Nuclear thermal propulsion: a key technology for space exploration

The history and future of astronautic nuclear technology

Over the past few years, organizations and companies including NASA, the U.S. Department of Energy, SpaceX, and the International Atomic Energy Agency have expressed interest in using nuclear technologies to make space exploration more efficient and economical. In particular, nuclear thermal propulsion (NTP) is emerging as a potentially key technology for transporting large masses from Earth to various positions in the solar system, while surface power systems using nuclear reactors could be used to provide energy to the first human inhabitants of Mars.

NTP is not a new technology: it was largely developed in the 1960s during the Space Race, culminating in the successful development of the Nuclear Engine for Rocket Vehicle Application (NERVA). The principle behind all NTP systems is the same: nuclear fission is used to heat helium or hydrogen to temperatures of about 2,200–2,800 Kelvin, accelerating these gasses to produce thrust. In the case of NERVA, the design was similar to that of a miniaturized nuclear fission reactor, containing components made of graphite and beryllium oxide and a liquid hydrogen-based coolant in the rocket propellant. These atoms, which have a low atomic mass, are used in thermal propulsion because they have the highest associated specific impulse — the ratio between the exhaust velocity of the particles and the gravitational field intensity at sea level. Essentially, higher specific impulses result in higher thrust and better mass efficiency: for the same thrust, the propulsion system mass is lower.

In 1973, NERVA was capable of generating significant amounts of aggregated power of exhaust particles, with a specific impulse more than double that of chemical rockets and a particle exhaust velocity close to 8,000 meters per second, almost double the particle velocity in chemical engines. NERVA had a thrust comparable to that of chemical rockets, but the main advantages of the technology were its high specific impulse and energy density. For maneuvers involving large specific transfer impulses, such as Earth-Mars missions, the maximum allowed mass of a spacecraft is the primary technical constraint, and at that point NTP results in higher economic efficiency for reaching the same transfer objective.

The operating parameters and performance of NERVA are considered state-of-the-art even in modern-day designs and illustrate the benefits of nuclear propulsion over chemical rockets: more compact energy sources and higher particle exhaust velocities. While the large size of NTP systems and inherent radioactive nature of ejected matter makes them unusable on Earth, they are a strong option in space conditions. 

However, after the success of the Apollo program and immediate public image benefits of its success in the Space Race, the United States government decided not to invest further in these projects. Politicians were no longer interested in investing in a program relevant for a manned mission to Mars, and although the technology had been developed and was ready to use, there was no public interest in continuing research in the field or making commercial use of it. Research and development of NTP was stagnant between the end of the Space Race and the 2010s.

Today, 50 years later, the landscape of space exploration is very different, as both China and Russia are willing to spend national resources to achieve strategic superiority in astronautics and the development of space systems, including preparation for the economic exploitation of space and resources in it. Economic competition over the last decade has also generated more interest in the field, and organizations and companies across both the U.S. and Europe have decided to invest more in it, with positive effects on the development, both physical and conceptual, of many technologies — including NTP.

Given the fact that NERVA weighed about 18 tons and Starship, the largest spacecraft currently produced by SpaceX, is capable of lifting more than 100 tons of payload to low earth orbit, it is evident for many specialists that we are currently capable of transporting such large propulsion systems in a single trip. Moreover, there exists the possibility of actually assembling large space structures while in the Earth’s orbit, or in nearly constant gravitational potential regions — space where the gravitational field intensity is close to zero, with almost no gravity observed.

Until we see NTP systems running in space, we may first witness the deployment of other nuclear technologies, like nuclear electric propulsion systems, which complement current work by NASA to miniaturize high-power nuclear systems for space missions. Using electromagnetic fields to accelerate ions, these systems produce less thrust than NERVA but are a step towards using nuclear technology in space and physically experimenting with propulsion systems that one day may be used on spacecraft heading to distant objects in the solar system. It is just a matter of time — and increasing economic competition between nations — until we will see such systems being used in transporting equipment and humans to Mars in order to make humanity a multiplanetary species.