Sub-light Speed in Star Trek: Can It Be Done?

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

Trekkies have helped define the science fiction universe, along with the technology that the Star Trek series, books, and movies promise. One of the most sought-after technologies from those shows is the warp drive. That propulsion system is used on the spaceships of many species in the Trekiverse to get across the galaxy in amazingly short times (months or years compared to the centuries it would take at "merely" the speed of light).

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

What is Impulse Drive?

Today, exploratory missions use chemical rockets to travel through space. However, those rockets 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.

That electricity supposedly 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. It's actually do-able, b ut not 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. It's probably only a matter of time before someone figures out how to build and deploy them.

 

Could We Someday Have Impulse Engines?

The good news about "someday", is that the basic premise of an impulse drive is scientifically sound. However, there are some issues to consider. 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. So, that's one problem to overcome.

Also, the shows often depict the impulse engines being used in planetary atmospheres and in nebulae, clouds of gas and dust. 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 or a cloud of gas and dust), the engines would be rendered useless. So, unless something changes (and ye canna change the laws o' physics, Captain!), impulse drives remain in the realm of science fiction.

Technical Challenges of Impulse Drives

Impulse drives sound pretty good, right? Well, there are a couple of problems with their use as outlined in science fiction.

One is 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 is 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. 

The other challenge for such engines is where they operate. They are most effective in a vacuum, but we often see them in Trek as they enter atmospheres or whip through clouds of gas and dust called nebulae. The engines as currently imagined wouldn't do well in such environments, so that's another issue that would have to be solved. 

Ion Drives

Not all is lost, however. 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 an interplanetary craft. That means only probes traveling to other planets would carry ion engines. There is an ion drive on the Dawn spacecraft, for example, which aimed at the dwarf planet Ceres. 

Since ion drives need only a small amount of propellant to operate, their 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 spaceship 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 the 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.

While we may not have futuristic drives yet, they could happen. 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.