First off, there are two broad classes of sensors: passive and active. Passive sensors just detect any emissions from the target, i.e., they passively look for the target. Passive sensors include telescopes and heat sensors. Active sensors emit various frequencies and detect their reflection off the target, i.e., they actively "shine a light" on the target. Active sensors include radar and lidar/ladar.
Active sensors are much better at detection, but have the annoying side effect of virtually placing a huge flashing neon sign on your ship that says: "LOOK AT ME! I'M HERE! SHOOT ME, SHOOT ME!!" . This not only lets all hostiles (detected and undetected) know where you are, but also gives their deadly radar-homing missiles some radar to home in on.
Passive sensors, on the other hand, are more blind but are undetectable. Much better if you are trying to hide. Passive sensors also generally can vaguely detect the presence of objects at a much greater range than active sensors. But active sensors can determine the precise location of an object with much greater precision.
Why? An active sensor emits "pings" of electromagnetic radiation in order to illuminate the target, the sensor "sees" the target if the energy returned by reflecting off the target is high enough to be detected. If the target has a small dimension compared to the angular and range resolution of the active sensor, the strength of the return signal is proportional to the inverse fourth-power of the distance to the target (i.e., signal fall-off is 1/r4).
Why this fall off is 1/r4 instead of the 1/r2 you'd expect from the inverse square law is explained here and here. Basically only a fraction of the initial pulse energy is reflected back. So the target acts as if it was an active sensor emitting pings with a strength of 1/r2 of the original pulse. These pseudo-pings travel back to the original ship, suffering a further loss of 1/r2. This combines to make an effective loss of 1/r4.
But on the third hand an active sensor uses tightly focused pings while a passive sensor has to make do with whatever unfocused radiation flux the target emits.
There is one cute real-world trick. If your active radar pulses mimic radio static, enemy radar detectors will filter the pulses out as random noise and fail to see them. This will make your active radar invisible. Until the enemy catches on to the trick and redesigns their detectors.
In some SF novels, passive sensors are called "sensors" while active sensors are called "scanners."
It would be a jolly science fictional idea to postulate a break-through that could detect passive sensors, keeping in mind that there doesn't seem to be any basis for this in reality. Wave your hands real hard, and vaguely mutter about "psionics", or something based on a Schrödinger's cat-like collapse of wave function (Captain, the wave function collapsed, it means somebody is peeking at us!) or specially trained experts who feel itchy sensations between their shoulders when somebody is looking at them. But to reiterate, this is strictly science fiction.
First off, as Ken Burnside explains, there is one major way that detection in space is different from detection on Terra's surface: There Is No Horizon. Since Terra is a sphere, the curvature means if you are of average height, the fact your eyes are about 1.7 meters off the ground means anything much further away than 4.7 kilometers will be invisible. That is the distance to the horizon, anything further (that is not outrageously tall) will be hidden below the horizon.
Space don't have no horizon, nohow. The range is pretty much to infinity (or 13.798 ± 0.037 billion light years if you want to be picky).
Yes, there will be a bit of a horizon effect if you and the target are in close orbit around a planet. The target will be hidden for about one-eighth of an orbital period. For something in LEO around Terra, this means it will be hidden for about 15 minutes, max. Which is not really a militarily significant amount of time.
Secondly, there are three different ranges:
- Detection Range: You become aware there is something out there, at that position in the celestial sphere. You may or may not know how far away it is (e.g., there is a bogey, a blip on the radar screen).
- Identification Range: You know there is an object of a certain type at range x (e.g., there is a Blortch CL-23 "FenderBender" light cruiser at x 135.2, y 17.3, z 325.1 ).
- Targeting Range: Your sensors have enough data for a targeting solution (Your casaba howitzers have a target lock on the enemy FenderBender, designated Target Tango 13. You may fire at will. ).
For a given sensor, these range are arranged above in order of decreasing distance.
In space, Detection (as opposed to Identification and Targeting) is basically a matter of time. You can purchase off-the-shelf software fully capable of processing a full spherical sky search and flag any bogeys. The processing power of an average PC graphics card is more than up to the task. Since it takes about three days to travel from Luna to Terra with current technology, it is not like there is any rush.
If the enemy is using torchships, then you can probably spot them with your naked eyes. At least if they are closer than a few astronomical units (1 AU = distance between Terra and Sol).
And thirdly, refer to the next section.
Wargames like GDW's STAR CRUISER describe interplanetary combat as being like hide and go seek with bazookas. Stealthy ships are tiny needles hidden in the huge haystack of deep space. The first ship that detects its opponent wins by vaporizing said opponent with a nuclear warhead. Turning on active sensors is tantamount to suicide. It is like one of the bazooka-packing seekers clicking on a flashlight: all your enemies instantly see and shoot you before you get a good look. You'd best have all your sensors and weapons far from your ship on expendable remote drones.
Well, that turns out not to be the case.
