SpaceX lowering orbits of 4,400 Starlink satellites for safety's sake | Hacker News Reader

SpaceX lowering orbits of 4,400 Starlink satellites for safety's sake

space.com

74 pointsthread_id102 comments

Won't this make running Starlink more expensive?

Lower orbits > Increased atmospheric drag > More fuel expended to maintain orbit > Heavier sats due to more fuel > Increased launch cost per unit

Or even: Lower orbits > Increased atmospheric drag > Quicker orbit decay > Shorter lifespan of sats > More frequent launches

Forgive my Kerbal-based space knowledge here.

Essentially, the atmosphere contracted due to the solar minimum and they want to make sure inoperative satellites decay within months rather than years. Ballistic decay is a safety mechanism in case their satellites break down. Also they will use less fuel for avoiding the crowded 550km band.

> "As solar mininum approaches, atmospheric density decreases, which means the ballistic decay time at any given altitude increases — lowering will mean a >80% reduction in ballistic decay time in solar minimum, or 4+ years reduced to a few months," Nicolls wrote in his X post. "Correspondingly, the number of debris objects and planned satellite constellations is significantly lower below 500 km, reducing the aggregate likelihood of collision."

From a comment :

>The first move in the coming WWIII, where the emperors try to expand their empires militaril,y will be to wipe out any orbit with Starlink satellites.

I find this highly unlikely, given Starlink is soon to reached 10k satellites and will continue to grow. Why expand 10 000 ballistic missiles to bring down one of many communications networks ?

Because Kessler syndrome means you don't need to hit all 10k yourself.

Lowering the orbits just means that we get back to normal faster, not that the it's impossible.

Does Kessler syndrome also mean ICBMs become nonviable?

No.

It's not a wall. The risk from going through a dangerous orbit is much much less than the risk from staying there.

That depends on how you define risk. If it means the probability of a collision, then you'd be correct. But if a collision does happen, the consequences will be worse than being in the same orbit. Based on an oversimplified model, debris in orbit is likely to have low relative velocities with respect to an intact satellite in the same orbit, since a large deltav would change the orbit. (It's not as simple as this, but it's good enough in practice.)

This is actually what asat weapons take advantage of. They usually don't even reach orbital velocity, just like ballistic missiles (of course, there are exceptions like the golden dome monstrosity). The kill vehicle just maneuvers itself into the path of the satellite and lets the satellite plough into it at hypervelocity.

I remember a short story about Canada preventing total global annihilation in WWIII, by deliberately triggering Kessler syndrome. My google-fu is failing me though.

I would love to read it:)

Stop trying to make Kessler syndrome a thing. Kessler syndrome isn’t a thing, and it will never be a thing.

PS The original paper expects the cascade to take decades to centuries. No one can afford to shoot down Starlink except SpaceX.

Stop trying to boss people around, and just make your point with some citations.

It's a reference to the "stop trying to make fetch happen" meme from the movie Mean Girls.

Ah, so I should have replied with "Yeahhh bitch, science!", because that's a reference to the Breaking Bad meme? That's how we're discussing things now?

> Kessler syndrome means you don't need to hit all 10k yourself

Kessler is useless for LEO constellations. The timeframes of the cascades exceed the useful lives and dwelling times at those altitudes.

I am not aware of a military solution to prompting a cascade over even a limited area. Instead, you’d use repeated high-atmosphere nuclear detonations to fry birds in a region.

If it's WWIII, and you're using ballistic missiles against satellite constellations, then either:

- You are not targeting individual satellites; you're setting off nuclear warheads in space, and relying on the EMP to disable all satellites within a large radius of the blast - https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse

- You're nuking the ground-based command & control centers for those satellites. Again, nothing like 10,000 missiles needed.

(Or both.)

To target 10,000 satellites directly, the "obvious" weapon would be a few satellite-launch rockets, lofting tons of BB's (or little steel bolts, or whatever) - which would become a sort of long-duration artillery barrage shrapnel in orbit.

The BB idea doesn’t really work either- if they are in orbit they circle with the satellites and don’t hit anything, if they are at different speeds they are in different orbits and fly above and below the satellites and miss, if they cross the orbit SpaceX just moves the satellites to miss.

"Circle with the satellites" is not how orbits work. Do a Google image search for satellite ground tracks, and observe how those tracks repeatedly cross each other. In LEO, a 90 degree orbital crossing represents a relative velocity of >10km/s. (Normally, collisions do not happen because the satellites are under control, and everyone is making ongoing efforts to avoid collision. Kinda like how cars & trucks normally don't hit pedestrians.)

