An Orbital House of Cards: Frequent Megaconstellation Close Conjunctions | Hacker News Reader

An Orbital House of Cards: Frequent Megaconstellation Close Conjunctions

arxiv.org

90 pointsrapnie46 comments

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...

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...

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)

This is confusing terminology in the field, but you generally talk about the cross sectional area in the plane of the conjunction (https://www.space-track.org/documents/SFS_Handbook_For_Opera...) to calculate the probability of collision.

It’s a conservative definition in the field. It’s generally defined as the hard body radius: take the smallest sphere centered at the center of mass that would entirely enclose the object, then use the maximum cross section of that sphere to define the potential “area” of the colliding object.

Maybe put more simply, it’s the worst case area size / orientation you could be looking at. So yes. Solar arrays have a narrow cross section from the side but looking at them head-on (which is the angle used for Pc calculations) they’ll be very large.

Shouldn't they try and take some kind of probabilistic average area, rather than worst-case? I assume this is a statistical analysis.

It depends on what you're going for.

Generally people really don't want collisions due to cascading effects, so they take the worst-case probability of collision found with bounding assumptions. Additionally, while often all these vehicles have active attitude (orientation) control, sometimes they go into safe mode and are spinning (often spin stabilized to point at the sun), so it will clear the entire potential radius while rotating.

Also how do you define the probabilistic average area for a space object that you don't know how it's control system works or what it's been commanded to do / point at. Yes we can make some pretty good assumptions for things like Starlink, but even those do take safemodes occasionally.

So It's an engineering judgement call on how to model it. It's hard to get a probabilistic average for attitude that you can confidently test and say is "right", it's a lot easier and conservative to take the worst-case upper-bound. That's at least not-wrong.

Worth adding that the actual collision avoidance manouevres Starlink (and other satellites with propulsion) makes are based on more conservative assumptions

The papers assumptions lead to the conclusion that with no manouevres, we'd see a catastrophic crash between two or more satellites in LEO within 2.8 days. To be on the safe side, Starlink did over 144000 in the first six months of the year (and based on historical doubling rate, will probably be doing 1000 per day by now)...

Yeah the solar array on Starlink is held perpendicular to the velocity vector, so the cross section relative to the colliding body will invariably be smaller than the worst case.

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.

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.

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.

"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.

That's why I mentioned the assumption about the velocity distribution. Sure, the velocities aren't Maxwell-Boltzmann, but that doesn't matter too much for getting a sense of the scale of the issue. The way an astrophysicist thinks (I am one) is that if we make generous assumptions and it turns out to not be a problem, then it definitely isn't a problem. Here they have determined it might be a problem, so further study is warranted. It's also a scientist strategy to publish something slightly wrong to encourage more citations.

Well, sure, they won't be thermally random, but they will be significantly perturbed from their nominal orbits, particularly at the lower orbital altitudes.

Solar flares cause atmospheric upwelling, so drag dramatically increases during a major solar flare. And the scenario envisioned in the paper is basically a Carrington-level event, so this effect would be extreme.

The current "carefully designed orbits" has a starlink sat doing a collision avoidance manuever every 1.8 minutes on average according to their filing for December 1 to May 31 of this year.

Interestingly, the report from which they draw that number is one of the few that they cite but do not link to. Here's a link:

https://www.scribd.com/document/883045105/SpaceX-Gen1-Gen2-S...

It also notes that the collision odds on which SpaceX triggers such maneuvers is 333 times more conservative than the industry standard. Were that not the case (and they were just using the standard criterion) one might naively assume that they would only be doing a maneuver every ten hours or so. But collision probabilities are not linear, they follow a power law distribution so in actuality they would only be doing such maneuvers every few days.

It is disingenuous to the point of dishonesty to use SpaceX's abundance of caution (or possibly braggadocios operational flex) as evidence that the risk is greater than it actually is.

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.

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

  >we introduce the Collision Realization And Substantial Harm (CRASH) Clock

The needless forced backronym is another clue. It's Cargo Cult technical writing.

