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As soon as that technology level is reached, humanity need not to worry about restoring dead ecosystems, but about preventing total destruction of the existing ones, including Earth. Throwing space rocks will be trivial for any terrorist cell, so any fragile single points of failure will be demolished first (like this magnetic shield, or possible Earth solar shade(because carbon capture doesn't work so far)). Then orbital stations will go, then pressurized domes, then planet surfaces in general.

I think we have no chance at the large space structures, and current age will be considered a fluke of the insufficient lifting capabilities.

Mars wouldn't instantly become uninhabitable once the shield collapses. Atmosphere loss is something that occurs over the timespan of millions of years, it would be entirely reasonable for a civilization with sufficient spacelift capacity to terraform Mars without a shield, with the understanding that the atmosphere would require active management over millennia.

Also "throwing space rocks will be trivial for a terrorist cell" makes a lot of presumptions about how social organization will work in orbit. IMHO there will never be the spacer society presupposed by so much sci-fi, because any spaceship with an interesting propulsion system is also always a weapon of mass destruction (and not just against orbital infrastructure). The solution to this is that weapons of mass destruction are only owned and operated by large, powerful states with things to lose. It's worked for nukes for 80 years soonish.

Fusion bomb is technologically hard problem even today. Fission is simple but required a of the material which can only be produced by technologically hard devices. This gates it for now. But even today we can see that countries wishing to destabilize neighbors can simply gift complete assembly to their allies. Like russia did with North Korea, and is now probably doing with Iran. So ownership of the tech by large political entities doesn't matter if one of such entities decides to defect and abandon common rules.

So, yes, there won't be some independent spacer terror cell. But what if one of the space faring countries will decide to simply sponsor some Hamas 2.0 to attack their opponents? Considering that this will be a new age of conquest possibly, where nothing in the Solar System is yet owned by anyone and there will be a lot of takers. Like a simplest question - who's are Luna and Mars? :)

Next issue is that chemical rockets are much simpler and cheaper, also they are not gated by any tech or material. Sure, there are important secrets around nozzles and stuff, but I suppose a simple crude rocket will work just fine. So assuming we are at the level when humans can travel to the asteroids to mine them (also a big if, due to it being economically unfeasible) they can setup such a kamikaze rocket. Attack will be from outside of the ecliptic plane, so likely undetected. Attack can be scaled to more than one rock, so that every single point of failure, every dome etc must have dozens of anti-spacecraft level defense rockets (nukes most likely). It is not feasible.

Of course all of this is about far far future, we can't predict what will happen.

> Attack will be from outside of the ecliptic plane, so likely undetected.

It won't be. There is no stealth in space, and telescopes are really good. Once there will be a lot of man-made objects in space, it will be trivial for a state actor to continuously track the exact position (and complete history) of every single man-made object large enough to have an engine in the solar system, and I would assume that everyone with major space interests would do that.

There is no stealth in space, but space is also very large.
If throwing space rocks is trivial (which I'm not sure I buy), surely knocking them off course will also be trivial.
an object in motion stays in motion

and inertia is a bitch

you have to choose to hit something, because the energy to completely eject that mass from being dangerous would be a lot

Seems like redundancy could solve this. Instead of a single huge magnet you create a constellation of smaller magnets with aligned fields that orbit the LaGrange point.

We're decent at tracking large space rocks, and small space rocks will likely have to be dealt with anyway since some occur randomly.

It's somewhat interesting that terrorism already isn't such a problem for huge skyscrapers. Skyscrapers can be destroyed with just a small fraction of the explosive energy required to launch an orbital rocket yet it rarely happens.

Your last paragraph is an excellent point that highlights the likely risk factor by using existing infrastructure as an example.
Skyscraper are not a good or a primary target. Why bother with a tower if the same number of people can be hit easier on a surface - a demonstration, Sunday market, transportation hub etc. Also I suspect that a lot of conventional explosives is needed to topple one, and it also requires to have a specific faults. You need at least half a ton of explosives to maybe do something (basing on the amounts russia uses in their ballistic missiles against cities), and that is not guaranteed.
"Technology exists" and "technology will be trivially deployable by any terrorist" are two different things.