BB's - https://www.amazon.com/bulk-bbs/s?k=bulk+bbs - run roughly 3,000 to the kg. And are far too small to individually track in orbit - https://clasp.engin.umich.edu/2023/12/06/tracking-undetectab...

Bottom line - a "3 tons to LEO" satellite launch vehicle could put ~10,000,000 untrackable little metal objects into orbit, crossing satellite orbits at lethal velocities. Trivial methods, such as dispersing the BB's with small explosive charges, could randomize their individual orbits.

The satellite operators have very good reason to be concerned about such "low tech" anti-satellite weapons.

> - You're nuking the ground-based command & control centers for those satellites. Again, nothing like 10,000 missiles needed.

With Starlink's peer-to-peer capabilities, hitting every single ground station and keeping the satellites from working through new ground stations may actually be quite difficult.

Starlink orbits close enough that they're looking into offering LTE coverage from "space". You don't need a giant dish to access the satellites, which means building new ground stations and reprogramming the network from an unassuming-looking ground device to use them is quite feasible.

The paths of the satellites are rather predictable, though, so your shrapnel attack executed with some precision should clear out enough of them.

The moment you launch a nuke (even if just to set off an EMP), you can expect nukes to come your way in retaliation before your nuke even detonates. Unless whatever war is going on has already gone full nuclear, I don't think nuclear weaponry is a viable move to take out satellites.

> With Starlink's peer-to-peer capabilities, hitting every single ground station and keeping the satellites from working through new ground stations may actually be quite difficult.

Yes-ish? I was thinking the command & control facilities - far scarcer than the (probably unmanned) StarLink-to-Internet Backbone connection ground stations.

> The moment you launch a nuke...

Yes-ish. The (great-)^n grandparent comment posited WWIII starting, and the nukes flying at scale. Between the widespread obliteration of ground-side infrastructure, ground-side EMP damage, and very likely EMP in space - I'd assume that Starlink would quickly go down. Plus, the ionosphere could become opaque to Starlink's radio frequencies. Finally, the ionosphere's upper layers might expand enough (due to nuclear detonations in or near space) that the orbits of the Starlink satellites started degrading very quickly.

With how easily any major space power could set off "small n" nukes in space during a major crisis, to knock out satellites - I would not rule someone doing so. The responsible parties need not claim responsibility. And sane leaders might hesitate to go full nuclear in response.

You don’t need 10k missiles. You need just one to blow up all of starlink satellites.

This is like bowling, you hit one, it hits the other one etcétéras.

You would likely need at least one per orbital plane, of which there are about 24.

Blowing up something in the same orbit as the targets isn't an effective strategy. The explosion disperses the fragments into different orbits that intersect the original orbit only at one or two points. And even if some of those fragments find their targets, the collision velocity will be low (relatively slow).

It will be like getting hit with with shrapnels from a grenade. Depending on how they collide, the target may survive. If you think that grenade shrapnels are fast, you need to understand the 'hypervelocity impact' that happens when objects in different orbits collide, or when an interceptor hits a satellite. Hypervelocity impacts are impacts where the impactor moves faster than the speed of sound in the solid target. What that means in practice is that the debris/interceptor may have hit one end of the satellite and vaporized already, while the other end of the satellite doesn't yet feel the shock and vibration from that impact. That end doesn't yet know about the carnage that's about to hit it in a few milliseconds.

I imagine you could just send a rocket to the specific orbit and just start metering out 100s of thousands of small steel (or whatever) BBs like seeding a yard; seeding a Kessler event.

Something just a bit bigger than:

https://en.wikipedia.org/wiki/Project_West_Ford

That is not how it works at all.

Imagine using a rocket and blowing up one car on a highway - how many other cars will actually be affected? How many cars on other highways will be affected?

https://en.wikipedia.org/wiki/Kessler_syndrome

What was that game on old PC's? ... Minesweeper ...

You could launch some missiles, blow a few satellites into smithereens, and gradually over the next few months they would take out the others. That's a poor kind of war weapon. An effective weapon is one where you can inflict damage continuously, and are able to stop immediately upon some concession. If you can't offer to stop in return for concessions, you won't get any.

Its not really that easy, to cause such a chain reaction, specially if the other person reacts.

And its also really expensive, each sat you take down costs you far more then what you hit. So unless you can actually cause a chain reaction its a losing proposition.

You don't take down satellites in order to force someone to negotiate, you take them down for denial of capabilities.

Not really. That’s more science fiction than reality. You should try some Kerbal Space Program and explore how orbits are affected by thrust = collisions, in different directions.

As soon as a satellite is hit the rest of the fleet can start thrusting and raise their orbits to create a clear separation to the debris field.