Why did this need to be a (badly done) acronym at all? It's a countdown to a collision, a collision clock, but of course "crash" (in all caps no less) sounds worse, and science writing needs sciencey acronyms don't ya know...

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

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.

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.

If the Starlink satellites all collided, the worst case is that we would have to ditch the space station (which is already planned in a few years) and wait a few years to launch more into LEO. The debris deorbits automatically due to atmospheric drag. And in the meantime we would still be able to launch through the cloud to higher orbits or escape velocity as it wouldn't be dense enough to hit something that only passes though for a couple of minutes.

IMO now that LEO communication satellites are feasible we should ban launching satellites into higher orbits. Collision debris up there is much, much worse because it's essentially permanent. It will not deorbit by itself for thousands of years or more, and there is no plausible way to clean it up even with technology much more advanced than ours.

There actually is one idea for cleaning up debris in high orbit: You launch tons of very fine powder into the orbits you wish to clear. These orbiting particles create drag on anything up there, so that their orbits degrade much faster. But the because the particles themselves are so tiny, they have a very low ballistic coefficient, and will deorbit quickly.

More: https://caseyhandmer.wordpress.com/2019/10/25/space-debris-p...

Hmm, seems like it would work for 800 km, but maybe not for 1000+ km? Just based on what he says there, which is that each 100 km increase is a factor of 10 in deorbit time, and it's 1 year at 800 km.

That’s a solid idea. Never heard that before. And it really seems like it would solve an otherwise extremely difficult problem.

It would not discriminate though. Everything in that orbit would be taken down - debris and any functional satellites.

You should read the linked post. You can tune the particle size to affect only objects below a certain size.

I'm not sure I believe that operational satellites would be unaffected by sustained bombardment with tungsten particles at orbital velocity (x2 for head on collisions), even if they are 10 microns.

a reasonable concern

If we assume there's some altitude that's so polluted by debris that we need to intervene, it might not have that many functional satellites left. Cleanup the orbit in 1 year might be something the world could agree to if the alternative is waiting 5 years for it to clear up by itself.

It's not just starlink up there, at minimum NRO will be sad and unable to track nuclear weapons and such, the US military will be down a satellite coms systems, and there are probably some people which use starlink for something important.

Yeah it would suck to lose Starlink for a few years. I wouldn't mourn spy telescopes. But most other satellites like weather satellites or ballistic missile detectors or GPS are in higher orbits and wouldn't be affected at all.

My point is that even the unlikely worst case scenario would be limited in time and extent. It couldn't possibly block us from reaching space or last for decades, as some people fear.

> the worst case is that we would have to ditch the space station (which is already planned in a few years)

There is more than one Space Station up there. "Tianhe space station module conducted preventive collision avoidance due to close approaches by the Starlink-1095 (2020-001BK) and Starlink-2305 (2021-024N) satellites on July 1 and Oct. 21 respectively." [1]. Wikipedia also has a long list of planned and proposed space stations.

1. https://www.n2yo.com/satellite-news/Chinas-space-station-man...

a collision can send debris into an orbit with a higher eccentricity. these orbits may not last very long as they would also have a lower perigee.

Even though most of the satellites affected would be other Starlinks, I don't think the many entities with other spacecraft in sub 550km LEO would be particularly delighted to lose them (and the ability to safely relaunch for 5 years)

Satellites in higher orbits do a lot that can't be done in LEO and typically have much lower collision risk (though GEO is fairly crowded). There are plenty of plausible candidate technologies for cleaning up debris, just few practical demonstrations (and even tracking smaller pieces is work in progress)

> Satellites in higher orbits do a lot that can't be done in LEO

This isn't really true anymore. Yes there's a lot of legacy technology still in use and even still being launched, but there's nothing in MEO or GEO that can't be done in LEO with today's technology. Doing it in LEO requires more satellites and better radios, but you get better performance.

The risk of collisions may be lower but the consequences last thousands or millions of times longer...

The consequences of "there's some debris in a little-used MEO orbit for enough decades for it to take for somebody to be bothered to deal with it" are a little less drastic than "the world can't use satellites for 5 years" with the kinds of LEO crashes you'd be encouraging by replacing single satellites in higher orbits with dozens in lower. So even if the unit economics of replacing a geostationary satellite with a constellation large enough to maintain that continuous line of sight from a 500km orbit were acceptable, it would still be the exact opposite of a safety-enhancing move.

