Assuming all these companies are interested in launching their own constellations of ~10K-100K satellites into L/MEO, how many companies could actually do this before cascading collisions starts becoming a real worry?
> how many companies could actually do this before cascading collisions starts becoming a real worry?
Twenty of them at 100,000 birds each to start approaching the density of planes in the sky [1]. Not around an airport. In all of the sky. Oceans and all.
Practically speaking, this is not a pressing concern for our generation.
It's interesting that people have a hard time visualizing this. The area in Earth's LEO is, definitionally, bigger than the Earth itself.
The SEA parking garage fits 12,000 cars in it. Two of those spread over the entire planet would be an imperceptible amount of space. You could drop a pin on a map your entire life and probably never hit one.
Not really. You're correct inasmuch as it increases collision energies. But it also increases momentum, which maintains orbital integrity within predictable bounds. Nobody is maneuvering around satellites, they–and their debris–stay where the math tells them to.
Thought experiment: Let's say you are simulating ten thousand satellites on your computer, and the simulation runs until there is a crash. Now let's say the simulation runs for an hour normally. If you increase the speed of the simulation, you get to a crash in a shorter time. Satellites move about 30x the speed of airliners. Hence, if everything else was similar, one would expect 30x the amount of collisions.
> Satellites move about 30x the speed of airliners. Hence, if everything else was similar, one would expect 30x the amount of collisions
Not how orbital mechanics work.
Planes maneuvers, get tossed around and have hubs they circle. A plane under my left wing can’t be relied on to continue in a straight line. The satellite can.
> your comment that speed is not a factor was incorrect
It would be if I had said this.
Speed doesn’t matter “a lot”. (Orbits around a small object get crowded quicker. The speeds are less. But the volumes are way less. Volume absolutely dominates speed when it comes to orbital cross sections.)
Orbits are predictable, but they intersect and decay [at different rates] and occasionally get perturbed by space weather. This already needs periodic conjunction avoidance manoeuvres, and whilst orbits are fast satellite manoeuvres are slow, so the notice you need to avoid a conjunction is measured in hours rather than seconds. Can't imagine a scenario in which it would be sustainable for LEO to even approach the density of commercial aviation, except perhaps for a hypothetical where a single entity actually managed all the satellites.
The other underestimated dimension is that satellite manoeuvres use up a finite supply of expensively-launched propellant. That's tolerable when Starlink is doing 50k conjunction avoidance manoeuvres in six months across its constellation, but once it becomes each satellite moving at least weekly you either need bigger satellites carrying more propellant or have to accept significantly higher collision risk than they currently do.
> and whilst orbits are fast satellite manoeuvres are slow
This is something people unfamiliar tend to misconceive in their limited thinking on the subject. You can't just tap the breaks to slow down. Changing altitude of satellites is done by speeding up to increase altitude and slowing down to lower altitude. Once you change the velocity and reach the desired altitude, you have to then undo that acceleration to get back to orbital velocity. Fuel is required in both directions. The less fuel used the better for the maneuver. Most satellites EoL is defined by remaining maneuvering fuel vs functionality of the hardware.
My first understanding of accelerating in space was from the old Asteroids game. To slow down, you had to rotate 180° and start accelerating in that direction. Others might learn it from Kerbal.
> This is something people unfamiliar tend to misconceive in their limited thinking on the subject
I have a background in astronautical engineering. While you can't tap the brakes to 'slow down', you can impart miniscule amounts of impulse which, over the course of hundreds of orbits, will change your plane by an imperceptible amount from a distance, but tens or hundreds of kilometers up close. OM being OM, you can predicts these collisions in advance.
I had a professor who referred to orbits not in altitude but in expected decay time. We're currently in the months to single-digit years orbits. (We will stay there for telecommunications due to latency.) If we were doing at decades or centuries what we're doing in LEO, this would be a problem. At LEO, it's a nuisance and barely more.