Following such an attack the rest of the fleet would of course spread out across orbital heights and planes to minimize the potential damage done by each hit, leading to maximum cost for the adversary to do any damage. Rather than like today where the orbits are optimized for ease of management and highest possible bandwidth.

Looking at the price of industrial lasers, right now the only thing stoping a random 3rd world terrorist cell from being able to afford to destroy all of them is the adaptive optics to compensate for atmospheric turbulence.

Well, that and the fact that so much of the stuff on Amazon etc. that's listed as "welding laser" is actually a soldering iron.

I think you severely underestimate the amount of power you would need to damage some Starlink satellites in the 4 minutes they would be visible while tracking them at ridiculous speeds.

Modern welding lasers are pretty powerful. Hard part is focussing them, especially given need for adaptive optics for atmospheric turbulence. Movement is almost irrelevant, they only deviate from a fixed path while their engines run, and even then those engines are pretty weak.

I'm saying the only real limit now is the adaptive optics.

When people attack satellites with lasers, what they're trying to do is blind surveillance sats. To actually physically damage a satellite would take enormous amounts of power and accurate tracking tied into powerful radars. That's something a state might do, but too many resources are involved for terrorists.

I don't think that's necessarily true. I think with modern lasers the power to get permanent damage is there.

I don't think powerful radars are required either. The satellites will probably reflect the laser. At 4000 km this is 26 ms, so you would probably be able to use the laser itself for the adjustments.

Previously, perhaps.

I'm saying the only limit to damaging them now is the optics.

Radar is pointless here. Both for crude and precise positioning. For crude, we already know roughly where the satellites themselves are because those are well-advertised.

For precise, you still wouldn't use radar, physics prevents high enough resolution ever. Even for tracking, angular resolution is k(λ/D), i.e. you care about aperture size in wavelengths, and radar uses wavelengths much much larger than visible or IR laser light. But even with arbitrarily large equipment, you get a spot size ~= wavelength, which wastes a lot of power as the wavelength is much larger than the necessary spot size for a critical component on a satellite.

So you'd use optical targeting and tracking, i.e. you'd look through the exact same system that the laser also fires through, with the exact same adaptive optics, and say "this specific point on this satellite".

The hard part is focussing a spot size order of cm scale (it can't be the same size as you find in a welding system for same reason radar is useless, k(λ/D) gets you ≥300m telescope and that's obviously a no). This requires adaptive optics (and also a wide telescope). Adaptive optics is the really hard part here.

Getting a 12 kW industrial laser is relatively easy, and putting that power into a spot on the joint between the PV and the main body, that has a decent chance[0] of weakening or severing it while also causing catastrophic loss of control, even with just a few minutes over the horizon. Weakening is still important, see all 9/11 memes about steel beams and why they miss the point. Severing is plausible but only because of space design constraints, see [0] again.

As I understand it, PV cells themselves have a much lower threshold for catastrophic damage, a 12 kW system is basically guaranteed to cause irreparable damage to that even in a few minutes even though the spot size here is much much larger than you'd find in a welding system.

The prices I see for 12 kW industrial lasers are significantly lower than the estimated cost per missile for most of what the Houthis used to attack shipping last year, and they fired quite a lot of those missiles.

[0] can't say for sure without detailed plans that it would be genuinely insane[1] for me to have access to; but do consider that everything in orbit is mass constrained, even with SpaceX pricing, and designed without expectations of e.g. wind or needing to support its own weight, so the thickness of structural elements is likely much lower than you'd expect from anything you see on the ground

[1] the world is currently going insane, so if it turns out they are available, either deliberately or via a leak, this is just more evidence of insanity rather than a contradiction

Or why try to shoot them down when you can also go to the command center and turn them off? Or do a targeted strike on said command center. The sattelites are plentiful and redundant, but the network will collapse very quickly when they're no longer controlled from the surface.

In fact, if SpaceX can no longer do any launches due to whatever reason, Starlink will no longer be feasible after a few year - if I'm reading it correctly, the sattelites have a lifetime of only 5 years, meaning they will have to continually renew them at a rate of 2000 new sattelites a year.

I think it's important to note that not all collisions are equally dangerous. Consider a sat on a polar orbit colliding with one on a equatorial orbit. Or two satellites on different directions. That is going to be spectacular. Otoh, these kind of collisions are unlikely and should be manageable by just assigning certain shells (say 5km) for every possible direction and orientation.

If two Starlink satellites collide that go roughly in the same direction, it's not exactly a huge problem.

I think the biggest issue is to coordinate this and potentially disallow some excentric orbits.

Not quite how it works, unfortunately.

Once you've got even hundreds of satellites in non-equatorial orbits, trying to provide global coverage - their ground tracks very frequently cross each other. Even if they're all at the same orbital inclination. While those mostly won't be 90 degree crossings - the great majority will involve several km/s relative velocity. And you'd run out of (say) 5km LEO shells very quickly.

