Rocket exhaust is free expansion of a gas if I understand correctly. It does not qualify for the effect of cooling in an external environment like the atmosphere because there is no environment to work against in a vacuum and thus work cannot be done and thus the energy that is kinetic cannot be dissipated into the environment. Nor does it qualify for the Joule-Thomson effect because it is not passing through the equivalent of a porous insulated plug (read the wiki on Joule-Thomson).

It is instead a free expansion of gas and that leads to NO cooling.
http://en.wikipedia.org/wiki/Free_expansion

There is the possibility that divergences from being an ideal gas allow some slight cooling to occur but it happens at nothing like the rate that cooling from expansion happens in atmosphere because positive work is being done there against the atmosphere and that takes kinetic energy out of the gas. In a vaccuum, there isn't that to work against.

So any cooling from a gas expansion behind a reaction drive will be far slower than rocket exhaust cooling in atmosphere.

Additionally, the scatter graph for particles in an in-atmosphere rocket is going to be constrained both by the effects of gravity (will tend to pull the particles downward which is often back along the rocket's trajectory) and by the effects of the surrounding atmosphere (which the gas collides with to cool but this also limits the spread of the molecules significantly).

This is not true of expansion in a vaccuum which is almost entirely unconstrained and thus will be wider and faster than the expansion behind a rocket in atmo. There will also be no significant gravity (for the most part) involved so the scatter will tend to be either spherical or conical and wider than the in atmo rocket generates.

Given the heats required to produce enough thrust to move significant ship masses, you can expect either a lot of ejected reaction mass (more collisions within the ejecta, more scatter as a result around the direction of the ejecta as particles move off energetically in orthogonal directions) or a lot of heat in a lower amount of reaction mass (more energetic ejecta, also likely to be driven out further and faster) as compared to terrestrial lift rockets which move limited masses.

This means the combination of slowly cooling ejecta in a vacuum and rapidly expanding ejecta clouds (as compared to the terrestrial equivalents) and the greater energy needed for larger space vessels to accelerate combines to mean that the shielding option will have a very short period of effect and it seems pretty much guaranteed that ejecta will still be hot when it passes beyond the shield (even if it is a few kms wide).

Effectively, within 10 km likely and definitely within 50 km of ejection point, the gas will have expanded beyond the shield and be hot enough to be at least quite a few degrees above cosmic background which is very near absolute zero. Hence, easily detectable. And the continued expansion of the cloud will make for an ever expanding and hence larger silhouette which any even moderate sized array will pick up.

Unless you somehow invent a drive using no propellant that can produce very high thrust efficiency that is scalable to large scale drives (not yet done by our science), you are stuck either with low power (and hence mass limited) vessels like small satellites or drones that don't need decent acceleration or with ejecting hot mass that will rapidly scatter.

Everyone seems to treat reaction mass ejection as if this somehow produces an infinitely tight laser beam out the back. That's not anything like how a rocket exhaust behaves in a vacuum. No drive we have envisioned has that kind of character that is also not a very low power drive.

Some of the drives we are trialing as satellite positioning tools may have low signature, but they are not capable of the kind of push a warship of any worthwhile size would require (even a small one).

But hey, if you want to cling to the notion of stealth in space, go ahead. Nobody can disabuse you of your notion if you aren't willing to take the time to understand little details like gas expansion or the other properties of gases in vacuum that are relevant to your arguments.

So your cunning rebuttal to the fact we have and do detect small asteroids is to say that because NASA has an understaffed and poorly organized and managed asteroid detection operation that this somehow has much to do with what the technology is capable of or what the science allows?

Really?

Don't bring business articles to a science discussion.... ....and you were accusing some other guy of being a humanities student.... ironic.

Sometime, do all the math for this.

Your laser photon drive has a very, very weak thrust. It might be suitable for accelerating small satellites or drones, but its thrust for a ship weighing *tens of thousands of tons* is negligible. That's a big ship being pushed by a snail.

So, you are going to either take forever to accelerate to a decent speed (months? years? and what about life support and consumables while you do this?) or you need another drive system.

If you try sinking that much heat into an internal heat sink (say for instance into a reservoir of fluid that is insulated), your efficiency won't be 100% and the fact it will take you a long time to get to a decent speed or else a long time to cross the significant tactical distance you might care about in a system means there will be a lot of heat that needs radiated.

This sink won't 'float in mid air' - The heat needs to be transferred from all of your systems throughout the ship with pretty much 100% efficiency for your plan to work. The heat sink will have physical connections to the ship and will (even if mostly surrounded by a near perfect vacuum or insulator) be not 100% efficient. Plus most machinery won't shed its heat 100% efficiently into the heat sink.

