PDA

View Full Version : Stealth in Space and solar sails: A solution?



rankrath
2009-04-13, 10:17 PM
I've read http://www.projectrho.com/rocket/rocket3w.html#nostealth, and the most of the Usenet discussion on the same topic, and while the consensus tends to be that a thruster based ship may be impossible to conceal, I seem to have noticed that one particular solution, solar sails seems to be left unmentioned.

As a method of propulsion, solar sails produce nothing in the way of emissions, making them perfect for stealthy interstellar travel. The one potential problem I see with the sails is reflections from sun light, however, current carbon nanotubes have achieved something within the magnitude of 99.999+% absorbency, and would seem to eliminate that particular problem. Such absorbent solar sails would serve a dual purpose, functioning as solar panels, powering the ship entirely.

This would reduce the emissions of a space craft down to the heat generated by the life support system, and any crewmen aboard. This heat could be dealt with in one of two ways, either have a double hull system, with the space in between the two hulls filled with water, or have a waste reclamation system that is endothermic, and siphon the excess heat into that. So, what say GITP? is this possible, or just another pipe dream?

Dervag
2009-04-13, 10:28 PM
A functioning solar sail would make a very large radar target. One large enough to propel a ship with a mass measured in hundreds or thousands of tons (large enough to have a significant crew and weapons capability) would be very large, and would tend to be highly visible at long range.

If the solar sail is highly absorptive, it will heat up when it gets close to the star and will be a very large, low-intensity infrared/microwave source.

Moreover, solar sails powered by sunlight alone have extremely low accelerations, which makes them unsuitable for combat maneuvers.

averagejoe
2009-04-13, 10:37 PM
I'm fairly sure I'll get something wrong. Thermodynamics has never been my strong suit. Oh, well, here goes.

Double hulling in that manner really doesn't work. Besides the fact that water is still much warmer than space (though, if to a significant degree I don't know, I don't know much about how heat is detected and how sensitive instruments are) it is, at best, a temporary solution. The water will eventually heat up and start emitting heat at the same rate. Anyways, even if you could devise a perfect heat shield, and keep any heat from leaving the ship, then the interior of the ship would just heat up to unlivable degrees.

Edit: This is also problematic in that, even if the sails function as solar panels, the ship's power system would still produce waste heat, and you can't convert all the heat from sunlight into power anyways, so there's another heat source.

Icewalker
2009-04-13, 10:53 PM
On the idea of avoiding heat signature as a method of stealth, I had an idea at one point for an interesting idea.

A short term stealth system would function as a giant block of some material with a very high specific heat capacity. It would heat up slowly. Pump all heat from the ship into this block, held in some sealed chamber. It could only hold short term before it would just be giving off too much heat to hold reasonably, and whenever possible a ship would drop off these heated blocks on a planet to let them be used to generate energy. Once the blocks are cooled they are resold to ships.

Not terribly efficient as a mechanism in use, but a good niche in an economy.

742
2009-04-14, 01:59 AM
well idk about the other stuff, but acceleration might not really be a problem, as the applications of stealth craft do not traditionally require quick acceleration.

Torchship
2009-04-14, 06:17 AM
I'm going to second (third? Fouth?) those who say this probably won't work. Life support is going to put out a lot of heat over the months or years you are coasting in oh-so-slowly (actually, life support won't put out that much heat; the fact you'll have to have your reactor ticking over to provide power for it will be far worse), and that'll have to go somewhere. Two hulls will help, but eventually they'll hit thermal equivalence and then you'll be radiating as if the second didn't exist (keeping vacuum between the hulls will actually be better for the purpose of stealth... but then you'll probably boil after a couple of weeks).

Thermodynamics isn't my forte either, but I don't think lugging around a high specific-heat block will solve the problem entirely. The block would tend to approach thermal equilibrium with the rest of the ship as well, and then, after a very long time, you're back where you started from (minus a great deal of usable, weaponisable mass). Might work if your trip is short enough, but since you're in a solar sail powered ship, there is no such thing as a 'short trip'.

Also, the fact that you're flying a couple of hundred thousand square k's of carbon fibre around won't help. You'll be occluding many, many times more stuff than an equivalent-mass ship would, and that will probably get you noticed quite fast.

warty goblin
2009-04-14, 01:18 PM
I think the problem with the solar sails isn't even one of waste heat. It's more one of strapping wacking great mirrors to yourself, then hoping nobody notices that you are basically flying around in a giant sequin. A giant, slow, and immanuverable sequin at that.