But the orbit is a minimum of about 50,000 kilometers, and the satellite is maybe a meter across. That's a very low probability of a collision per crossing.

I get that 'probably safe' or '0.001% chance of destruction per day' is not very satisfying for an investment that cost millions, but everything always comes down to odds. None of these satellites are eternal, even if they're the only thing in their orbit.

Is it still a small number when multiplied by the square of the number of satellites and the number of times they orbit each day?

Don't know. But I'm sure that people at NASA and other such places have done that calculation. I just wanted to point out that orbital space is big, so you have to do the math to see if there's an actual problem.

They have, and it is. It's a big problem.

3 week old news OP?

Previously: https://news.ycombinator.com/item?id=46457454

Can anyone explain how does one technically lower a satellite?

Eject mass in the forward direction of its current tangent of motion. Slow down to go down.

So, for this they have a bit of expendable extra mass on board? What material is it, would it not cause even more debris then?

https://starlink.com/technology

> Efficient argon thrusters enable Starlink satellites to orbit raise, maneuver in space, and deorbit at the end of their useful life. Starlink is the first argon propelled spacecraft ever flown in space.

And you can see "How Ion Engines Work in Under 60 Seconds" https://www.youtube.com/shorts/_MUv28Yf_4g

The 'expendable mass' is almost never a solid or liquid. It's the gaseous combustion exhaust or plasma exhaust from the satellite's thrusters. The advantage of gases is that they just expand and disperse fast enough to be too wispy to cause anything on impact.

However, there are a few systems that do use solid masses for obtaining a reaction force. A remarkable example is called a 'Yo-yo despinner' [1]. It was used in missions like Phoenix (Mars mission) and Dawn (Asteroid belt proto-planet mission). And yes, it does create space debris. But those space debris are probably somewhere in orbit around the sun. Nothing that those guys are going to be too worried about.

[1] https://en.wikipedia.org/wiki/Yo-yo_de-spin

Satellites need thrusters for station keeping. Otherwise they drift out of their desired orbits over time.

Yes though the smallest ones like cubesats don't have them. They do tend to have rotation wheels for keeping themselves aligned but they can't actually affect their own orbit.

You're correct. I'm just going to add a bit more technical context here. The process of keeping a steady orbit is called 'station keeping'. Similarly, the process of maintaining the correct orientation/alignment is called 'attitude control' (attitude is the technical term for orientation).

Attitude control can be achieved to a finite limit using momentum wheels or reaction wheels. But at some point, it will reach its maximum speed and its control capability saturates. You will need to 'desaturate' the wheels and restore its control capability. One method is to produce a counter torque using special reaction control thrusters (RCT) called 'attitude control thrusters'. That needs propellants. Smaller satellites don't have that luxury. So they exploit Earth's magnetic field by using a 'magnetic torquer' to produce the counter torque against it. That needs only power, not propellants.

Let me see if I can. Before we go to space, let's try something on the ground. Imagine pitching a ball horizontally. What do you expect if you pitch it too slow? The ball will curve more towards the ground and meet it early, won't it? (In other words, it doesn't go very far and doesn't stay airborne for long). Going from ground to space, this action remains the same. You need to 'lower an orbit'? Reduce its forward velocity. It will curve more towards the planet and reach closer to the ground.

However, there is a bit more detail involved here. Why doesn't the satellite just fall to the Earth? (Please excuse me and disregard this part if you know this already. I'm trying to maintain conceptual continuity.) So, when something is flying horizontally (no aerodynamic forces), we know that its trajectory will curve towards the Earth due to the pull of gravity. If the ground (on Earth) curves as fast as, or even faster than the trajectory's curve, the object will never get an opportunity to even reach the ground. This is 'orbiting'.

Now assume that the satellite is initially in a circular orbit. The gravitational force acting on the satellite at any point in the orbit is perpendicular to the satellite's velocity vector and tangential to the orbit. The satellite will maintain a constant speed at this point, since its velocity and the force are always perpendicular [1]. So, what happens when we reduce the satellite's forward velocity? Just as we've seen with the ball, the satellite's trajectory (orbit) starts to curve more towards Earth. Now a subtle, but important change occurs. The velocity and the gravitational pull are no longer perpendicular! They start to align! And when that happens, the speed MUST increase. So, the satellite is now losing altitude and speeding up simultaneously [2]. At some point, the satellite will pick up enough speed again to 'straighten its curve' and avoid falling to the ground. In effect, the satellite had to compensate for the lost velocity in order to remain in orbit, and it did so by exchanging some of its altitude (gravitational potential energy) for velocity (kinetic energy) [3].

