Motives for interstellar warfare

[Darth Smiley]

I think we can pretty much agree that resources aren't a good enough reason to invade. It probably isn't even good enough for wars to break out between interstellar powers (ie one side wants to strip mine the other's belt) unless some kind of handwavium FTL makes interstellar flight dirt cheap, though there is potential for conflict within a solar system (ie Mars wants to reserve the asteroid belt for it's own projects, Earth disagrees, and space navies duke it out to see who gets the resources), but only if resources are in extreme demand and short supply (not likely).

However, colonizing presents better potential (Lebensraum, anyone?). After all, take a good look at what the Europeans did to the Native Americans. Granted, one of their main goals was resources, which obviously wouldn't work in an interstellar environment, but the idea of alien outcasts coming to live on the 'New World' (aka Earth) has storytelling potential. Indeed, fully habitable worlds are very, very useful, even to a spacegoing civilization. Unlike a habitat, a planet comes with its own life support, mineral resources, recycling. In addition, with enough initial colonists, a habitable world can grow much faster than a habitat (which must carefully control biomass and population growth, so there is always enough room), and eventually become a major power in its own right. In addition, it provides a 'backup' - someone nukes/asteroids/bio-weapons your homeworld, and your species is still in the running. Also, there is a lot of storytelling potential if two or more sides want the same planet (uninhabited or not).

As for religion...we can file that under Cultural/Irrational. There a lot of totally irrational 'reasons' to fight an interstellar war. Consider: the government needs to stay in power, and the population is getting restless. Suddenly, SETI signals are detected. Well, nothing brings people together like a common enemy. What about the aliens in Footfall, where it is simply their custom, whenever two herds meet for the first time, to fight?

[Rye]

I think a cool cause for a sci-fi villain would be to not just ensure dominance of a multi-system empire for its own sake, but to unite the resource base to protect them all against a bigger threat, whereas without invasion they would've kept to themselves and the area would've either succumbed to the other force or fought amongst themselves for religion/paranoia/unobtainium etc.

[Patrick Degan]

However, colonizing presents better potential (Lebensraum, anyone?). After all, take a good look at what the Europeans did to the Native Americans. Granted, one of their main goals was resources, which obviously wouldn't work in an interstellar environment, but the idea of alien outcasts coming to live on the 'New World' (aka Earth) has storytelling potential. Indeed, fully habitable worlds are very, very useful, even to a spacegoing civilization. Unlike a habitat, a planet comes with its own life support, mineral resources, recycling. In addition, with enough initial colonists, a habitable world can grow much faster than a habitat (which must carefully control biomass and population growth, so there is always enough room), and eventually become a major power in its own right. In addition, it provides a 'backup' - someone nukes/asteroids/bio-weapons your homeworld, and your species is still in the running. Also, there is a lot of storytelling potential if two or more sides want the same planet (uninhabited or not).

I should think a civilisation which is capable of large-scale interstellar flight and the fielding of sizable fleets of vessels is not going to be too much daunted by the challenge of establishing large-scale orbital habitats at any star they wish to.

[fgalkin]

Not that sci-fi lacks many religious zealot races. Though it's usually more of the "destroy destroy glass glass" sort.

But I'm not so sure if it's "touchy." I think it'd just be harder to develop a realistic alien culture and religion and make the race relatable to us. And make the conflict more interesting than "most people see the light except for a few oldliner fundies lol what ludds!" Just why did some people hold on to traditional religion in B5 even after they found out about the role of the Vorlons and the Shadows, anyways?

Becase, according to "Lost Tales," God and demons are literally around.

Have a very nice day.
-fgalkin

[GrandMasterTerwynn]

However, colonizing presents better potential (Lebensraum, anyone?). After all, take a good look at what the Europeans did to the Native Americans. Granted, one of their main goals was resources, which obviously wouldn't work in an interstellar environment, but the idea of alien outcasts coming to live on the 'New World' (aka Earth) has storytelling potential. Indeed, fully habitable worlds are very, very useful, even to a spacegoing civilization. Unlike a habitat, a planet comes with its own life support, mineral resources, recycling. In addition, with enough initial colonists, a habitable world can grow much faster than a habitat (which must carefully control biomass and population growth, so there is always enough room), and eventually become a major power in its own right. In addition, it provides a 'backup' - someone nukes/asteroids/bio-weapons your homeworld, and your species is still in the running. Also, there is a lot of storytelling potential if two or more sides want the same planet (uninhabited or not).

No. A fully habitable world may come with its own life-support and recycling, but they're tuned to the biochemistry of the natives. Trying to get your ecology to mesh with a mature alien ecology is going to be more trouble than it's worth. Frankly, if you're going to settle a planet, settle one where the most sophisticated alien life is single-celled algae. You can work around those much more easily than disgruntled natives. Furthermore, a planet comes with a beefy energy penalty in getting things up into orbit. There's also substantial penalties involved in extracting a planet's mineral resources that simply don't exist on a small, low-gravity asteroid.

