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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.

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 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

> 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.

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.

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?

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.

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?

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)

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

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.

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

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

>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.

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.

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.

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.

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?

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

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

What’s the plan as the solar maximum returns?

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:)

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.

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.

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.

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.

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

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.

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.

Starlink has already been used in Russian's war against Ukraine. Of course the satellites can take photos as a bonus.

It's a massive spy network, if weaponized.

What kind of pictures can starlink would take? When I look at pictures of starlink satellites, I don't see a camera. Maybe they have one, but if we can't see it, it is most likely useless for observation, except for taking pretty pictures of the Earth, or maybe other passing satellites.

Spy satellites are more like space telescopes, but pointed at the Earth. As an example, Hubble is designed after a spy satellite, the "camera" is pretty massive and obvious.

Starlink can probably be weaponized for a variety of thing, like for communication, obviously, but I don't think earth optical observation is one of them.

It's also been used for regime change attempts - part of the internet that's harder to shutdown, though apparently jamming GPS currently appears to be quite effective.

https://www.independent.co.uk/news/world/middle-east/iran-in...

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

or

- 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.

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