Sure you will probably escape notice for the several thousand years it'll take to get between stars at sailing speeds, but the closer you get to your target, the greater the chance of you occluding a star, at which point the game is basically up, and you had better hope that your new 'hosts' have nice prisoner of war camps. At the very least, the instant you pass an enemy sensor you are totally screwed, since the backside of your ship is going to be shining like, well, a star (remember, solar sails work by reflecting light).

So yeah, I'm not seeing this one.

rankrath
2009-04-14, 02:28 PM
Thanks for all the responses, however, I need to clarify a few things. Firstly, I'm not talking about using this technology for interstellar travel, rather interplanetary, such as between Earth and Mars. And second, kind of a big mistake on my part, sorry, but I should have put magnetic sails as opposed to solar sails. As Warty Goblin pointed out, solar sails are mirrors.

Dervag
2009-04-14, 02:45 PM
Thanks for all the responses, however, I need to clarify a few things. Firstly, I'm not talking about using this technology for interstellar travel, rather interplanetary, such as between Earth and Mars. And second, kind of a big mistake on my part, sorry, but I should have put magnetic sails as opposed to solar sails. As Warty Goblin pointed out, solar sails are mirrors.You'd still have the heating problem. You can't really get your ship's skin temperature down into the cryogenic range, and if you can't do that and don't have a technomagic method to deal with your hull's infrared signature, you're going to be quite visible even at long ranges.

Especially if the target knows they might be attacked and keeps a far-infrared telescope trained on the region of space you'd have to fly through to reach them.

______


On the idea of avoiding heat signature as a method of stealth, I had an idea at one point for an interesting idea.

A short term stealth system would function as a giant block of some material with a very high specific heat capacity. It would heat up slowly. Pump all heat from the ship into this block, held in some sealed chamber. It could only hold short term before it would just be giving off too much heat to hold reasonably, and whenever possible a ship would drop off these heated blocks on a planet to let them be used to generate energy. Once the blocks are cooled they are resold to ships.

Not terribly efficient as a mechanism in use, but a good niche in an economy.You'd need a technomagic material to make this work- something with a specific heat capacity higher than any material now known to science. In my book, it's hard to see how such a material could be made out of atoms.

rankrath
2009-04-14, 03:17 PM
You'd still have the heating problem. You can't really get your ship's skin temperature down into the cryogenic range, and if you can't do that and don't have a technomagic method to deal with your hull's infrared signature, you're going to be quite visible even at long ranges.

Especially if the target knows they might be attacked and keeps a far-infrared telescope trained on the region of space you'd have to fly through to reach them.

My knowledge of thermodynamics is limited, to say the least, but my understanding was that radiating heat into space was a difficult thing to do. So with a small crew, and a short voyage, with some sort of endothermic reaction occurring on the ship, (say, in the waste reclamation system), could the heat levels be kept low enough that radiating heat into space was needless?



You'd need a technomagic material to make this work- something with a specific heat capacity higher than any material now known to science. In my book, it's hard to see how such a material could be made out of atoms.

Such a material doesn't have to be quite so technomagicky. IIRC, Current theoretical chemistry predicts that synthetic elements in the noble gas group become stable, and since none of these have been made, (except Ununoctium), you can assign them needed properties for whatever bit of science fiction you happen to be writing, and keep it some what plausible.

Meltemi
2009-04-14, 04:16 PM
My knowledge of thermodynamics is limited, to say the least, but my understanding was that radiating heat into space was a difficult thing to do. So with a small crew, and a short voyage, with some sort of endothermic reaction occurring on the ship, (say, in the waste reclamation system), could the heat levels be kept low enough that radiating heat into space was needless?
Radiating heat, as in the colloquial and extremely imprecise definition of any thermal transfer, is rather difficult in space, because neither convection nor conduction exist in a vacuum. Radiating heat, as in specifically the process of thermal radiation (by contrast to convection and conduction), is not difficult, occurs naturally along any thermal gradient, and relies only on thermal differential, surface area, Stefan-Boltzmann's constant, and assuming you aren't a blackbody, a correction factor based on material and shape. In fact, it's the only way to dump heat in space, and the problem is usually radiating enough heat to do the job, given how much thermal energy is generated by internal processes.