The "bazooka" part is accurate, but not the "hiding" part. If the spacecraft are torchships, their thrust power is several terawatts. This means the exhaust is so intense that it could be detected from Alpha Centauri. By a passive sensor.
The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto. The Space Shuttle's manoeuvering thrusters could be seen as far as the asteroid belt. And even a puny ship using ion drive to thrust at a measly 1/1000 of a g could be spotted at one astronomical unit.
As of 2013, the Voyager 1 space probe is about 18 billion kilometers away from Terra and its radio signal is a pathetic 20 watts (or about as dim as the light bulb in your refrigerator). But as faint as it is, the Green Bank telescope can pick it out from the background noise in one second flat.
This is with current off-the-shelf technology. Presumably future technology would be better.
Read the essay in the Rocketpunk Manifesto entitled Stealth Reconsidered.
Now I know you do not want to accept the fact that stealth in space is all but impossible. This I know from experience (Every day I have new email from somebody who thinks they've figured out a way to do it. So far all of them have had fatal flaws.). The only thing that upsets budding SF writers more is Albert Einstein denying them their faster than light starships. But don't shoot me, I'm just the messenger. The good folk on the usenet newsgroup rec.arts.sf.science went through all the arguments but it all came to naught.
If you are bound and determined to have stealth in space, you will have to postulate some sort of hand-waving technology. Popular in science fiction are "cloaking devices" and stealth as a side effect of the faster-than-light propulsion used by starships ("We can't detect the Zorg ship until it comes out of warp, sir!"). Much more rare is something like a heat radiator, where the radiator sticks into hyperspace to make the heat invisibly go away into the fifth dimension.
It is not like the absence of stealth in space takes all the fun out of things. Sometimes things are more interesting this way. For example, John Reiher shows how to incorporate this in to the tabletop role playing game Diaspora (incidentally, Diaspora has been awarded the Atomic Rocket Seal of Approval).
If you want to really argue on this topic, I'd advise you to cut out the middle man and go directly to rec.arts.sf.science and lay your case out before the experts. You might also want to review the section on Respecting Science.
This is true. Take my word for it, I know from bitter experience.
First off, the answer is NO, you cannot solve the problem by using a thermocouple to convert the heat into electricity.
Ken Burnside said:
For reference purposes, here follows some brief summaries of the more common arguments and their rebuttals.
If you are hoping to lose your tiny heat signature in the vastness of the sky, I've got some bad news for you. Current astronomical instruments can do a complete sky survey in about four hours, or less. Presumably future technology can do it even faster.
Ken Burnside said:
Rd = ( 17.8E6 * sqrt( Ms*As*Isp*(1-Nd)*(1-Ns) ) ) * (sqrt(0.04 * π))
- Rd = maximum detection range (kilometers)
- Ms = bogey spacecraft mass (tons)
- As = bogey spacecraft acceleration (G)
- Isp = bogey drive specific impulse (seconds)
- Nd = bogey drive efficiency (0.0 to 1.0)
- Ns = bogey "stealth efficiency", i.e. fraction of waste energy which can be magically shielded from enemy detectors. (0.0 to 1.0)
- π = 3.141593...
This assumes about one hour for a full sky scan. Current chemical rockets have Nd of roughly 0.95. Ion drives get about 0.50, and steady-state plasma thrusters 0.65 or so - both can in principle be pushed to 0.90 with some difficulty, but not much beyond that. For realistic rockets, Ns = 0.0. There really isn't any way to hide your waste energy from your opponents, short of science fiction.
"Well FINE!!", you say, "I'll turn off the engines and run silent like a submarine in a World War II movie. I'll be invisible." Unfortunately that won't work either. The life support for your crew emits enough heat to be detected at an exceedingly long range. The 285 Kelvin habitat module will stand out like a search-light against the three Kelvin background of outer space.
The maximum range a ship running silent with engines shut down can be detected with current technology is:
Rd = 13.4 * sqrt(A) * T2
- Rd = detection range (km)
- A = spacecraft projected area (m2 )
- T = surface temperature (Kelvin, room temperature is about 285-290 K)
If the ship is a convex shape, its projected area will be roughly one quarter of its surface area.
To keep the lifesystem in the spacecraft at levels where the crew can live, you probably want it above 273 K (where water freezes), and preferably at 285-290 K (room temperature).
Glancing at the above equation it is evident that the lower the spacecraft's temperature, the harder it is to detect. "Aha!" you say, "why not refrigerate the ship and radiate the heat from the side facing away from the enemy?"
Ken Burnside explains why not. To actively refrigerate, you need power. So you have to fire up the nuclear reactor. Suddenly you have a hot spot on your ship that is about 800 K, minimum, so you now have even more waste heat to dump.
