natev wrote:
There kind of is. Even by manipulating space in the way you're describing, causality ends up getting broken, unless you institute unimaginable restrictions like, "You can go FTL, but only due east." Either causality isn't as hard of a rule as we think it is (wouldn't our future selves have let us know by now?), special relativity is bogus (when it's been tested to the extent that I have as much faith in it as I have in Newton's laws), or FTL travel, even functionally FTL travel, is not possible.

Well, there's enough reasons why an Alcubierre drive shouldn't work. You'd need negative energy, it'd have massive energy requirements to get anywhere, arriving at any place would send very powerful particle rays ahead in the direction of propagation... It works with general relativity, but the superluminous speed in general would allow for time travel arguments to break causality. It doesn't specifically break special relativity, because this particular way of going effectively faster than light doesn't do so in spacetime. It makes the space itself go faster than light.

If there's any effect making this completely impossible, my guess would either be on the negative curvature of space requirement or on quantum effects due to particle self-interactions when bending space. Sadly, gravity is the one thing that particle physicists have not a clue about and there's no unification between the theory of relativity governing gravity and spacetime with the standard model of particle physics describing electromagnetic, weak and strong forces in sight.

On that particular problem, http://www.nature.com/news/astrophysics-fire-in-the-hole-1.12726 is a nice overview.

natev wrote:
I would not at all be surprised to see us achieve human immortality via one method or another before we get good at traveling through space. (Before I was a nurse, suspended animation always seemed like a cool SF idea-- since, though, the idea of extending lifespan by ceasing activity seems less and less credible.) Consciousness uploading, even if lossy, seems likely within the next few hundred years.

Once one managed uploading, the next question is if one can manage copying a consciousness. Then, the final question is where and how to download it into artificial (non-)biological bodies interfaced to the consciousness. I'm not sure whether the final result would look more like the Horatio or the Sowers.

natev wrote:
Yeah, but you have to get your weapons out of the well sometime or other. If you're able to most economically build them someplace with less gravity (hard, because you need workers, you need tools, you need a wide variety of materials and expertise that you have to get out of your gravity well, and then you need continuing life support that you probably need to get out of your gravity well also), then of course you launch them where you build them.

The best solution to that problem is still to have some kind of space elevator. If you have a counterweight, that means reducing the problem to one of strain and friction. Making the elevator tall enough even solves the problem of reaching the escape velocity for you, since the tangential velocity far enough away from earth is large enough to launch anything from up there with minimal energy expenditure.

I don't see a massive space-based armament or economy to thrive without this prerequisite. The one missing technology for us on earth is a strong enough cable material.

natev wrote:
I think the bigger difference is whether you apply your acceleration over thousands of kilometers or whether you apply it over tens of meters. The latter sounds more difficult.

If the main advantage sought is the projectile speed, you can either increase the gradient of acceleration or reduce the mass of the projectile. Going the latter way and simply shooting more seems to be the way to go to me. Shooting particles might even be the most cost-effective ammunition, as hydrogen is extremely cheap, easy to accelerate (let's hope for plasma wakefield acceleration to work with staged setups until then) and hard to block for the target. The best bet for the victim of such weaponry would be to have a minmum density, so the energy absorption would be minimal.

Im sure that we could just use Orion Drives for everything, right?



What is a little Gamma Ray between friends!

natev wrote:
That seems a lot more reasonable to me than setting up a production facility millions of km from earth. The whole project can be automated relatively easily, very long-term. Probe an asteroid, attach an ion thruster to it, direct it into Earth orbit where I suppose you'd chip off spaceship-sized bites and send them down.



But there's been enough comet-strike apocalypse porn out there that you might have some serious social pressure against shooting an asteroid at Earth. You know that if the US is for it, Russia will be against it, and vice versa. It's the sort of thing with a global impact if you will forgive the pun Even though I'm sure you can come up with a way to make it completely safe, can you come up with a way to convince everyone in the world that it will be safe? That the risk, however miniscule, is balanced by a potential benefit to everyone on Earth and not just to Elon Musk? Would be an interesting scenario, one that I might live to experience.

If you wanted to make it safest, do it by proxy. Simply use a stellar body with a more shallow gravitational potential (Mars or Moon) for delivering and mining the asteroids. Installing a spacelift there is far easier and could, in some cases, already be realized with current technology. Here's an example. Just deliver the asteroids to the far side of the moon and chip them off. https://en.wikipedia.org/wiki/Lunar_space_elevator

Nosferatiel wrote:
Well, I agree with Adventurer that rare earths and precious metals like platinum should be more abundant on asteroids, making them good targets for their resources. I disagree that one should inhabit them, though. Instead, one can tractor an asteroid to a spacelift, thus making going downwards full of asteroid ore a lot more efficient. You simply don't want to use a spacelift just for bringing things out of the gravity well, it's just as useful the other way around.