So our satellite 'fell' from where we slowed it down, until it had enough velocity again to maintain orbit. At that point, the gravity and the velocity are parallel again, since it will keep falling otherwise [4]. But since it 'fell from a higher altitude', it's speed is now too high for it to remain at that altitude. The orbital curvature is a bit 'too straight' now and it starts to curve away from Earth. So now we're in the exact opposite situation of what was explained in the last paragraph. The satellite is now climbing back up again! As it happens, the satellite actually climbs back up to the point where we slowed it down! And when at that point, its velocity is exactly the same as what it was, after we had slowed it down! [5] So the satellite did the inverse of what it did earlier - it exchanged kinetic energy to get back its altitude (potential energy). The satellite is now living in cycles juggling kinetic energy and potential energy back and forth. The final effect is that the point in orbit that's diametrically opposite to where you slowed it down, is now at a lower altitude. And thus you've effectively 'reduced the orbit'!

One more detail to pin down. How do we slow down a satellite in the first place? Easy! Push the satellite in the opposite direction of its velocity [6]. This is called 'retrograde thrusting' or 'retro burn'. But that's about as easy as it gets. Remember that unlike on Earth, you don't have a surface (a wall or the ground) to lean against. Imagine pushing something heavy on an ice rink. The good news is that you can still push things on an ice rink. The only catch is that the push force will set both the item and you in motion in opposite directions [7]. And that's exactly what we do in space. We throw out mass from the satellite in the form of super-fast gaseous of plasma exhaust. The key is to throw out the mass with as much momentum as possible. But the mass is limited by how much you can carry - it's a depleting resource. So you're basically left figuring out how to throw it out with ever increasing speeds. And that's how we slow down the satellite in space - fire your thrusters!

And finally to lower an orbit entirely, instead of just one point on it, you have to do multiple firings. There are bunch of these 'orbital maneuvers'. The most common one is the Hohmann Transfer [8]. If you could understand what's given above, most orbital maneuvers including Hohmann Transfer will feel very intuitive to you.

[1] Speed is the magnitude of velocity and it remains steady in a circular orbit. However, the perpendicular force will keep bending the velocity vector, thus constantly changing its direction.

[2] This is the from-the-first-principles explanation of conservation of angular momentum. This is how the ballerina spins faster by pulling in her arms.

[3] If this sounds like a 'negative feedback' phenomenon to you, that's because it is. Feedback is a mathematical construct. Nobody ever said that a feedback mechanism must be implemented separately. Some systems have them inherently built-in.

[4] This is the lowest point of the orbit - the periapsis.

[5] Yes. There is quite a bit of hand waving here. I didn't explain why the satellite went back to its original position with the exact same speed. But that's what actually happens. It might take a lot more 'mathematical sense' to explain just using words. One thing I know is that this has something to do with the fact that the gravitational field is one of those 'conservative fields'. If you take a trip inside a conservative field, and return to the location where you started, you will be left with the exact same (kinetic) energy as you started with. You may exchange your energy during the trip, but you always regain it back when you get back to the starting point, no matter what path you took. As far as I understand, the 'conservative' part refers to the part that the energy is conserved and stored, and never lost. Unfortunately, the force field that we're most familiar with - frictional force - isn't conservative at all. If you're going on a trip, be ready to spend some energy!

[6] One matter that confuses a lot of people is why the satellite's position changed at the opposite side of the orbit, instead of the point where we applied the force. The answer is in the Newton's second law. Force changes momentum, not position - at least not directly. The direct effect of application of retro thrust is that the velocity reduces at that point. The change of position on the other side of the orbit is only a consequence of that velocity change.

[7] Yes, the Newton's vengeance law.

[8] https://en.wikipedia.org/wiki/Hohmann_transfer_orbit

[9] Every so often, someone comes along and argues that gravity is not a real force and all these explanations are wrong. If you want to deal with this in terms of relativity and space time curvature, be my guest. But for all practical purposes, the old faithful Newtonian physics works just fine, even as a special case of relativity.

[10] This should probably have been a blog post. Please don't shout at me if it annoys you. This is one of my favorite subjects and I just got carried away. I used to teach and train many students and junior professionals in these topics.

>[10]..

From the looks of it, you still are teaching. Very informative read!

Extra points for non-referenced footnotes! =)

Thank you for your kind words! Glad you liked it! It's during instances like this that I wish I really wrote things down and preserved them.

What’s the plan as the solar maximum returns?

Adjust them again as needed ..

There are so many satellites in orbit that there is a pretty good chance that if even one was to be hit by something and explode in many pieces, it would crash another one and then another one until there is nothing left.