This means a larger radiator surface to dump all the heat, which means more mass. Much more mass. It will be either a whopping two to three times the mass of your reactor or it will be so flimsy it will snap the moment you engage the thrusters. It is a bigger target, and now you have to start worrying about a hostile ship noticing that you occluded a star.
Dr. John Schilling had some more bad news for would be stealthers trying to radiate the heat from the side facing away from the enemy.
If you are actually trying to apply thrust, the upper equation comes into play, and they can see you all over the solar system. What's worse, they can measure the spectrum of your drive to estimate the thrust and use a telescope to observe your acceleration. Simple division will reveal the mass of your ship.
"Well fine!", you say, "I'll just burn once and drift silently"
But now you will be months in getting to your target. The extra time increases the chance that the enemy will spot you. It will be harder to keep your directional radiator aimed away from any enemy observers. And if you are spotted, so much of your ship mass will be radiators instead of weapons, so that the enemy ships will out-gun you by an obscene margin.
Not to mention the fact that once your initial burn is spotted, the enemy will be able to calculate your future position anytime in the future. They can set a computer controlled telescope to track your current calculated position, and will quickly spot any future course correction burns.
So much for being ambushed by a space pirate appearing out of nowhere. And everybody on a cruiser would know that the hostile bogey would be within combat range in two months, three days, five hours, and thirty-three minutes. You might as well take it easy and get your rest before the battle. You know the cliché: long stretches of boredom punctuated by brief moments of stark terror.
And to forestall your next question, decoys do not work particularly well either. More specifically, a decoy capable of fooling the enemy would wind up costing almost as much as a full ship.
Just to make sure that we are both on the same page here, I am talking about time frames of weeks to months. Such as found when a task force weeks or months away from their target, attempting to fool the enemey observers into thinking that your are a force of twenty warships, when you are actually a force of one warship and nineteen decoys.
I am not talking about time frames of a few seconds. Such as found when a combat spacecraft, with a hostile heat-seaking missile attempting to fly up its rear, dumps off a couple of decoy thermal flares hoping the missile will be confused.
First off, a decoy needs to emit a similar amount of radiation and heat as the ship it is pretending to be. This means each decoy needs a power source comparable in size to a full ship, the same goes for radiator area.
If the decoy and the real ship thrusts, it becomes worse. The exhaust plume has to be the same, which means both the decoy and the real ship has to have the same thrust. This means the decoy has to have the same mass as a real ship, or it will accelerate faster, thus giving itself away. If you down-rate the decoy's thrust, the dimness of the exhaust plume will give it away.
So if each decoy needs a spaceship sized engine in a spaceship sized hull with a spaceship sized mass isn't much of a decoy. Why not add weapons an make it an actual spaceship?
And you'd better add defenses as well. Otherwise the decoy is nothing more than an unusually expensive, unusually easy to destroy missile.
Isaac Kuo points out that all of this assumes that the decoy and the warship are using rocket propulsion. It does not apply if they are using solar sails, laser light sails, magnsails, or other non-rocket propulsion.
But I repeat: while it is more or less impossible to use decoys to fool distant observers, it may be possible to use something like decoys in a dog-fight to protect your ship from enemy short-range antiship missiles. In the latter case, you are not trying to make a fake image of your ship so much as you are trying to break the target lock the hostile missiles have on your ship's vulnerable posterior.
Dr. John Schilling discusses why the exhaust plume of a decoy will have to have the same thrust as a real ship:
When the enemy spots your ship by the exhaust plume, it not only knows that a ship is there, it also knows the ship's exhaust velocity, engine mass flow, engine power, thrust, acceleration, ship's mass and ship's course. Not only can it tell a warship from a cargo freighter with all that information, but it can also tell the class of warship, and maybe make a good stab at determining which particular member of that class it is.
In more detail: as mentioned above, propulsion system's exhaust velocity is revealed by the doppler shift in the emission lines, mass flow is revealed by the plume's luminosity, the thrust is exhaust velocity times mass flow, acceleration is revealed by watching how fast the plume origin changes position, ship's mass is thrust divided by acceleration, and ship's course is revealed by plotting the vector of the plume origin.
This means that painting the ship with camouflage in an attempt to disguise its identity is pretty pointless.
Remember the light-speed lag. Light moves quickly, but not at infinite speed. It takes about eight minutes to travel one astronomical unit. So if you are in orbit around Terra and you observe a spacecraft near the Sun with a telescope or radar, you are actually are seeing where the ship was eight minutes ago. By the same token, if you change course it will be eight minutes until the Sun-grazer ship will know.
In C.J. Cherryh's Company Wars universe, ships use both radar and something called Longscan for detection and tactical information. Longscan helps cope with the lightspeed lag of radar.
James Huff is experimenting with plotting something similar to a Longscan display. He is trying to make a "probability plot" of where to aim your guns, given the target's acceleration, maneuvers, and lightspeed lag due to the range to the target. Mr. Huff generated these plots with a custom C++ program he wrote for generating iterated function systems.