It's interesting to try to create a metric of collision avoidance "stress" and resiliency to outages. I don't think this is a particularly useful one (and the title is alarmist/flamebait), but it is a first cut at something new. A more nuanced aggregate strategy for different orbital altitudes would make sense. Maybe some can suggest (or has already suggested) a comprehensive way to keep the risk of cascading debris events low (and measured) that is useful for launch planning.

Complete loss of control of the entire Starlink constellation (or any megaconstellation) for days at a time would be an intense event. Any environmental cause (a solar event) would be catastrophic ground-side as well. Starlink satellites will decay and re-enter pretty quickly if they lose attitude control, so it's a bit of a race between collisions and drag. Starlink solar arrays are quite large drag surfaces and the orbital decay probably makes collisions less likely. I would not be surprised if satellites are designed to deorbit without ground contact for some period of time. I'm sure SpaceX has done some interesting math on this and it would be interesting to see.

Collision avoidance warnings are public (with an account): https://www.space-track.org/ But importantly they are intended to be actionable, conservative warnings a few days to a week out. They overstate the probability based on assumptions like this paper (estimates at cross-sectional area, uncertainty in orbital knowledge from ground radar, ignorance of attitude control or for future maneuvers). Operators like SpaceX will take these and use their own high-fidelity knowledge (from onboard GPS) to get a less conservative, more realistic probability assessment. These probabilities invariably decrease over time as the uncertainty gets lower. Starlink satellites are constantly under thrust to stay in a low orbit with a big draggy solar array, so a "collision avoidance manuever" to them is really just a slight change to the thrust profile.

Interesting stuff in the paper, but I'm annoyed at the title. I hate when people fear-bait about Kessler syndrome against some of the more responsible actors.

By now, I'm convinced that Kessler syndrome exists solely to be fear bait. Almost no one knows what it is or what it does - people just know the stupid "space is ruined forever" media picture.

tangent conspiracy thought

was watching a video about ICBM detection/taking them out in boost phase, and needing a lot for coverage if you had these LEO satellites ready to go but need a lot of delta v (fuel), star link... plenty of em but nah it's for internet/basic navigation/not much fuel

This is exactly what Golden Dome proposes to do for missile defense. It improves the math of missile defense a lot compared to past proposals and it’s not completely crazy but it’s still not certain if the technical capabilities would be there even with a fully operational Starship.

these videos are saying Golden Dome (or at least missile defense) is currently not good enough

the one above and this one https://www.youtube.com/watch?v=KdPTpRfhdWM

which you know credibility who knows

edit: the "not good enough" part I mean kill vehicle trying to lock onto the right target not decoys

but I've only started looking into this, not that I really have a say in it as a civilian just along for the ride

Yeah it’s a long and complex topic. Perun takes the same view as those people you linked but I think there’s several interlinked developments driving this, one of which is the pace of advancement in space access. Even if it won’t work today this is realistically a 20 year project regardless of Trump’s desire to have it by 2028, and if launch costs fall another order of magnitude between now and then and you can make really small interceptors it changes the math a lot. There’s also the broader game-theoretic and strategic stability (or really strategic metastability but this is a whole long and complicated digression) issues which tldr means that if you think such a thing could be possible it would allow any actor who could accomplish it to alter the fundamental MAD equilibrium we have lived under for 80 years and this would come with immense first mover advantages.

The others would also build their own I imagine but yeah, the tech is cool despite the purpose

Checking out Perun

My first thought about that is you'd need a lot of satellites already nearly co-planar with the ICBM's inclination and there probably aren't enough Starlinks in any given inclination to make that realistic (granting secret dV and a sporty enough TWR). Boost phase is pretty short.

yeah the guy I was watching was talking about needing 3Gs of acceleration to intercept if it's not too far away

This is the context I was thinking about https://youtu.be/XDXKRQCkvms?si=1P8eLrZcPiP_ZSHw&t=353

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