That seems a lot more reasonable to me than setting up a production facility millions of km from earth. The whole project can be automated relatively easily, very long-term. Probe an asteroid, attach an ion thruster to it, direct it into Earth orbit where I suppose you'd chip off spaceship-sized bites and send them down.



But there's been enough comet-strike apocalypse porn out there that you might have some serious social pressure against shooting an asteroid at Earth. You know that if the US is for it, Russia will be against it, and vice versa. It's the sort of thing with a global impact if you will forgive the pun Even though I'm sure you can come up with a way to make it completely safe, can you come up with a way to convince everyone in the world that it will be safe? That the risk, however miniscule, is balanced by a potential benefit to everyone on Earth and not just to Elon Musk? Would be an interesting scenario, one that I might live to experience.

Well, I agree with Adventurer that rare earths and precious metals like platinum should be more abundant on asteroids, making them good targets for their resources. I disagree that one should inhabit them, though. Instead, one can tractor an asteroid to a spacelift, thus making going downwards full of asteroid ore a lot more efficient. You simply don't want to use a spacelift just for bringing things out of the gravity well, it's just as useful the other way around.

Personally I believe the attraction of asteroid mining is the large amount of material available in asteroids that is usually very rare on Earth. As for your above concerns, diet problems could easily be solved by having large agricultural sections, as well as keeping animals, atmosphere could easily be kept in using airlocks in all entrances, sunlight can be mimicked using various kinds of artificial light *though I don't know if it is sufficient for the human body's vitamin D needs), and finally gravity in a larger space station could be be artificially created by just revolving at a fixed speed. As I understand it astronauts mostly suffer bone and muscle loss from zero gravity so artificial gravity seems like the perfect way to help prevent that as well as making it easier for astronauts to exercise to keep their muscle mass from deteriorating.What do you think?

Adventurer_Blitz wrote:
Would it be possible to achieve perhaps a rudimentary space economy just by building cities and bases on asteroids for instance? The materials would be plentiful, workers would be available from the cities and bases, and asteroids would hardly be difficult to escape from from the perspective of even large and slow ships.

I think that this is possible, but much more difficult than science fiction typically makes it sound.



Asteroids have materials, but not as much material as Earth, neither in terms of quantity nor in terms of diversity. Even with perfect reclamation tech for life support, you're going to have losses, because, well, no gravity well to hold on to your atmospheric gasses and water vapor. It's hard to predict the health issues asteroid colonists are going to face without gravity, with reduced sunlight, without much variety in their diet, living in permanent climate control.... We know that astronauts suffer health problems, and there's still a lot of research to be done with regards to that. You're almost certainly going to have to pay your workers more than workers on the most remote oil rig imaginable



But ultimately, all of your workers and life support and tools (and probably some materials that you're short on-- plastics are extremely useful, and there's almost certainly no petroleum on any asteroid anywhere) are all going to have to be shipped out of a gravity well originally. If you can imagine the most inhospitable place on Earth to set up a construction facility-- Antarctica, the bottom of the Marianas Trench, the top of Mount Everest-- it's still cheaper and easier to set up there than on an asteroid.



There's a misconception that asteroid belts are like that scene in Star Wars, dense, dangerous to pass through. The reality is that asteroid belts are very sparse. Each space rock is separated by hundreds of thousands of kilometers. So you're not just going to zip workers or materials around from site to site. That's a doable distance, but it's still going to be expensive to traverse. A little like going to the moon, if there wasn't as much gravity to overcome.



But I know a lot of people are really pumped on asteroid mining, so maybe there's something I missed, and I'm always happy to learn something new

"The best solution to that problem is still to have some kind of space elevator. If you have a counterweight, that means reducing the problem to one of strain and friction. Making the elevator tall enough even solves the problem of reaching the escape velocity for you, since the tangential velocity far enough away from earth is large enough to launch anything from up there with minimal energy expenditure.

I don't see a massive space-based armament or economy to thrive without this prerequisite. The one missing technology for us on earth is a strong enough cable material."



Would it be possible to achieve perhaps a rudimentary space economy just by building cities and bases on asteroids for instance? The materials would be plentiful, workers would be available from the cities and bases, and asteroids would hardly be difficult to escape from from the perspective of even large and slow ships.

natev wrote:
Putting together what you're saying and what I've read elsewhere: you basically alternate shooting protons and electrons in order to maintain a neutrally charged gun?



The things I'm reading are saying that this would be range limited. But I don't trust everything I read. What would be the problems with range?