ICBMs have existed for a long time, as have nuclear weapons. Terrorists cannot "trivially" deploy them.

These kind of extremely powerful magnets will not be trivial, and doing anything in space even less so.

While they are not ICBM, the mid range, nuke capable, ballistic missiles are used daily against Ukraine and Israel cities. Imagine that these two countries had domed cities on Mars. Even a single rocket would kill whole population immediately.
The small dingy rockets fired by Hamas are utterly incomparable to ICBMs. It's not the same thing at all. Theoretically almost everything is "nuke capable" and that means nothing in this context.

You were talking about the "total destruction" of the ecosystem on Earth, because "throwing space rocks" will be "trivial for any terrorist cell". Talk about hypothetical rockets fired on hypothetical domes on a hypothetical Mars colony bears absolutely zero relation to that.

hive mind is like 40 years out, we’ll all live to see it, we’re watching early stages happen in real-time. once we’re all truly connected there won’t be war
Do you have any evidence to back this up? It seems like it was the SV's dream in the nineties, yet so far it failed. Economical and intellectual connections doesn't seem to prevent us from being divided and ultimately waging war...
What does this even mean? What is your definition of truly connected?
That will go against climate change (aka global warming) narrative. At this point, doing that is considered blasphemy to those hardcore left. I would reckon your take will only happen after carbon AND methane taxations are well in place and regulated like income taxes per business entities and per individuals basis. Total destruction is like economy public goods. No one will value it unless you can "fence" it as private responsebilities. Leonardo in Look Up was a great exposition of that indifference.
Such a project would see no tangible benefits for tens of thousands of years.

Unfortunately, humans tend not to put much effort into anything that has no benefits within ~100 years. "If neither me nor my children will benefit from something, I don't want to invest my time/money."

I’m not sure I understand why you think this would take 10’s of thousands of years to yield benefit. The most important benefit is shielding the planet surface from solar radiation and cosmic rays. This would be immediately beneficial to anyone living on the surface and would be essential in any terraforming effort of any duration, long or short. There are proposals that terraform mars in the span of a few generations (orbital solar powered large lasers evaporating the iron oxide and melting the ice for instance). But even without terraforming making the surface less hostile has immediate benefits.
All of the terraforming business always seems like just digging around in your belly button.

We can't even fix the problems on our first planet. How (and why) can we even begin to think about another one?

> We can't even fix the problems on our first planet. How (and why) can we even begin to think about another one?

By this logic we never go. There will always be giant seemingly insurmountable problems. If we solve climate change there will just be another, and another, and another. Life is a never ending fight against entropy. There will never be a time when we have it all figured out.

Secondly it’s entirely possible that the stuff we learn trying to survive off world will be applicable to making life here more sustainable. It could force us to figure out how to run industry efficiently with a very high percentage of material recycled, and how to run an economy without a lot of low entropy ecosystem to draw from. Also no fossil fuel. They don’t exist. We’d only have nuclear and solar really.

We have solved countless problems on our planet. What are "the problems" you are thinking of?
Green house gases emissions and the collapse of biodiversity are two reasonably conservative examples
We have solutions for those problems on Earth, the difficulty is implementing the solutions. And that's a political and sociological problem, not an engineering and logistics problem. The engineers and logistics people can work on Mars while the politicians and sociologists work on Earth.

(Not to mention that solving Mars problems are likely to yield novel solutions to Earth problems, ones which we might never discover without thinking about things with an unusual perspective.)

I can think of a few: Petroleum replaced whale hunting, cars replaced horses pooping on streets, we finally eliminated lead in gasoline, we significantly reduced acid rain (NOx and SOx), we eliminated CFCs once they were shown to damage the ozone layer.
I guess if we do develop terraforming technologies I'd rather we test them on Mars first.
> We can't even fix the problems on our first planet. How (and why) can we even begin to think about another one?

For precisely this reason?