So all of that thermal inefficiency will go into the air inside the ship and to the hull (the hull does not just float there without any thermal connection to the inner cabin). Over the length of time a slow laser drive will take to either get you to a decent speed or that will take to move you at a lower speed through a system of interest, enough heat will be emitted via inefficiency to your hull to ensure that you show above cosmic background.

And the idea of a ship weighing tens of thousands of tons tons (if your heat sink is thousands of tons, not counting the equipment to move heat to it and the drives and all other parts of the ship including your own sensors assuming you don't plan to run entirely blind) being not noticed on visual occlusion checks is ridiculous.

The technology exists today to do full sky scans that can detect as little as a 0.6 Kelvin variation from cosmic background. The movement of the background and the system you are traversing makes it unlikely you will be able to align with a single background source for camouflage for your entire transit and any monitoring system will eventually figure out you are an anomaly worth looking at, even in your magical spaceship.

The last time I spoke with my friend who works on NEAR as an instrument scientist about detection and stealth in space, he was fairly confident full sky surveillance was within near term grasp and that as processing power, synthetic arrays and detector sensitivities kept improving and so did our stellar catalogues/models, stealth was going to be less and less likely to the point of not being at all practical over meaningful intra-system distances.

I will agree you might be able to sneak a low power, round, blind, nearly inert satellite through a system. It would need to generate little internal heat (hence nearly inert), not have a big array of sensors of its own (which would be detectable) (hence blind), and low power in order to avoid heat, and small to avoid occlusion and to further limit heat issues (hence no crew). That you might be able to do but it would for the most part be a worthless effort.

If you started doing stuff with an array from such a satellite, you get detected. If you start running systems and thrusting, likely to be detected eventually. If you make it much bigger, your odds of detection go up and your time until being detected goes down.

My friends in the space sensing community could be wrong. Nobody knows 100% (except some trolls) how these theoretical exercises will unfold when they meet real engineering challenges. It's just interesting to me that most of the detection tech is available today (faster processing, bigger synthetic arrays and a bit more sensitive detectors will help, but aren't absolutely necessary) but most of the things for the stealth are thought exercises that have yet to meet any real engineering - massive heat sinks, shielded exhausts with incredible focus or unlikely rates of cooling, etc.

So I tend to back the people who say stealth won't happen in any but the smallest packages.

1) Exhaust particles will strike each other and radiate off in what I imagine will be a conical dispersion. They are very hot and will take a fair length of time to cool (not much cool mass to transfer their heat to and wave radiation will take time). I don't know for certain but I'll bet that even a kilometer wide bell will not stop particles rapidly going beyond your ship's silhouette.

2) If you ever have to change course, you'll have hot particles still that are no longer shielded, further intensifying the problem.

3) A kilometer wide shield may in fact visually occlude things that will allow optical spotting.

4) We already use synthetic arrays. Why does the article limit the array to 24m? I'd say 100m+ is feasible and that sizably increases detection distance. If we assume tech progresses, we can have dispersed arrays (and should have given the possibility of attacks) using various satellites deployed in varying orbits around the system (including perhaps ever 60 degrees from your satellite in question plus some further out in the system). In fact, a coordinating system may be able to process data from every friendly sensor in the system. So both the assumptions about detection threshold/range and the assumptions about how many different perspectives at different angles might be available vastly changes the chance of your ship from sneaking in with a hot exhaust.

5) Real engineering means perfectly spherical ships that are thermally identical in all facings are pretty much just not going to happen.

Where you have a distributed detection network, as you will around any system of note (and around any fleet of note because they will tend to distribute satellites to extend their synthetic array and increase its sensitivity), you will find it very hard to sneak up with the ship the article mentions.

That article makes less likely assumptions about heat, about detection capability, and about the nature of any array trying to detect the ship. You can fully expect distributed arrays (even with one moderately large ship and a number of satellites it launches, let alone a fleet or any ground installations or a larger system data synthesis system). You can definitely expect synthetic arrays bigger than 24m. I mean, why can you drag a km wide heat shield and the other side not drag a km wide lightweight array? Change the math on the array to 1 km instead of 24m and tell me what the detection distance is!

My best guide on this is an instrument scientist I know who worked for NASA and who worked on thermal detection. His opinion, with his knowledge of current state of the art and what's likely in the near future, is that thermal stealth is going to be nearly impossible in short order. His opinion was real time processing was within grasp for full-sky in close to real-time within the next decade or two if we wanted to invest in it.