For an interstellar civilization capable of overcoming the technical challenges and energy requirements for interstellar travel, orbital habitats are going to be child's play in comparison. And really, for such a civilization, colonizing planets would probably be a stupid idea. After all, planets are big targets and are depressingly fragile. They also can't utilize the energy coming from the host star quite like an orbital habitat can.

[Gigaliel]

No. A fully habitable world may come with its own life-support and recycling, but they're tuned to the biochemistry of the natives. Trying to get your ecology to mesh with a mature alien ecology is going to be more trouble than it's worth. Frankly, if you're going to settle a planet, settle one where the most sophisticated alien life is single-celled algae. You can work around those much more easily than disgruntled natives. Furthermore, a planet comes with a beefy energy penalty in getting things up into orbit. There's also substantial penalties involved in extracting a planet's mineral resources that simply don't exist on a small, low-gravity asteroid.

For an interstellar civilization capable of overcoming the technical challenges and energy requirements for interstellar travel, orbital habitats are going to be child's play in comparison. And really, for such a civilization, colonizing planets would probably be a stupid idea. After all, planets are big targets and are depressingly fragile. They also can't utilize the energy coming from the host star quite like an orbital habitat can.

Wouldn't all that dirt make a nice shield against relativistic attack, though? I mean, it's pretty easy destroy the ecosystem, but as a mass shield sort of thing. Digging deep enough to do that would be technically difficult, but having an unassailable control center of some kind would be valuable. Kilometers deep, by the way, so the asteroids can't break it.

I imagine this might work better for moons, however, do to gravity and 'radius to core'. The geothermal heat and deep resources would make it fairly self-sufficient.

In this regard planets might become the habitats of vagabonds as well, one of the few places with enough dirt to hide you. Slightly off topic, that.

I'm going to lend my support for the 'benevolent conqueror' scenario as morality is about the only other motivator besides religion or extreme resource depletion to make it worth it in any scenario.

Ironically the latter was the motivation in the War of the Worlds. If we take Welles' other work, I suppose the Solar System could be packed with hidden, advance life so that Earth would be the easiest target. Not sure if he ever mentioned the asteroids or how much was known about them back then.

[Sriad]

As for the possibility of interstellar war in general, it will most occur during the initial colonization phase to determine who gets rights to the star. I say this because interplanetary warfare is vastly easier, so it would probably continue until one side has been kicked out of the system.

Not really. In any semi-plausible interstellar warfare, it's always the invader who holds the high ground, as there's little you can do to stop him from parking out at the edge of a system's Edgeworth-Kuiper belt and slinging fractional-c projectiles into the system, or just boosting comets inward, forcing the defender to spend his time swatting incoming projectiles. (Well, I suppose if the defender has any mass-drivers of his own, he could shoot back at the invading ships. And this may, in fact, discourage invading a system if your starships are expensive enough. Although, you could drop off your small fry well outside the system and hope you've done it in a region of space the defenders aren't watching... since a bunch of maneuvering spaceships with the power generation necessary to launch fractional-cee projectiles are going to stick out like a sore, and very bright thumb. And since a space habitat, planet, or asteroid is going to be a big, obvious IR target on a predictable trajectory, you could probably smash all the meaningful infrastructure in the starsystem you're trying to invade hours, days, weeks, maybe even months before the defenders can get their navy out to you. Sure, it'll suck to rebuild all that infrastructure, but if you've got the sort of industry that can afford to build spaceships that could possibly get blown up, then rebuilding a space habitat isn't going to be much of a problem for you. And if the defending system has any sort of unified government, their tolerance for casualties and collateral damage may be such that they'll be begging for mercy before you get around to smashing everything.

Interestingly, Charles Stross takes exactly the opposite view in Singularity Sky.

Using only a few tera-tons of mass (it sounds like a lot until you realize a cube of iron 2km to a side (Or: a reasonable generalization of about a hundred thousand asteroids in our system) masses 56 trillion kg), it's fairly trivial for a system to build trillions of quiet sentinels to scatter around the outer solar system, passively scanning for invaders. As you said, fleets of invading starships are relatively easy to spot.

As soon as someone gets within a couple thousand AUs without broadcasting friendlyness, the news spreads through the star system at the speed of whatever communication is present in that fictional 'verse, and a trivial percentage of drones (mere hundreds of thousands) unload their one-shot payloads of kinetic impactors, anti-matter missiles, nanite-hackers, or whatever nastiness the attackers might reasonably have into said foolish invaders.

[Lisa]

The only reason I could see to come to a planet like earth is if you needed something that's not on other planets.

[Ypoknons]

If you have FTL travel and communications and therefore an interstellar community, I don't see why it'd be different from current reasons. Differences in ideology, power projection, trade routes ...

Trade routes are interesting, of course. There'll still be comparative advantage in production between societies, so I'm sure trade could be lucrative.

[GrandMasterTerwynn]

If you have FTL travel and communications and therefore an interstellar community, I don't see why it'd be different from current reasons. Differences in ideology, power projection, trade routes ...

Trade routes are interesting, of course. There'll still be comparative advantage in production between societies, so I'm sure trade could be lucrative.