No matter whether you shoot electrons or protons, you end up with a packet of charged particles. Without magnets keeping these in line, the charges will repel each other, diverging the beam.

A solution to such behaviour could be to directly use a free electron laser. The concept is similar like using a linear accelerator with the added stage of using undulators (rapidly alternating magnets) at the end, forcing the particles to emit synchrotron radiation in the same direction they're propagating. As the photons don't have charge, you can kick out the electrons shortly before leaving the apparatus, getting a high power high energy photon/laser beam that does have less drastic beam divergence properties, as these are then purely geometrical in nature.



The question is whether one could have two mostly parallel accelerators for the protons and electrons and then fire the electrons into the proton bunch shortly before leaving the apparatus at a very shallow angle and similar speed, so that in their rest system the protons and electrons can recombine to neutral hydrogen. I'm not sure that'd even be possible but it would help wonders with beam divergence problems...

Nosferatiel wrote:
If you got a linear accelerator, using either protons or electrons is equally efficient.

This. I have one under my estate, and using either protons or electrons is really fun. lol

Putting together what you're saying and what I've read elsewhere: you basically alternate shooting protons and electrons in order to maintain a neutrally charged gun?



The things I'm reading are saying that this would be range limited. But I don't trust everything I read. What would be the problems with range?

natev wrote:
Particle guns sound interesting, I'll have to read more about them. What's the easy way to accelerate hydrogen? What do you do with the electrons, or are you shooting un-ionized H? It'd be nice to be able to snatch up bullets from anywhere in space If you've got a gun, you're interested in maximizing velocity, but it's a trade off with energy delivered to target, which decreases as mass decreases.

Typically, one simply uses protons stripping the electrons away. If you got a linear accelerator, using either protons or electrons is equally efficient. Once you need some kind of synchrotron for storage or acceleration purposes, it gets the messier the lower the particle mass is (synchrotron radiation radiates radially outward and is a function of 1/M^4, therefore being extremely different depending on the particle mass -> synchrotron based particle guns are most likely to fire heavy ions like lead). In any case, if you're accelerating charged particles, you need an electromagnetic field gradient. The simplest solution to that problem is to have e.g. for a positively charged proton a negative charge ahead and a positive charge behind it. Your ring or line consists of cavities which can change their surface charge rapidly and you do it at accelerating frequency, driving your proton bunches around. That concept is an RF-cavity accelerator (e.g. the LHC).

There are other concepts around, the most interesting ones to me being either a drivebeam accelerator or plasma wakefield acceleration:

A drivebeam accelerator uses a high current low energy beam which is far easier to generate to decouple the radiofrequency needed for a second parallel accelerator to hit the really high energies. You are still stuck with RF cavities and their shortcomings, but it's at least being experimentally tested as a concept as we speak.

A plasma wakefield accelerator fires a particle bunch into a gas chamber where a highly energetic laser ignites the plasma. The particle bunch then starts "surfing" on the electromagnetic wave generated by igniting the plasma giving rise to extremely high field gradients. The whole concept is very promising, but yet too volatile to control. It's still maybe 20-30 years away until a larger accelerator with that kind of concept is built, I guess.

Nosferatiel wrote:
Once one managed uploading, the next question is if one can manage copying a consciousness. Then, the final question is where and how to download it into artificial (non-)biological bodies interfaced to the consciousness. I'm not sure whether the final result would look more like the Horatio or the Sowers.

It's kind of an interesting question. I can't imagine any circumstances where a consciousness could be uploaded but not copied. I think any self-interested being with this kind of functional immortality would be supremely concerned with on-the-fly error correction You don't want to go to a backup because that's more like death than like continuing consciousness-- you lose everything you've learned since the backup. Over billions of years, or however long you might imagine existing, every conceivable part is going to fail, sometimes subtly, sometimes catastrophically. You need as much redundancy as possible to extend the lifespan of your consciousness. You'd probably much prefer something modular like computer memory over biological memory because it's easier to add redundant silicon than it is to add redundant brain.



If you wanted a soft body for some reason, remote control seems like the way to go. I can still imagine long-distance travel as being risky: you're mass-limited, so there's an incentive toward giving up redundancy, giving up error-correction. Every time you do that, you increase the mean time to failure.

If the main advantage sought is the projectile speed, you can either increase the gradient of acceleration or reduce the mass of the projectile. Going the latter way and simply shooting more seems to be the way to go to me. Shooting particles might even be the most cost-effective ammunition, as hydrogen is extremely cheap, easy to accelerate (let's hope for plasma wakefield acceleration to work with staged setups until then) and hard to block for the target. The best bet for the victim of such weaponry would be to have a minmum density, so the energy absorption would be minimal.

