[ricardobeat]

That’s one single estimate, and the problem is much more nuanced.

For example, Starlink satellites orbit so low, that even if every single one of them collides and becomes dust, it will all decay and burn up in a matter of months, a couple years at most. The debris cannot physically move to higher orbits to affect other “normal” satellites, though it might impair launches.

Conversely, collisions at much higher geosynchronous orbits can’t possibly create a dense debris field as the total area is immense, deorbit will take millions of years, and everything is usually moving at the same speed (the synchronous part).

[SiempreViernes]

The debris that ends up with equal or lower orbital energy than one of the satellites started with doesn't move up, that is true.

But all the bits the bits that end up with more energy than the orbit the satellites were on obviously do move up, and some bits will move up very substantially as we know from Mission Shakti debris: debris from that event at 300 km got apoapsis of up to ~2200 km.

[nicce]

> For example, Starlink satellites orbit so low, that even if every single one of them collides and becomes dust, it will all decay and burn up in a matter of months, a couple years at most.

That is way too long. The threshold we are speaking of cannot allow any fragments, because they start chain reaction and destroy more satellites. And there is always one which is on the highest level. What if that gets destroyed?

[JumpCrisscross]

> threshold we are speaking of cannot allow any fragments, because they start chain reaction and destroy more satellites

Kessler cascades are localised to specific orbits. In low-earth orbit, they're a problem for a few years.

They're going to be annoying. But not catastrophic.

> there is always one which is on the highest level

Highest level?

[nicce]

> Kessler cascades are localised to specific orbits. In low-earth orbit, they're a problem for a few years.

> They're going to be annoying. But not catastrophic.

I think there is a misunderstanding about the whole term. If it is not a big problem, then it does not meet the definition. So there must be some threshold where they aren't problem. What is that threshold? Because certainly there isn't space for infinite amount of objects. Primary question is that whether that threshold matters on practice. If it is 70k, then it is certainly a problem, but who knows the exact number yet.

> Highest level?

There is always the one which is classified orbiting on the highest level in LEO. Also that object can get destroyed; which means it will start deorbiting and with a chance to hit some other object below.

[JumpCrisscross]

> What is that threshold?

Way beyond anything we can currently achieve with current and planned launch capacity or radio technology.

> that object can get destroyed; which means it will start deorbiting and with a chance to hit some other object below

Got it, altitude.

Yes, in theory. In practice, the odds of that happening are vanishingly low. If it did happen, the volumes we're talking about are still so big that you'd struggle to come up with a way to cause a third collision even if we remove satellites' abilities to marginally change their orbits.

[nicce]

> Way beyond anything we can currently achieve with current and planned launch capacity or radio technology.

How are you so sure, when scientist have been debating this for decades?

> Got it, altitude.

Quibbling isn't an argument.

[JumpCrisscross]

> when scientist have been debating this for decades?

They have been. That's what I'm basing my arguments on.

You've been mentioning a ca. 70,000-bird limit. I think that comes from Bongers & Torres [1]. Their paper runs LEGEND (LEO-to-GEO Environment Debris Model). It does not distinguish between LEO and GEO. That's material because the natural decay period for an object in LEO is on the order of months to years, for LEO, to decades to centuries, for GEO.

Kessler in GEO? Real problem. If you wanted to be a space terrorist, you could probably engineer a cascade today that would make large sections of GEO unusuable for decades if not centuries. The point is that isn't possible for LEO, where you may make a mess in a few orbits for a few years at best.

> Quibbling isn't an argument

Sorry, wasn't quibbling. I genuinely couldn't tell what you meant by "highest level." (I was picturing a food chain, where big clouds of debris "eat" smaller satellites in their way.)

[1] https://www.sciencedirect.com/science/article/pii/S092180092...

[Dylan16807]

> If it is not a big problem, then it does not meet the definition.

It's still a big problem to wipe out low orbit, but it's not a long lasting one.

> What is that threshold? Because certainly there isn't space for infinite amount of objects.

