We have come a long way since the days of Jules Verne, who imagined the first spaceship being launched out of a cannon in his novel From the Earth to the Moon. The Space Age began nearly a century later in 1957 with the launch of Sputnik I by the Soviet Union, and, arguably, reached its climax with the Apollo Moon landings conducted by the United States. Since then, further successes in our efforts to conquer space have included the construction of the orbiting International Space Station, the deployment of the Hubble Space Telescope, and, most recently, the launch of the James Webb Telescope in 2021.
In addition to the US and Russia, both China and India have joined the space race in the last twenty years, with lunar probes that landed on the Moon, orbiters around Mars and even a separate space station in Earth orbit. Now, 50 years after the first Apollo mission, we are more poised than ever to return to the Moon and reach Mars within the next two decades.
Challenges of Space Exploration
That said, there still exist many major obstacles in the way of a potential manned mission to Mars and beyond, with the most significant limitation being the amount of time astronauts can spend in space. The human body is not designed to function in the microgravity of space. Time spent in such an environment leads to the degeneration of our muscles and bones and damage to our immune systems.
There is also the factor of psychological issues that can arise from being confined to a small space-faring capsule for long periods of time. In the case of a journey to Mars, astronauts will have to spend a minimum of 14 months in space for a two-way journey, assuming the fastest speeds achievable with our current chemical propulsion technology.
It stands to reason that the best way to remedy the issue of time is to make spacecraft that can travel faster. This is where nuclear powered engines come into play.
The Promise of Nuclear Engines
The human conquest of space is currently stuck in second gear due to our reliance on chemical propulsion systems. Chemical rocket engines work by combusting a liquid fuel and oxidizer (most often liquid oxygen) together, then shooting the resulting hot gas out of a nozzle. This emission pushes the rocket in the opposite direction, by Newton’s Third Law of action-reaction.
The speeds that can be achieved with chemical propulsion systems are limited by how much energy can be generated from a given mass of fuel. The operational efficiency of a rocket engine is indicated by a metric known as the “specific impulse”. Where the fuel efficiency of a car can be gauged in litres per kilometer, a rocket engine’s specific impulse measures how long a kilogram of fuel can last while providing a constant thrust of one Newton. Conventional chemical rocket engines can usually generate impulses up to 460 seconds.
Nuclear thermal engines are not too different in that they also work by expelling hot gas to generate thrust. The difference here lies in the type of fuel used. In nuclear engines, a type of gas, like hydrogen, flows over a nuclear reactor operating on fissile material like uranium-235 or plutonium. The process generates extreme heat (in the range of 2000 to 4000 degrees Kelvin), expanding the hydrogen gas and expelling it out of the engine’s nozzle at extremely high pressure, propelling the engine forward.
Because of the incredibly high temperatures achievable in a nuclear reaction, nuclear fuel engines can generate a higher specific impulse than any chemical combustion rocket. Nuclear engines are theoretically able to reach specific impulses of between 850 and 1000 seconds, which is twice the highest impulse attainable by a chemical rocket engine. Additionally, the pure hydrogen gas ejected from nuclear thermal engines is pure hydrogen, which is much lighter in mass per molecule than the waste generated by chemical engines, meaning it can achieve higher velocities – and by Newton’s Third Law, the higher the velocity of the ejected material, the higher the velocity that would be reached by the rocket.
The high energy density of nuclear fuel also means that less fuel needs to be carried by a nuclear-powered spacecraft compared to a chemically propelled one. This enables more efficient mass budgets and, in turn, faster travel times for future interplanetary missions. Reaching Mars on a nuclear spacecraft could take two weeks instead of the current seven months. Journeying to Jupiter, which could take up to ten years with a chemical propulsion system, could theoretically be done in only two with a nuclear rocket, which would finally put the gas giant within humanity’s reach.
Past and Future Prospects
Several nuclear rocket engines have already been tested so far. Between 1959 and 1973, a total of 23 engine tests were performed by the United States. The U.S. Department of Defense took the lead with its NERVA program (Nuclear Engines for Rocket Vehicle Applications). Researchers in the program were able to develop an engine which sustained a maximum impulse of 850 seconds for 90 minutes and achieved a maximum temperature of 2750 K. The NERVA program had a total cost of around 2 billion USD (about $6 per person in the U.S. at the time).
Newer projects for nuclear thermal engines, such as a 2023 DARPA-NASA joint contract and another independent endeavor by Lockheed Martin, are already under way. The costs for these projects are estimated to be between 10 and 20 billion USD (about $62 per person in the US). Despite the seemingly high price tags, investing in nuclear rockets could actually pay off in the long run in the form of reduced travel times for interplanetary journeys and reduced costs for the transport of materials to and from the Earth, such as shipments to the Moon for the construction of a future lunar base.
At present, there are still several challenges standing in the way of building commercially viable nuclear engines. Building an engine able to withstand the extreme heat of a nuclear fission reaction, for instance, is a major challenge. The materials used to build Earth-bound nuclear reactors are difficult to transport and would be hard to use in space. This is compounded by the necessity of using chemical rockets to transport building materials into space, as igniting a nuclear engine on the Earth’s surface would be extremely dangerous.
Any nuclear spacecraft would also require extensive shielding to protect astronauts and electrical instruments on board from the nuclear radiation, adding to its mass budget. There is also the problem of public opinion around the idea of a nuclear reactor in orbit above our heads – nuclear accidents over the years (such as Chernobyl and Fukushima) have cast a pall of fear over the use of nuclear power which would undoubtedly carry on to the public perception of new nuclear-powered spacecraft.
Nonetheless, nuclear engines hold great potential for a new generation of spacecraft. Despite the current logistical challenges that persist, the technology to build nuclear- powered spacecraft is already well within our reach. The relevant engineering hurdles are expected to be overcome in the next 10 to 20 years. Thorough testing by reliable organizations and a strong enforcement of safety standards may be able to sway public opinion and interest in the use of nuclear energy in space. Nuclear propulsion could be vital for our forays into deep space, and I, for one, am excited for the engineering marvels we are likely to witness in the coming decades.