How Fusion Power Will Give Us the Keys to the Solar System
Since before interplanetary space travel was even conceivably possible within the bounds of human technology, we have dreamed of crisscrossing the Solar System in the search for new scientific knowledge, new avenues of wealth and technology, or even new homes for the huddled masses of humanity yearning to break free from the atmosphere of our native world and seek new horizons amid the far-flung worlds of our native star.
For most of the last century, this vision has swam in the minds of great thinkers and fortune tellers, of great captains of industry and great scientific minds alike. But also for much of the last century, this vision has remained solely that: a possible future, a pipe dream, locked beyond the intervening walls of insufficient technology, insufficient will, and above all else, high risk.
For what is the primary risk to the human exploration of the Solar System, and its eventual development into an interplanetary hub of civilization? The answer is simple: the risk held within this promise, is death.
With these ultimate stakes hanging over our heads as we begin to test the waters of Carl Sagan’s “cosmic ocean”, first by venturing into the wading pool of the Sol System itself, we must seek out ways to mitigate this risk to life and limb, to reduce the likelihood that interplanetary and intra-system travel will be fatal to those who embark upon it. For if we cannot, it is likely that the simple calculus of risk over reward will maintain the present status quo, in which the other worlds of our system — and yes, even the worlds of other stars altogether — remain locked beyond our reach for all time.
The first and perhaps most lethal danger to overcome is space radiation, which specialists have debated since before the astronaut program even got off the ground, so to speak. Simply put, the longer humans travel in space, the more cosmic rays and solar radiation will have to be accounted for.
Now, there are many proposed methods to deal with this risk, but perhaps the most simple way to reduce it is to bypass the factor which makes the risk of radiation exposure so great in the first place. Simply put, if we are able to shorten the length of trips between the worlds (which would be on the scale of several months from Earth to Mars, and several years from Earth to the Jovian system), the risk of radiation exposure drops commensurately.
So how do we reduce the travel time between worlds? The answer is simple: make our ships go faster. To do so, we will need to access new technologies, many of which are still under development, and some of which may be so for some years. Before we cover those technologies in depth, however, a brief primer on the current status of interplanetary spaceflight is in order.
Space Travel — A Brief Summary
In the present day, most people would date the beginning of the Space Age to early October of 1957, when the former Soviet Union launched Sputnik I. In the sixty-four years since, the rate of advancement in the space sector has been nothing short of explosive, sometimes even literally so.
From early successes and setbacks in launching satellites and pioneering missions to nearby worlds in the Solar System, through the heady days of Apollo and the Shuttle program, and on to contemporary successes in the privately-run space industry, the record speaks for itself, and points to a continued upward slope of human achievement in space.
With this in mind, however, we must consider some of the setbacks, trials, and tribulations with which we have been met along the road to our current upward trajectory, for they have certainly been as frequent as the milestones.
From the very earliest days of human space travel, the risks have always been high, always testing our resolve to seek out the rewards. Between the first manned spaceflight — carried out by Yuri Gagarin in April of 1961 — and the present day, around 30 astronauts and cosmonauts are known to have died in various stages of the preparation to enter space, with three of them having died beyond the Kármán line (the nonphysical boundary, roughly 60 miles up, at which the atmosphere of Earth is generally held to give way to space).
So far, however, the jury is still out on just how much danger contemporary spacefarers face in terms of space radiation. That being said, the risks are clear: according to NASA-provided information on the subject, the average astronaut serving in a contemporary or near-future space mission may face anything from 50 to 2000 milli-Sievert (mSv) of radiation during their trip. This is equal to about 150–6000 chest x-rays, a dosage which, received on a relatively truncated timescale such as a roughly year-long space mission, is certainly enough to increase lifetime cancer risks down the line.