Remember that space has an average background radiation of around 3 degrees Kelvin, more commonly referred to as "really, really ruddy cold." More than that, the amount of energy output per unit time by a radiating material, any radiating material, goes by the thermal difference between the material and its surroundings to the fourth power, so a small difference will go a very long ways towards making you highly visible. Simply maintaining the interior of the ship at survivable temperatures, excluding all other essential things like powering whatever's keeping your ship alive, will create a tremendous thermal footprint, and that heat will naturally leak outward through the physical material of the hull, if nothing else. For an example of scale, as I recall, it took less than a week for the Apollo 13 LM and CM to approach freezing temperatures after they lost power; it may not have been designed specifically to keep heat in, but on the scales of months or years necessary for a ship without acceleration in an interplanetary scale, one can still extrapolate.

Basically, even if you had a sufficiently endothermic process that absorbed all waste heat and thus negated the need for radiation to keep your ship cool, or a miracle material that absorbed all of the heat, you still have the problem with how to prevent radiation outward in the first place. A double-hulled ship would not help overmuch, because of conduction through the fixtures. I always thought Mass Effect's Normandy was a decent effort, but nowhere near good enough.


Such a material doesn't have to be quite so technomagicky. IIRC, Current theoretical chemistry predicts that synthetic elements in the noble gas group become stable, and since none of these have been made, (except Ununoctium), you can assign them needed properties for whatever bit of science fiction you happen to be writing, and keep it some what plausible.
That actually rather feels rather technomagic to me, though it's naturally a very subjective criterion. The beauty of the periodic table is that you can extrapolate these sorts of things for atoms that haven't yet been discovered or empirically observed. Moreover, there are chemical laws and models that fit observed data so far for both atoms and molecules: Dulong-Petit actually predicts a limit to heat capacities for "any" (quote-unquote important) material of 3*R per mole of atoms using the equipartition theory that is also used for the ideal gas law, where R is the universal gas constant, which rules out the possibility given certain criteria (it breaks down for very high temperatures or non-crystalline/non-solid materials, but the former is irrelevant and the quantum interactions that occur in the latter generally conspire to lower specific heat capacity below the Debye predictions).

Still, the Dulong-Petit law and the Debye model are empirical rather than theoretical in derivation - that is, scientists observed the reality and drew up the theory to make predictions based on the data, rather than deriving from first principles and then obtaining data to confirm or deny. If a material that breaks the Debye model is found, there isn't a fundamental theory of science that would be shattered; still, there would have to be a very good reason why this material and no other has violated the model. Newtonian motion was found to be invalid in the same manner, after all, but it took sub-atomic motion, special relativity, and extreme gravity to manage it.

More than that, as mentioned before, your stealth ship cannot accelerate, decelerate, or otherwise maneuver with known technology, because all of these require energy, and that energy can be easily detected. Without any ability to accelerate in any direction, anyone who sees you getting up to speed, assuming you do it under your own power, will be able to predict where you are at any given point with reasonable confidence and arrange for something nasty to get you there. If you use some external installation to slingshot yourself to speed and thereby avoid strapping a huge torch to your ship, someone who can observe that installation can also determine where it's pointed.

Anyone who observes a couple occlusion points by your ship while it's in transit (because anyone encamped at a single location with a known need for security will know what the stars around them are supposed to look like, and be able to spot for one that's missing, assuming competence), will be able to manage the same, and we already, even today, have these sorts of telescopes set up to spot asteroids far smaller than a ship. It's always seemed to me that future technological advances will generally make it easier, not more difficult to detect a ship visually, especially once we get a large number of telescopes out from under the smothering atmosphere and into space.

Dervag
2009-04-14, 04:29 PM
I vouch for the overall truth of the previous post. I may have missed some specific point on which Meltemi is mistaken, but it's true in general.

However, Meltemi should probably break that post up into shorter paragraphs that are easy to digest.

