Comment by rzimmerman | Hacker News Reader

Comment by rzimmerman | Hacker News Reader

I think this is generally a good thing to do, with some caveats that I hope the FCC considers.

For most satellites below ~450km, this is really a non-issue. Atmospheric drag will generally be sufficient to deorbit the satellite in a few years. For big Earth-observing constellations, Starlink and friends, and many small missions, this doesn't change anything. Debris don't really accumulate dangerously in this zone and collision avoidance is a well-studied problem with a lot of people doing great work. The 18th SDS with the US Space Force, LeoLabs, Starlink/SpaceX, and a lot of other constellation operators take this very seriously and do a good job.

For satellites in the 800km+ range, this is also not a meaningful change. The 25-year rule meant that these satellites needed a deorbit plan anyway. 800km is a rough estimate, in real life it depends on the satellite mass + geometry and the solar cycle.

For satellites in the 450-800km range (again - hand waving here) this is actually a big deal. Satellites that would decay naturally between 5 and 25 years now need an active deorbit plan or need to launch lower and keep themselves up with propellant. Small satellite propulsion is becoming cheaper and more available, so this isn't that onerous for commercial operators. But I do worry a little about educational and non-profit launchers of satellites. They'd be excluded from using launches to this orbital range unless the FCC allows waivers (which I'm hopeful they will).

Overall I don't think this has a particularly big impact. It's a sensible revision of an old rule that was a little too lax. But there aren't that many satellites launching at the high end of the 450-800km range without propulsion. The 1000km-2000km area is a more important area for debris mitigation and that was really already covered.

As for the FCC having this jurisdiction vs. the FAA or the USSF, as Spock would say - "it is not logical, but it is true."

> Debris don't really accumulate dangerously in this zone and collision avoidance is a well-studied problem with a lot of people doing great work.

I think this line of thinking is dangerously incorrect. Sure it’s not a problem right now but we’re poised to have a lot more stuff in orbit than we’ve ever had before. Plastic waste didn’t initially “accumulate dangerously” either but once it became more widely used (and did accumulate dangerously) it was much more difficult to try to reign it back.

Debris does not accumulate in LEO, it reenters naturally. This isn't like plastic waste, it's more like organic waste that naturally decays. Sometimes the law says you have to clean up organic waste, but sometimes it's better to let it rot on its own.

The timeframe varies widely based on the shape of the object and the altitude. There are something like 5 orders of magnitude pressure difference between an object at 100km and one at 500km which is the difference between deorbiting in days vs years.

yes, but if deorbiting in years is sufficient, the difference in time is not relevant.

What? Starlink eventually wants 40,000 satellites - with 5 year lifetimes that'll be 8,000 per year... there are numerous other planned constellations, so figure something like 15,000 LEO per year within a few decades.

15k satellites per year with a 5 year lifespan and a 1-year decay means you'll be stable at 75k satellites. 5 year lifespan with a 3-year decay means you'll eventually be stable at over 100k satellites in orbit with 30,000 of them 'dead' and decaying. That's a massive difference in collision risk.

>That's a massive difference in collision risk.

Is it? It's a big difference in absolute numbers but that doesn't mean it's a meaningful difference in Risk.

If both numbers are quite small relative to the level of concern, the difference can still be irrelevant.

My point is that it takes more than just looking at the number of satilites to understand risk. You need to do the work to show how the collision chance compares to what an acceptable limit might be. Both cases could be very acceptable (or both cases could be unacceptable).

The rule is 5 years. You've given 2 examples under 5 years. If everything is going to re-enter within 5 years without boosting then your point is moot, because even in your "massively more collision risky" example, it still abides by the rules.

If something was going to deorbit within 1 minute, or was going to deorbit within 4 years, 6 months... It doesn't matter to the creator, because they don't have to change their design at all to meet the rules.

If something was going to deorbit within 8 years (because they previously had a 25 year allowed limit), they now have to rework the design.

There's plenty of room for debate about if 5 years is adequate, but as it stands, _most_ things (under 500km) will naturally deorbit within the legal time frame anyway even without special consideration.

Just responding to a comment that said "If deorbiting in years is sufficient, the difference in time is not relevant" where it's obviously relevant.. this doesn't seem remotely controversial.

You're arguing a different thing. The topic was satellite design for satellites under 450km.

If deorbiting in years is sufficient, the difference in 1 minute vs 4 years is NOT relevant -> to a satellite builder worried about the law.

If everything deorbits within 5 years, the only way for more things to accumulate is to launch things faster. But that's a separate discussion. If everything launched today is deorbited within 5 years, then in 5 years, all satellites will be new satellites launched after today. If everything launched today is deorbited within 5 months, then in 5 years, all satellites will be new satellites launched after today. Deorbit speed under a threshold has no bearing on accumulation beyond that threshold.