Not really. Unless you lived in a universe where the FTL was significantly cheaper and faster than the FTL radio, trade of bulk goods makes approximately zero sense. If you've got the sort of fabrication techniques, manufacturing capacity and energy utilization that would permit you to have an interstellar civilization, you'd probably either have the ability to store stupendous amounts of data in very compact volumes and/or be able to transmit that information over long distances with little appreciable degradation. You'd be better off trading information . . . blueprints, fab techniques, even entire genetic sequences.

Seriously, every kilogram of crap you move in space requires insane amounts of energy to move around, given the distances you have to move it. Add more energy for every gravity well you have to pick it up and drop it down. That energy has to come from somewhere, and unless you've got a magic energy tap, it probably comes from something you've either got to produce or mine from somewhere. There is simply nothing beyond starships, starship crews, small things, data and initial colonists that's going to be worth the cost of transporting. Anything you need, you can extract locally for much, much cheaper. Even exotic shit like, say, alien life, could have its genome sequenced and possibly have its eggs fabricated from scratch by a sufficiently advanced civilization (or cloned from small samples if you're lazy.) Or data could be collected the results fed into the stupendously powerful computers of some holographic zoo in another starsystem, replete with accurate simulcra of alien creatures.

Comparative industrial advantages won't matter so much since, discounting freakishly (meaning we essentially open the window and pitch reality out as hard as we can) cheap FTL and normal space travel, it will always be cheaper in the long-run to trade terabytes and build up the refinements in your manufacturing processes yourself than it would be to import the goods from Alpha Centauri. (And even if it were cheap enough to make transporting bulk items between stars economical, then it likely follows that transporting those same items between habitats in a single starsystem must be the next best thing to gratis.) The exception to this rule would be the founding of colonies, and even then, the flow of bulk goods would be almost strictly one-way.

That leaves power projection and ideological differences as possible motivators for interstellar war, but even then, if we take the above assumptions to be true, there's still a lot of seriously questionable shit two competing powers can pull via subterfuge, sabotage, misinformation, and propaganda before having to resort to something as crude and extravagantly wasteful as breaking out the space battleships. Unless a given species' ideology involves the wholesale extermination of other species, though such a species would tend to live only as long as it takes for them to encounter someone much older and much, much more powerful who objects to such behavior.

[Ypoknons]

What you say makes sense, but there's still a lot of universe specific "ifs".

[GrandMasterTerwynn]

What you say makes sense, but there's still a lot of universe specific "ifs".

A lot of those are conclusions reached if one thinks on the economic implications of interstellar travel. I'll trot out the oft-stated figure of 62 megajoules/kilogram of energy required just to get a given object into Earth orbit. Doesn't matter what that object is, be it spaceship, crew, fuel, or payload . . . everything that goes into orbit requires that same 62 MJ/kg of energy expenditure. It's why we're still nowhere near the fanciful images of space travel and space vacations for the Everyman, because both building the vehicles to carry shit into orbit, and safely harnessing and releasing that much energy in a short enough timespan to overcome gravity without causing aforementioned shit to be scattered all over the landscape in very, very tiny pieces is an enormously difficult undertaking.

Interplanetary travel is worse still. You have to design a spacecraft and associated systems capable of withstanding the rigors of deep space (hostile thermal environment, cosmic radiation, angry Sun tantrums, etc) for months or years at a time . . . and you have to pay the energy costs of moving this spacecraft from one planet to another. Right now, we generally use gravity to do most of our heavy lifting. And that's fine, but a trip to Mars still takes at least half a year to accomplish. You can get to places faster, but you have to expend a lot of energy to boost the spacecraft's velocity, and then expend more energy to reduce it again so you don't overshoot your destination and go sailing off into interplanetary space. And this is just for a distance of a few tens or hundreds of millions of kilometers. Billions of kilometers for the outer Solar System. One light-year is 9.46 trillion kilometers, and the only thing of interest at a light-year out are the most distant stragglers of the Oort Cloud. You need to go three and a third more light-years after that before you reach the closest star. Something like six more past that to reach the closest star known to have planets . . . and another ten past that to reach the closest known potentially habitable star-system. That's a distance of close to 200 trillion kilometers . . . roughly a couple or a few million trips to Mars and a couple hundred billion trips into Earth orbit.

Unless you've got a magical free energy device that also repeals Conservation of Energy/Momentum while it's on, you're going to have to pay a lot to travel interstellar distances, equally magical FTL drive or not. This has the potential of severely limiting what your starship's mass budget is going to be after you subtract out the vital things like shielding, engines, fuel and crew support stuff. Which tends to limit what you can do with the starship. Most of the soft sci-fi staples regarding trade and war tend to require that most of reality go straight out the nearest window.

[bz249]

If you have FTL travel and communications and therefore an interstellar community, I don't see why it'd be different from current reasons. Differences in ideology, power projection, trade routes ...

Trade routes are interesting, of course. There'll still be comparative advantage in production between societies, so I'm sure trade could be lucrative.