The nasa is pretty scared of it, so is SpaceX.

There is huge increase of orbital launches in recent years [1] done mostly by SpaceX and China is also planning to double its numbers in the coming years. The risks will be even higher.

[1] https://spacestatsonline.com/launches/country

That's the Kessler Syndrome. But it's better if it happens in a lower orbit, irrespective of what assets are present there. Space will be free for exploration again in a few years since all the debris there would eventually decay and deorbit.

The article mentions a few months at 480 km. I'm a little skeptical about this figure though, because the last tracked piece from an NRO satellite that was shot down at ~250 km by SM-3 missile in operation burnt frost, lasted 20 months in space before reentry. SpaceX is probably using a statistical cutoff percentage of fragments to calculate the time. But all the pieces are dangerous uncontrolled hypervelocity projectiles. Spain lost a military communications satellite a few days ago from a collision with a tiny undetermined space debris.

Two objects colliding can send debris into different orbits. Combined kinetic energy and mass differences can send debris to many different orbits.

A golf ball hitting a bowling ball or basketball, both traveling at 30 units of speed can produce quite a fast golf ball. Not all of the debris will safely burn up.

At the speeds we're familiar with, basketballs and golf balls have elastic collisions. At orbital speeds, satellites are nearly inelastic. So fragment exit velocities lie between the two initial velocities, kv1 + (1-k)v2 for some k that depends on where each fragment came from. If they're colliding, the velocities must be somewhat different, so the weighted average speed has to be lower than orbital speed. So fragments usually don't survive many orbits.

That's what I was thinking, Kessler syndrome should be impossible for objects in LEO since all debris orbits decay rapidly (probably 99.9% enter the atmosphere and burn up in minutes, the rest in hours)

I guess if a collision ruptures a pressurised tank, or causes an actual explosion then you could end up with a higher-than-orbit speed?

Possibly. But more likely the thrust from escaping gas will push it in a direction to either slow the orbit down or make it more eccentric and unstable.

Right, if there's something like a small hole in a pressure tank, it's very unlikely to be aligned exactly with the CG, so the tank will spin around and the net thrust will be near zero.

If a pressure tank splits in half, both halves will fly away but that's a very inefficient way of using the energy in the gas, so the added velocity will be a small fraction of the speed of sound in the gas, which is 1/6 of orbital speed for hydrogen, less for any other gas.

You can't really get much of a chemical explosion because the fuel and oxidizer both disperse very quickly in space.

Very well put. It also seems like there's a limit to how bad Kessler syndrome can get. The more debris there is the more collisions, but the more collisions the quicker the debris collides with itself and de-orbits.

The periapsis will always pass through where the collision happened.

To circularize at a higher orbit you would need secondary collisions on the other side of the earth.

You're right that all the fragments will pass roughly through the impact point in orbit. But it's not always the periapsis.

1. The normal or anti-normal delta-v imparted by the explosion/fragmentation (i.e, the velocity imparted perpendicular the plane of initial orbit) will cause the orbital plane of the fragment to change. The new orbit will intersect the old orbit at the impact point. Meanwhile, the eccentricity (the stretch of the orbit), semi-major axis (the size of the orbit) and displacement of periapsis from the impact point (the orientation of the orbit) remains the same as the initial orbit.

2. The prograde and retrograde delta-v (velocity imparted tangential to the orbit) will cause the diametrically opposite side of the orbit to rise or fall respectively. Here too, the new orbit intersects the old orbit at the point of impact. But since the impact point isn't guaranteed to be the periapsis or apoapsis, the above mentioned diametrically-opposing point also cannot be guaranteed to be an apsis.

3. The radial and anti-radial delta-v (this is in the third perpendicular axis) will cause the orbit of the fragment to either dip or rise radially at the point of impact. Again the impact point remains the same for the new orbit. So the new orbit will intersect the old orbit either from the top or the bottom. The new orbit will look like the old orbit with one side lowered and the other side raised about the impact point.

So none of three components of delta-v shifts the orbit from the impact point. You can extrapolate this to all the fragments and you'll see that they will all pass through the impact point. The highest chance of recontact exists there. However the perturbation forces do disperse the crossing point (the original impact point) to a larger volume over time.

Edit: Reading the discussion again, I get what you were trying to say. And I agree. The lowest possible altitude of the fragments in orbit (i.e the periapsis) is the same that of the impact point. So if the impact point is low enough to cause drag, the orbit will decay for sure. There is nothing that demonstrates this better than a Gabbard plot [1][2] - the best tool for understanding satellite fragmentation.

[1] Gabbard Plot Discussion (NASA Orbital Debris Program Office): https://ntrs.nasa.gov/api/citations/20150009502/downloads/20...

