The Interstellar Ramjet Conundrum
And a possible solution…
As a writer with the goal of bridging the wonders of science fiction with the realities of science, I am often asked whether or not certain aspects of peoples’ favorite sci-fi TV shows or movies hold true to the torch of real science. One such example is the popular sci-fi show The Expanse, which readily flaunts its creative adherence to physics and science on screen with its representations of zero-G, its industrial ship designs, and of course, its futuristic depiction of space travel. When I explain to people that I am not a huge fan of the show, they usually recoil in disappointment, confused how someone with my interests could not be in love with something so “scientifically realistic”. While the show does depict several elements of space travel and technology in a relatively authentic light, science fiction as a genre involves a lot more than just wowing the audience with the surprising realities of science and mathematics. A discussion of what exactly The Expanse lacks for me could constitute an entirely separate article, but the technology depicted in the show does tie in nicely with the topic of this article.
The Expanse is unique in one sense, that it is the first popular sci-fi show to accurately depict travel in space between two distant cosmic bodies. In space, a ship requires not only to accelerate towards its destination, but also to slow down upon approach. Today, our spacecraft are restricted to relatively small thrusts and velocity changes, relying on the natural physics of orbital mechanics to carry our spacecrafts to their destinations along what are known as transfer orbits. The Expanse imagines a future where space propulsion has advanced to the point that ships can simply aim towards their destinations and initiate a continuous burn of fuel, accelerating at a precise rate to generate artificial gravity on board the ship. At the halfway point, the ship simply spins around and again activates her thrusters in order to decelerate towards the target, maintaining artificial gravity for the duration of the voyage. Because the velocities involved would be so enormous compared to anything we have launched to date, the trajectory of such a spacecraft would hardly be warped by the gravitational pulls of the planets, or even the Sun, allowing trip times between the Earth and Mars or Jupiter in a matter of days, instead of months or years.
The idea is logical on paper. Seemingly all you would need is a powerful enough engine and you could travel to any planet in the solar system and back in less than a calendar month. The problem, however, is fuel. Let’s imagine a vessel with a mass of the space shuttle flying a crew of 7 to Mars utilizing a constant acceleration thruster. The vessel would require to accelerate halfway to Mars, swivel around, decelerate the remainder of the distance, and then mirror this maneuver on its way home. At its closest pass, Mars is just 0.52 AU away from the Earth. This would allow for a rapid fly-by to the planet and back in just a little over 4 days, achieving a peak velocity of 875 kilometers per second; a staggering 15 times the current velocity of the New Horizons probe (which, by the way, has no intentions of stopping, let alone coming back!) Even with advanced fusion rockets (estimated to yield optimal exhaust velocities of ~700 km/s), the ship would require 147 times more fuel than the mass of the shuttle itself. So much for maneuverability in space dogfights…
In 1960, physicist Robert Bussard postulated a potential solution to this. He noted that even “empty” interstellar space is filled with primordial atomic hydrogen, and that, due to its charged nature, it could be captured by a spacecraft which could generate a powerful enough magnetic field. Once acquired, this hydrogen could be used to fuel a hydrogen fusion propulsion system to accelerate through space. Because no fuel is actually carried on-board the spacecraft along the journey, the vessel only needs to generate enough thrust to accelerate its own mass, as opposed to a traditional rocket which requires exponentially more fuel the farther you want to travel. The idea inherited its creator’s name, and the Bussard Ramjet became a staple in science fiction, as in Poul Anderson’s novel Tau Zero.