With this in mind, and considering that long-term space missions (such as those which may take human scientists, explorers, and other pioneers to Mars in the not-too-distant future) would also face substantial danger simply from the risk of engineering faults during the flight or time at the destination, possible breakdowns of control caused by unforeseen human error or mental health crises, and so on, it is crucial that the overall time scale of space missions be reduced in order to commensurately reduce the risks those missions face.
Why Long Missions are Long
As the image above clearly shows, modern spaceflight is neither quick nor direct. For the Mars 2020/Perseverance rover mission to arrive at Mars took just over seven Earth-standard months (from 14 August 2020 to 18 February 2021, to be exact), and although prior missions have varied from anywhere between 128 and 330 Earth days to reach the Red Planet, an average estimate of about six to nine months is usually held as the standard.
The reason trips take so relatively long, compared even to sailing ship travel in the previous Age of Exploration here on Earth, is the same reason why trips between Earth and Mars — or Earth and any other body in the Solar System, with the possible exception of our moon — is not a direct process.
Simply by looking again at the above image, we can see that the vessel carrying Perseverance had to almost literally chase Mars in its trajectory, and only by traveling faster than Mars’ own orbital speed was it able to catch up and enter an insertion orbit to deposit its cargo. I will brush over most of the heady science of orbital dynamics in this article, but the basis premise is obvious: planets, or any other body in orbit around the sun, or indeed the center of the Milky Way, are never quite at rest relative to one another.
While the implications of this fact are numerous and in some cases almost literally Earth-shaking, the main detail (the one I will focus on, anyway) is that the dynamics of planetary orbits around the sun, coupled with the limited power of contemporary chemical-powered rocket engines, limits our ability to reach other Solar System bodies in a timely manner, relative to the human need to reduce exposure to harmful space radiation, that is.
With this in mind, how can we go about making a trip of six or seven months, during which time the astronauts tasked with completing a milestone first manned mission to Mars would likely be assaulted with enough cosmic and Solar radiation to dangerously increase their lifetime cancer risk in the long-term and possibly even present severe mental and physical health risks in the short-term, into a trip of only a few weeks — or even days?
This may seem at first glance to be a preposterous proposition, and one which nothing short of purely miraculous technological development could attain. The truth, as it often is, is much more dramatic than even this seemingly straightforward assessment — for the technology of which I speak is, in fact, nothing short of miraculous. It’s the same technology which will someday power our society, drive our cars, provide energy for our day-to-day life, and yes, likely even provide the means with which we will reach out and touch the countless other worlds of our home star system — and possibly those beyond. It is also the same power which drives the core of our Sun, and provides us with all the light and heat that power every other aspect of our lives. The name for this power, the power that makes the stars go, is Fusion.
How Fusion Power Will Make Long Trips Short
Before getting into the details of how fusion travel will actually open up the rest of the Solar System to human exploration, it should be noted that this article is not intended to be a primer on fusion power itself. This subject has been written on and covered extensively by much more passionate researchers and journalists, and the subject matter continues to develop as new breakthroughs occur on the road to its full-scale implementation.
What we will be focusing on, rather, is the ways in which this near-future technology may very well advance the cause of space exploration into a new age, perhaps even a true Second Age of Exploration — one in which the many worlds of the Solar System are the uncharted new lands waiting to be seen and landed upon by previously Earthbound travelers.
The basic answer to the question of “how will this happen?” is really quite straightforward. Simply put, with fusion power at our disposal, the days of powerful but relatively slow chemical rocket engines are all but over. While such engines may well remain useful for a long time to come in the area of surface-to-space launches of material cargo and personnel, the truth is that fusion rockets allow for much longer specific impulse than any chemical rocket we can even conceive of with our contemporary technology.
What is specific impulse? Simply put, it is the efficiency a propulsion system (in this case, a rocket engine) can provide a certain delta-V, which is the unit that designates how much thrust that engine can provide. For a classic example the Rocketdyne-made F-1 engines produced for the Apollo-era Saturn V first stage boosters had a specific impulse of approximately 263 seconds at sea-level. This meant that, upon launching from Cape Canaveral in the late 1960s and early 1970s, they could burn at maximum thrust for nearly four and a half minutes before shutting down.