If SpaceX launches a trillion Starlink satellites, and they all deorbit within a year, then yes. it's going to be a very crowded year, and we'll have to drastically rethink how much stuff we have in LEO, but at the same time SpaceX would not be in violation of the 5 year deorbit window, so the issue is about how much stuff we're sending up, and not how fast it de-orbits.

"Amount of junk below 450km, total" and "Amount of junk below 450km, that hasn't deorbited after 5 years" are very different things. You're making points about total, while the original point was about deorbit speed.

Let me put this another way, if I use my computer to send emails, it doesn't matter to me if my ping is 10 milliseconds or 10 femtoseconds.

One time is 10^12 times longer, but the difference does not matter to me. My emails are still sent and received faster than I can possibly perceived.

There's probably more than 5 orders of magnitude difference between throwing an apple core into the woods and a city's worth of sewage, but both are organic waste. I think the analogy is fairly sound.

While true, there’s also an asymmetry here in that a city can expand their landfill or sewer system capacity- orbital space has a fixed capacity

WTF? Space is incomprehendibly big. The number of potential landfill locations are very limited.

Isn't this thread discussing a very specific orbital range? The 450k-800k range is big, but I'm not sure I'd classify it as "incomprehensibly big".

It’s at least bigger than the surface of the earth :)

Landfills can go underground though! Although I agree that it's unlike we can dig several hundred thousand kilometers :)

Anything orders of magnitude larger than the Pacific Ocean deserves that title IMO.

That's not really true. The "capacity" such as it is is defined by current technological ability to track the assets and respond to changes quickly. There's a whole ton of work to be done there as satellite operators only update their position every few days usually. It should be at least daily and ideally hourly or even by the minute.

Huge constellations with stable orbits under active control are irrelevant. It’s really the a pair of uncontrolled satellites colliding that’s a meaningful risk, and that risk grows exponentially with age. To the point where the difference between 1 day and 1 year is effectively meaningless while the difference between 5 years and 50 years is huge.

If we're launching 15k satellites per year as some people expect, the difference between 1 day deorbit and 5 years deorbit is something like 75,000 satellites.

If the risk of collisions in both scenarios is sub 0.0001%, is it a meaningful difference? Very quickly the risk is dominated by being launched into the wrong orbit.

The risk grows quadratically in decay time, not exponentially.

Actually less than that, as orbits with more decay time have more vertical space.

It seems like it should be quadratic in that double the satellites should get 4x the number of collisions, but satellite orbits don’t start out random. Without stationkeeping initially clear orbits become chaotic in that slight differences in initial position and velocity result in wildly different orbits. That transition results in an exponential increase in risk.

This is most obvious with geostationary orbit. Over a single year you could have say 10,000 satellites in geostationary orbit with essentially zero risk of collision, but without station keeping risk continues to grow every year those 10,000 satellites drift around.

I see your point that it’s not random but an exponential increase wouldn’t result from chaotic changes in orbit. That randomizes, making it quadratic.

It looks like inactive geostationary satellites will drift to the same longitudes, and tilt their axis away from the pole. If you have a set of geostationary satellites in that situation, the risk of collision is still quadratic in growth.

The risk is the sum of the pairwise probabilities of collision. So the only way you get super quadratic is if incrementally added satellites have increased pairwise risk with other satellites. It is possible that could be the case, because if you have to pack satellites more tightly, then maybe each satellite has higher pairwise risk with its direct neighbors.

That would still be sub-exponential, unless the collision risk between two satellites got exponentially higher as they get more closely packed. For example, suppose you have a ring of satellites, and you double the number. Then immediately neighboring satellites are twice as close. By basic inequalities you can show that considering the non-neighbor pairs of satellites, they have less than quadrupled the total risk of a collision. So by the master theorem we know the risk of collision would only grow exponentially if there were an exponentially higher risk of collision between two neighboring satellites in terms of the reciprocal of their distance. But that is not the case, is it? You’d expect it to be polynomial. Fewer orbits before the satellite drifts past its neighbor, a smaller expected distance to the neighbor as it drifts past… as with most physics problems these are linear components, multiplied together.

Thinking about it more, it could probably be modeled by the intersection of random walks that are initially unable to come into contact. I was thinking that was exponential but it’s presumably some polynomial function with a large exponent that’s eventually bound by a different quadratic function for sufficient values of time.

It would seem to be exponential after the first collision, at least.

https://www.youtube.com/watch?v=mQT5aMa_7iI

"Gabbard Diagram for Low Earth Orbit 1959-2021"

See how the stuff in the lower left corner speeds up towards the lower left corner? That's low flying debris falling out of space.

I am generally allergic to video, but that was fascinating to watch. Thanks for sharing it.

Everybody interested in the topic should watch this video (it's just an animated graph, no music). You will get a better intuition for the lifetime of orbital debris, and the impact of various incidents, in a few minutes than in hours of reading.

Wow, what a beautiful video. I really enjoyed trying to figure out what events and phenomena the different shapes/patterns in the diagram represent.