The theory was tantalizing; because of the time-dilating effects of relativity close to the speed of light, such a spacecraft could utilize this technology to essentially explore the entire observable universe in human timescales. A passenger on board a 1-g continuously accelerating spacecraft could make a round trip to Neptune in 16 days, visit our nearest celestial neighbor and come back in 8 years, and take a 45 year vacation to the center of the Milky Way. Since all of interstellar space is filled with atoms of hydrogen, the universe itself would provide all of the fuel a Bussard Ramjet would ever need to travel anywhere within it. At first glance, the opportunities seem limitless…
But the Bussard Ramjet suffers from one fatal flaw that no other space propulsion system experiences. Traditional rockets can travel at speeds that are multiple times their own exhaust velocities because the exhausted fuel exerts a force on the spacecraft regardless of the spacecraft’s current speed. To conceptualize this, imagine you are in orbit around the Earth travelling at 7.5 km/s, and you toss a bowling ball (your fuel) away from you in the direction opposite to your orbit. If you could throw it pretty fast, say 10 meters per second, your orbital velocity may increase by 1 meter per second. Though this seems insignificant, it is important to note that you have still increased your orbital velocity, despite you not being able to throw the ball faster than 7.5 km/s. A typical rocket can do the same.
A Bussard Ramjet, however, is different. At first, continuous acceleration is achievable for a Bussard Ramjet, collecting slow moving hydrogen atoms and ejecting them out of its thruster at high speeds. However, as the vessel accelerates, the velocity of the incoming hydrogen atoms being collected begins to approach the velocity of the propellant being ejected by the thruster. This confines the ship’s maximum velocity to that of its own propulsion system. A good analogy for this conundrum is an airboat, which uses a fan to force air backwards in order to propel the craft forward. An airboat can never travel faster than the airspeed generated by its own fan because the fan would start to produce drag if it could not keep up with the velocity of the air encountering it. This effectively stunts its maximum speed to the same as the exit velocity of the air from the fan itself (ignoring air resistance on the boat).
As was previously discussed, even an optimistic fusion rocket would only generate exhaust speeds up to about 700 km/s (0.23% of the speed of light). This would restrict a one-way trip to Proxima Centauri to a blistering 1,800 years. But before we start toppling our chalkboards and snapping our pencils, there may be a way in which to overcome the Interstellar Ramjet Conundrum. If we had the technology and energy available to create a magnetic field large enough to capture millions of square kilometers of interstellar hydrogen, why not simply use some energy to speed it up as well? Instead of using a traditional propulsion system, which requires the fuel to be burned or fused in order to generate a thrust, an interstellar ramjet could simply add extra energy directly to the incoming stream of hydrogen atoms using a powerful array of lasers or microwaves. If tuned to the absorbance frequencies of hydrogen, this array could potentially increase the velocity of the interstellar atoms to within a fraction of the speed of light. The vessel’s speed would still be capped by its maximum exhaust velocity, but this limit would be much, much greater.
Unfortunately, the Sun is currently travelling through a lower density region of interstellar hydrogen, 10 times more tenuous than the Milky Way’s average. This “Local Bubble” was likely created by a violent supernova thousands of years ago, which generated a bow shock that pushed all of the interstellar gas and dust away from our region of space. This means that any future attempts at interstellar travel involving a Bussard Ramjet design will require an energy source at least 10 times as powerful as we would need elsewhere in the galaxy. Still, the benefits reaped by a propulsion system that doesn’t need to transport its own fuel shouldn’t be ignored as a possibility. With our current understandings of physics and science, the Bussard Ramjet may yet be the most practical means of exploring the stars that we have conjured to date.
An engineering project such as the Bussard Ramjet should not be undertaken lightly. There is still so much we have yet to understand about space travel and the universe before this technology can ever become tangible. Still, humanity has shown a particular resilience to the hurdles within engineering and science in the past, and that drive for understanding has only increased with our level of knowledge. Nothing, however, encourages us more to reach for the unknown than the wonder of what may lie beyond. Science and science-fiction share a symbiotic relationship. Each one encourages the other to stretch the limits of our logic and imagination, so that we may develop solutions to problems that we may not have otherwise seen. Space is a nearly infinite domain. If in our endeavor to explore it, science begins to restrict us more than we had hoped, it may take a little bit of fiction to remind us of how wonderful the universe really is, so that we may push on towards exploring its endless expanse.
