[TGower]

Dissapointing that the paper is full of simplifying, and seemingly unreasonable, assumptions instead of simulation based on the known orbital elements of all these tracked satellites. For example, collision cross section of 200 square meters when discussing starlink even though the satellites are about 4 x 3 meters. Assuming random distribution of trajectories. I'm also unconvinced that "how fast would a collision occur if all the electronics got fried" is a useful metric, in that scenario I'm much more worried about the situation on the ground and commercial avaition...

[deddy]

Need to do a full read in more depth but it looks like they used a collision cross section of A=300 m^2, which is a little conservative but not insane given that the current Starlink v2 mini has about 90-120 m^2 of total surface area on its solar arrays. [1] The solar arrays are the largest part of these spacecraft by far and what defines the “collideable” area. A combined hard-body radius of 2 x 120 = 240 is in the ballpark for starlink-on-starlink collisions.

However most of collisions of concern are going to be starlink-on-debris, which is back down at the 120 m^2 level. Starlink already self screens for collisions and uplinks the conjunction data messages over the optical intersatellite link backbone or over their global ground station network.

If they aren’t able to talk to their satellites regularly from somewhere, you’re right we have MUCH bigger things to worry about on the ground.

[1] https://spaceflightnow.com/2023/02/26/spacex-unveils-first-b...

[brookst]

And wouldn’t the solar panels have less cross section than the satellite bodies, so even an apparent collision might just be a very near miss? (Honest question, not rhetorical, could be I’m wrong)

[SiempreViernes]

They do verify their analytical calculation using a N-body simulation, that's section 4.4

> We verify our analytic model against direct N-body conjunction simulations. Written in Python, the simulation code SatEvol propagates orbits using Keplerian orbital elements, and includes nodal and apsidal precession due to Earth’s J2 gravitational moment. [...] The N-body simulation code used in this paper is open source and can be found at https://github.com/norabolig/conjunctionSim.

[Sanzig]

The cross section isn't actually all that outrageous, it corresponds to a hardbody radius of 4.5 meters. Hardbody radius is equal to the sum of the radii of the two colliding bodies, so 2.25 meters - which seems about right for Starlink.

[queuebert]

They did an n-body simulation based on the known Keplerian orbital elements. That's exactly what you're asking for, right?

Also, the formalism is the standard way astrophysicists understand collisions in gases or galaxies, and it works surprisingly well, especially when there are large numbers of "particles". There may be a few assumptions about the velocity distribution, but usually those are mild and only affect the results by less than an order of magnitude.

[MarkusQ]

"N-body simulation" doesn't mean what it's normally taken to mean here.

And the colliding gasses models have the huge assumption of random/thermal motion. These satellites are in carefully designed orbits; they aren't going to magically thermalize if left unmonitored for three days.

[MarkusQ]

Agreed. This reads more like a hit-piece than a good-faith effort to quantify the risks. They make long-tail pessimistic assumptions, explicitly ignore possible mitigating factors, and act as if this "worse than worst case" scenario is a reasonable description of the world we live in.

[hughes]

Even the title "Orbital House of Cards" is unnecessarily editorializing.

[philipwhiuk]

200 might be more reasonable for the next gen Starlink satellites.

[M3L0NM4N]

Yeah they seem to have gotten excited to do the probability math (with bad assumptions, conflating a 300m^2(!) cross section collision with an actual probable collision), and with no consideration that this can actually be trivially simulated.

[bpodgursky]

Also, if a solar storm actually wiped out all satellites in LEO (a huge assumption), who really cares how long it takes them to collide? Realistically it's all dead space until they de-orbit in a couple years.