In the heydays of the early space race, both the Americans and Russians toyed with various concepts involving nuclear propulsion for human space exploration within the solar system.
In the United States, this work focused on three major areas of study: nuclear pulse propulsion (NPP) – literally exploding atomic bombs behind a space vehicle, propelling it forward, as exemplified by Project Orion; Nuclear-Thermal Propulsion (NTP) – the use nuclear motors in place of chemical rockets either from launch or once in orbit as seen with Project NERVA; and Nuclear-Electric Propulsion (NEP) – the use of nuclear energy to power low-thrust ion propulsion motors.
NPP was effectively (and perhaps fortunately) abandoned over both the fear of fallout from the vehicle’s atomic explosions during its ascent through the atmosphere and the signing of the Partial Test Ban Treaty in 1963. NTP, using a nuclear reactor to heat liquid hydrogen (LH2) propellant to create ionized hydrogen gas (plasma) which can be expelled via engine bells, has continued to be researched, although its use from launch was overruled dues to the radioactive exhaust plume (thus requiring liquid-fuelled rocket to lift the propulsion units to orbit where they might be used), and remains a solid concept for propulsion that could help reduce the journey time to Mars by weeks.
Nuclear-Electric Propulsion (NEP), relies on a nuclear reactor to provide electricity to an ion engine using an inert gas (like xenon) to create thrust (rather than spewing a radiative exhaust). The resultant thrust is less than that of either NTP or chemical propulsion, but it has the advantage of being able to be maintained for far longer periods, potentially allowing a crewed vehicle to gently accelerate to the half-way point to Mars before trying around and using that same thrust to decelerate gently and achieve orbit around Mars. This could cut a 6-month journey to Mars in half.
Experiments in NEP have continued through until recent times, including space-based test; NTP, however, only reached the stage of ground-based testing before being curtailed. However, it has remained the preferred approach to crewed deep-space missions, should nuclear propulsion on crewed vehicles again be seriously considered. The interest is now re-awakening in light of Project Artemis and America’s stated desire to both return to the Moon and reach beyond it to Mars, with a focus on new approaches to methods of propulsion.
One of these new approaches is the rather tongue-twisty Bimodal NTP/NEP With A Wave Rotor Topping Cycle. The “bimodal” references combining NTP fission to generate the electricity required to power a NEP ion engine, while the “wave rotor” effectively meaning a “supercharger” which further compresses the reaction mass to deliver greater power to the NEP. Research into the approach suggests a transit time to Mars could be reduced to just 45 days.
Based on conventional propulsion technology, the most fuel-efficient Mars crewed mission profile offering the longest period for surface exploration is the Opposition Mission. This requires crews to spend between 6 an 9 months each way in transit between the two planets, with a surface stay of up to 23 months. However, a bimodal nuclear propulsion system could both reduce the transit time each way to 45-60 days, allowing crews to spend more time on Mars, whilst also potentially releasing a mission for the 26-month launch windows, enabling a crew to make an emergency return to Earth if required.
As well as propulsion, NASA is looking at ideas using nuclear power systems for long-duration surface missions when solar and wind power cannot be used / relied upon, These include KRUSTY, the Kilopower Reactor Using Sterling Technology, a joint venture between the space agency and the US Department of Energy’s National Nuclear Security Administration (NNSA) successfully demonstrated in 2018. Then there is a new take on the hybrid fusion / fission reactor, first selected by NASA for development in 2013 and which has recently seen renewed investigation, and which is now showing promising signs for future use.
Conventional fusion methods generally comprise either inertial or magnetic confinement, using extreme pressure or a powerful magnetic field to compress a fuel such a deuterium (hydrogen-2), forcing fusion to occur. Both require significant energy input and the generation of significant amounts of heat – around 15 million degrees centigrade. As such, both require large, heavy systems and associated cooling – although this hasn’t stopped the likes of Boeing developing concepts for hybrid systems to propel crew-carrying interplanetary spacecraft to rival biomodal NTP / NEP powered craft.
Hybrid fusion / fission utilises high-energy fast neutrons from a fusion reactor to trigger fission in non-fissile fuels. It is still a complex method, but it has the advantage of being capable of of generating multiple fission events from a single neutron, rather than a single reaction per neutron, requiring less fuel feedstock, and as the fuel is non-fissile, output from the reaction is not radioactive. In fact, such a reactor could even use waste from other fission reactions, disposing of it. Even so, the systems required for hybrid fusion / fission reactors have tended to be extensive and mass-heavy, competing directly with bimodal NTP / NEP systems in size, complexity and mass.
However, a team from NASA’s Glenn Research Centre, Ohio, have developed a potential way in which the complexity (and mass) of a hybrid propulsion system could be significantly reduced.