Particle guns sound interesting, I'll have to read more about them. What's the easy way to accelerate hydrogen? What do you do with the electrons, or are you shooting un-ionized H? It'd be nice to be able to snatch up bullets from anywhere in space If you've got a gun, you're interested in maximizing velocity, but it's a trade off with energy delivered to target, which decreases as mass decreases.

Hey, I always like a bit of discussion about what space war could realistically be like-- never know when you're going to learn something! I'm curious what everybody else thinks is realistic? With the announcement of ES2, it seems like a good time.



I'll offer my thoughts first:



FTL. As far as we can tell, being able to exceed the speed of light (in a chosen direction) breaks causality. And if causality is broken, there's no real telling what the world looks like. So I don't expect any kind of FTL travel.



Once you eliminate FTL, warfare is either limited to immortal-ish beings (maybe AI?) or to intrastellar conflict. Timespans for interstellar conflict are just too big for humans.



Hulls. There's always been a lot of argument about this. I think the ideal hull for a spaceship is-- a planetary body. In other words, I don't think it's worth building military spaceships. The simplest way to phrase the argument is, "Why launch a ship to launch missiles when you can just launch the missiles directly?"



Planets have built-in armor, life support, radiation shielding. They have an unparalleled capacity to sink and radiate heat. They have their own nearly inexhaustible supply of repair materials. And they're cheap to build



The drawback, of course, is that you can't accelerate them at all. But really, any "war" ship that couldn't better be described as a missile is going to require so much reaction mass to move that it's not going to have much in the way of acceleration either.



Guns. I figure bullets are projectile weapons that are accelerated entirely by a force that isn't accelerated along with the bullet, to differentiate them from missiles. Gunpowder blows up, the bullet is accelerated down a barrel. Or a railgun accelerates a projectile. Or whatever. The nice thing about bullets is that they get all of their velocity relatively quickly; the crappy thing about them is that that's all they get. That means that missiles end up reaching their target more quickly at longer range. And space is all about long range. There's a reason that we deliver our biggest warheads via ICBM, not howitzer, and the ranges involved in space combat are just going to exacerbate the advantage that missiles have over bullets. We might see combination projectiles (accelerating a self-propelled missile via a gun), but I don't think classic bullets are going to cut it in space combat.



Missiles. The nice thing about projectiles in space is that the range is infinite. Give your missile some acceleration and nothing is going to slow it down. Sure, it might take years for a missile to reach Pluto from Earth, but it'll get there eventually. (Practically speaking, eventually you run into the inability to hit your target. We can hit Pluto with a lot of work invested, but we'd have a hard time hitting Kepler 442b.)



Lasers. A lot of people see lasers as these perfect, infinite beams of energy, but they're not. Even with perfect collimation, lasers need to be focused onto a point in order to deliver their peak energy, and the distance with which they can be focused depends on the frequency. Missiles can hit a target at any distance, but lasers can only really reach tens of thousands of kilometers. (They can be refocused, however, if you have a series of mirrors along the path you want to fire the laser.) So if you're fighting interplanetary wars, you've really got to be firing missiles. But lasers aren't totally worthless....



Point defense. Classically, point defense is considered a losing proposition, suitable only to the Goliath in an asymmetric war. It's always cheaper to fire more bullets than it is to hit their bullets with your own. At the ranges of space combat, however, this stops being true. Eventually the guy firing the gun is far enough away that it's easier to hit his bullets than it is to hit him. Lasers might be good for point defense. (Or, missiles might be better. I'm not sure.)



Propulsion. I've been reading a lot about laser ablative propulsion lately, and it strikes me as a fantastic way to accelerate missiles. Basically, you fire at a laser at a part of the missile, which turns into plasma and bounces off the missile. The plasma goes one way, the missile goes another. The cool thing about this is you don't need your missiles to have any sort of engine-- they only need to bring the gas tank, so to speak. This limits the range with which you can accelerate your missiles to the range of the laser, at least, until you start firing thousands of missiles, at which point some of your missiles can can carry a refocusing payload instead of explosives. You also manage to take advantage of a lot more of your laser's power than if you were using it offensively, because your missiles aren't trying to evade or defeat the laser.



Warheads. People often like to talk about how you don't need warheads if you can get your projectiles going fast enough. That's kind of true (although the speeds and/or masses need to be very large, and it's not an either/or proposition-- a nuclear explosion at high speed is more energetic than one at low speeds!) When people say that a billiard ball going at some speed is more energetic than a nuclear warhead, they're also ignoring the fact that the cost of either is actually very similar. The cost of stuff in space is mostly a matter of how much kinetic energy you've got. The nuclear warhead isn't appreciably more expensive than the billiard ball.



