[Bobboau]

heat in space is radiative, so if you put something cold between a sensor and something hot all the sensor will see is cold, it's not like it's going to see 'hot' vacuum.

heat generated by the refrigeration system for the stealth propellant and the magnetic impulse system will be directed away from hostile sensors

you turn your ship in the general direction of your base, turn your thrusters to max burn until you reach a good speed, obviously evading attack that might be launched at you, once you reach a cruising speed you turn your stealth system on, that disables all high energy systems and refrigerates the back side of your ship. you use a mass driver that fires small same-temp-as-space pellets to apply course corrections.to get you at the very least not on the same trajectory you were on when you were doing a high thrust burn away from your pursuers. the only thing not currently feasible with our level of technology is a refrigeration system that can  cool the hull of a ship down to 2K, but it is not physically imposable.

[Aardwolf]

What about laser propulsion? If you do it right, it could only be spotted from (almost) directly behind (and then you'd be dead)

[Klaustrophobia]

well being as how lasers are massless, i'm not really sure how that would work.

[Aardwolf]

well being as how lasers are massless, i'm not really sure how that would work.

Oh, you poor fellow. 

Don't you know how a solar sail works? It's not by solar wind (common misconception, that), it's by light pressure. Light has momentum.

[Klaustrophobia]

granted, but GENERATING the light with the miniscule momentum doesn't sound practical to me.

[NGTM-1R]

Even so, you need to cool your hull to around 2 Kelvin, and I simply don't see any way to do that.

Since I have friends up at Palomar Observatory, I can state that this isn't really true. 50 or 100 Kelvin is a much more realistic figure for obscuring your radiated heat from instruments that could be practically mounted on a manned or unmanned missile truck for space combat. Anything else is too large, too sensitive, and far too easily burned out by an opponent with IR-frequency laser pointer (I'm not joking about that, someone wrecked a 50k infrared camera once with a laser pointer from a couple miles away) and half a brain.

If you go ultrasensitive thermal, a few minutes after mutual detection you both go blind. They can afford to saturate your possible courses with infrared "spots" because it's not very power-intensive to damage a sensitive instrument. Granted, cooling to 100 or 50 Kelvin is still a big problem.

There's also tradeoffs that would have to be made between field of view and effective detection range if you want to look deeper than a couple AU.

[General Battuta]

That's in a ship-to-ship engagement, but anyone smart is going to have a lot of passive thermal detection on drones or whatever.

And like you said, cooling the ship to 50 or 100 is not going to be easy either. Overheating is a huge problem in space.

[General Battuta]

heat in space is radiative, so if you put something cold between a sensor and something hot all the sensor will see is cold, it's not like it's going to see 'hot' vacuum.

Yeah, it's just going to see the glare of the radiated heat through your screen of cold metal pellets, which are never going to be thermally opaque.

heat generated by the refrigeration system for the stealth propellant and the magnetic impulse system will be directed away from hostile sensors

See Project Rho. I recall it doesn't work.

you turn your ship in the general direction of your base, turn your thrusters to max burn until you reach a good speed, obviously evading attack that might be launched at you, once you reach a cruising speed you turn your stealth system on, that disables all high energy systems and refrigerates the back side of your ship. you use a mass driver that fires small same-temp-as-space pellets to apply course corrections.to get you at the very least not on the same trajectory you were on when you were doing a high thrust burn away from your pursuers. the only thing not currently feasible with our level of technology is a refrigeration system that can  cool the hull of a ship down to 2K, but it is not physically imposable.

Yeah but then you run into a wall of sand somebody puts in front of your incredibly limited future position cone.

[NGTM-1R]

That's in a ship-to-ship engagement, but anyone smart is going to have a lot of passive thermal detection on drones or whatever.

Which have to transmit somehow or they're useless, and are equally susceptible to being blinded for that very reason. Unless you want to put them on a really long wire.

[General Battuta]

That's in a ship-to-ship engagement, but anyone smart is going to have a lot of passive thermal detection on drones or whatever.

Which have to transmit somehow or they're useless, and are equally susceptible to being blinded for that very reason. Unless you want to put them on a really long wire.

Yeah, but again, you only have to vector the target once to get a pretty good paint (most 'realistic' ships don't have anywhere near the kind of delta-V to make rapid course changes) and your remotes can use laser signals, which are pretty hard to pick up.