Selected for Phase I development by the NASA Innovative Advanced Concepts (NIAC) programme, the team has focused on the development of a special lattice into which deuterium can be packed in densities around a billion times greater than a within the core of a conventional hybrid reactor. This, combined with the ability of the fusion process to generate multiple fission reactions, means that overall, less deuterium fuel needs to be carried for feeding into the reactor, thus also reducing the mass of all the associated tanks, piping, etc., required to handle it. Further, the nature of system means that reactions can occur at far lower temperatures than a standard bimodal system, further reducing mass and complexity by eliminating much of the thermal control mechanisms and radiator surfaces required to remove the heat needed to generate the fusion reaction, and the heat it also generates.
The upshot of this is that the research team’s work could result in propulsion / power systems which could be designed and scaled for a range of both crewed and automated space vehicles. In particular, the use of a hybrid reactor system is being considered for potential robot missions to the outer solar system – notably to the “ice worlds”.
The latter are a number of Moons – Jupiter’s Europa, Callisto and Ganymede, and Saturn’s Enceladus – together with places like Ceres and potentially Pluto, where it is believed they might have liquid (or semi-liquid) interiors. This is particularly true of the four moons mentioned, as they are subject to tidal forces induced by interactions with their parent planets and other moons, which generates heat deep within them, and this heat allows for the potential of liquid water oceans to exist under their icy surfaces.
This theory is given particular credence with both Europa and Enceladus because our robotic visitors to Jupiter and Saturn have gathered photographic evidence of both moons venting vapour and dust.
In the case of Enceladus, for example, NASA’s Cassini space probe directly passed through two such plumes in 2008 and 2009, tasting them with its on-board systems to find they were a mix of hydrocarbons, carbon dioxide and water vapour. These, coupled with further studies of the moon during Cassini’s long sojourn around Saturn (from orbital insertion in 2004 to its demise in 2017) gathered data consistent with the idea of some form of salty ocean existing under the crust of Enceladus which might be fully liquid in nature, or liquid very close to the moon’s rocky interior, and becoming more and more icy-slushy as it rises towards the crust.
There are already missions to study Europa in in greater detail – the European Space Agency’s (ESA) Jupiter Icy Moons Explorer, due for launch in April of 2023, and NASA’s Europa Clipper mission, set to launch in October 2024. Both are designed to determine whether there is a liquid ocean under Europa’s icy surface and whether it has conditions suitable for the development of microbial life (with JUICE also examining Callisto and Ganymede).
Both of these missions are potential precursors to NASA’s in-development Europa Lander mission. Using a conventional approach, this requires a spacecraft massing up to 16 tonnes – the majority of which would be utilised in reaching Jupiter and delivering a relatively small lander to the surface of Europa. Battery-powered, this lander would have a lifespan of around 22-days, meaning it will (literally) only scratch the surface of the moon, obtaining samples from a maximum depth of 10cm below the surface of the ice.
However, a hybrid fusion / fission propulsion / power system could enable the delivery of a larger, more capable lander to Europa in the future, one itself nuclear powered by one or two radioisotope thermoelectric generators (RTGs). With proper shielding around the lander’s electronics, the RTGs could power it for multiple months or years, potentially enabling it to deploy a drilling system capable of slowly boring its way down through Europa’s icy crust (estimated to be between 15 and 25 km and comprising varying compositions such as ammonia and silicate rock, at different depths, pressures, temperatures, and densities), to reach the surface of any underlying ocean to obtain samples the lander can directly analyse – thus offering the potential to obtain results that could prove to be more far-reaching than those obtained via Europa Lander.
The Europa Lander mission concept requires a complex multi-system vehicle massing up to 16.5 tonnes, culminating in a small, battery-powered lander with a surface duration of 22 days. A compact hybrid fusion / fission power / propulsion system could simplify the design (potentially eliminating the de-orbit stage, allowing a future mission to carry a larger / heavier RTG-powered lander capable of a more extended surface mission. Credit: NASA
Nor does the Glenn team’s research need to be limited to space-based applications. It could provide a new kind of nuclear energy and medical isotopes for nuclear-based medical treatments and offer a less radioactive means of nuclear power production than nuclear fission whilst also, as noted above, potentially offering the means to dispose of existing stockpiles of nuclear waste by re-using it as fuel.
But all of that lies somewhere in the future. Like the recent “fusion breakthrough” achieved in December 2022 at the National Ignition Facility at the Lawrence Livermore National Laboratory (LLNL) in California – the hybrid fusion / fission system defined by the Glenn research team has a long way to go before it is ready for practical application.
As to the development of NTP / NEP bimodal power systems, these are far closer to realisation / deployment, although they do face their own challenge. Even if only powered-up once outside of the Earth’s atmosphere, the fear of what might happen should a chemical rocket carrying a NTP system explode on the pad or during its ascent and leave a cloud of (low-)enriched uranium fuel to possibly drift back over populated area to be inhaled / contaminate crops is unlikely to sit well with people.
Even so, when it comes to the likes of crewed missions to Mars, nuclear propulsion (and power) is a far more realistic option than nonsensical ideas of trying to push 100 at a time into stainless steel cans, then lobbing those cans into orbit where they need to be refuelled by three additional cans before being fired towards Mars (a concept that is almost dead in the water even before the first of those cans has achieved orbit, for reasons I’ll cover in a future Space Sunday).