The bigger problem is that it's hard to hit small, unpredictable things unless you create a blast. So what kind of blast can you get from a nuclear warhead vs a bunch of miniature sand? If you're trying to hit 1m objects (missiles) with a kill range of 1km, they're pretty similar, although at other values, nuclear comes out ahead. And the mass of the nuclear warhead goes up linearly with the volume it covers. To my mind, that makes the ideal payload the smallest nuclear warhead one can build. If these can be designed as shaped charges, they will. You want them capable of destroying point defenses without destroying their neighboring missiles, after all.



What about antimatter? It's incredibly expensive and incredibly dangerous to store and incredibly difficult to turn into useful destruction. But if these issues are addressed somehow, it would be better than nuclear. What about really big space junk? Like asteroids? The problem is that asteroids take either a lot of energy to accelerate to good speeds or a lot of time to reach their target, and we're already on the lookout for them and researching defenses. Anything you can do to accelerate an asteroid is something your target can do to decelerate it. No, you're probably better off with a lot of small missiles than with one really big one.



Orbit. If powerful lasers end up being used, you've got to put them someplace. Firing through atmosphere is wasteful. The first thing to come to mind is orbit-- but think again. Things in orbit are very vulnerable. Once a few of your orbital lasers are destroyed, your planet's orbit is going to be full of tiny shrapnel that will eventually destroy anything else you've got in orbit.



The better place for a planet like Earth to store a laser battery would be on the Moon. It gives you a good heat sink for your lasers and any debris will eventually fall back to the surface. (Don't worry about the big chunk of rock that might be between you and your target(s)-- you're going to need to rely on refocusing stations to increase range and redirect the beams anyways. Some of these stations may be in orbit or Lagrange points, but you'll probably have several expendable stations that you put on rockets with escape velocity in case of war.)



Crew. Not going to happen. Humans, their living quarters, and their life support just cost too much mass. As distances and light speed lag increase, we're going to leave more and more decisions to computers. Which is just to say that we're going to give more and more responsibility to our coders instead of our lieutenants.



Motive. The motive for war always tends to fall to the wayside in these conversations. The truth is that war stopped being profitable for anyone a hundred years ago. Putting it in space just makes that more true than ever. The cost of delivering any payload, much less a military one, through the solar system is astronomical. If space war ever occurs, it's not going to be for rational motives. Of course, humans seem to have few limits to their potential for irrational violence....

Like why we can't use ion engines to actually get out of our atmosphere?

Thanks for the well thought out replies!

Nosferatiel wrote:
Two things to consider here. Firstly, it's true that the vacuum speed of light is a barrier, but only within spacetime. If you can move a spacetime bubble around, there's no theoretical argument anymore, why the bubble itself would be limited to the speed of light.

There kind of is. Even by manipulating space in the way you're describing, causality ends up getting broken, unless you institute unimaginable restrictions like, "You can go FTL, but only due east." Either causality isn't as hard of a rule as we think it is (wouldn't our future selves have let us know by now?), special relativity is bogus (when it's been tested to the extent that I have as much faith in it as I have in Newton's laws), or FTL travel, even functionally FTL travel, is not possible.

Secondly, the question is whether cryogenic systems are realistic to achieve. In such cases, you can still transport personnel from A to B. When they arrive, maybe everyone they knew is dead, but that's not really something to worry about when you traveled too far to ever get back, anyways. Still, there's other ways to make wars achievable in more or less stretched human lifetimes. You could just as well build generational ships and have people live their whole lifecycles in transit.

I would not at all be surprised to see us achieve human immortality via one method or another before we get good at traveling through space. (Before I was a nurse, suspended animation always seemed like a cool SF idea-- since, though, the idea of extending lifespan by ceasing activity seems less and less credible.) Consciousness uploading, even if lossy, seems likely within the next few hundred years.

Moreover, planets have a few downsides. Their gravitational potential makes launching any kind of massive weaponry from them unnecessarily costly

Yeah, but you have to get your weapons out of the well sometime or other. If you're able to most economically build them someplace with less gravity (hard, because you need workers, you need tools, you need a wide variety of materials and expertise that you have to get out of your gravity well, and then you need continuing life support that you probably need to get out of your gravity well also), then of course you launch them where you build them.

Well, you're comparing in-atmosphere combat to out-of-atmosphere combat, here. A missile has a limited amount of fuel, as a bullet has a limited amount of nozzle-velocity. In space, the nozzle-velocity stays more or less constant, because there's no atmosphere to brake it. The missile can accelerate until it's out of fuel, then it stays at that velocity. So given a good enough firing apparature, you can make both equally effective in 1D-combat

I think the bigger difference is whether you apply your acceleration over thousands of kilometers or whether you apply it over tens of meters. The latter sounds more difficult.