Not really. Unless you lived in a universe where the FTL was significantly cheaper and faster than the FTL radio, trade of bulk goods makes approximately zero sense. If you've got the sort of fabrication techniques, manufacturing capacity and energy utilization that would permit you to have an interstellar civilization, you'd probably either have the ability to store stupendous amounts of data in very compact volumes and/or be able to transmit that information over long distances with little appreciable degradation. You'd be better off trading information . . . blueprints, fab techniques, even entire genetic sequences.

Seriously, every kilogram of crap you move in space requires insane amounts of energy to move around, given the distances you have to move it. Add more energy for every gravity well you have to pick it up and drop it down. That energy has to come from somewhere, and unless you've got a magic energy tap, it probably comes from something you've either got to produce or mine from somewhere. There is simply nothing beyond starships, starship crews, small things, data and initial colonists that's going to be worth the cost of transporting. Anything you need, you can extract locally for much, much cheaper. Even exotic shit like, say, alien life, could have its genome sequenced and possibly have its eggs fabricated from scratch by a sufficiently advanced civilization (or cloned from small samples if you're lazy.) Or data could be collected the results fed into the stupendously powerful computers of some holographic zoo in another starsystem, replete with accurate simulcra of alien creatures.

Comparative industrial advantages won't matter so much since, discounting freakishly (meaning we essentially open the window and pitch reality out as hard as we can) cheap FTL and normal space travel, it will always be cheaper in the long-run to trade terabytes and build up the refinements in your manufacturing processes yourself than it would be to import the goods from Alpha Centauri. (And even if it were cheap enough to make transporting bulk items between stars economical, then it likely follows that transporting those same items between habitats in a single starsystem must be the next best thing to gratis.) The exception to this rule would be the founding of colonies, and even then, the flow of bulk goods would be almost strictly one-way.

That leaves power projection and ideological differences as possible motivators for interstellar war, but even then, if we take the above assumptions to be true, there's still a lot of seriously questionable shit two competing powers can pull via subterfuge, sabotage, misinformation, and propaganda before having to resort to something as crude and extravagantly wasteful as breaking out the space battleships. Unless a given species' ideology involves the wholesale extermination of other species, though such a species would tend to live only as long as it takes for them to encounter someone much older and much, much more powerful who objects to such behavior.

If FTL travel isn't fast and cheap enough to allow interstellar trade... then what is the motive for interstellar organizations? I see no peaceful ways to unite stellar communities, because they are fine by themselves and they gain nothing from cooperation.
Military conquest is another matter, a warmonger can always find some 'really good' reasons to attack the others... however since this 'interstellar empire' costs a shitload of money without any benefits in return it will collapse soon. (Just like the colonial empires here in earth).
So I think without assuming that FTL travel is very-very cheap we lost the reason for anything beyond stellar level.

[Coalition]

If you have FTL travel, that allows you to hold together a small empire. Essentially each planet would be self-sufficient, but would focus on one specific technology. When a breakthrough is made, that data is transmitted or sent via FTL courier to other planets.

Essentially each planet donates a single piece of technology, and gets the benefit of other worlds' research in exchange.

Since even the closest planets would be 4-5 ly apart (Alpha Centauri is 4-5 ly away, IIRC), these projects would be along the lines of multi-decade projects, not just a better computer technology.

[bz249]

If you have FTL travel, that allows you to hold together a small empire. Essentially each planet would be self-sufficient, but would focus on one specific technology. When a breakthrough is made, that data is transmitted or sent via FTL courier to other planets.

Essentially each planet donates a single piece of technology, and gets the benefit of other worlds' research in exchange.

Since even the closest planets would be 4-5 ly apart (Alpha Centauri is 4-5 ly away, IIRC), these projects would be along the lines of multi-decade projects, not just a better computer technology.

Well, if a planet specialize on some field of research it will just waste a good part of it's talents. So I think it's not a good idea. Multi decade projects have at very best minimal effects on everyday life. Sometimes they think in an outdated paradigm, so they practicallty have no meaningful results, other times they requires so advanced technology/extreme conditions...etc that it will only have beneficial effects centuries after...

Another thing that scientific cooperations requires cheap and fast communication methods and transportation also. Without the possibility to exchange emails daily/weekly and visit the partners at least yearly the effect of co-working is negligible. And if FTL is so expensive than I think the society will not finance the frequent exchange of some guys who are trying to find the fifteenth useless boson.

[Ender]

But, if you have the technology to launch an interstellar war, surely robotics and other machines are within your grasp?

Never read Pandora's Planet? Aliens have starships. Earth has everything else. As soon as we learn how to build starships...

You could try collecting it from random interstellar objects like comets, though you're not going to get anywhere near the kind of load you'd get off a water world.

Exactly the opposite. The mass of water locked in the Oort cloud far exceeds what is on earth, and you don't need to drag it out of a gravity well.

If FTL requires a map that matters (stargates, fixed wormholes, etc), systems have strategic value.

Certain systems will have strategic importance regardless. Logistics being what they are, it is always bettr to concentrate at certain points, even if they are arbitrary, to make it easier to ship and distribute supplies. And these bases will need to be engaged so that you force their fleet into an engagement - that way you either pound the fleet to scrap or sever their logistics chain and starve them.

