Sub-light Speed in Star Trek

Is Impulse Drive Possible?

ion thruster test
NASA's 2.3 kW NSTAR ion thruster engine being tested at JPL. It was used on the Deep Space 1 vehicle. While this does not provide impulse drive, it is the next step in long-distance propulsion within the solar system. NASA

Are you a Trekkie? Anxiously awaiting the new series, the next movie, playing the games, reading the comics and books, and re-enjoying the older series and videos?  If so, you know that in Star Trek, humans are part of an intergalactic federation of races. They all travel the galaxy exploring strange new worlds. They do this in ships equipped with Warp Drive. That propulsion system gets them across the galaxy in amazingly short times (months or years compared to the centuries it would take us at "merely" the speed of light).

 However, there's not always a reason to use warp drive, and  so, sometimes the ships use impulse power to go at sub-light speed.

What is Impulse Drive?

Today, we use chemical rockets to travel through space. However, they have several drawbacks. They require massive amounts of propellant (fuel) and are generally very large and heavy.

Impulse engines, like those depicted to exist on the starship Enterprise, take a slightly different approach to accelerate a spacecraft. Instead of using chemical reactions to move through space, they use a nuclear reactor (or something similar) to supply electricity to the engines.

The electricity powers large electromagnets that use the energy stored in the fields to propel the ship or, more likely, superheat plasma that is then collimated by strong magnetic fields and spit out the back of the craft to accelerate it forward. It all sounds very complex, and it is.

And, it is not impossible! Just difficult with current technology.

Effectively, impulse engines represent a step forward from current chemical-powered rockets. They don't go faster than the speed of light, but they're faster than anything we have today.

Technical Considerations of Impulse Drives

Impulse drives sound pretty good, right?

 Well, there are several problems problems with them, at least how they're used in science fiction:

  • Time Dilation: Any time a craft travels at relativistic speeds, concerns of time dilation arise. Namely, how does the timeline stay consistent when the craft are traveling at near-light speeds? Unfortunately there is no way around this. That's why impulse engines are often limited in science fiction to about 25% of the speed of light where relativistic effects would be minimal. Note: this is not a concern for warp drive technology as technically the craft is stationary relative to the space-time continuum and therefore would not experience relativistic effects.
  • Acceleration Effects: On science-fiction shows, the crew of a starship never seems affected by the craft's acceleration. If you've ever ridden a bus you know what it's like to try to stand up while the bus is accelerating. It would be the same on an accelerating spacecraft. Yet, it's never a problem in the shows, expecially when the acceleration rates are extremely high. One explanation is that the starships are fitted with acceleration compensators to correct for this effect. However, it never seems to work when the ship is hit by an enemy weapon. That's a huge problem in storytelling, never mind the actual physics. 

    Could We Someday Have Impulse Engines?

    Even with those problems, the question remains: could we build impulse drives someday? The basic premise is scientifically sound. However, there are some considerations.

    In the films, the starships are able to use their impulse engines to accelerate to a significant fraction of the speed of light. In order to achieve those speeds, the power generated by the impulse engines has to be significant. That's a huge hurdle. Currently, even with nuclear power, it seems unlikely that we could produce sufficient current to power such drives, especially for such large ships.

    Also, the shows often depict the impulse engines being used in planetary atmospheres and in regions of nebulous material. However, every design of impulse-like drives relies on their operation in a vacuum.

    As soon as the starship enters a region of high particle density (like an atmosphere), the engines would be rendered useless.

    So, unless something changes (and ye canna change the laws o' physics, Captain!) on the surface things may not look promising. But, NOT impossible. 

    Ion Drives

    Ion drives, which use very similar concepts to impulse drive technology have been in use aboard spacecraft for years.

    However, due to their high energy use, they are not efficient at accelerating craft very efficiently. In fact, these engines are only used as primary propulsion systems on interplanetary craft. Meaning only probes traveling to other planets would carry ion engines.

    Since they need only a small amount of propellant to operate, ion engines operate continuously. So, while a chemical rocket may be quicker at getting a craft up to speed, it quickly runs out of fuel. Not so much with an ion drive (or future impulse drives). An ion drive will accelerate a craft for days, months, and years. It allows the space ship to reach a greater top speed, and that's important for trekking across the solar system.

    It's still not an impulse engine. Ion drive technology is certainly an application of impulse drive technology, but it fails to match the readily available acceleration ability of the engines depicted in Star Trek and other media.

    Plasma Engines

    Future space travelers may get to use something even more promising: plasma drive technology. These engines use electricity to superheat plasma and then eject it out the back of the engine using powerful magnetic fields.

     They bear some similarity to ion drives in that they use so little propellant that they are able to operate for long periods of time, especially relative to traditional chemical rockets.

    However, they are much more powerful. They would be able to propel craft at such a high rate that a plasma powered rocket (using technology available today) could get a craft to Mars in little over a month. Compare this feat to the nearly six months it would take a traditionally powered craft. 

    Is it Star Trek levels of engineering? Not quite. But it is definitely a step in the right direction.

    And with further development, who knows? Maybe impulse drives like those depicted in movies will one day be a reality.

    Edited and updated by Carolyn Collins Petersen.