I don´t quite have a college-level knowledge of physics, I barely scraped by the few subjects I took.



That said, I personally don´t think we´ll ever see the kind of space war we imagine right now. I don´t believe in humans flying in ships through space and shooting one another. Mainly because - what is this space? The close universe inside a solar system? Which dynamics could lead to two coexisting groups inside a solar system to have an attrition which would lead to extended warfare? Anything that can besolved inside earth, will be. And any civilization capable of comfortable space travel is capable of remotely causing massive destruction.



There will naturally be someone who is control of the technology. Is it possible for two groups to have hold of similar technology before one wipes out the other completely? Wouldn´t the security issue demand that the first to come up with an applicable use of the technology would have to eliminate the other before such technology can be applied to peace?





If you consider systems that are too far between one another, "space" becomes too large for manned conflict to happen. I can see defensive satelites and automated battle stations, I can imagine technology that "foresees" any event close to space-bending in order to send massive preemptive energy blasts in a general direction. I cannot see the human race occupying the media of war like it has inside earth, space wars imo would be exclusively remote. Even because if there´s any democratic power that needs public approval for such war, it´s always easy to get it by alienating the war from perception.



Interesting read though, however uninformed I am.

natev wrote:
Rotation is a great way to maximize your armor vs laser-based point defense. Your spin is basically just limited by the structural integrity of your missile, it will never decay nor cost much energy, and it limits the amount of time that a laser can focus on any particular part of the hull-- or another way to put it is to say that the laser has to penetrate a larger area of the hull to get through. You can make long skinny missiles with a large roll and the smallest cross-section you can to maximize the number that get through point defenses. But it's not as efficient as just making a sphere.

Just keep in mind that giving your missiles a spin has the downside/advantage that steering them off-course becomes more difficult. In simple terms, it becomes a gyroscope and it's precession stabilizes it. You could still hit it on top of the rotation axis, though.

I'll just comment on a few things in chronological order:

natev wrote:
FTL. As far as we can tell, being able to exceed the speed of light (in a chosen direction) breaks causality. And if causality is broken, there's no real telling what the world looks like. So I don't expect any kind of FTL travel.



Once you eliminate FTL, warfare is either limited to immortal-ish beings (maybe AI?) or to intrastellar conflict. Timespans for interstellar conflict are just too big for humans.

Two things to consider here. Firstly, it's true that the vacuum speed of light is a barrier, but only within spacetime. If you can move a spacetime bubble around, there's no theoretical argument anymore, why the bubble itself would be limited to the speed of light. You can look up the Alcubierre warp drive concept, although that would require extreme advances in physics and engineering and quite possibly too much energy to ever be viable... You can find a very extensive list of space propulsion concepts here: https://tauzero.aero/discoveries-log/getting-there/starship-designs/propulsion-ideas/ At least fraction of speed of light travel is realistic, e.g., with an antimatter propulsion concept. We produce that stuff regularly, just not in the required quantities yet and we don't have a good storage concept, to date.

Secondly, the question is whether cryogenic systems are realistic to achieve. In such cases, you can still transport personnel from A to B. When they arrive, maybe everyone they knew is dead, but that's not really something to worry about when you traveled too far to ever get back, anyways. Still, there's other ways to make wars achievable in more or less stretched human lifetimes. You could just as well build generational ships and have people live their whole lifecycles in transit.

natev wrote:
Hulls. There's always been a lot of argument about this. I think the ideal hull for a spaceship is-- a planetary body. In other words, I don't think it's worth building military spaceships. The simplest way to phrase the argument is, "Why launch a ship to launch missiles when you can just launch the missiles directly?"



Planets have built-in armor, life support, radiation shielding. They have an unparalleled capacity to sink and radiate heat. They have their own nearly inexhaustible supply of repair materials. And they're cheap to build



The drawback, of course, is that you can't accelerate them at all. But really, any "war" ship that couldn't better be described as a missile is going to require so much reaction mass to move that it's not going to have much in the way of acceleration either.

