probably would) still occur but, for the most part, any fragments would decay out of the environment quite quickly. The effect of the atmosphere is one of the key justifications given by SpaceX to the FCC for the change in altitude of the constellation

Based on Kessler's & Anz-Meador's stability model (presented at the 3rd European Conference on #SpaceDebris btw) the number of Starlink satellites proposed does not exceed the critical number of objects needed for a runaway population.

Sure, we need to do more work & Starlink is still a genuine cause for concern for many reasons, but not really because it is a potential 'trigger' for the #KesslerSyndrome. That's a view based on some flawed thinking & we can do better.

That's the end of this PSA. If you made it this far - thanks and well done!