> you can impart miniscule amounts of impulse which, over the course of hundreds of orbits
right. this is what is counter-intuitive for those that are not familiar with space. they don't just light the burner and boost to a new altitude. the part about stopping the acceleration with an opposite burn is often not considered. most think you can fly a space ship like a jet fighter, but in space. can't blame them since that's how sci-fi portrays it. real life space flight is really boring in comparison. jumping out of FTL to land in orbit around a planet makes me laugh every. single. time.
> whilst orbits are fast satellite manoeuvres are slow, so the notice you need to avoid a conjunction is measured in hours rather than seconds
I'm not arguing against collisions becoming more likely. I'm arguing aginst it becoming commonplace to the point that it becomes a commercial concern.
> satellite manoeuvres use up a finite supply of expensively-launched propellant
Nobody is plane changing out of a collision. And for the foreseeable future, in LEO, the birds are not propellant constrained. (And launch is getting cheaper.)
> you either need bigger satellites carrying more propellant or have to accept significantly higher collision risk than they currently do
We're decades away from this being a problem. That gives ample runtime to developing e.g. magnetic station-keeping (if we go reactionless) or more-efficient engines.
> what's the state of solar powered magnetorquers these days?
Academic. We don't currently have a pressing need for reactionless thrust in the magnetosphere. Each of semiconductors, launch vehicles and telecommunications standards are moving faster than satellites last.
> Each of semiconductors, launch vehicles and telecommunications standards are moving faster than satellites last.
That's certainly a pragmatic cost based argument for not using them in the fast moving world of commercial magnetosphere constellations.
> Academic.
I feel they've moved past academic and transitioned to deployed .. at some evolution of implementation. Not commercially relevant is certainly one state of play.
I guess I was more interested in the nonlinear control issue in a field of highly variable intensity.
A bit pedantic here.. I think you might be thinking about space tether propulsion. I don't know if that has been deployed yet. Magnetorquers, as in a device that uses magnets to rotate the satellite are very common in cubesats, you can buy it off the shelf
I first encountered space tethers in 1980 reading an Introduction to Engineering text where the example was given of unrolling a flat spool of thin metal through shaping rollers to extrude a very long boom with a spring on the end to stabilise the orientation of a satellite.
That was one of the first times I noodled about with the dynamics of a pendulum in a potential field.
These days, of course, there's a few more tricks that can be done with a dangling lasso, including interacting with the magnetic field via a looped current.
That aside, I was curious about traditional magnetorquers and their variations actively providing force in the magnetosphere.
The Earths magnetic field has a lot of diurnal pulsing .. the gravitational field is lumpy but stable.
There's a control challenge in getting a smooth desired response from a choppy field.
Cheer's for the lookout though, it hadn't occurred to me that some would be talking about magnetic force against the field using "space tether" as the base description - my background was more about the field equations than the physical implementation.
( Magnetorquers are also used in the US Navy for twisting controls inside a fully sealed container. )
> I'm not arguing against collisions becoming more likely. I'm arguing against it becoming commonplace to the point that it becomes a commercial concern.
Minimising collision risk already is a commercial concern, and the number of conjunction avoidance manoeuvres SpaceX takes in order to achieve this has been growing exponentially (which presumably is a major factor driving their move of 4k satellites to a lower orbit which involves more station keeping) Obviously this gets harder when most of the satellites avoiding their orbits coming too close don't have the same owner, particularly if some of the other megaconstellations aren't even particularly cooperative (hi China!)
> Nobody is plane changing out of a collision. And for the foreseeable future, in LEO, the birds are not propellant constrained. (And launch is getting cheaper.)
No which is why I mentioned the fact that constellations pre-emptively plane change to avoid conjunctions. The frequency with which they have to do this scales superlinearly with the number of satellites operating in or intersecting the orbital plane. Ultimately propellant use for those manoeuvres and station keeping defines the satellite lifetime: agree it's not a huge problem when a satellite is only making small orbital changes a handful of times a year and its got a decent sized delta-v budget for station keeping and EoL deorbiting anyway, but another 70k satellites in the same plane would require quite a lot more adjustments, never mind them operating at aircraft density as proposed earlier.
> We're decades away from this being a problem. That gives ample runtime to developing e.g. magnetic station-keeping (if we go reactionless) or more-efficient engines.
