My first order of business this time around is to offer a disclaimer. I am not a scientist, though I am married to one. What I am is an author of science fiction and fantasy who understands the importance of research in creating verisimilitude in fiction.
In my last science column, I detailed the perils of conventional space travel as related by astronaut Stan Love.
Conventional space travel is accomplished by means of chemical rockets, as it has been since the inception of space programs around the world. Chemical rockets use hydrogen and oxygen as fuel and, after 90 years of development, scientists have reached the limits of the efficiencies they can achieve through chemical rockets. Ultimately, we’re limited by the energy contained in the chemical bonds of these elements, so any rocket’s payload for the foreseeable future is going to be mostly fuel.
Scientists are now focusing their efforts on methods of extracting fuel at the rocket’s destination, thereby reducing the need to carry so much fuel on launch. For example, NASA engineer Ronald Litchfield says the ice caps on Mars could be mined for hydrogen and oxygen. It’s not a far leap to see how comets or other icy bodies can also be sources of fuel.
The limits of chemical rockets mean it will be difficult to reach high speeds, even with massive amounts of fuel, which would be necessary for longer manned missions. They will, however, continue to be the primary means of launching and landing missions from Earth.
If a space station around Earth or Mars, or a base on the moon or Mars (which have lower gravity than Earth) could be used to prepare spacecraft for longer missions, it opens the possibilities to use other kinds of engines. If your science fiction story features deep space travel or colonization efforts, you might want to consider one or more of the following methods of propulsion.
Alternate Propulsion Currently in Use
Electrothermal
These engines have been in use by Russian and US satellites since the 1970s. Using hydrazine as fuel, they create electrical energy, which is then fired through a supersonic nozzle. Unfortunately, the thrust produced is low and so their use would be limited to satellites in orbit and maneuvering thrusters in space.
Ion Drive
An ion drive uses molecules of a non-reactive fuel, such as xenon, ionizes them by giving them positive or negative charge, and then accelerates them using electrical fields. Like electrothermal engines, ion drives provide lower thrust, but are great for maneuvering. The Dawn space probe, launched in 2007 to explore the asteroid belt, uses an ion drive. In 2016, Dawn was orbiting Ceres and has used its ion drive to enter and leave the orbit of several celestial objects.
Solar Sails
Back in the 1600s, Johannes Kepler thought about harnessing the energy of the sun to propel an object. This concept has been converted for use in space travel by unfurling a “sail” to capture photons and use their energy to fuel spacecraft.
In 2010, Japan’s Interplanetary Kite-craft Accelerated by the Radiation of the Sun (IKAROS) proved the concept on its mission to Venus. In 2015, the Planetary Society launched the Lightsail 1 and Lightsail 2 should launch on a SpaceX Heavy Falcon Rocket this year.
Though solar sails can be used for steady acceleration over long periods of time, the farther they travel from the sun, the fewer photons they can capture. Scientists working on these projects are currently tackling the problem of alternative energy sources, like lasers, to supplement solar sail craft in the doldrums between stars.
Alternate Propulsion in Development
Plasma Propulsion Engine
These things are like ion drives on crack. Instead of a non-reactive fuel, magnetic currents and electrical potentials accelerate ions in plasma (super-heated gas) to generate thrust. The Variable Specific Impulse Magnetoplasma Rocket (VASMR) in development by the Ad Astra Rocket Company in Texas is the most powerful plasma rocket in the world. Ad Astra estimates that it could get a spacecraft to Mars in 39 days (compared to a minimum 18 months for a chemical rocket with astronauts and payload), but VASMR has yet to make it to space.
Thermal Fission
A fission reactor (the kind we use to generate power) can heat a propellant to sufficiently high temperatures to produce thrust. A nuclear rocket was in development in the 60s and 70s, but the Nuclear Engine for Rocket Vehicle Application (NERVA) was shelved by the Nixon administration before it could be tested.