To have any kind of war, you need two factions in reach of each others' weaponry. So when you use planets as warcraft, then you need to take into account that either both have to be in the same system or the "bigger than a lifetime" argument you did applies to hitting the launch button and having the twentieth generation of your enemy feel the consequences. Moreover, planets have a few downsides. Their gravitational potential makes launching any kind of massive weaponry from them unnecessarily costly and their atmosphere may be a good radiation shield, but kicks in as a means to also not being able to fire radiation downwards up, as well. Of course you can get around that if you invest in an orbital lift and launch all weaponry from orbit. Best case, you even make a complete ring around a planet's equator and fasten it with new materials to the planet surface. As it will fall towards the planet's surface equally on all sides, there's quite a lot of strain, but still a stable configuration possible.

natev wrote:
Guns. I figure bullets are projectile weapons that are accelerated entirely by a force that isn't accelerated along with the bullet, to differentiate them from missiles. Gunpowder blows up, the bullet is accelerated down a barrel. Or a railgun accelerates a projectile. Or whatever. The nice thing about bullets is that they get all of their velocity relatively quickly; the crappy thing about them is that that's all they get. That means that missiles end up reaching their target more quickly at longer range. And space is all about long range. There's a reason that we deliver our biggest warheads via ICBM, not howitzer, and the ranges involved in space combat are just going to exacerbate the advantage that missiles have over bullets. We might see combination projectiles (accelerating a self-propelled missile via a gun), but I don't think classic bullets are going to cut it in space combat.

Well, you're comparing in-atmosphere combat to out-of-atmosphere combat, here. A missile has a limited amount of fuel, as a bullet has a limited amount of nozzle-velocity. In space, the nozzle-velocity stays more or less constant, because there's no atmosphere to brake it. The missile can accelerate until it's out of fuel, then it stays at that velocity. So given a good enough firing apparature, you can make both equally effective in 1D-combat, but: The missile has the advantage of being able to correct it's course. If it reaches significant fractions of the speed of light, or the target does, it get's problematic to even detect what you're hoping to hit, though. In a vacuum, it might be easier to fire a bullet at .x of c and hope that your projectile is so fast that it cannot be dodged. This will require very big and easy to hit weapons, though, so I see advantages and disadvantages to either method.

natev wrote:
Missiles. The nice thing about projectiles in space is that the range is infinite. Give your missile some acceleration and nothing is going to slow it down. Sure, it might take years for a missile to reach Pluto from Earth, but it'll get there eventually. (Practically speaking, eventually you run into the inability to hit your target. We can hit Pluto with a lot of work invested, but we'd have a hard time hitting Kepler 442b.)

Just take note that you need to exactly know everything that's going to happen in between. If you're firing a swarm of superkiller missiles to the next starsystem and it needs 2k years to get there, the enemy might see them coming, fire one heavy missile full of fuel there and gravity tractor your missiles off course. Or you hit an uncharted molecular cloud, some mini asteroids, etc... Long range = long time means large risk of losses, as well.

natev wrote:
Lasers. A lot of people see lasers as these perfect, infinite beams of energy, but they're not. Even with perfect collimation, lasers need to be focused onto a point in order to deliver their peak energy, and the distance with which they can be focused depends on the frequency. Missiles can hit a target at any distance, but lasers can only really reach tens of thousands of kilometers. (They can be refocused, however, if you have a series of mirrors along the path you want to fire the laser.) So if you're fighting interplanetary wars, you've really got to be firing missiles. But lasers aren't totally worthless....

Beam divergence is definitely a problem and I agree that any high energy beam is unlikely to work well for intra-system warfare. For medium ranges, the speed of fire is unrivalled, though. I'd just like to add that lasers don't stop in the visible range and you could just as well use free electron lasers to deliver energy to massive mirrored targets that might otherwise be largely immune to heat accumulation by laser fire (although they'd be driven off course by radiative pressure, if they cannot steer). I see any kind of beam weaponry in medium to short range inter-vessel and especially point defense capacities.

natev wrote:
Point defense. Classically, point defense is considered a losing proposition, suitable only to the Goliath in an asymmetric war. It's always cheaper to fire more bullets than it is to hit their bullets with your own. At the ranges of space combat, however, this stops being true. Eventually the guy firing the gun is far enough away that it's easier to hit his bullets than it is to hit him. Lasers might be good for point defense. (Or, missiles might be better. I'm not sure.)

If you've got no FTL drives, the laser beats missiles by accuracy and speed at short ranges. You just need to have sufficient spatial and timely coherence and small beam divergencies, as well as high power output. "just" might be a big understatement, although even in the atmosphere the first laser weapons for defense against missiles and grenades are being employed as we speak.

natev wrote:
Propulsion. I've been reading a lot about laser ablative propulsion lately, and it strikes me as a fantastic way to accelerate missiles. Basically, you fire at a laser at a part of the missile, which turns into plasma and bounces off the missile. The plasma goes one way, the missile goes another. The cool thing about this is you don't need your missiles to have any sort of engine-- they only need to bring the gas tank, so to speak. This limits the range with which you can accelerate your missiles to the range of the laser, at least, until you start firing thousands of missiles, at which point some of your missiles can can carry a refocusing payload instead of explosives. You also manage to take advantage of a lot more of your laser's power than if you were using it offensively, because your missiles aren't trying to evade or defeat the laser.