This in turn was what allowed the Saturn Vs carrying the Apollo astronauts to leave the Earth’s gravitational pull behind and enter a transit orbit to insert for the lunar landings. It is also among the primary factors that made the Saturn V’s F-1 engines the most powerful single combustion chamber rocket engines in the world — a record they still hold to this day.
But let us contextualize this factoid: while the F-1 engines were certainly powerful, and powerful enough to launch the lunar modules into an orbit that made the moon landings possible in the first place, how powerful were they really? Because in space travel, simply having muscle under your proverbial hood is not the only way to get the job done. For another example, the Dawn mission to asteroids Ceres and Vesta, launched in September of 2007 and ended in November of 2018, took a more circuitous route to reach its two rocky destinations even than the Perseverance mission took to reach Mars.
This is because Dawn used a different method of spaceborne propulsion than either the Apollo missions or the Perseverance mission. Whereas both of those flights relied on more traditional chemical propulsion, launching from Earth’s surface on high-powered but low-specific impulse rocket engines, launching with almost all their power used in the very first minutes of flight, Dawn flew in space with a revolutionary type of engine: Ion propulsion.
While traditionally a factor in science fiction stories such as the Star Wars films, ion propulsion is in fact very much real. The simplified summary is that, by using a very small electrical charge, a spacecraft utilizing this method of propulsion can propel a stream of ionized particles out of its thrust nozzle, giving a small but critically very sustained period of forward thrust.
This means that, while an ion drive-propelled spacecraft may only accrue a tiny fraction of total thrust per unit of time, as compared to the F-1 engine for example (which reaches maximum speed in seconds and stays there only a few minutes), it is able to maintain this thrust for a very long time indeed: in the case of the Dawn spacecraft, NASA data indicates it was under powered spaceflight not for mere minutes, hours, or even days — rather, it remained under powered flight for roughly six years, or more than 50% of its total mission length. This result means that, despite the low thrust provided per unit of time, as the thrust provided over the total history of the mission was extreme — for another example from the NASA data mentioned above, it reached a maximum of 25,700 mph in its journey, a solid 2.7 times faster than any prior mission powered by solar-electric propulsion.
What does this mean, though? It may seem to indicate that space travel propulsion systems have only two real options: high thrust power but low specific impulse; or high specific impulse but low individual thrust. The reason it seems this way, to be frank, is that for the entire history of space travel up to this point, it has been. Space missions must balance the options between a high-powered launch that then cruises under little or no power on a complex orbital trajectory to intercept its destination, or a long, slow buildup of thrust over months and perhaps years. Either way, we run into the same problem: long-term human spaceflight is frankly dangerous to the humans in question.
But there is a way around this, and its the same thing mentioned above that would make fast space travel possible in the first place: fusion.
The gist of this solution to the travel time problem is that fusion would allow scientists and engineers to build spacecraft capable of striking much more of a happy medium between thrust and specific impulse. The way this type of propulsion works is by heating energy sources such as deuterium or helium-3 to the point at which they become plasma.
If you’ve heard of plasma before, it was probably in relation to it being pointed out as the actual fourth physical state of matter, a state at which matter becomes little more than a burning mass of energized particles.
But why is this beneficial to spaceflight? Well, the answer is again deceptively simple: fusion rockets would allow spacecraft to reach moderately high thrust and maintain relatively high specific impulse while fulfilling the third major necessity of any spacecraft propulsion system — it would bypass the need for the weight of the spacecraft in question to be made up mostly of the fuel. The reason that is even a concern is called the Tsiolkovsky or “rocket” equation.
Simply put, the Tsiolkovsky equation (named after the Russian scientist who discovered it in the early 1900s) is the method by which the final thrust of a spacecraft can be determined, based upon its possible weight and the ratio of that total weight to the amount of said weight comprised of fuel.