By the time we're out colonizing space, it'll be commonplace. The stealth can't keep up.

Modern instrument science disagrees. Even now, we can detect fractions of a degree off the cosmic background and it is very hard to sink heat (and impossible to stop generating it with power systems, electronics, batteries, life support, etc. aboard) and impossible to hide particle ejecta that are heated if thrusting.

Even an inert ship will be detected as it heats up. I can think of ways to *temporarily sink heat* but not that last long enough for closure of even interplanetary distances.

Stuff will get seen. The fight will be about overwhelming the other side's defenses likely. And with lightspeed point defense, that'll be one heck of a challenge. Or else just exchange coherent light at high energy densitites.

Of course, you might make a smarter system....

If the missile is ballistic, you simply use point defenses to destroy the missile. There might be shrapnel or not depending on how you defend exactly.

One option would be interceptors that close with an enemy missile and then use a laser to shoot a hole in one side of its rocket exhaust. Guess what? Your missile can sail off into space.

One option would be to have your own large anti-ship missile masses on the stationary target. Once you've destroyed a missile, project one of these into the path of the debris. You ought to deflect most of the debris. Or have your anti-anti-ship missile defence launch missiles that have weighted nets or the like that they deploy when approaching your missile's debris field.

There are a number of ways to deal with the missiles that either redirect them away from you without shredding them, that intersect them with a massive enough object (obviously if your missile can be coming in with much delta-V, then if they can slug it with another missile of similar mass with as much delta-V, they can likely deflect or stop it dead before it becomes a debris could), or that can vaporize them (sufficiently energetic explosion such as a nuke with very proximate detonation).

I'm not saying defending the static target is simple. I think the deterrent will be less about tactical defense than strategic - those with such installations will not want to fight because both sides' installations can be blown to hell. The danger might be more from fringe players (non-governmental enemies with some sufficient backing).

But there will be some partially effective defensive measures. Even if it means flying defense drones into every incoming anti-ship missile, that's still an economically viable defense. They probably cost less than the anti-ship missile.

There is rarely any form of weapon that is, for any length of time, without counters. Sometimes, like modern nuclear weapons, the counters have to be more of same on the other side or a context of battle that precludes their use. But those are counters and they work.

The exhaust plume is a dead giveaway for thermal detection though.

First, that 20m array and full sky can would likely be fairly possible in short order with today's tech. I know scientists who worked with Hubble and on NEAR doing instruments for scanning space, so i'm pretty positive I trust their judgement. Certainly such things will be militarily feasible with some investment within 10-20 years if that long. So realtime full sky imagining to high accuracy will soon be within our grasp.

Second, any defense system that relied solely on a single array is .... ridiculous. You'd have various arrays scattered on satellites and mobile platforms and on other system bodies. Yes, they couldn't share data in realtime if they were on other system bodies or very far away, but even a momentary look at your 'cold stealth missile' from a non-frontal angle would clearly give it away and they would report its location and vector at detection within a few seconds (near space) or few minutes (near planetary bodies) to the target installation which would *really* reduce the amount of space they have to check. Then that system would put up some interceptors and array-extenders into the general cone the incoming missiles would be coming from and widen out their array in real-time and thus give them targeting for lightspeed or near lightspeed point defenses.

If your attack was construed to threaten from multiple directions, its detection odds go up dramatically. If it had countermeasures for their array-extender satellites or interceptors, then you again are revealed.

Plus, the 'keep at 2.7K' task is a lot harder than you think over the longer term especially if married to a thrust motor, batteries, electronics, etc. that generally don't love 2.7 K temperatures and that tend to emit heat.

So the attack isn't nearly as viable as you think and will be much less viable within 20 years.

Detection isn't that hard. Thermal can get you simply because it is hard to appear, over any lengthy period of hours or days, as if you have the same signature as the cosmic background (some fractions of a Kelvin above absolute zero). An modern optical detection is pretty good on its own right, but it pales compared to the detection of your heat versus the background. It's almost impossible to mask over any length of time required to close from interplanetary distances at non-relativistic speeds.

Mind you, weapons ranges are practically no more than a few thousand kms.

So for two fleets to meet, they have to want to and work at it by bending their vectors to intersect. If they are coming from opposing directions with any speed that gets them there sooner than days or weeks, the conflict will likely be a single pass, then a rethink on whether to revector to meet again. Momentum from decent amounts of interplanetary speeds is a huge issue.