Depends how fast the satellites get put up there (and also whether orbital megastructures become a reality, although non-trivial numbers of them actually might be decades away). There's some scope to improve propulsive efficiency (hi colleagues!), but within the power/mass constraints of a smallsat, you're not likely to see orders of magnitude more improvement in specific impulse over current gen EP, and we are forecast to need orders of magnitude more avoidance manoeuvres, which is generally going to mean more reaction mass. Sure, if we get reactionless propulsion suited for precise orbital changes in LEO then we can forget all about the tyranny of the rocket equation, but hey, if we perfect flying cars we won't have to think about the implications of congestion on the roads!
Randomly place 50,000 shoe boxes up and down the entire eastern seaboard.
Randomly place 50,000 shoe boxes up and down the entire western seaboard.
Send them in straight lines towards the other side of the country.
See if any collide. Almost certainly none of them will. Edit: They will almost certainly
For reference, if you placed all 50k boxes next to each other on the same beach, it would be about 10 miles wide. The total shoreline on either side would be ~1800 miles wide.
By my calculations there will be an average of 500 collisions, no? Each shoebox has an effective width of 2 feet, and with 50k of them that's about 1% density. With 50k in the other direction, and about a 1% collision rate, that's 500 collisions.
If they put their sats low enough (like Starlink already mostly does) any collision debris should be quickly deorbitted by drag, before a cascade can happen.
Presumably, there would be a corridor for traveling through elevations whether that was for reaching orbit or de-orbiting. The people placing things in orbit are not doing this with out coordination.
There are in fact many objects that deorbit in an exceedingly uncoordinated manner. It's a statistical inevitability that kessler syndrome is in our very near future if we allow higher orbits to be polluted.
Debris moves in 3D. Debris moving up will continue moving up. There is no force acting on it to bring it back down. Your comment makes it sound like an explosion would only be in 2D along the same orbit as the original object.
It may seem counterintuitive, but if something in orbit gets a push that isn’t strong enough to make it totally escape orbit, it will stay in a new elliptical orbit. That new orbit will pass through the point where the push happened, so it will come back through that location again, just with a different speed and direction.
> There is no force acting on it to bring it back down.
Gravity?
But also orbital dynamics (at least as I understand it) means debris that debris that is flung up is going to have a more oval orbit, so the high point (apogee) increases and the low point (perigee) decreases. And a lower perigee means more atmospheric drag, which will help deorbit the debris.
>means debris that debris that is flung up is going to have a more oval orbit, so the high point (apogee) increases and the low point (perigee) decreases. And a lower perigee means more atmospheric drag, which will help deorbit the debris.
Not quite.
If you are at apogee and accelerate, your perigee will be raised. If you are at perigee and accelerate, your apogee will be raised. You can't increase your apogee and perigee at the same time.
If the impulse is in the direction of orbit, then the altitude of your orbit 180 degrees from your current position will raise. If the impulse is against your orbital direction, your height 180 deg away will be lowered. Once you complete an entire orbit (360 degrees) you will pass through your current position again.
If you wish to move to a higher, circular orbit two impulses are required, 180 deg apart.
... It is a very real possibility, but less of a problem below 550km altitude because the decay time is much shorter (and why all of these mega constellations tend to stay at lower altitude, even though ~1000km is generally better for a communications satellite).
It's really not. Not in the popularly-portrayed manner. Militaries have been researching how to intentionally cause such a cascade in even a limited orbit. To my knowledge, there isn't a solution.
It's a very real possibility, it's just a very slow exponential.
If you have 100,000 satellites, and each collision produces 50,000 pieces of shrapnel with some distribution of altitudes and atmospheric drag, it's not that hard to do the math. The cinematic portrayal of cascading failure (ala the movie Gravity) is completely insane, but that doesn't mean this isn't a real problem on a 100 year timescale.
> If you have 100,000 satellites, and each collision produces 50,000 pieces of shrapnel with some distribution of altitudes and atmospheric drag, it's not that hard to do the math
One, it is. And two, you need insane distributions to get over the energy requirements of plane changes.