The simplified explanation for this relationship is that, given a certain total weight for your payload (i.e. your spacecraft and any cargo/passengers it carries), a certain other amount of the total weight for the spacecraft itself will need to be comprised of fuel. The classic explanation for this is that, if you want to propel a spacecraft, you need to not only propel the weight of the spacecraft but also its fuel — as well as the fuel needed to propel that weight, and so on. Therefore, the heavier a spacecraft an its payload are, not only is more fuel weight needed to push that weight around, but more fuel is needed to push the weight of that fuel, and so on, in an exponential growth situation that swiftly adds up to monumental costs in terms of weight.
With a fusion-driven rocket, however, the central problem of that equation is all but removed: in short, fusion-powered engines require less fuel to make them go, meaning that the “tyranny of the Rocket Equation” can be more or less bypassed. Therefore, once fusion energy can be harnessed, the technology of fusion rocket engines will allow for moderate-to-high thrust, moderate-to-high specific impulse, and moderate-to-high fuel economy.
As such, a spacecraft propelled by this type of engine system could create a high enough combination of thrust and specific impulse to allow for much faster interplanetary transit orbits. For example, the Ad Astra company, known for its continual development of the VASIMR rocket design (“Variable Specific Impulse Magnetoplasma Rocket”; pictured above), touts that its design could one day reduce the length of an Earth-to-Mars trip from six months to only around forty days. While this is clearly remarkable in and of itself, it isn’t the only trick that fusion engines have up their sleeve.
The Realities of the Brachistochrone Burn
The short answer to “how can we reduce spaceflight times between the worlds from months to day?” is the Brachistochrone trajectory. Contemporary science fiction programs such as The Expanse have popularized this type of travel, which is referred to as “flip-and-burn” in their parlance. While the realities of such a method of getting from world A to world B are of course more complicated than this, they are still remarkably straightforward.
This is in fact the real meat and potatoes of our discussion, the bit all the information thus far has strove to set up. Simply put, with a powerful and efficient enough fusion drive, one might be able to utilize what is referred to in mathematics as the Brachistochrone curve — namely the method by which you might move from said world A to world B in the least amount of time.
The reason this works (or could work in future) is that given a spacecraft with enough fuel economy and a powerful enough engine, you could do what is called “constant acceleration” travel. In reality, the spacecraft would only be accelerating for half of the trip in this scenario (as seen above) and decelerating for the other half, as obviously you must slow down to come into a standard orbit around your destination if you actually want to do anything there but wave at the world or space station in question as you zip on by.
One of the secondary benefits of constant acceleration travel is that it could be the key means by which the microgravity problem inherent in space travel is solved. For a primer, clear data from NASA indicates that spending extended periods in the microgravity of outer space is harmful to the human body in a number of ways, including but not limited to causing bone loss, muscle wasting, eye and vision problems, and more. We can see the evidence of this just by looking at ISS astro- and cosmonauts when they return to the Earth — they are weakened by their trips in space to such a degree that many have trouble even standing up upon emerging from their descent capsules.
With constant acceleration, however, we may be able to provide a steady force of downward gravity through the forward thrust of the spacecraft itself, similar to how going up in a fast elevator makes you feel like you’re being pushed into the floor. Likewise, the deceleration in the second half of the journey would provide an equal but opposite force of faux-gravity.
With all this in mind, we come to the math section. Now, I won’t bore you with the methods by which this equation was reached, but the gist of the thing is that you can calculate the travel time between two planets under the Brachistochrone or “straight line” burn using the following equation:
[Time = 2 * SQRT (Distance / Acceleration)]
With this equation, we can calculate at least a rough estimate of how long it would take to reach Mars from Earth using the Brachistochrone trajectory. By plugging in the current distance between the worlds (about 393.25 million kilometers as of this writing) and the acceleration we would be using (in this case, either Earth-standard gravity of 9.81 m/s² or the gravity on Mars, 3.72 m/s² could be used), the resulting travel time is somewhere in the neighborhood of one week — a far cry from the six to nine months of the average Earth-Mars spaceflight using current chemical rockets.