Out of curiosity, what is the probable number of habitable world's in the Milky Way?

That sort of depends upon your definition of "habitable."

If by habitable you mean a world possessing an environment comfortable for a human being in their shirt-sleeves, Stephen Dole derived a figure of about 1.15 x 10^-5 human habitable solar systems per cubic light year.

If by habitable you mean a world that it is not quite impossible for some kind of exotic extremophile life form to exist, my calculations with the HabCat database suggest a figure of about 5.14 x 10^-4 alien habitable solar systems per cubic light year.

An acquaintance of mine did some calculations based upon The Cambridge Atlas of Astronomy, and concluded that the milky way's disk plus the bulge had a volume of approximately 8.17 x 10^12 cubic light years. The disk, bulge, and halo had a volume of approximately 5.24 x 10^14 cubic light years.

I'm curious as to what definition was used for "habitable" - did you take into account the proximity of nearby nova and supernova capable stars? Interstellar gas density (a dense cloud could disrupt the heliopause and start reacting with the atmosphere, which would be bad juju)? Neutron star collisions? Orbital stability (binary systems will be very unstable)? Did you factor in that those that are in areas of areas of higher supernova activity will have too many heavy metals to be habitable? What about the probability of left handed and right handed amino acids? Worlds where right handed ones are dominant will essentially be useless to us. Long terms stellar stability? Only yellows and red dwarves look to be good there. Did you subtract everything not inside the galactic habitable zone (that far enough from the core that the GRBs from the galactic black hole won't zap everything on the face of the planet)? I'm not trying to be overly critical if you didn't, but all these would be major factors for a galactic civilization. Yeah, 100,000 years is a very long time for us, but if the blue giant next door is going to explode after that there is no point in colonizing the system - you will get so entrenched that evacuation will be impossible. Things like this will have to be a concern for a K2+ civilization - thinking big also means thinking long term.

Furthermore, a planet comes with a beefy energy penalty in getting things up into orbit. There's also substantial penalties involved in extracting a planet's mineral resources that simply don't exist on a small, low-gravity asteroid.

After investigating mining, I am extremely skeptical of the assertion that it is better to get it out of an asteroid. When we mine on earth, we find veins of the material we want, haul out huge chunks of it, and melt away the few impurities - its a prety easy process for most things because it is so concentrated. But those veins are the result of huge colonies of microbial life and algae filtering out the material, then dying and roting away. Asteroids won't have the material we need concentrated, in fact extracting it will be far harder then mining gold or uranium from seawater. You will need to disassemble and sort it with some kind of ubernanotech, or carve it up, melt huge chunks, and spin the slag in a centerfuge to get the elements you want. That is a huge energy debt itself, though I haven't run the numbers to see how it compares to terrestrial mining and hauling it up.

Interestingly, Charles Stross takes exactly the opposite view in Singularity Sky.

Using only a few tera-tons of mass (it sounds like a lot until you realize a cube of iron 2km to a side (Or: a reasonable generalization of about a hundred thousand asteroids in our system) masses 56 trillion kg), it's fairly trivial for a system to build trillions of quiet sentinels to scatter around the outer solar system, passively scanning for invaders. As you said, fleets of invading starships are relatively easy to spot.

As soon as someone gets within a couple thousand AUs without broadcasting friendlyness, the news spreads through the star system at the speed of whatever communication is present in that fictional 'verse, and a trivial percentage of drones (mere hundreds of thousands) unload their one-shot payloads of kinetic impactors, anti-matter missiles, nanite-hackers, or whatever nastiness the attackers might reasonably have into said foolish invaders.

Indeed. The standard "victory tohe who holds the high ground" is true only if the invader is willing to use WMD as their standard ammunition. If they actually want something other then global extinction then they are fucked by the rules of logistics. The defending force may need to pay 62 mj/kg to go fight them, but the invader needs to pay 62mj/kg + energy used to ship it from his homeworld to the battle. And when you consider the average trooper conducting active operations will burn through 100 pounds of supplies per day it becomes very clear that the invader is fucked. He'd have to bring all his factories and supporting infrastructure with him, which then begs the question of why he would bother taking the planet in the first place.

[Sikon]

But those veins are the result of huge colonies of microbial life and algae filtering out the material, then dying and roting away.

Actually, past hydrothermal processes are a more common source of concentrating less common elements.

With that said, the source varies somewhat. For example, around 10% of world nickel production comes from mining at a basin in Sudbury, Ontario where a past asteroid impact produced a large concentration of nickel, copper, and platinum.

Asteroids won't have the material we need concentrated, in fact extracting it will be far harder then mining gold or uranium from seawater.

What specific materials in particular?

The bulk of mass needed is that for structure and radiation shielding. Though the latter may optionally be satisfied even with just bags of dirt, nickel-iron asteroids are an exceptional source of metal, as they are almost like giant blocks of stainless steel in regard to the availability of iron, nickel, and some other metals. They are not even oxidized like almost all available metals are in earth's crust.

Aluminum and titanium oxides are common on some other bodies, such as them being a prime component of the lunar surface.

Other bodies have kerogen hydrocarbon tar, others are mostly ice, etc.