Even if you crash a billion objects together at 300km, they're all going to go away in a few years. There is no threshold for semi-permanently ruining low orbit.

[JumpCrisscross]

> still a big problem to wipe out low orbit

You're not wiping out LEO, but a particular LEO.

[lxgr]

Why would there be a single numeric threshold?

You can pack many, many satellites into the same orbit without any danger, for example – as long as they move in the same direction. Let's make it 1000 for this thought experiment.

On the other hand, just two moving in opposite directions are obviously going to crash.

So is the number of "safe satellites in all of LEO" 1000 or 1?

[bryanlarsen]

No it's not. Kessler simulations show those chain reactions happening over multiple decades.

[nicce]

It purely depends of the density of objects. The whole definition of the Kessler syndrome is about the estimation when the density is too much to handle.

[zevon]

Maybe this is helpful: https://aerospaceamerica.aiaa.org/features/understanding-the...

[sidewndr46]

I don't know the specifics of starlink satellites but a rupture of any pressurized line has a chance of causing an unintended ascent. Thankfully in most cases the satellite is stabilized, so there is a good chance the satellite just gets a huge amount of rotational velocity added to it with no increase in altitude.

[nradov]

You seem to have a misunderstanding of basic orbital mechanics. That wouldn't cause an "ascent" like with an airplane or something. There will be a change in orbital parameters but a permanent change in orbital altitude isn't really possible in that scenario.

[Dylan16807]

Whatever you do to launch an object higher, it will return to its original altitude once per orbit. If you want to stay high you first have to boost up and then you have to boost again half an hour later, which will happen just about never with debris.

[lxgr]

You'd still have an eccentric orbit intersecting some "higher" ones periodically, no?

Certainly less dangerous than something "going the wrong way" in a given orbital shell, but not sure if it's completely negligible either.

[Dylan16807]

For the lucky few pieces that launch at just the right angle, they'll spend a few months or years intermittently intersecting higher orbits.

But almost all the debris will either stay close to the original orbit or burn up within hours.

[observationist]

It's a mass problem. Instead of imagining the gravity well as something moving away from earth out into the vacuum of space, think of it as a ball that needs to be rolled uphill - even if you give it a huge burst of energy, it's not going to go as far as you think from that one big push, and it's still going to roll back downhill. In order to make it out of the gravity well, you need a lot of focused, continuous energy over huge distances.

There are other factors, too - imagine you're trying to send a penny around the entire equator of the earth, and think of the largest possible explosion you could subject it to without vaporizing it. A stick of dynamite could launch a penny only around a half mile's distance around the equator, assuming ideal conditions, which is about .0025% of the circumference of the earth, which is 10% of the distance between the earth and the moon, and the moon is about 25% of the distance from which earth's gravity stops being a significant factor.

If you carefully deployed a large number of well timed series of dynamite sticks precisely located so that each blew up perfectly beneath the penny at its apex following each previous explosion - you'd need 150-300 sticks to get the penny out past the edge of the effective gravitational well, the point at which other factors in the solar system have the dominant influence - it'd effectively leave earth and start falling toward the sun. At any point closer to earth than that, it will slowly and inexorably return back to earth, reaching up to 25,000 mph before vaporizing itself in the atmosphere (if it fell from the outer edge). If you had no atmosphere, a clear shot, and the "ideal" penny cannon to launch it, you could hypothetically reach escape velocity with only a quarter stick of dynamite.

Incidental bursts of gas, or even outright exploding objects in space are not going to launch a bunch of stuff into much deeper orbit. There's a constant downward pull, and gas and dust creating drag and downward acceleration the closer in you get, and just vast, incomprehensible distances to travel under the influences of gravity. Getting things to go faster than 25,000mph, or reaching escape velocity, without vaporizing the thing you're trying to make go fast, requires as big a continuous explosion as you can make over as long a time period as possible.

I love that AI can whip up an xkcd style "What-If?" type scenario for these questions.