From there, the whole rest of the Solar System is more or less opened up to us: Jupiter, for instance, which took the Voyager probes (themselves among the fastest man-made objects to ever fly) around two years to reach, and even then only on a fast flyby trajectory, could also be reached within the span of about two weeks using this style of travel, even when compensating for the fact that Jupiter would be a moving target as the spacecraft headed towards it.
This is all of course glossing over some rather glaring problems with the concept (the foremost of which being that the requisite technology does not yet exist), including but not limited to radiating the huge amounts of heat that a suitably powerful fusion drive would create (and yes, that IS a problem in space, too) and managing the immense power of the propellant exhaust, referred to by noted sci-fi enthusiast Winchell Chung (known for helping writers in the genre to keep “science” in their “fiction”) as being strong enough to “evaporate Rhode Island” in some speculative cases.
But with these problems in mind before the aforementioned technologies even take off, either literally or figuratively, we can formulate plans to mitigate them. The potential rewards, after all, far outweigh the risks.
Conclusion: “What Then May We Make of this Solar System?”
So, now that we have covered the basics (at some length, that is), what are we to make of the possible future that fusion-driven space travel could open up to us? Well, first and foremost, we must realize that such a future is not necessarily a distant one — according to one Space.com article, it could even arrive as early as the end of the current decade.
While some might argue that this is a gross overestimation of our current level of progress on the overall fusion attainment problem (especially given the old line about how fusion travel has been “just 20 years off” for the last sixty years), it is easy to contend that the current state of technological development is in keeping less with a steady, easily predicted series of milestones, and more akin to a rapid upswing of start-and-stop black swan events that can affect everything from global financial markets to the course of history itself.
For examples of this fact, we need only look at history: perhaps the greatest example of this trend is the rapid emergence and explosion of the Information Age, as first computers, then the Internet, then the World Wide Web, and more contemporarily global media and online marketplaces, and everything that goes with them. Although this series of events may seem obvious in hindsight, at the time they occurred, each step in the trajectory of the rising Communication Age seemed a chance happening. In any case, the outcome is clear: the dawn of this new age brought about swift and sometimes dangerous changes to human culture, economics, and civilization as a whole.
So if or rather when the interplanetary travel revolution happens, and assuming that fusion-powered spacecraft play the key role many believe they will, what does that mean for humanity? Where will our children and their children find themselves fifty to a hundred years from now, provided that the dawn of the Fusion Age arrives in the next decade or two as so many predict it will, and that this new age opens the door to interplanetary trade?
Honestly, no one knows. I haven’t come here to extrapolate the possible futures awaiting us as we begin to wade further out into the cosmic ocean, for many others have done so in far grander ways than any simple prose of mine could attempt. Suffice to say that this future, or any potential future that may emerge as a result of fusion-powered space travel opening the metaphorical door to full-scale human exploration — and settlement — of the Solar System at large, is more wondrous and awe-inspiring even than some o the best authors and futurists of the last hundred years have been able to do justice.
But one thing should be clear, both from the headiest speculations of fiction writers and the most thought-provoking science coming out of groups like NASA and its various associated institutions, and that is this: some day, likely much sooner than later, humans from Earth will set foot on Mars, on the rocky bodies of the Asteroid Belt and the icy moons of Jupiter, Saturn, and the other gas giants. When they do, they will likely have travelled to their destination on a ship propelled by some form of fusion energy.
From those new distant vantage points, spread far and wide across this Solar System (and yes, possibly even those beyond), they will surely look back at where Earth hangs in the sky, wondering at its distant, pristine fragility, and marvel at how a people who once scurried in the trees and roamed the scrubland of the East African rift valley could have come so very far indeed.
[Note: all credit for artworks and resources goes to their original creators]
