I haven't found any red backlight in Enterprise NX-01:
Enterprise NX-01

I have also not found it in initial USS Enterprise NCC-1701-A, but this image tells it had one red backlight (not sure its legitimate image or not):
USS Enterprise NCC-1701-A

I have found three red backlights in USS Enterprise NCC-1701-D:
USS Enterprise NCC-1701-D

Out of these three red backlights, two were turned off in initial intro of ST:TNG TV series:
USS Enterprise NCC-1701-D

USS Enterprise NCC-1701-E also had two red backlights:
USS Enterprise NCC-1701-E
USS Enterprise NCC-1701-E

Now, my question: NX-01 didn't have red backlights. Why did they put them on later versions? What was the purpose of red backlights on a starship which had nothing to do with traffic standards? On what conditions, they turned off red backlights (as displayed in initial intro of ST:TNG)?

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    I have got many new wallpapers for my desktop, thanks to you. :) – user11147 May 3 '12 at 13:57
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    uuuhhh ... shiny pictures, let's upvote :) – bitmask May 3 '12 at 14:08
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    NX-01 did have red backlights, but only when braking. – Reinstate Monica - Goodbye SE May 3 '12 at 14:40
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    @Wikis: ...and white ones when backing up? – Mason Wheeler May 3 '12 at 20:04
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    I'd love to see the light flashing on and off right before the Enterprise banks right... – Chris B. Behrens May 4 '12 at 21:44

They are the impulse engines. The fact that they weren't on the original is an omission, as they are huge plasma vents. It's noted elsewhere that the impulse engines of other ships glow blue.

  • Actually they are on the NCC-1701 in about the same place as the 1701-A, they just don't light up when the ship is in orbit, which is most of the time we see the rear of the ship. – Tyson of the Northwest Feb 10 '13 at 18:14

More specifically, the red "lights" on the more advanced ships are the impulse engines.

On the NX-01 they were positioned differently, and are blue. I would simply attribute this to more advanced technology. (Or possibly Starfleet becoming more "edgy" in the intervening hundred years ;)

enter image description here enter image description here

  • But, there wasn't any need of locating impulse engines in NX-01. Design of NX-01 is smart enough to tell those are impulse engines (unlike other ones on which impulse engines really look like backlight). – Harley Quinn May 4 '12 at 2:01
  • @SachinShekhar I have to disagree with that. Rocket exhaust usually has a slight red/orange coloration, but it's pronounced enough - and red is simply associated with heat - that I would expect most people to see the "red lights" as exhaust ports, if not the impulse drives themselves. – Izkata May 4 '12 at 2:14
  • Where did you find red/orange color in the linked photo? Also, remember... Yellow, blue and white come above red when it comes to represent such things. Plus, NX-01 design clearly generates depth perception in viewer's mind. See the blurring there too. It shows something hot is coming out. But, in other starships, it looks like there's a glass cover over the port. It looks clearly in question image of Enterprise E. – Harley Quinn May 4 '12 at 2:30
  • Also, radiating red light is cooler then radiating blue light. So the exhaust from the later Impulse drives is less energetic than the NX-01, possibly an indication that newer technology provides more thrust at lower temperature. – Tyson of the Northwest Feb 10 '13 at 18:13
  1. They are the Impulse Engines of the ship.

  2. The color of the Impulse Engines are related to the heat generated.

In simple terms the blue impulse color means that it is hotter, and the red on the more advanced starships means its cooler. Think of it like stars, cooler stars (Like our sun) have a red color while stars that burn much hotter glow a blueish white color. There's a sciency reason behind the relation of the color of heat.

So all the Impulse engines are is a Deuterium-powered fusion reactor, an accelerator generator, a driver coil, and a vectored thrust nozzle to direct the plasma. Now there are many more parts to the Impulse Engines, but those are the basic ones. The Fusion Reactor generates the plasma, the accelerator accelerates the plasmas speed, the driver coil simply helps propel the plasma even more, and the thrust nozzle directs the plasma out to propel the ship. On the NX the plasma vent is blue because it is hotter because it is less advanced technology, and runs hotter. Now, on the more advanced ships it's red because they've developed the technology to get better propulsion with cooler plasma. Benefits of that would be you wouldn't need as hefty of a cooling system, and you'd burn much less fuel (In this case Deuterium, a heavy Isotope of Hydrogen).

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    Welcome to SFFSE! Thanks for your detailed answer, but could you please provide some references to back up your claims? This would further your answer's credibility. – Often Right Feb 29 '16 at 0:46

its the impulse engines, which use a sort of fusion combustion as power, the reason on the NX-01 and other species craft have blue is the type of fuel used NX series used a sort of cold nuclear fusion, by the time of the Origonal series they have moved away from Nuclear to some other energy, the Romulans used a Plasma based fusion reaction so their impulse engines glowed green, the Ferengi have the color as yellow moving up into the orange spectrum the Klingons went from red to green and back to red again

  • This is a nice answer, and correct as it aligns with the others. However, you could make it a bit better if you edited it to include evidence (show quotes, out of universe material, etc.) to back up your main points: that they are the impulse engines and why they used to be blue. – TheLethalCarrot Dec 29 '20 at 11:45

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