On August 20, 2019, U.S. President Donald Trump issued a memorandum about research on a potential launch of spacecraft containing nuclear powered engines. He urged government entities to continue their research on this matter and he explained that “the United States of America shall develop and use space nuclear systems when such systems safely enable or enhance space exploration or operational capabilities“. His justification was that “the ability to use space nuclear systems safely and sustainably is vital to maintaining and advancing U.S dominance and strategic leadership in space“.
However it is not a technology like the others and it has been made clear in the memorandum that “all U.S. government entities involved in the launch of spacecraft containing space nuclear systems shall seek to ensure safe operation“.
Nuclear propulsion is a well-known technology which is already used in the military sector: submarines, aircraft carriers and cruisers already use this type of propulsion. But it is also used in the civilian maritime field on board of some Russian icebreakers and transport ships.
Furthermore on December 16, 2020, Donald Trump signed Space Policy Directive-6 on space nuclear power and propulsion, a roadmap for the responsible and effective development and use of space nuclear power and propulsion systems.
To support the development of such technology, the American congress released in 2018 and 2019 a special budget of 225 million American dollars.
In view of the exponential speed of the development of space nuclear power and propulsion, one can wonder how is the use of nuclear energy sources in space regulated.
Over the past century, many disasters have involved nuclear energy: Hiroshima, Nagasaki, Tchernobyl or Fukushima, to only name the best known. It is therefore not without surprise that the international community has taken up the subject to regulate the use of nuclear energy in space, especially in the context of the cold war.
As of today, international treaties formally prohibit objects carrying nuclear weapons, whether in orbit or on another planet. For instance, the treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and other Celestial bodies provides in article IV that: “States Parties to the Treaty undertake not to place in orbit around the Earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies, or station such weapons in outer space in any other manner“.
But the main treaty governing the use of nuclear energy in outer space is the 1992 Principles relevant to the Use of Nuclear Power Sources in Outer Space.
The first principle establishes that States do not have their sovereignty in this area: “activities involving the use of nuclear power sources in outer space shall be carried out in accordance with international law“.
Then guidelines and criteria for safe use are described in principle three. It is clearly explained that nuclear power sources shall only be used for “space missions which cannot be operated by non-nuclear energy sources in a reasonable way“, thus in order to minimize the quantity of radioactive material in space and the risks involved. Thus, engines using nuclear propulsion will be mainly used in the case of interplanetary missions or in orbits high enough to avoid any risk on Earth.
This third principle sets out in particular several safety objectives such as the protection of individuals, the community and the biosphere against the risks associated with radioactivity. It then established technical criteria. For instance, “nuclear reactors shall only use highly enriched uranium 235 as fuel” and “nuclear reactors shall not be made critical before they have reached their operating orbit or interplanetary trajectory“.
Then the fourth principle sets the condition for a thorough and detailed safety assessment before the launch of any device with a nuclear energy engine. The fifth principle set an obligation of information on behalf of States if the damaged object risks causing the return into the earth’s atmosphere of radioactive materials.
Eventually, principles eight and nine lay down the conditions for engaging State responsibility and liability as well as the compensation to which potential victims would be entitled in the event of damage. Accordingly, States have an obligation to ensure that national activities which result in the use of nuclear power sources in outer space are carried out in accordance with the treaty.
Another point is that any State which launches a space object or causes it to be launched and any State whose territory or facilities are used for the launching of a space object may be held liable if damage is caused by this space object or its constituent elements.
International law, aware of the risks associated with nuclear energy, has therefore provided a good framework for its use in space and has limited the field of action of States in this area.
As of today, almost all of the engines used are chemical reaction engines. This is a technology that is still used for projects under development.
For the development of its Space Launch System (SLS), NASA is using RS-25 liquid rocket engines which contain liquid hydrogen fuel and liquid oxygen oxidiser. SLS is set to be the primary launch vehicle of NASA’s deep space exploration program and that includes the Artemis program and probably a human mission to Mars.
Even Space X’s Raptor engines, intended to equip the lower and upper stages of Starship super heavy launcher, use engines powered by liquid methane and liquid oxygen.
The development of nuclear powered engines for heavier rockets is still in its early stages.
The Current Researches on Nuclear Powered Engines
Research on the development of this type of engine was first carried out by the United States of America in the 1950s with the Orion project. This project was about using pulsed nuclear propulsion, that is to say triggering small nuclear explosions outside the spacecraft near a thrust plate so that it recovers each shock wave and translates it into movement. However the project was abandoned mainly because of the Partial Test Ban Treaty that prohibited test of nuclear weapons, except if they were conducted underground.
At the same time, the Nuclear Engine for Rocket Vehicle Application (NERVA) program was developed from 1960 to 1972 as to study thermal nuclear propulsion. It is a technique which is based on the high speed ejection of hydrogen heated by a nuclear reactor. But again, the project had to be abandoned when NASA had to curtail its targets in the 1970s.
However, NASA did not abandon the idea of using nuclear propulsion and in 2003 the Prometheus project was launched to develop nuclear propulsion systems for long duration space missions. At the end this project was abandoned in 2005.
The space agency had to resolve not to work on this subject for a while. But it was still able to develop this type of engine on space probes. Thus, interstellar probes Voyager 1 an 2 and New Horizons Pluto spacecraft used radioisotope thermoelectric generators which convert to electricity the heat generated by the radioactive decay of plutonium-238.
Currently the aim would be to use a conventional chemical engine for the departure from Earth and then to use nuclear propulsion once in space. This represents a real technical challenge since it would be necessary to develop an engine capable of withstanding the extreme heat produced by nuclear fission and avoid any risk of contamination. It would be a question of real nuclear reactors where uranium-235 fuel blocks quiver, enter into a chain reaction and release a thermal power of approximately 500 million Watts.
Pros and Cons
There are many advantages and disadvantages, but let’s start with the pros of nuclear powered engines.
The main interest of the use of nuclear power in the exploration of space lies in the speed that spacecraft could reach, that would be very useful for missions to Mars and beyond. For exemple, even when you launch a mission to Mars at the moment when the two planets are the closest to each other, the trip would still take six to eight months. With a nuclear powered engine, it would be easy to halve this time and therefore reducing the trip to only three to four months.
This reduced travel time has significant benefits on radiation received by the human body: shorter transport time means less radiation exposure. Even if technologies are being developed to counter these effects as much as possible, the best solution is still to reduce the duration of the trip.
But a shorter travel time also means less stress on the astronauts’ minds and therefore less psychological stress, which is just as bad as radiation. NASA studied the effects of seclusion and promiscuity over long periods of time and observed that it could cause the crews to mentally crack.
Now let’s see the cons using nuclear powered engines.
The main drawback is the risk of a nuclear accident as a result of reactor failure. This would cause both the irradiation of the crew but also radioactive fallout on Earth if the explosion occurs at the start of the trip. Besides the development of this type of reactor is expensive. It requires very long certification deadlines and it produces toxic waste that we do not yet know how to manage.
As expected of what the Space Policy Directive-6 on space nuclear power and propulsion set, the priority of NASA is to design a fission surface power system of the Moon as to supply any permanent bases in order to meet all electrical needs: water purification, generation of oxygen, recharging of rovers, heating of habitats. It will also allow further testing of the system for a potential use on Mars.
Although nuclear-powered engines are in development, it is highly likely that the first human mission to Mars will be using liquid rocket engines. But this new impetus given to research offers us many perspectives.