Meltemi
2009-04-14, 04:42 PM
Done. Hopefully it's easier to read now. I don't have much of a background in thermodynamics, to be honest, just what was necessary for my major. I certainly didn't learn about things like the Debye model and the such-like from the uni; that's advanced thermodynamics, and far beyond the scope of any of our classes, so I know little beyond the names and a few basic snippets.

warty goblin
2009-04-14, 07:02 PM
Wow that's...that's a lot of thermodynamics up there. Most cool.

To expound a bit, stealth in space is even worse off than the simple thermodynamical problems of propulsion and heat dissappation indicated. I'd think it would be reasonably possible to make hard to see passive detection satillites for one thing. Since these wouldn't have to move, they could have truly minimal power usage for the most part, and hence would not need to be that much hotter than the cosmic background radiation. This will make seeing them subsantially more difficult than them seeing you.

In short I'd imagine approaching an enemy planet to be something like driving through a field filled with motion detectors, vibration detectors, heat detectors, and every other sort of relevant detector with your engine roaring and headlights blazing. You could drive a quiet little helpless car, or say 'screw it all' and take a tank, since you'll almost certainly bee seen even if you took a Segway.

Eldan
2009-04-14, 07:58 PM
Hmm. Actually, the passive satellite thing gave me an idea... what about projectiles without propulsion? How hard to spot would, say, a large nuclear warhead be?

warty goblin
2009-04-14, 08:22 PM
Hmm. Actually, the passive satellite thing gave me an idea... what about projectiles without propulsion? How hard to spot would, say, a large nuclear warhead be?

At interstellar distances? For all practical purposes, completely and utterly impossible. Inside the solar system, difficult but probably doable. Inside of say, Jupiter's orbit, getting easier. By the time it reached the moon's orbit, I'd imagine it would be fairly trivial for any power capable of projecting power into space to see, and hence stop.

The nuke could possibly be made invisible with proper construction and materials, but that's not the real problem. The real problems are those of time and accuracy.

The sort of launch you could hide within the solar system would produce a really low velocity projectile- you'd be looking at years upon years of flight time easily. Pretty much the same thing goes for launches in nearby star systems, except you could get the projectile faster, but it would take even longer to reach target. Pretty much the only way to get your ordnance on target in a reasonable timeframe is to launch it fast enough that the launch will be seen anyways.

Accuracy is another killer- there's simply no way that I can concieve of to fire a ballistic projectile across interplanetary, let alone interstellar distances and actually hit anything. You would have to attach guidence rockets, which would, as previously noted, make the projectile non-stealthy.

Meltemi
2009-04-14, 08:35 PM
Hmm. Actually, the passive satellite thing gave me an idea... what about projectiles without propulsion? How hard to spot would, say, a large nuclear warhead be?

Slightly more difficult than the spaceship, but still fairly easy depending on the nature of the computer and sensor suite on board - you could actually take advantage of space ambient temperatures to create superconductive circuits for a computer to control the system and, say, make it explode if it detects an enemy close enough via some passive sensor array on the outer surface, but that computer would itself require power and that would generate at least some heat. It'd probably be best to disguise it by making it appear to be something else, like a piece of debris from some long-ago battle or construction project or a semi-metallic asteroid, rather than simply hide it entirely. For that matter, a remnant from some long-ago battle might also explain any radiological signature a nuclear warhead would exhibit.

Of course, a nuclear warhead runs into the inverse square law, since its effectiveness drops by the square of the distance, so it'd have to be very close to the enemy to do any damage at all. You could use something like Teller's bomb-pumped X-ray warheads, but those are far more expensive for a single-shot weapon. Moreover, any mine would run into the non-trivial problem that space is a volume rather than an area, and one that is effectively without terrain. There are no natural chokepoints in space to block, no typical approach vectors that could be reliably mined to defend against an enemy approach. If your in-verse FTL is reliant on jump points, wormholes, or other point sources, and you have a sufficently strong industrial base, you could conceivably use mines in a defensive capacity, but even then it'd be unlikely to pay off.