Different asteroids have different elements concentrated due to past history. For example, some large bodies that formed in the past were molten at one point, with heavy metals sinking to their cores. When shattered by impacts long ago, pieces of their metal cores became metal asteroids known as nickel-iron asteriods. As a consequence, some asteriods have relatively exceptional concentrations of some heavy metals in the platinum group like iridium. Indeed, finding a layer of iridium was used as evidence of the apparently dinosaur-killing asteroid because iridium is so rare in earth's crust due to being a heavy metal that mainly sank down to earth's inaccessible molten metal core when the planet was molten billions of years ago.

Meanwhile, many bodies (particularly in the outer solar system) are undifferentiated, having never been melted since the solar system formed billions of years ago, and those can have plenty of ice, hydrocarbons, nitrogen compounds, and more.

[LaserRifleofDoom]

The ID4 aliens. They don't need to strip mine earth. They don't need slave labor. Actually, we never really get a good reason why they have to attack our planet.

But there also seems to be no indication that they have FTL capabilities. And Earth, which realistically has no chance to defeat them, apparently has something they need. They might be stocking up on food for the next leg of a long trip. Food is something that might be hard to continually produce on their mothership, and which can't be taken ready-made from a planet.

This is also suggested by Dave's rage, where he is hoping to pollute the planet- something that doesn't matter to strip miners, only aliens who want damageable goods. This also explains why they attacked civilization instead of just tearing up/burning to a cinder the planet's surface- Earth food being compatible with their biology is too rare a thing to risk losing it.
---

But for other aliens, I would say either religion or fear that if we don't kill them, they'll kill us first. OR both.

[Patrick Degan]

The ID4 aliens. They don't need to strip mine earth. They don't need slave labor. Actually, we never really get a good reason why they have to attack our planet.

But there also seems to be no indication that they have FTL capabilities. And Earth, which realistically has no chance to defeat them, apparently has something they need. They might be stocking up on food for the next leg of a long trip. Food is something that might be hard to continually produce on their mothership, and which can't be taken ready-made from a planet.

This is also suggested by Dave's rage, where he is hoping to pollute the planet- something that doesn't matter to strip miners, only aliens who want damageable goods. This also explains why they attacked civilization instead of just tearing up/burning to a cinder the planet's surface- Earth food being compatible with their biology is too rare a thing to risk losing it.

Except that makes no sense. If you don't have FTL travel, you must plan for inflight food production to sustain the ship/fleet population indefinitely, as there is little guarantee of finding a world with compatible foodstuffs in range. If you do have FTL travel, you can return within a convenient time before your rations run out.

[GrandMasterTerwynn]

I'm curious as to what definition was used for "habitable" - did you take into account the proximity of nearby nova and supernova capable stars?

Both events are relatively rare, and need to occur really close to a planet to disrupt the evolution of life there. Which means one generally needs to be closer to the center of the galaxy, where stars are packed closer together . . . and generally older. This part of the galaxy generally lies inside what is thought of as the Galactic Habitable Zone.

Interstellar gas density (a dense cloud could disrupt the heliopause and start reacting with the atmosphere, which would be bad juju)?

Even the densest interstellar dust and gas clouds are still the next best thing to hard vacuum. This isn't as much of a problem as one might think.

Neutron star collisions?

This is an event that, while surprisingly common (occurs between once every couple of years to once every couple of decades) you have to include a huge volume of space (something like a sixty million lightyear radius from Earth) to see such commonality. The odds of this affecting a given system's habitability go down as a result.

Orbital stability (binary systems will be very unstable)?

Not really. If you've got planets close enough to their parent stars, the fact that the stars exist in a binary system won't matter so much. This is true for Alpha Centauri, for example. The problem comes in the initial formation of the system, when the gravitational interaction between the two stars could serve to clear away their protoplanetary discs before planets have a chance to form. But most calculations of habitability tend to exclude multiple-star systems for this very reason.

Did you factor in that those that are in areas of areas of higher supernova activity will have too many heavy metals to be habitable?

Most of these issues are irrelevant to an orbital habitat. And when you colonize a system, there's far more habitable surface area to be had in building Dyson swarms than there is colonizing planets.

Yeah, 100,000 years is a very long time for us, but if the blue giant next door is going to explode after that there is no point in colonizing the system

Blue giants have lifespans that are far longer than a scant 100,000 years. Ten or tens of millions of years is more like it.

After investigating mining, I am extremely skeptical of the assertion that it is better to get it out of an asteroid. When we mine on earth, we find veins of the material we want, haul out huge chunks of it, and melt away the few impurities - its a prety easy process for most things because it is so concentrated. But those veins are the result of huge colonies of microbial life and algae filtering out the material, then dying and roting away. Asteroids won't have the material we need concentrated, in fact extracting it will be far harder then mining gold or uranium from seawater. You will need to disassemble and sort it with some kind of ubernanotech, or carve it up, melt huge chunks, and spin the slag in a centerfuge to get the elements you want. That is a huge energy debt itself, though I haven't run the numbers to see how it compares to terrestrial mining and hauling it up.