[2] Satellite Breakup Analysis (Australian Space Academy): https://www.spaceacademy.net.au/watch/debris/collision.htm

No worries. I think I could have been more precise in my wording. :)

My comment is based on the hunch concerning physical calculations and interactions from an engineering physics degree and way to many hours in kerbal space program a decade ago.

Thanks! I figured that you had a reasonable understanding in this subject. But I still couldn't help just laying it out. I have some background too - as a professional.

>But it's not always the periapsis.

>But since the impact point isn't guaranteed to be the periapsis or apoapsis, the above mentioned diametrically-opposing point also cannot be guaranteed to be an apsis.

You're correct on the generalized case of the math here, no argument at all, but this also feels like it's getting a bit away from the specialized sub-case under discussion here: that of an existing functional LEO satellite getting hit by debris. Those aren't in wildly eccentric orbits but rather station-kept pretty circular ones (probably not perfectly of course but +/- a fraction of a percent isn't significant here). So by definition the high and low points are the same and which means we can say that the new low point of generated debris in eccentric orbits will be at worst no lower then the current orbit of the satellite (short of a second collision higher up, the probability of which is dramatically lower). All possible impact points on the path of a circular orbit are ~the same. And in turn if the satellite is at a point low enough to have significant atmospheric drag the debris will as well which is the goal.

Solar pressure would be a small factor too, though I assume it's not a big deal compared with orbital speeds.

Just to elaborate the correct reply given by the others, the perigee of all fragments will be less than or equal to the altitude at impact point. If that's low enough, they will all eventually decay and deorbit. Even the fragments in elongated high-eccentricity orbits will have their orbits circularized by lowering apogee (the perigee is never going to rise) due to air drag. It will eventually spiral into the atmosphere. Here is the best visualization for this phenomenon - the Gabbard plot.

[1] Gabbard Plot Discussion (NASA Orbital Debris Program Office): https://ntrs.nasa.gov/api/citations/20150009502/downloads/20...

[2] Satellite Breakup Analysis (Australian Space Academy): https://www.spaceacademy.net.au/watch/debris/collision.htm

It's one reason why space should be regulated (but globally / internationally), the systems in place are kinda loose and more of a gentleman's agreement insofar as I understand it. A plan for decomissioning / de-orbiting stuff should definitely be mandatory. I know there's an area for geostationary sattelites to park themselves after their lifespan, for example.

But the LEO ones like Starlink will see their orbit decay in about five years (if I'm reading things correctly) even if they run out of fuel / can no longer be controlled, according to e.g. https://space.stackexchange.com/a/59560. But it's exponential, at 600 km it takes 10 years, at 700 25 years, at 800 100 years, etc. Between 500-600 km seems to be ideal for things to naturally decay in case of issues.

But also, it won't be a hard and fast "we are confined to the earth now"; the simplest model is a "the risk of being hit by debris is now x%", more advanced is "there are debris clouds in these altitudes / inclinations so best to avoid those at these times of day".

Given that the previous world police are presently treating international law as toilet paper, how do you propose global regulation of space would work or be enforced?

There are tentative signs that this is happening right now. As in: each collision causes debris that on average causes more than one additional collision, causing collision rates to go up exponentially.

But so far it's not anything like in Hollywood movies, it's just a graph slowly going up. There are about 12000 satellites orbiting earth. That looks like a lot on a map, but 12000 objects spread over an area larger than the surface of the earth isn't all that much

Like all exponential processes it will become a major issue if we don't address it, but this is one that starts pretty slow and is well monitored

Yep. That's the things about exponential curves, it's a graph slowly going up until it's no longer "slowly".

https://www.thefrogdoctrine.com/p/the-29th-day

This can't go up forever. There is only so much mass up there in orbit, and much of it is in low earth orbit so will fall back into the atmosphere quickly as it's trajectory gets knocked off course.

> 12000 objects spread over an area larger than the surface of the earth isn't all that much

People keep saying this, but the only way to assure there is no collision is to have non-intersecting orbits, but that is not going to work: not enough space.

It's a tell that SpaceX is now lowering the orbits, even though their satellites mostly move in flocks that maintain a formation relative to each other: because the other ways are exhausted.

Of course if they do cause a (low orbit) Kessler syndrom, then they don't have a business any more, and SpaceX will have achieved the opposite of its stated goals.

The major reason to lower these orbits is likely the risk of a terrorist state turning these constellations into a weapon, by willingly causing the Kessler syndrome. SpaceX isn't going to tell you that, just as it doesn't tell you it's the USA's most important military asset.

> The major reason to lower these orbits is likely the risk of a terrorist state turning these constellations into a weapon, by willingly causing the Kessler syndrome.