EDIT:
Ah, I missed the whole "projectile" part of the post. Well, conceivable, but you wouldn't be able to hide the firing. There is one thing that I don't believe Warty mentioned, though, which is that there is one potential target that could be easily hit on even interplanetary scales - a planet. Of course, any spacefaring society worth its salt is going to have some way to protect itself against anything ranging from small-strike to planet-killer asteroids, and something like a barrage of nuclear warheads isn't going to be that much more difficult to stop, especially given the times involved. If you do something like this and you have the power for it, you might as well go relativistic - at sufficiently high fractions of the speed of light, not only do you no longer need a warhead, but their warning (assuming the light-speed limitation holds) will be effectively negligible. That would be better suited as MAD than it would be an actual weapon, though, because you are not going to be able to hide a launcher capable of firing projectiles at near light-speed; it simply needs too much power.

warty goblin
2009-04-14, 09:07 PM
EDIT:
Ah, I missed the whole "projectile" part of the post. Well, conceivable, but you wouldn't be able to hide the firing. There is one thing that I don't believe Warty mentioned, though, which is that there is one potential target that could be easily hit on even interplanetary scales - a planet. Of course, any spacefaring society worth its salt is going to have some way to protect itself against anything ranging from small-strike to planet-killer asteroids, and something like a barrage of nuclear warheads isn't going to be that much more difficult to stop, especially given the times involved. If you do something like this and you have the power for it, you might as well go relativistic - at sufficiently high fractions of the speed of light, not only do you no longer need a warhead, but their warning (assuming the light-speed limitation holds) will be effectively negligible. That would be better suited as MAD than it would be an actual weapon, though, because you are not going to be able to hide a launcher capable of firing projectiles at near light-speed; it simply needs too much power.

True enough, it would be possible to hit a planet at interplanetary ranges, but by no means trivial- depending mostly on the planets one is thinking of. Hitting something about as far away as Mars or Venus is fairly easy, but if you are firing from Pluto?

Launching reletivistic projectiles does to a lot to solve the 'having them shot down' problem agreed. Of course then you have to hide the bit where you are building a reletivistic cannon in your enemy's backyard, more or less. My guess is the instant somebody on planet A gets news that the next doors on planet B are building Ye Olde C-Fractional Raille or Coile Gunne is the instant that those invasion ships start making their way over.


Space warfare- it's like putting on a sequined Elvis suit, grabbing a shotgun, and charging across a few miles of wide open, completely flat terrain towards another dude in an Elvis suit with a 12 gauge. Also, the ground is made of glass and you are wearing shoes coated in grease.

Eldan
2009-04-14, 09:23 PM
Right. I forgot the launch. Now it's getting difficult. Perhaps hiding a launch station near the (a) sun might help disguise energy signatures, but that's just grasping straws, since the heat would once again be a problem...

Meltemi
2009-04-14, 09:46 PM
True enough, it would be possible to hit a planet at interplanetary ranges, but by no means trivial- depending mostly on the planets one is thinking of. Hitting something about as far away as Mars or Venus is fairly easy, but if you are firing from Pluto?

Launching reletivistic projectiles does to a lot to solve the 'having them shot down' problem agreed. Of course then you have to hide the bit where you are building a reletivistic cannon in your enemy's backyard, more or less. My guess is the instant somebody on planet A gets news that the next doors on planet B are building Ye Olde C-Fractional Raille or Coile Gunne is the instant that those invasion ships start making their way over.


Space warfare- it's like putting on a sequined Elvis suit, grabbing a shotgun, and charging across a few miles of wide open, completely flat terrain towards another dude in an Elvis suit with a 12 gauge. Also, the ground is made of glass and you are wearing shoes coated in grease.
Well, we can even with modern computers tell exactly where a planet will be at a given point in time, since they cannot exactly manoeuvre, and we can already hit a planet with only minor course correction - New Horizons, heading for Pluto, only required two of its three planned corrections due to a surprisingly high level of accuracy, though hopefully it isn't as "accurate" as the Mars Climate Orbiter. Regardless of that little snafu, Moore's Law won't hold forever, but if it holds for long enough, we could probably computationally solve an eight-body problem fairly trivially with simple mainframes in a decade or two. Including supercomputers, which I suspect the military uses as well even today, we've already done 10-billion and 70-billion body problems for the Millenium Run and Horizon simulation respectively to investigate how the Universe has evolved over time.

Oddly, this is beginning to feel like nuclear proliferation politics. Direct examples aside, there are plenty of examples of nations doing or refusing to do precisely this today, though a C-weap facility would be closer to a centralized breeder reactor than it would be the dispersed centrifuges, and like Osirak, would be very vulnerable to a preemptive attack.