As has been pointed out, energy in space is essentially free. All you need is a giant mirror to slag a given asteroid. You then use relatively little energy moving this slag into your solar-powered centrifuges. Not to mention the leftover slag itself is useful, because you can use it to build orbital habitats with. And you don't need nanotech, all you need is chemistry, and to choose your asteroid carefully.

Indeed. The standard "victory tohe who holds the high ground" is true only if the invader is willing to use WMD as their standard ammunition. If they actually want something other then global extinction then they are fucked by the rules of logistics. The defending force may need to pay 62 mj/kg to go fight them, but the invader needs to pay 62mj/kg + energy used to ship it from his homeworld to the battle. And when you consider the average trooper conducting active operations will burn through 100 pounds of supplies per day it becomes very clear that the invader is fucked.

All of this assumes that the invader cares about the planet. Which an interstellar invader probably won't, because your planet is going to be populated with ecosystems using different biochemistry from his own that he's going to have to wipe out anyway if he wants to colonize the planet. And that's a lot of trouble to go through for what is ultimately gong to be a tiny, tiny fraction of the system's total potential habitable area. Unless the invader happens to be the same species as you, in which case punching out your infrastructure until you surrender is a perfectly viable tactic. Sure you could sucker him if he decides he cares more about your tiny ball of rock than he does about the entire starsystem . . . but that's just going to tell him that you don't actually intend to be reasonable, so his next move will probably be to crater your planet with fractional-cee projectiles and learn to live with his conscience.

He'd have to bring all his factories and supporting infrastructure with him, which then begs the question of why he would bother taking the planet in the first place.

Indeed. If he's advanced enough to build starships, and he's got the energy to waste sending them out to make interstellar war, then the cost of shipping a few fabs along is going to be downright trivial in comparison. There's pretty much no reason for an interstellar invader to want to take a planet, except as a demonstration of dominance, but that part can wait. He simply lock you onto your planet by seeding low planetary orbit with enough debris that launching through it would be suicidal, drop a few fractional-cee projectiles on your major infrastructure to give you something to do that doesn't involve preparing for the invasion to come, while he makes the rest of the system his own and brings in, or breeds the soldiers he needs to drop onto your planet. End result, he holds your strategically important starsystem, and gets to humiliate you by forcing you to sit on your planet and watch him do it for the next fifty or a hundred years, or however long it takes him to finally get around to invading you, or for you to genuinely give up and surrender to him.

[Eris]

Since interstellar trade has been brought up and largely attacked for plausibility reasons, I feel like sharing my thoughts on the matter.

First, the general idea that trade would be fairly uneconomical beyond the scale of maybe intra-system trade between colonies is fairly sound. It'd take enormous amounts of energy to actually happen - much more than it would to fabricate whatever you were shipping in situ.

However, I find that in context, no trade is much much worse than implausible trade. My reasoning is generally that since we're in the business of writing interesting fiction, or at least that's the assumption, we need trade. Without trade, there's nothing much interesting that comes out of relations between different star systems, and consequently introducing interstellar travel becomes fairly unfruitful from a story development point of view, and probably should be jettisoned entirely for being clunky and distracting the readers from what's important.

This is boring. Let's admit it, although fiction in one solar system can be fascinating and interesting in its own right, there's something just really wicked cool about writing about galaxy-spanning empires and exploring worlds so exotic we can't put them in our own solar system. Since without economic incentive not much really gets started, we presuppose that we have economics that support interstellar colonisation and trade and invent justifications after the fact.

This tells us two important things. One, the tech level's going to be absurdly high and energy is going to be absurdly cheap. Let's assume that our interstellar society has things like Dyson spheres, or at least the potential for millions or billions of square kilometres of space solar panels or somesuch. At the scales we're working at this isn't too implausible, since we're dealing with people who can travel light years and do so on relatively small fuel tanks for modest pricing (otherwise colonisation would never have occured, even if FTL were possible). Hook them up to anti-matter production or something and you get tonnes and tonnes of energy. So we'll say getting the energy isn't a problem, and just for kicks we'll say you can travel about in reasonable amounts of time - no more than weeks or months between star systems. We can also invent some workarounds. Beanstalks are going to make orbital transfer much cheaper, and moreso if you get really good regenerative breaks. It'll still cost a lot, but it'll be less, and downright trivial compared to the sheer amount of potential energy lying around space.

All very space opera-y, but you need some space opera to make things like interstellar war plausible. Hard scifi for the most part rules it out. This leaves, though, one big question which has been talked about already, in rather disparaging terms.

Bulk goods are still ruled out, since while we're going to make space travel cheaper by making energy all but free, that also means it's going to be cheaper to make things. So long as it takes less energy to make something at a place than it does to drag that thing there, it won't be imported. We're left, I think, with three options. Some of which have been already mentioned by Terwynn and others, but I'll go over them all anyway.

The first of course is handwavium. Exotic matter that makes people live twice as long or powers FTL drives and for some peculiar reason only appears in certain places and can't be fabricated. Those particular women whose chanting in Welsh has magical properties which oddly only appears in certain individuals and can't be replicated genetically or otherwise. Crap like that which exists solely to give a reason for interstellar relations, conflict, and trade. Not the most elegant solution, but it'd work.