Once you get the laser out of the atmosphere so that nonlinear optic effects aren't a problem, anymore, that might be an option worth pursuing.

natev wrote:
Warheads. People often like to talk about how you don't need warheads if you can get your projectiles going fast enough. That's kind of true (although the speeds and/or masses need to be very large, and it's not an either/or proposition-- a nuclear explosion at high speed is more energetic than one at low speeds!) When people say that a billiard ball going at some speed is more energetic than a nuclear warhead, they're also ignoring the fact that the cost of either is actually very similar. The cost of stuff in space is mostly a matter of how much kinetic energy you've got. The nuclear warhead isn't appreciably more expensive than the billiard ball.



The bigger problem is that it's hard to hit small, unpredictable things unless you create a blast. So what kind of blast can you get from a nuclear warhead vs a bunch of miniature sand? If you're trying to hit 1m objects (missiles) with a kill range of 1km, they're pretty similar, although at other values, nuclear comes out ahead. And the mass of the nuclear warhead goes up linearly with the volume it covers. To my mind, that makes the ideal payload the smallest nuclear warhead one can build. If these can be designed as shaped charges, they will. You want them capable of destroying point defenses without destroying their neighboring missiles, after all.



What about antimatter? It's incredibly expensive and incredibly dangerous to store and incredibly difficult to turn into useful destruction. But if these issues are addressed somehow, it would be better than nuclear. What about really big space junk? Like asteroids? The problem is that asteroids take either a lot of energy to accelerate to good speeds or a lot of time to reach their target, and we're already on the lookout for them and researching defenses. Anything you can do to accelerate an asteroid is something your target can do to decelerate it. No, you're probably better off with a lot of small missiles than with one really big one.

Then let's use antimatter propulsion missiles, so the fuel doubles as the warhead. If a laser point defense destroys the antimatter-carrying missile at half the speed of light, there will be a big radiation burst with a sizeable boost into the direction the missile was going. So if you hit it too late, you're still hit. Interesting thought.

Antimatter is already regularly produced for research purposes. The tevatron was a proton-antiproton collider built in the nineties, for example. The remaining problems are that the input of energy to get some output antimatter is very much disproportional and that it's very difficult to store. On the other hand, as I tried to point out: If your missile's fast enough before it explodes, the radiation will spread mostly in a conelike fashion in the original direction of propagation of the exploding missile. You just need to get close enough to do damage, not even to hit on the spot.

natev wrote:
Orbit. If powerful lasers end up being used, you've got to put them someplace. Firing through atmosphere is wasteful. The first thing to come to mind is orbit-- but think again. Things in orbit are very vulnerable. Once a few of your orbital lasers are destroyed, your planet's orbit is going to be full of tiny shrapnel that will eventually destroy anything else you've got in orbit.



The better place for a planet like Earth to store a laser battery would be on the Moon. It gives you a good heat sink for your lasers and any debris will eventually fall back to the surface. (Don't worry about the big chunk of rock that might be between you and your target(s)-- you're going to need to rely on refocusing stations to increase range and redirect the beams anyways. Some of these stations may be in orbit or Lagrange points, but you'll probably have several expendable stations that you put on rockets with escape velocity in case of war.)

The main advantage of an orbital lift is the vast loss of expenditure for getting things into the orbit in the first place. Once you've fired enough missiles into the orbit that were shot down in your gravity well or once the enemy decides to simply fire a vast amount of useless shrapnell into your orbit, you face the exact same problems.

natev wrote:
Crew. Not going to happen. Humans, their living quarters, and their life support just cost too much mass. As distances and light speed lag increase, we're going to leave more and more decisions to computers. Which is just to say that we're going to give more and more responsibility to our coders instead of our lieutenants.

If humans do anything, it'll likely be on planets, moons or generational ships, I agree.

natev wrote:
Motive. The motive for war always tends to fall to the wayside in these conversations. The truth is that war stopped being profitable for anyone a hundred years ago. Putting it in space just makes that more true than ever. The cost of delivering any payload, much less a military one, through the solar system is astronomical. If space war ever occurs, it's not going to be for rational motives. Of course, humans seem to have few limits to their potential for irrational violence....

Well, it is only profitable for a possible third side selling weapons to both warring parties. Otherwise, I agree.

About missiles... If you are to launch missiles from land, the shape would have to be aerodinamic... Not too different from our current missiles... If the missiles are to attack targets in space, launched from space, I would probably opt to get close to a sphere... Lowest surface per volume implying in A big propulsor at one side, strong maneuvering thrusters, mirror reflexive (to resist point defense laser fire), pairs of lenses so it can see very well where it is heading to (after all it would be self-guided)...