Right. I forgot the launch. Now it's getting difficult. Perhaps hiding a launch station near the (a) sun might help disguise energy signatures, but that's just grasping straws, since the heat would once again be a problem...
It could help wash out the signature of a launch, but then you'd run into design problems. Radiation works both ways, and as you say, near a sun, you'd start absorbing a lot of heat. It helps, of course, that the interior of a star is still not very well-understood, which leads to a certain unpredictability of stellar phenomena that could be used to mask a launch even if the signature is detected, but that still leaves aside the possibility of visual detection via occlusion. Assuming that there are measures like a 24-hour star watch in effect to protect against the potential of incoming asteroids, the same measures should work to detect any other incoming projectiles as well.

Dervag
2009-04-14, 09:55 PM
Warty goblin, your analogy about the car and the tank was great, and I was amazed that you managed to top it with your battling Elvis impersonator analogy.


Well, we can even with modern computers tell exactly where a planet will be at a given point in time, since they cannot exactly manoeuvre, and we can already hit a planet with only minor course correction - New Horizons, heading for Pluto, only required two of its three planned corrections due to a surprisingly high level of accuracy, though hopefully it isn't as "accurate" as the Mars Climate Orbiter. Regardless of that little snafu, Moore's Law won't hold forever, but if it holds for long enough, we could probably computationally solve an eight-body problem fairly trivially with simple mainframes in a decade or two. Including supercomputers, which I suspect the military uses as well even today, we've already done 10-billion and 70-billion body problems for the Millenium Run and Horizon simulation respectively to investigate how the Universe has evolved over time.The problem isn't so much targeting planets as it is targeting anything smaller than a planet. It's not just your ability to calculate where the target will be, it's the limit of precision of your telescopes and your launcher. The laws of probability are not your friend in this case. I talked about the problem in greater length in this post (http://www.giantitp.com/forums/showthread.php?p=5389078&highlight=pluto#post5389078).

Meltemi
2009-04-14, 11:38 PM
The problem isn't so much targeting planets as it is targeting anything smaller than a planet. It's not just your ability to calculate where the target will be, it's the limit of precision of your telescopes and your launcher. The laws of probability are not your friend in this case. I talked about the problem in greater length in this post (http://www.giantitp.com/forums/showthread.php?p=5389078&highlight=pluto#post5389078).

True enough, and even targetting the planet only works as long as you don't mind where on the planet you actually hit; I did mention that it was the one thing missed of sufficient scale to be hit. Generally, though, diffraction limits are proportional to wavelength and focal length and inversely proportional to lens diameter. Since you didn't actually say why you can't measure so precisely in that post beyond a two-decade-old single-telescope satellite that had some pretty serious weight limitations even going up, I think I want to look at the math underlying it, if that's fine.

We'll use the Rayleigh criterion formula on Wiki to resolve a city like Ouagadougou, say, treating it as a 6-km-long object resolved as a distinct Airy disc "point" identifiable through some arbitrary means, probably by being in the right rough spot of Africa. We'll use visible light at...call it 580 nm, though I expect we could work higher to blue, violet, or UV if we wanted more precision. We'll disregard astronomical reality and assume Earth and Pluto are at their very, very closest possible approach (28 AU difference) and also neglect diffraction problems enroute (another possibly bad assumption, by the way - you'd expect this to be rather significant for the light minutes of separation between Earth and Pluto despite the vacuum) to get a best-case scenario. With that, you require a focal resolution of 8.07E-8 degrees, or 0.00029 arcseconds, or 1.41E-9 radians. A single telescope with given approximation would require an objective lens diameter of 411 metres, and probably an ungodly focal length on top of that. We can use a telescope array via aperture synthesis, which is known to give resolutions in the thousandths of arcseconds, but we'd require one that is 411 metres at its widest separation (baseline). And this is with our very best-case assumptions - Pluto is always further out than this. For its furthest distance, around 50 AU (now also neglecting the sun in the way), it would be closer to 735 metres baseline.