[Aardwolf]

That's in a ship-to-ship engagement, but anyone smart is going to have a lot of passive thermal detection on drones or whatever.

Which have to transmit somehow or they're useless, and are equally susceptible to being blinded for that very reason.

Well yeah, in order to share their findings they have to transmit. That transmission is probably omnidirectional, rather than being beamed directly toward some command post (then you'd have the advantage of not revealing your sensor's position as much, but you'd be revealing where your command post is when they manage to detect it), so the sensor is now a target.

So make them as cheap as possible.

And maybe set them up so they can be remotely switched from a passive mode to active mode once you figure they've been spotted anyway.

Edit:
Oh right, lasers for transmitting the data.

[NGTM-1R]

Edit:
Oh right, lasers for transmitting the data.

Laser comms are probably a bad idea in a situation where you are required to A: manuver radically to avoid destruction and B: light lag is a factor.

In essence, if you are manuvering enough to avoid being skewered by enemy lasers, you are also manuvering enough your drones won't be able to reliably signal you with a directional beam. It has to be omni. Even a semi-directional transmission cone would require you to squawk yourself on a fairly regular basis and invite dire consequences if your cryptology isn't up to the task.

[Aardwolf]

Ah, but this is a sensor drone/platform/probe.

It only ever needs to send (well, almost), and if it's using a laser, when it does send, it (probably) doesn't reveal its position. And since it's unmanned, it doesn't need to keep humans at livable temperatures. So it's cold. Stealthy. No need to be maneuvering and avoiding enemy lasers if they don't even know where you're watching them from.

[NGTM-1R]

You misunderstand me. I'm not talking about the drone. I'm talking about your weapons platform to which the drone is actually transmitting, which has either been fried by enemy lasers or to which the drone cannot transmit using a form of completely undetectable communications like laser comms because it can't predict where it's supposed to be transmitting to.

So the drone has to squawk. And then it will be vunerable to either blinding or destruction.

[General Battuta]

Your whole scenario is silly. If you're evading lasers you know exactly where the enemy is because he's shooting lasers at you, so you don't need the drone at all!

[Thaeris]

Again, the implications so far have relied on the "super sensor" scenario, which also seems to imply the given region is perfectly observed or defended. Over a truly massive region, this simply is not feasible, at least not on a scale with "sub-FreeSpace tech," which of course does not have access to magic or unobtanium.

Being "stealthy" in space really comes down to avoiding major "control points," such as planetary orbits, etc. The problem is thus that to engage the enemy, you will need to encroach upon such positions...

[General Battuta]

Again, the implications so far have relied on the "super sensor" scenario, which also seems to imply the given region is perfectly observed or defended. Over a truly massive region, this simply is not feasible, at least not on a scale with "sub-FreeSpace tech," which of course does not have access to magic or unobtanium.

Being "stealthy" in space really comes down to avoiding major "control points," such as planetary orbits, etc. The problem is thus that to engage the enemy, you will need to encroach upon such positions...

As Lex Luthor would say, wrong!

Something as teensy as the Space Shuttle's main engines can be detected, by modern sensors, from a range of several AU. Something realistic like a fusion torchship could be detected from Alpha Centauri.

As for the drone issue - your remote sensor platforms should simply fire whisker lasers at stationary (well, predictably moving) repeaters near your own ships, which could then rebroadcast omnidirectionally.

Again, that's with modern tech.

[headdie]

Here is a related problem how much effort in computational power and staffing would it take to monitor a defence of our home system? also how many sensor locations would it take to have total coverage?

[Flaser]

Here's a thought: unless we're talking about an unmanned drone, your ship will need to have sections at least 293 degrees Kelvin hot (20 degrees Celsius), so the hairless monkeys on board don't die.

...you'll have to do something with that heat.

As to "directional radiators": How the hell do you know which direction is safe? Your enemy can be just as stealthy as you.

...and the bottom line: if you run your radiators cool, they'll be massive. Bye-bye dodging incoming shots as you'd be too heavy to do so.

...and all the enemy vessels need is a separation of a couple thousand kilometers to get some scattered IR. Not too hard to do.

So:

Absolute stealth is impossible in space and even "practical stealth" is impractical as the benefits are far outweighed by the costs.

[BloodEagle]

at least 293 degrees Kelvin hot

lolwut?