[aucisson_masque]

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.

[wongarsu]

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

[spiritplumber]

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

[indoordin0saur]

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.

[childintime]

> 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.

[notahacker]

> 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.

[ben_w]

> 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.

[notahacker]

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"

[childintime]

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".

[ben_w]

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.

[childintime]

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

[fireflymetavrse]

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

[goku12]

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.

[Cthulhu_]

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".

[vermilingua]

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?

[_factor]

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.

[tlb]

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.

[indoordin0saur]

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.

[WithinReason]

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)

[perilunar]

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?

[indoordin0saur]

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.

[tlb]

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.

[goku12]

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

[ViewTrick1002]

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.

[FranOntanaya]

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

[goku12]

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

[xoa]

>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.

[ViewTrick1002]

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.

[goku12]

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.

[inglor_cz]

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.

[NetMageSCW]

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

[laughing_man]

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

[JumpCrisscross]

> there is a pretty good chance

No;, there is not, particularly in LEO.

[tonyhart7]

small price to pay for global internet

[ben_w]

When it happens, it no longer provides global internet.

[tonyhart7]

interstellar internet ???

[ben_w]

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.

[tonyhart7]

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

[ben_w]

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.