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Air laden with sulfur, boiling seas of lava, acid rains on areas that aren’t molten and when you think you’re safe, you hear the screams of the damned coming after you. This is Hell.

Or is it?

High above the acid clouds, two forces battle it out in the sky.

One is a flying sword of steel that unleashes strange purple flames at its rear and unleashes bolts of light against its foe. The other is a hideous abomination truly worthy of this hell. Though it may not have wings, it flew through the air with the same grace as its opponent. It used fantasy-like powers to keep it aloft.

The sword of steel went on the offensive, it unleashed long flames in the shape of diamonds and shot upwards, even higher than the demon, into the air. The sword of steel aimed its tip downwards and shot down towards the demon at tremendous speed, unleashing a boom as it went.

The demon foresaw its impending doom and dove downwards as well. The two of them disappear from view, into the clouds. From an outsider’s point of view, only the strange whine of the sword and the occasional roar of the demon could be heard. Brrt, rods of light punched holes through the clouds and out came the demon followed closely behind by the sword.

To evade the sword, the demon flew even lower, below the clouds and over a sea of lava. The heat emanating from the lava made the air shimmer. The one piloting the “sword” wasn’t about to give up, he followed the demon even closer, as close to the lava as his engines would allow him to.

Firing the 20mm rotary cannons didn’t work, they somehow keep missing the demon, some shots hit but they ricocheted on some sort of invisible barrier. He only has one more missile. Unfortunately, it is a heat-seeking missile. One that may work on the demon, given his incredibly high body temperature. If they were in normal conditions, that is.

Unless?...

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    $\begingroup$ Creative question, you may want to check the myths of Vimanas. They were the Flying Chariots of the Gods as per Bhagavad gita. Maybe your pilots aren't the first to storm Hell. $\endgroup$ – Gustavo Apr 2 at 13:11
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It depends on the missile's capabilities and on the demon's physiology. However, if the missile is capable of detecting any heat gradient, and the demon's biology is roughly restricted by the laws of physics (that is, based on some kind of chemistry and incapable of breaking the second law of thermodynamics), it should be able to.

The reason is as follows: In order for any creature to function, its body temperature must be at a constant level. Depending on its chemical makeup, this body temperature may be higher than that of Earthly life, but it will die if it gets too cold or too hot.

Now, it stands to reason that demons are warm-blooded. Cold-blooded animals can only function well under fairly specific temperatures, but this demon is capable of fighting both in and out of Hell. So it probably has some mechanism for controlling its body temperature regardless of the surroundings.

If the demon's normal body temperature is higher than Hell's ambient temperature, the demon will use some kind of mechanism to keep its temperature high (like shivering). This means the heat-seeking missile can track it.

If the demon's normal body temperature is lower than Hell's ambient temperature, the demon must use some kind of mechanism to move heat away from its body. Earth animals use panting or sweating. Demons probably use a more efficient method. But the thing is that the heat has to go somewhere, and the mechanism used to move the heat around must generate heat, because of the second law of thermodynamics. Think about the part of an air conditioner that sticks outside - if it able to cool off the air inside, the outside will always be hotter than the ambient outside temperature.

This means that the heat radiating from the demon must always be higher than the surrounding temperature, even if the demon's own body temperature is lower than its surroundings. That means a heat-seeking missile can track it.

Note that this only applies to the demon's temperature relative to the surrounding air. If there are smaller sources of heat that are hotter than the medium through which the demon flies, like lava pools, the missile may get confused and target them instead.

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    $\begingroup$ I'd like to add, that with modern heat seeking missiles like the AIM-9X or IRIS-T, even the lava pools probably would distract the seeker, due to their locking onto a specific IR-image $\endgroup$ – OneSaltyAceTanker Apr 2 at 10:04
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    $\begingroup$ the heat radiating from the demon must always be higher than the surrounding temperature - this doesn't even makes sense, thermodynamically speaking. The way Earth animals keep their temperature lower is by forcefully evaporating water in sweat, iff that's possible at all - by using the mechanical work of air currents. In which case both the evaporated water and the body temperature are lower than the ambient. (to be ctnd) $\endgroup$ – Adrian Colomitchi Apr 2 at 10:25
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    $\begingroup$ (ctnd) The only way to lose energy against the heat gradient without additional work (the case of using a heat pump) is if your body has an energy level distribution that does not follow the thermodynamics - this happens in lasers, on which higher energy states are in higher number than the lower energy states - the "population inversion"; from the point of view of thermodynamics, lasing media have a negative temperature $\endgroup$ – Adrian Colomitchi Apr 2 at 10:31
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    $\begingroup$ 2nd to last paragraph seems wrong to me. If it maintains a lower body temperature, then it is not constantly shedding heat as though all of its molecules were simultaneously and instantly trying to return to ambient every single second. I also don't buy that a demon would be warm blooded. If there's so much heat around you, why not use that? From an evolutionary perspective, it makes perfect sense that a demon would be less suited to fighting outside of hell. And aren't demons stronger in hell? Cold blooded-ness seems like a good explanation for why that is. $\endgroup$ – Harabeck Apr 2 at 13:59
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    $\begingroup$ @OneSaltyAceTanker Those ancient missiles? :-) . It's not all that hard to lock onto a specific spectral signature rather than a more general integrated range of IR. You just need a decent grating, or even different IR-bandpass filters on each pixel. $\endgroup$ – Carl Witthoft Apr 2 at 17:33
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It depends.

A very simple heat seeking missile will just chase any hot spot its IR sensor will detect. It can be then easily fooled by spurious hot spots, like fires, flares and the like. In an environment like hell, unless the target is way hotter than hell, its sensor will probably be saturated and won't detect any specific target.

If it instead has more technology on top of the "find a hot spot and point", it might still detect the intended target, as it will be increasingly difficult to fool all of the them in the same moment.

I.e. some doppler sensors to detect also relative motion (a moving target will almost invariably have a velocity different than the background), or some shape recognition routine.

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