Hard to see how the repositioning appreciably alters this risk, since there are still thousands of satellites in the original plane to get hit by shrapnel from intentionally caused collisions, and the satellites in the lower orbit aren't invulnerable to it either

Suspect there's a rather more practical calculation that the extra thruster firings needed to main position in a lower orbit with more atmospheric drag are offset by the smaller number of conjunction avoidance manoeuvres they need to undertake in less congested space (the cost of lowering the orbit is simply deducted from their original delta-v budget for end of life deorbiting). In simple terms they get lower accidental collision risk without operations in the lower orbit shortening satellite lifetime.

> Hard to see how the repositioning appreciably alters this risk, since there are still thousands of satellites in the same plane to get hit by shrapnel from intentionally caused collisions, and the satellites in the lower orbit aren't invulnerable to it either

Yes, but the lower the orbit, the faster atmospheric drag (which isn't zero, just low) cleans up a cascade.

Exactly. And this is likely to be the only valid reason for the orbit change.

Feel like I'm repeating myself here, but they're moving less than half of them, which is going to have a negligible impact on a state with sufficient ASAT weapons' ability to create a massive mess with the many thousands of Starlink satellites operating in their original plane. Not even like the satellites in the lowest orbit are insulated from the effects of debris cascades set off in higher reaches of LEO either

Plenty of operational reasons to want a large fraction of your constellation in a slightly lower orbit, none of them involve "terrorist states"

It is the difference between optimization and unacceptable risks. Unacceptable risks make you act, they create a leitmotiv, and you don't seem to get that. So you talk about other motives, while ignoring the potentially main one.

Space no longer is a friendly place, it is the battle field of the future. SpaceX is a major military power in ascent, Musk is richer than many nations already, and he'll be in the supervillain category soon, and alone in actual physical power. He sits between the nations as a different entity in nature. It's happening in front of your eyes but you don't see it. That's why you are repeating yourself, as if we didn't see what you see. You speak about a technical motive we all see, as if only you see it. We all see that, it's trivial given the subject, and I mentioned it. Maybe we're not as stupid as you think we are, "teacher".

If that's the only risk, sure. But there's modelling and observation that suggest the required number of ASAT weapons to cause a Kessler cascade is currently zero, i.e. we're already in the early stages of one.

Because physics of orbital dynamics is less dramatic than shown in film, lowering orbits of satelites is an effective way to mitigate this.

> there is a pretty good chance

Source?

You're quoting something I didn't write; but to the point that we may already be in the early stages of a cascade, here's the modelling behind it:

https://conference.sdo.esoc.esa.int/proceedings/sdc9/paper/3...

This paper is brought up elsewhere in this thread and responded to. TL; DR It raises interesting modelling strategies, and develops on cool numerical methods from Kessler (2001), but it's far off from "pretty good chance."

small price to pay for global internet

When it happens, it no longer provides global internet.

interstellar internet ???

If you smash up your router, your router does not magically get better, it simply fails to provide any internet.

The same happens with orbiting routers, e.g. Starlink satellites.

and we would fix that shit, you acting like its impossible problem

A Kessler cascade necessarily requires the density of shrapnel destroys basically everything in that orbit. Below the density where this happens, it isn't a Kessler cascade in the first place.

You would be forced by (currently around) 2000 tons worth of bullet-mass shrapnel to wait for that shrapnel to de-orbit. Depending on the orbit, this takes months to millenia, because it's determined by atmospheric drag.

The lower the orbit, the better. Starlink's orbits got lowered, this is better vs. that particular issue.

I think the maths is counterintuitive here and that 10-20-40 thousand objects, give or take, isn't that much. The volume of space around our planet is HUGE.

Let us say that you had 10 thousand people running around on Earth, including all the oceans and Antarctica, and that collision of any two would release a hail of small deadly darts into the troposphere lasting, for, at 2 years or so. Which is approximately how long debris will last on LEO, though the actual values vary.

You still wouldn't expect all those 10 thousand people to obliterate themselves like that, as the Earth's surface is pretty darn big.

The volume of the LEO-relevant space is much bigger than the volume of the entire troposphere on Earth, because a) it is further away from the Earth's center than the troposphere, b) it is much deeper.

Now, 10 million objects, that would be a different story. So would be some specific peculiar orbit which is overcrowded. But tens of thousands of objects spread all over the entire planet isn't that much. That would be like 2-5 people in total roaming the entire Czechia, how often would they come into contact? Not very often.

There is no chance at all of that happening, and especially not at the orbital height of Starlink.

At that altitude the pieces will deorbit in less than a few months.

> there is a pretty good chance

No;, there is not, particularly in LEO.

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