We can't stop asteroid impacts. We can't stop a well-adapted plague, either (Ref: Covidiots). We have dictators with nuclear weapons that we can't stop. etc.

Having a self-sufficient colony somewhere other than Terra is almost certainly the first step toward avoiding a "Great Filter" event.

Because life deserves redundancy?
There’s a lot why’s to do that. How is the question.
That makes sense though. I can hardly imagine any effort put 10.000 years ago that would be valuable now, with the technological difference.
Only 5,000, but look at the pyramids. They're cool to look at, and the construction is fun to think about, but they're utterly useless. For any sort of 10,000 year space project, there's little (1%) harm in having a 100-year cool-off period to see if it still makes sense and how far technology has come.
Yeah humans can measure progress afterwards but are terrible planners for anything beyond ourselves and next of kin.
I mean, how would you even do maintenance over 10,000 years?

At some point your descendants will be too far to remember the reason but not far enough to need it and they’ll stop wasting resources on a nothing activity.

People still do some maintenance on the 8k year old Afghanistan water system (underground aqueducts) because they are still used today. I’m failing to find some good sources, but when I was there in 2008, it was something the Army Core of Engineers was investigating on my base.
That solves an immediate need though, not a once in a few ten thousand year disaster.
this[1] places them at 3k years old: https://en.m.wikipedia.org/wiki/Qanat
You could write exactly what this is and what you’re supposed to do with it on a big stone plate outside…

Then in 10,000 years, find that an entire religion, culture, and wars were built around the simple maintenance instructions.

Outside, on Mars.

So we have to assume we will maintain the ability for human Mars travel in ten thousand years, that anyone can read English or whatever in ten thousand years, and that the supporting religion didn’t lose any of its wars.

To put this in perspective our first evidence of any human agriculture was 12,000 years ago.

Yeah, maybe, ever heard of the Georgia guidestones? They lasted ~ 30 years...
Progress is exponential until "surprise it is a sigmoid!" I'd argue we are there already, but even if we aren't planting seeds is rarely a waste in the long term.
Road networks in cities are probably the most durable creations.
A hundred years? I feel like 90% of what happens is done because of election cycles and quarterly reports
People build bridges today that are expected to still be standing in 100 years.

They could have made it a bit weaker, not bothered with paint or inspection hatches, and saved a little money, and had it only last 25 years...

The time-value of money says that any benefit 25+ years from now is almost worthless, so it probably wasn't worth spending anything on those things to make the bridge last longer.

Yet in most cases we still tend to build it to last 100 years, despite the economist saying it isn't worth it.

Probably not the best example since once a bridge is built you can immediately use it, the benefits don't only start rolling in after a 100 years. Making sure something lasts is correlated with natural disaster safety margins anyway so if it has to be built to survive near term extremes it'll generally also survive a while as a side effect. Older constructions also seemingly last longer through survivorship bias and the lack of computer modelling at the time meant that safety margins had to be higher.

A better example are maybe tree avenues that take decades to grow into anything useful, or maybe long term river redirection projects or reactors. ITER's still in construction after 16 years. But there's just about nothing we do that would take more than a few decades to start showing returns.

I don’t think we really have most bridges built to last 100 years. What I see most is built to last 50 years and then usually it is stretched by patching/fixes or ignoring the issues as those will be passed on to whoever is in charge politically when a bridge finally fails.
I'm not sure what the rules are in the US but in Australia the infrastructure service life is set by regulation. It's not an economic / financial argument or even a benevolence / moral argument that causes this, it's a legal requirement. There are significant real world advantages of having a bridge last more than 25 years that are not reflected by a financial return on investment.
To fix that, we need humans that actually expect to live for tens of thousands of years.
Wouldn't it still protect humans who visit (or stay on) Mars from the solar radiation?
I've seen this idea before, and someone did the math showing that while it would be possible it would take an exceptionally large amount of energy to keep the magnetic field running. Not impossible, but I recall it being equivalent to some significant fraction of the energy produced by all power plants on Earth or something like that. Have any advances been made to lower the energy needs for something like this?