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.
> "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."
>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 ?
- 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.
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.
Lowering the orbits just means that we get back to normal faster, not that the it's impossible.
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.
PS The original paper expects the cascade to take decades to centuries. No one can afford to shoot down Starlink except SpaceX.
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.
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.
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.
This is like bowling, you hit one, it hits the other one etcétéras.
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?
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.
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.
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.
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.
Previously: https://news.ycombinator.com/item?id=46457454
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.
From the looks of it, you still are teaching. Very informative read!
Extra points for non-referenced footnotes! =)
The nasa is pretty scared of it, so is SpaceX.
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
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 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.
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".
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.
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.
No;, there is not, particularly in LEO.