I didn't mean to imply orbit debris isn't a real problem. But for the low end of LEO:

- Most debris deorbits naturally in a few years. Any debris causing events or accumulation naturally clears out in a reasonable time frame. It's not like debris at 800-2000km which is the real "Kessler Syndrome" concern where it takes decades or centuries.

- Debris mitigation efforts that are already in place are effective. Limiting debris release during launch and deployments has had a huge positive impact over the last few decades.

- "Traffic control" is a lot easier at these altitudes and debris in this range is well tracked. Obviously this doesn't extend to small stuff (<5cm), but due to active mitigation and natural decay this is less of an issue. Also ground radars are getting better and can actually see a lot of these objects now.

In this case it's policy reflecting Little's Law:

    population = arrival rate X duration of visit

If suddenly someone wants to launch twice as many satellites, you either have to reduce tenancy or accept a higher population in the system. If you have a policy that's at most 25 years and a lot of satellites de-orbit in say 15 years on average, you can still triple your launches by pulling that down to 5.

I am worried though, do we really build satellites that are expected to only work for 5 years? Are we disincentivizing people from building 20 year satellites this way?

I assume it means deorbit within 5 years after the satellite is decommissioned.

The article says "5 years after completing their mission", not 5 years after launch.

time to assign 25 year missions to satellites built to last 10 years!

Every mission has to pass FCC approval, and we're just discussing a super high level gloss of the rules. Thee's no way that would pass, so that's really not an issue.

Wait, really? What orbital mechanics take five years to achieve? Lunar assisted orbital insertion burns only take a year or two, right? Why wouldn’t the deorbit burn start the day after decommissioning?

If it takes 5 years, there is no deorbit burn.

Several companies have been developing deorbit add-on modules, and I hope this requirement spurs on this growing industry. I would love it if it became the norm that satellites have a passive failsafe deorbit system that activates at after a set amount of time or if ground communications fail for a set duration.

https://www.nasa.gov/smallsat-institute/sst-soa/deorbit-syst...

How high can the debris loft after a particularly nasty collision?

Two satellites hitting at an acute angle should produce a cone of debris in front of them, of which about a third deorbits and a third goes up into a higher orbit that's an average of the two.

Two satellites that hit at an obtuse angle, well, they pancake and produce a donut of debris. The stuff headed straight up is on a parabolic orbit that will hit the atmosphere on the way back down, but in the meantime any other satellite that crosses paths with it is effectively hitting a wall of stationary debris, creating a new cone that ladders up higher. Is there enough space that the ladder stops, or does it just keep building?

I don't think debris 'going higher' isn't much of a problem. Whenever this happens the orbit is going to be more eccentric - meaning a lower periapsis, and consequentially lots of drag that will cause a rapid deorbit.

On the second point about parabolic orbits I also find that probably relatively low risk because we are only talking about a fraction of an orbit for a collision to occur so unless the debris field was massive the chance of another collision is probably still low. Remember when we are modelling orbit collisions normally we are often talking over 25+ years - 100,000 + orbits.

I think the main problem is busy orbits (e.g. sun-synchronous polar orbits at popular altitudes) where most of the debris remains roughly in the same orbit following an acute collision but has a lot of other potential collision targets. Also as satellites are disabled by a collision they lose the ability to avoid other objects already in the same crowded orbit - i.e. the fraction of objects able to take avoidance decreases increasing the chance that future collisions are from 2 incapacitated satellites, removing the possibility of avoidance.

Angular momentum is subject to a conservation law.

What does angular momentum have to do with Kessler syndrome?

After a collision, the periapsis is at most as high as the point of collision, and the apoapsis is at least as high as the point of collision. In plain English, if two satellites collide with each other at an altitude of 450km, it doesn't matter what the original orbits were; 100% of the debris will have a low point in their orbit at or below 450km, and 100% of the debris will have a high point in their orbit at or above 450km.

One of the quirks of debris deorbiting is that drag is exponentially higher the lower you are in orbit, and any drag that the debris experiences at any point along its orbit will manifest in a reduction of altitude 180 degrees along is orbit on the other side. So if you have a piece of debris with an eccentric orbit, let's say 300km at periapsis and 1,000km at apoapsis, after a fairly brief amount of time you'd expect the debris to have a periapsis of 299.9km and an apoapsis of 500km. Then perhaps 299km periapsis and a 350km apoapsis. etc. I'm making these numbers up but you get the idea: the high point in an orbit is the part that drops the quickest.

Even if you were to launch 100,000,000 full sized satellites into orbit at 400km altitude and deliberately orchestrated Kessler syndrome, space would be unusable for a few years, but would be 100% back to normal after 5 years.

The dangerous orbits are those in the 500-2,000km range. Satellites whose orbits never bring them low enough to experience significant atmospheric drag. Those are the satellites this new rule is targeting.

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