We all want to reach the stars. But how realistic it for near-future humanity to actually reach them? I am defining near-future as within 50 years. That’s just the technology time; not the travel time. There is little realistic way to reach the nearest stars without a multi-decade travel time.
The nearest star is Proxima Centauri is about 4.2 light-years away. One light year is approximate 9.4 Trillion KM! So 4.2*times that is almost 40 trillion kilometers to Proxima Centauri. Using Voyager as a benchmark, it left the solar system doing ~61k km/h or approximately 16.985 km/s. Suppose we use some unspecified “new technology improvements” and increase the solar-system exit speed 100x ( two orders of magnitude). How long will it take such probe to reach the nearest star moving at 1698 km/s?
Well this Super-Voyager would travel this incredible distance per year: 53.6 Billion KM / yr. {16.985 km/s * 31.5M Seconds / Year * 100 technology-factor}. So with no more velocity being added in, (nor any subtracted out once we get to the star) then humanity would need 744! years of travel before a souped-up Voyager-esque spacecraft would get to another star. To give a little more context to those years, the cathedral Sagrada Família started construction in 1882 and is expected to be completed in a decade. Thus society would need to commit to something at least 5-times longer than a century spanning cathedral. A city in German has completed three decades of a 1300 year public art installation called the time pyramid. Both are examples of society committing resources to projects that they will never see realized.
So clearly we need a system that both accelerates during the flight-time in order to get a spaceship to another star in 100 time frame. If we are able to continuously accelerate during the trip, we might also have the ability to also decelerate when we get to the star.
I will not be covering warp drives as I am trying to keep this down to realistic levels even though the math works.
Using a Reaction Drive
A reaction drive is just another name for a normal rocket engine that you would normally associate with space travel. Pushing a bit of mass faster one direction results in a proportional amount of mass being pushed the opposite direction. More information linked here, but this Wikipedia image is easy to comprehend for visual thinkers:
Long story short (but very meta): In order to accelerate the ship, you need to accelerate the fuel to accelerate the fuel, you later need to accelerate the ship. So to accelerate the ship from 0 to 1 on the chart below you need a mass ratio of ~4:1. 0 to 2 ~8:1, 0 to 3 ~20:1.

Society has been able to accomplish human spaceflight with this geometric problem, but in order to realisticly travel to another star (with less than a few-hundred year life-time) we need something else.
So why a Reationless Drive?
First of all, I have come across the term “Propellentless Propulsion” as a synonym to Reactionless Drive. I have to admit I love the term. Catchy names and alliteration go a long way to selling an idea. For the sake of this blog I will keep using the original term.
A reactionless drive is a space propulsion device that does not require any fuel carried on board to be propelled. Proposals include solar sails, laser powered sails and other more exotic drives. So how close is humanity to creating a true reactionless drive that is capable of traveling interstellar distances? Not very close, but some engineering work has been accomplished (even in space).
Proposals using known Physics (or studied extensively)
Bussard ramjet:
This is one of the more popular proposals and has been studied for decades. This isn’t truly a reactionless drive, but achieves the same result. Instead of bringing the fuel with us, can we collect it in a magnetic scoop and burn it along the way? I won’t go into much detail, but instead point you the this article on Centauri Dreams. Short answer is ‘no’ that the vehicle would not be able to accelerate into the solar wind.
Photon Rocket:
This rocket is almost like a flashlight in space. Okay it is a little more complicated than that, however it is the most realistic proposal as it relies on no speculative science and uses established technology. One problem though is that the maximum speed that can be achieved right now (no fusion drives, just nuclear fission) is about 0.02% the speed of light or 60 km/s. Which is like a Jupiter assist at the edge of our solar system. Not enough for interstellar speed.
Laser Power and Solar Sails:
This concept is one of the most realistic proposals for interstellar flight. However, I was originally mistaken when I first thought about the idea. At first impression, a true solar sail would not reach interstellar speeds with any reasonable time frame (~100 years). Even if it did, it seemed like it would need another star of equal size to travel to if it wanted to stop. Laser powered craft could have accelerated it just enough to have it then decelerate by the continual pressure of the star, but the long deceleration time did not make the flight realistic. After researching it a little more, there were interesting proposals for reflecting the laser light back on the spacecraft using a mirror. The mirror would continue to accelerate, but it could slow down the spacecraft half way.
So that makes these proposals more most realistic for interstellar travel. Another check int heir box is the fact that they have been studied and tested in actual space. Everything from the Kepler Space Telescope to the IKAROS spacecraft to a future proposal used or plan to use solar radiation pressure to maneuver spacecraft.
The science works, the engineering has been tested, and it propels spacecraft! Take us to the STARS!
Electric Sail / Dipole Drive.
This proposal is currently in the development stage. From my research I have only read of small one satellite to use this proposal. “Space” is not empty. There are numerous charged particles flying around. These proposals use designs to propel the particles flying through space a certain direction and thus propel the spacecraft in the opposite direction.
Most of the information I am using comes from this excellent video by Dr. Robert Zubrin:
A second updated podcast (June 2020) is available here (~30 minutes in). Basically with a double-sail electric device we can propel the neutrons in space in one direction, and the electrons will be propelled in the opposite direction. However since the mass difference is almost 2 orders of magnitude (an electric is about 2% of a neutron) and interstellar space has a relative equal amount of both, a spacecraft can push against the interstellar medium. The double grids can be maintained at their proper spacing with spin alone, though I suspect it is possible that some force will be needed to maintain revolutions over the vast interstellar distances. There might be a method to deflect in such a way to impart spin (not sure how that would work without adding more mass to deform the grids.
Now the biggest roadblock is not the ability or the physics, its power. Dr. Zubrin mentions this one of the videos, but he considers beamed power to be a valid option. Still, even with a small mass and unlimited power from beaming, I needing to increase the speed and decrease the spaceship’s mass by at least 10x -100x each to achieve a ‘more realistic’ ~100 year performance.
Another Proposal using Unknown Physics:
EM Drive:
I have added this to let people know that yes sometimes engineering (aka reality) can inform pure science. Is it possible that engineering can create something without the science being confirmed prior? Yes absolutely. History had many inventions before science could explain how they worked. A few years ago this drive came up in the ‘news’. The initial research produced a very marginal effect. However this was not duplicated. In addition one claim was that any propulsive force measured was really just heat escaping from the drive, and is basically just an inefficient photon rocket at that point. The apparent thrust is far smaller than any testing margin of error. Thus even if it is real, it’s not going to propel us to the stars.
I have put in this speculative drive, to remind the reader that there are still unknown forces in nature. Intuition about the unknown unkowns could be shown in anomalies in thrust from some of our distant probes, and unexplained gains during flybys. Since I am writing in the hard sci-fi genre drives like this are too speculative for me to consider.
Conclusions:
So from the proposals I outlined above, I have hopefully shown why I chose the Electric Sail when traveling to another star. Even though I am taking great liberties with the performance in my upcoming book, I still have it taking over 100 years! And that is with a +50x performance over currently existing technology.