[HN Gopher] An Orbital House of Cards: Frequent Megaconstellatio...
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An Orbital House of Cards: Frequent Megaconstellation Close
Conjunctions
Author : rapnie
Score : 73 points
Date : 2025-12-11 15:01 UTC (7 hours ago)
(HTM) web link (arxiv.org)
(TXT) w3m dump (arxiv.org)
| TGower wrote:
| 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...
| philipwhiuk wrote:
| 200 might be more reasonable for the next gen Starlink
| satellites.
| M3L0NM4N wrote:
| 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.
| deddy wrote:
| 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 wrote:
| 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)
| deddy wrote:
| 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.
| HPsquared wrote:
| Shouldn't they try and take some kind of probabilistic
| average area, rather than worst-case? I assume this is a
| statistical analysis.
| deddy wrote:
| 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.
| rzimmerman wrote:
| 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.
| MarkusQ wrote:
| 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 wrote:
| Even the title "Orbital House of Cards" is unnecessarily
| editorializing.
| Sanzig wrote:
| 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.
| bpodgursky wrote:
| 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.
| queuebert wrote:
| 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 wrote:
| "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.
| Sanzig wrote:
| 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.
| queuebert wrote:
| 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.
| SiempreViernes wrote:
| 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.
| SiempreViernes wrote:
| 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.
| modeless wrote:
| 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.
| SiempreViernes wrote:
| 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.
| modeless wrote:
| 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.
| emtel wrote:
| 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...
| oofbey wrote:
| 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.
| emtel wrote:
| You should read the linked post. You can tune the particle
| size to affect only objects below a certain size.
| modeless wrote:
| 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.
| modeless wrote:
| 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.
| hidroto wrote:
| 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.
| infinet wrote:
| > 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...
| notahacker wrote:
| 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)
| ge96 wrote:
| 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
| procflora wrote:
| 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.
| ge96 wrote:
| 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
| ls612 wrote:
| 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.
| ge96 wrote:
| 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
| ls612 wrote:
| 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.
| ge96 wrote:
| The others would also build their own I imagine but yeah,
| the tech is cool despite the purpose
|
| Checking out Perun
| rzimmerman wrote:
| 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.
| ACCount37 wrote:
| 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.
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