Wait, is my math right? Certainly, the very largest single telescope will only have a objective diameter of 10.2 metres, but the theoretical Overwhelmingly Large Telescope has one of 100 metres. For arrays, the Very Large Array already uses a baseline of 36 kilometres, though it's radio instead of optical, CHARA (optical) has a maximum baseline of 330 metres already, and the optical at Magdalena Ridge is even larger at 400 metres planned. It must be my assumption to neglect decollimation and general light scattering, but these equations are used for interstellar observation telescopes on scales far beyond this, and NPOI managed similar resolutions (slightly worse) despite operating under the equivalent of a heavy blanket - the Earth atmosphere. But on the other hand, these telescopes are not used for planetary surveying, so there must be a fundamental reason for that...lovely. This is so far beyond my level of experience that I can't make heads or tails of it anymore. It must be a computational bottleneck in handling the data, or the apertures of the individual telescopes, or something else I'm not understanding that isn't in the simplistic theory I have access to.

Well, either way, the necessary scales are obvious now, and they are not at all insignificant. I suppose there's nothing stopping one from replacing Charon with a massive telescope to aim that person's Plutonic Cannon...though the infrastructure investment is so massive, the logistics chain so tenuous, and your projectile ETA so abysmally long that one might as well put that Plutonic Cannon someplace a bit closer, anyways. That turned out to be a really inspiring read, though, and it did remind me of the old joke that ends with "Apologies for next three incoming." ^_^

Dervag
2009-04-15, 05:44 PM
True enough, and even targetting the planet only works as long as you don't mind where on the planet you actually hit; I did mention that it was the one thing missed of sufficient scale to be hit. Generally, though, diffraction limits are proportional to wavelength and focal length and inversely proportional to lens diameter. Since you didn't actually say why you can't measure so precisely in that post beyond a two-decade-old single-telescope satellite that had some pretty serious weight limitations even going up, I think I want to look at the math underlying it, if that's fine.What I was getting at was that even with what we now call truly good astronomical telescopes (and the Hubble was good), resolution limits make targeting a problem over interplanetary ranges. You need a really impressive gunsight to resolve a military target at those distances.

Moreover, you need to be able to point your gun to the same precision you need to have in your gunsight resolution. That isn't easy, either. So unguided projectiles may be stealthy, but at ranges long enough that their firing can go undetected, they'll be extremely difficult to aim... and the infrastructure needed to aim them will be hard to move and easy to destroy.


...though the infrastructure investment is so massive, the logistics chain so tenuous, and your projectile ETA so abysmally long that one might as well put that Plutonic Cannon someplace a bit closer, anyways.Good name: "the gun from Hell." I like it. It's an accurate description of the project on so many levels.


That turned out to be a really inspiring read, though, and it did remind me of the old joke that ends with "Apologies for next three incoming." ^_^Never heard it. Can't find it conveniently. Care to share it?

Meltemi
2009-04-15, 08:32 PM
What I was getting at was that even with what we now call truly good astronomical telescopes (and the Hubble was good), resolution limits make targeting a problem over interplanetary ranges. You need a really impressive gunsight to resolve a military target at those distances.

Moreover, you need to be able to point your gun to the same precision you need to have in your gunsight resolution. That isn't easy, either. So unguided projectiles may be stealthy, but at ranges long enough that their firing can go undetected, they'll be extremely difficult to aim... and the infrastructure needed to aim them will be hard to move and easy to destroy.

Good name: "the gun from Hell." I like it. It's an accurate description of the project on so many levels.
Fair enough. Installing the gun itself is a significant investment, though, so the infrastructure to build the observatory necessary to build it is not that much greater, and I did only bring up the sight problem. Still, the math says we can build the observatory already with what we have today, though it's be fairly massive, unless I made a mistake. I did neglect both gun design and atmospheric forces that would affect the projectile as it enters Earth's exosphere, which would be the real killers. A single arcsecond's inaccuracy would translate to several thousand metres for an unguided projectile. About the size of a decent-sized planet, but not anything smaller.


Never heard it. Can't find it conveniently. Care to share it?
I heard it a while ago, so I'm not sure if I have it exactly right here, but...

A small British corvette is on patrol late at night off Newfoundland during early World War 2, and the lookout spots the silhouette of a ship near the horizon. They ID it as an Iowa class battleship, and get ready to contact it when a shot from the ship suddenly splashes into the water mere metres in front of their bow. They start signalling, "This is the HMS Juno. Cease fire." After a moment, the battleship radios back, "This is the USS Iowa. Apologies for next three incoming."