The British Interplanetary Society launched Project Daedalus in 1973 and determined that a nuclear-powered engine could reach speeds of 10,000 kilometres per second. In those plans, a sophisticated autopilot system would carry passengers and supplies across the galaxies, but a working prototype was never designed.
Continuous and Pulsed Fusion
A fusion reactor combines, or fuses, nuclei, releasing energy (as opposed to fission, which splits atoms to produce energy). Our sun is a massive fusion reaction and that’s the kind of energy scientists hope to harness. It would be a carbon-free energy source, and, in terms of propulsion, it would be hundreds of times more efficient than chemical rockets. Unfortunately, decades of research and billions of dollars have not managed to produce a sustained fusion reaction.
Companies like Lockheed Martin, General Fusion, and Tokamak Energy are working toward fusion reactors and representatives say that they could achieve the goal by 2030. A fusion engine might not be far behind.
A pulsed fusion engine might be easier to achieve, using the controlled detonation of miniature fusion bombs to power a spacecraft. Such a design is in development at the University of Washington. The problem is that testing the design would violate nuclear test ban treaties.
Antimatter
Antimatter is created by particle accelerators and is the most efficient and energy dense fuel proposed for space travel. In 2006, the NASA Institute for Advanced Concepts (NIAC) funded the research and design of an antimatter-fuelled craft. Researchers calculated that ten thousandths of a gram of antimatter would be enough to send a ship to Mars in 45 days.
The problem is that even if all the antimatter created in particle accelerators to date were amassed, it would barely be enough to boil water. Also, antimatter reactions emit dangerous gamma radiation.
Bussard Ramjet
In 1960, US physicist Robert W. Bussard conceptualized an interstellar spacecraft that would be able to travel at a significant fraction of the speed of light by harvesting hydrogen from space and using it to fuel a fusion engine. Unfortunately, the scoop, or ram, used to collect sufficient amounts of hydrogen for the reactor would have to be 10,000 square kilometres in diameter, making the concept infeasible.
Taking It to the Page
Though each of these potential means of interstellar propulsion come with limitations, that doesn’t mean you can’t use them as the basis for your science fiction space travel. Poul Anderson, Larry Niven, and Vernor Vinge have used the Bussard ramjet (one of the most speculative methods discussed) in their fiction, and Carl Sagan even referenced the concept in Cosmos.
What would make the method of space travel possible? The discovery of a new element? A boom in particle accelerators? Does Tokamak Energy beat Lockheed Martin and General Fusion to a sustainable fusion reaction, and thus a fusion engine? If you want to play with alternate history, what if the Nixon administration never shelved NERVA? What if Kepler developed a means of solar propulsion back in the 1600s?
And remember, you only have to include enough science in your fiction to make it plausible. It’s not necessary to “show your work” like you did in school. The best measure for the inclusion of any research is whether it furthers the plot or deepens the reader’s understanding of a character. Look to the golden age authors and see how they did it. How hand-wavey can you be before you cross the line into fantasy?
I wish you the best with your space travel creation and I hope you let me know how it works out.
References
- https://cosmosmagazine.com/technology/antimatter-ion-drives-nasas-plans-deep-space-propulsion
- https://futurism.com/here-future-interstellar-spacecraft/
- https://www.seeker.com/space/exploration/the-future-of-deep-space-propulsion-may-soon-be-radically-altered
- https://www.scientificamerican.com/article/nasa-funds-22-futuristic-ideas-for-space-exploration/
- https://www.nasa.gov/topics/technology/space-travel/index.html
Melanie Marttila creates worlds from whole cloth. She’s a dreamsinger, an ink alchemist, and an unabashed learning mutt. Her speculative short fiction has appeared in Bastion Science Fiction Magazine, On Spec Magazine, and Sudbury Ink. She lives and writes in Sudbury, Ontario, Canada, where she spends her days working as a corporate trainer. She blogs at https://www.melaniemarttila.ca and you can find her on Facebook and Twitter.
