Following on from the answer to this question: What was the purpose of adding red backlights in USS Enterprise? I am curious about a new thing:

Impulse engines of Enterprise NX-01 glowed blue unlike impulse engines of later Enterprise versions (in which they glowed reddish). Why? Is there a canonical explanation of it?

  • 2
    My non-canonical guess is different fuel in the reactor.
    – Xantec
    Commented May 3, 2012 at 14:48
  • 2
    So that other federation starships can see when Travis has to slam on the brakes to avoid hitting a squirrel. Commented May 3, 2012 at 22:21
  • 1
    Blue is the colour of Phlebotinum. Commented May 4, 2012 at 6:39
  • Burning hotter and less efficiently than more advanced engines, maybe? Blue stars are hotter than red ones, after all. Commented May 17, 2015 at 4:35

5 Answers 5


As mentioned by Spock in Undiscovered Country:

                         Under impulse power she expends
                         fuel like any other ship. We call
                         it "Plasma" - I do not know the
                         Klingon name for it, but by any
                         name it is merely ionized gas.

The Impulse Drives are a combination of Fusion Reactor, Drive Coil, and a vectored nozzle for plasma exhaust. As those technologies shift and advance the properties of the plasma exhaust that provide the thrust will change. We know that the visible color of an object is related to the atoms that make up the object and the temperature of the object. As the impulse reactors are fusion reactors that use Deuterium a heavy isotope of hydrogen they change color along the light spectrum depending on temperature of the fusion reaction. Red indicates a cooler, less energetic reaction, blue is hotter. This is seen in astronomy where hotter stars give off blue light, and cooler stars give off red (baring color shifts due to relative motion).

The NX-01 was early technology, probably using a primitive drive coil and accelerator-generator technology. As thrust is the flow rate of exhaust and the speed of the exhaust, hotter more energetic exhaust will provide more thrust. So on the NX-01 they run the fusion reactor hotter than later ships to get more thrust out of them, hence the higher frequency color of the plasma. As Drive coil technology improves and the ship can get more thrust out of cooler plasma, the fusion reactors run cooler, hence the lower frequency red impulse drives. An a cooler fusion reaction is safer, more fuel efficient, and easier to control and maintain.

As for the Accelerator-Generator mentioned in the memory alpha article, I assume that is what they use to shunt plasma from the reactor to either the drive coil or the EPS conduits as needed. As far as I can tell they are not mentioned by this name outside of the tech manuals. It may be another name for the impulse Relays which relay power from the impulse reactors to critical systems.


So far as I know, it has not been directly addressed in canon.

Warp plasma can also burn green, as referenced in another SciFi.SE question.

Real plasma can't be ignited in the same sense as it's often used in Star Trek, but the accepted answer over on Physics.SE gives us this nice picture of plasma leftovers from a supernova explosion: enter image description here

Now, here's a slight run-down of the power systems on Federation Starfleet ships:

  • The Warp core generates power by way of antimatter reaction.
  • The Impulse drives (at least in TNG+) are powered by fusion reaction.
  • The Electro-Plasma System (EPS) relays plasma generated by those two reactions around the ship.
  • The majority of the power generated from the Warp core is shunted to the Warp drive.
  • The excess is relayed around the rest of the ship for use in lights, life support, and everything else.

I propose that it the EPS relays had a different layout in the NX-01 era. Take a look at the Warp cores from that time - the NX-01's was red, while Voyager's was blue - the exact reverse of their impulse drives:

enter image description here enter image description here

  • The question is floating around impulse drive and your answer is floating around warp drive. Impulse drive isn't powered by Warp plasma. Probably, it holds fusion reaction (not sure)..
    – user931
    Commented May 4, 2012 at 2:40
  • @SachinShekhar In the TNG/DS9/VOY era (and likely TOS, and maaaaybe ENT) Impulse drives work through a fusion reaction, and Warp drives through an antimatter reaction. Both bleed off excess power into the EPS relay, where it can be used in the rest of the ship. Because it's not mentioned in canon, I'm proposing that it's caused by the specific configuration of Warp drive, Impulse drive, and EPS relays, and that it changed at some point between ENT and TNG.
    – Izkata
    Commented May 4, 2012 at 4:11
  • @SachinShekhar Attempted to clean up the bullets in the answer
    – Izkata
    Commented May 4, 2012 at 4:16
  • 1
    TNG warp drive was blue, also. Don't recall what TOS had.
    – Iszi
    Commented May 4, 2012 at 16:04
  • @Iszi I thiiiink the area behind Engineering was some sort of amorphous combination of Warp core and Warp drive. It's been a while since I've watched TOS.
    – Izkata
    Commented May 5, 2012 at 1:51

According to Mr. Scott's Guide to the Enterprise, the refitted movie version of the original series Enterprise features a "deflection chrystal" on the top of the saucer section immediately in front of the impulse drive. This chrystal can be seen to be glowing blue from the outside in the movies and the warp intermix chamber extends up to it. Under normal situations where the "mains are online", the warp antimatter reaction feeds into the chrystal to also power the impulse drive. When the warp drive is offline, fusion reactors located adjacent to the impulse drive provide auxiliary power for the impulse drive and other systems.


It just depends on the type of plasma you use maybe the red plasma is more abundant or more efficient in the other ships times you see plasma is jut heated gas like neon or xenon light are usually red orange or blue so you see it is just different types of plasma.


I think this might have to do with the type of sources they were using...perhaps the blue impulse drive plasma is only found rarely while the red plasmas can be found more regularly...as the federation expanded and the number of ships grew bigger it became obvious that blue plasma was not going to be sustainable for mass production and naturally they swapped to red plasma impulse drives.

  • This seems very speculative.
    – Valorum
    Commented Sep 22, 2014 at 13:39
  • Speculative answers aren't inherently bad. If you could provide some source or quote that helped lead to your conclusions, it would really go a long way.
    – phantom42
    Commented Sep 22, 2014 at 14:00

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