Secondly is information. If for some reason your ansibles can't do the job (or you just don't have any), you can run ships around carrying genetic samples, blueprints, academic journals, and just plain old news. Given the vast volume of information an interstellar society would produce, this could actually be a highly sustainable and very lucrative business, even if you can only sell one piece of information once at any given place. Look at how much information mostly undeveloped Earth with a piddly six and a half billion people produces. It would take untold fleets of couriers to cart around all the interesting information produced across even a small star cluster.

Finally, you've got luxuries. While it's true that paying 200,000 dollars for a flying car made on Beta Cephai IV that you could produce locally for 50,000 dollars is not at all economical, you can then brag to all your friends that you own a 200,000 dollar flying car from Beta Cephai IV and they don't. And drinking expensive, rare, and exotic alcohol is a practice nearly as old as society itself. Anything then with intrinsic value not held for purpose, but for some kind of emotional or similar attachment.

Art is another good example. Sure we could just print out a near perfect copy of a famous painting, but that doesn't stop people from spending millions of dollars on the originals. In related ways, the wealthy (and maybe even the less than wealthy seeing the level of wealth we have to posit as the baseline just to get the whole thing going) could be avid tourists. You thought the Grand Canyon was impressive? Wait till you get up close to the Crab Nebula, or the fifty mile deep singing gorge of Altair.

There are plenty of things you could trade between stars, provided you had scads of energy sitting around and travel times that were relatively short, they're just not going to be much like Earthly trade. No one's going to war over iron imports, but they will so they can buy the luxury flying cars of Beta Cephai IV and have access to the research centres at Barnard's Star. Most importantly it means we can still write stories about having adventures in space.

Some of this has already been covered, and a lot of the rest is orthogonal to the main discussion of the thread, but I thought I'd pontificate on the potential for trade anyhow as it's a topic that interests me, and I think an issue that's vital to settle since it's presupposed that there is an answer before you can go any further in interstellar scifi.

[Cykeisme]

Is Star Wars trade unrealistic?

[ThatGuyFromThatPlace]

Would nebulae be worth fighting over?

They seem like a good contender, easy access to all sorts of useful elements and just rare enough to be possible sources of contention between interstellar societies.

[ThatGuyFromThatPlace]

not dense on the average sure, but you get in around forming stars and other stuff of that nature and things can get dense enough to mine fairly easily, and even the 'little more than vacuum' areas would make easy waypoints for Bussard-Ramjet style ships (or other vessels fueled at least in part by the interstellar medium).

[Adrian Laguna]

The only reason why there would likely be interstellar warfare, aside from maybe religious fanatics, is that shown in The Killing Star, as mentioned by the OP.

This review details the line of thinking:

"In 2076 A.D., humanity has started spreading into the solar system. There are bases and space habitats on and near several planets, moons, asteroids, and comets. With the breakthrough technology of self-replicating robots, humanity wields a powerful tool for large scale engineering, allowing Earth herself to slowly recover from the damage done in previous decades. Humanity has found tentative peace, and antimatter-powered Valkyrie rockets, unarmed explorers, are sent on first expeditions to nearby stars.

When the relativistic bombs strike, they eradicate every inhabited surface in the solar system. With a single blow the surface of Earth is sterilized. The followup ships of the Intruders start cleaning out the solar system, destroying returning Valkyries and any habitat they can find. The remaining patches of humanity fight a desperate, losing struggle, running on the lowest power output possible to avoid detection, with no contact to other survivers, trying to hide in unlikely places or to entirely escape Sunspace in order to preserve the species.

In recent years it has become fashionable to assert that any species advanced enough for making physical contact over interstellar distances will have a peaceful attitude towards other beings. A bold assumption, considering that we know nothing about any extraterrestrial species. What can we expect? Paraphrased from the book:

1. Any species will place its own survival before that of a different species.
2. Any species that has made it to the top on its planet of origin will be intelligent, alert, aggressive, and ruthless when necessary.
3. They will assume that the first two rules apply to us.

Add to this the facts of relativistic bombardment. A missile approaching at a speed close to that of light is hard to detect, leaves very little time to react, is next to impossible to intercept, and is utterly devastating on impact. In short, once a civilization has achieved the technological level necessary for relativistic bombardment it can erase a neighboring civilization in a single strike. The beginning of the books depicts just such an attack on our solar system. The victims will not just suffer enormous losses as in your run-of-the-mill disaster novel, they will in all likelihood be exterminated. No species can accept the risk of its utter destruction. Do you know that your neighbor is friendly? If you are mistaken, you will go extinct. The only way to be sure is to lead the first strike. And the neighboring civilization will know this, too, once it has reached a sufficiently advanced stage.

The Killing Star is one of the most terrifying books I have read in a long time. It paints a frightening picture of civilizations exterminating their interstellar neighbors, not from malice, but simply because it is the most logical action. A universe, where successful genocide is the norm, the "right" way. The novel illustrates its premise in frightening ways."