If you want to be able to enter weapons range fast enough to prevent an enemy getting too many shots, then your closure has to be fast. That means the engagement is short. If you want a longer shootout, then you can go a bit slower and have more manouver options. In either case, you can manouver a few hundred meters or a few kms either way in a random erratic way around your main vector of motion and make fine weapons targeting a bit more challenging as well as rotating ship to bring different faces to bear versus enemy beams or weaponfire.

Defenders have an interesting conundrum from static locales:

a) They are an easy target
b) They can likely mount larger weapons arrays than mobile vessels and will have a fair bit of time to engage any incoming ships or missiles

In the long run, installations can be attacked by stand off or ballistic means. But any fleet closing in is going to get a rough ride.

Assuming the atmosphere doesn't preclude that.

The US has spy sats now, drones in the air, etc. and yet insurgencies continue to present problems. Most areas where you might engage have civilians and sorting out civilians and enemy combatants is at best a tough prospect and at worst a horrible disaster when you get it wrong.

This is why over the horizon missiles aren't much use in most smaller scale conflicts on Earth. ROE requires you to have eyes-on and know who you are shooting. If you don't, then you shoot down unassociated airliners for instance.

Whoever holds the high ground, likely mostly the US, China and maybe a few other nations for the forseeable future, will have an advantage in any conventional conflict. That merely means less capable enemies will not engage in conventional conflict.

If forces in orbit decide they have an issue to resolve at laser-point, then that gets into actual space combat.

Many of the fights on earth are about a) not being able to live the way you think is right or b) not having the resources do live the way you'd like.

If we can get to space on any large scale, it is possible that there is enough space for a) for a long time. You don't need to live anywhere near the people you dislike or have anyone up in your grill. b) will possibly go away too by the time we can meaningfully colonize other planetary bodies even in our own system. Energy could be harvested from the sun and resources from various places around the system. If we go interplanetary, having one more Earth-ish planet would allow us to vastly reduce populations rubbing elbow to elbow on Earth.

Of course, that's all in theory. All of Israel could move to Nebraska and have better neighbours, but that land is sacred to many of them. So of course, it'll be fought over. That same logic will make Earth a battleground for as long as I can imagine.

Similarly, I happen to believe getting out of this system is hard to envision ever and that our biospheres capable of supporting human life on a large scale are limited to exactly ONE. We can create other populations, but they will end up like the space stations in the TV series 'the 100' - too small to be truly viable independent of Earth.

So I do expect the wars you talk about. But it is at least possible that they might not be necessary.

It's also possible Earth becomes a massive sinkhole for the ignorant, violent, overpopulated underclass and the intelligentsia and the rich (two very different groups) end up living off-earth and harvesting enough resources from elsewhere while keeping the bulk of humanity in quarantine on Earth. Earth can then have nasty little wars, but the only space conflicts will be some small scale scuffles because the rich and wealthy and the scientists will want to keep their limited ecosystem intact.

Lots of different possible scenarios.

Prove such a thing is possible and that the bomb won't consume the laser. This particular idea is trotted out and used, but I don't think we are anywhere near seeing one even if this idea is possible (which I still debate).

With modern optics, it is possible to make out any sizable object at rather long ranges visually. With thermal, even moreso. It's hard to hide against cosmic background temperature if you have any sort of engine, electronics, etc.

So, let's posit a best case missile:
a) launched from a railgun (no initial thermal heating, kept near 0K before launch, dark outer skin, decent launch velocity)
b) no manouvering until final attack phase (requires a stupidly non-manouvering target for a long time so your missile can close without having to use thrusters)

b) is pretty unlikely. The minute you know an enemy ship is out there (see my first observation - provided to me by a NASA instrument scientist of my acquaintance who worked on the NEAR mission), you'd start some form of manouvers just to eliminate the blind ballistic launch.

a) This may make the missile harder to spot. The chaff cloud would make it very easy to spot the cloud and the chaff cloud can't manouver like the missile (and the missile might hit the cloud to its detriment) so evade the cloud with your ship and you force the missile to manouver and thus reveal it and remove its cover.

The amount you'd have to manouver (how hard) is inversely proportional to engagement range. At longer ranges, a relatively mild set of course altering thrusts would rapidly ensure that no chaff cloud would come anywhere close and any missile would have to start manouvering to be in any position to hit you.

So your long range attack scenario is pretty unlikely unless your opponent for some reason has their sensors (passive) off.

Modern telescopes (not even future arrays) married to a decent computer can do fast sweeps of the sky and spot optical and thermal differences. Marry this to computers 20 years in the future and it'll be realtime (if today's take a few minutes). And once you've made the initial spot of an enemy ship at a distance, then you can focus your array and processing time drops further as you don't need a full sky sweep, so missile spotting becomes very likely even if they are trying to be within a few degrees of absolute zero (background thermal level) and not manouvering. If they manouver, you have them sighted.

If a missile has to manouver a lot to keep up with ships, it has to carry lots of reaction mass and burn it. Hotter signature, ejected hot trail, and more weapon mass devoted to propellant.

Covering areas of space with any sort of projectile saturation (or even a chaff cloud) is impossible at any range. The dispersion rate of the cloud and the manouvering rate of an enemy target at any range to speak of will be enough to render the amount of projectiles you'd have to lob as 'vast'. For ballistic projectiles, you'd have to predict the enemy location at time of intersect and set your scatter to attain your chosen volume at that particular point. Any enemy manouver to speak of ruins that option.

So, this sort of long ranged attack might work versus planets or stationary targets, but never against a military vessel that was aware of an enemy presence and most of the time they would be. Stealth in space is a fiction that's hard to justify given modern sensor tech and computer power and that available in the near future.

Instead, just simply closing under manouver and using carefully aimed energy weapons and perhaps close enough in some secondary batteries using railguns with high speed projectiles might work. Missiles could be used in swarms at short range.

For obfuscation, you can jam active sensors with EMPs or emissions, put modern passive thermal and opticals will largely render much of that EW useless. Passive targeting will be enough to hit cruisers and destroyers and so on.

It's funny - real space warfare with near future tech is horrible to game out. It is boring. Much of the action is closing and the nature of fights dictated by relative closure rates (a joust with one pass? an orbital gunfight at short range?). Stealth doesn't work. Planets and stations aren't defensible (the latter more defensible than the former but still hard to save). Fleets closing will be able to engage at medium distances hoping for a lucky hit and then at close ranger ranges looking to score significant damage with some statistical luck and being able to put enough joules on the enemy for enough time to get burn through or mess up their sensors arrays and so on. Fighters won't have much point except as added defenses for stations where they only have to manouver in self-defense and are just extra weapons platforms (probably drones not manned).

If you ever try to design semi-plausible ships in any of the more realistic space combat sims, you realize you have a lot of trade offs. Then you realize tactics are a bit limited as are the types of effective weapons. Then you realize the geometry of fights is everything and many engagements will take hours or days to unfold and boil down to whose gunnery tech works better because the sides will be using similar systems. Whoever brought the biggest fleet has the best chance of winning barring one side having a tech advantage in targeting or energy delivery downrange. Fuel (reaction mass) is a big factor in what you can do on the larger scale and in ship design. Energy constraints become an issue too.

It's not like the movies. There aren't 100 strategies nor boatloads of different technologies and ship designs. Time, distance, energy, fuel, and the ability to see a long way but only engage effectively over a much much shorter distance really shapes what can happen.

Not sure the flight dynamics of White Stars and Shadows was properly newtonian. They looked like they used an inertia-less drive. Yes, the White Stars rotated and strafed, but the way they and the Shadows generally manouvered seemed inertia-reduced at the very least.

Mostly true, except stealth isn't nearly as possible as you believe if current existent technology or reasonably-expected technology is considered.

The 'near c rock' is the big bugaboo in in space game setting. You might even see it coming but stopping it isn't trivial. And if you have a lot of targets (populations on different planetary bodies or in orbit over them), then this form of defense becomes fraught and unlikely to succeed.

Even jump-drives of sci-fi often have issues - jumping a ship in an atmosphere might well liberate enough energy to significantly damage the ecosphere. Or create some sort of singularity that would consume the planetary mass.

Space wars will either be:
a) Genocidal conflicts involving planetary decimation
b) Small political conflicts that adjust borders, resources, etc. but don't endanger the larger polities enough to make them switch to mode a)
c) Conflicts between groups who can't do a) yet, so are fighting with more limited means (and likely not an interstellar civilization, just intrasystem).
d) War on Terror style conflicts with NGOs (not usually fought with fleet battles)

Defending large biospheres is ridiculously resource consumptive. Attacking them less so.

This makes for the interesting point that mobile Dyson Sphere type population movers might be the way to go versus actual planetary populations if you get the wazzoo imagine-tech of the space opera universe. Take your populations with your warfleets and give the enemy nothing to attack except perhaps some remote resource gathering operations. Then there may actually still have to be a fleet battle - usually when one fleet wants a shot at the other fleet's 'worldships'. A decisive fleet victory for the attackers allows a genocide but at least they have to fight for it, unlike shooting fish in a barrel (rock bombing panets).