In February, it was revealed that Russia has been developing a nuclear-armed anti-satellite (ASAT) weapon. Officials quickly issued reassurances that the technology had not been deployed and that it did not pose an imminent danger. While it is still not publicly known how far Russia is in the development of this capability, the news nevertheless underscores that trends are pointing to a future in space that is nuclear. But whether this future will involve the weaponized use of nuclear power in space remains an open question. As the United States seeks to curtail the proliferation of nuclear weapons in space, it must do so with a clear vision for the sort of nuclear future it would like to see in space. This article examines three possible such futures and the questions on arms control, nonproliferation, and extended nuclear deterrence that arise from them. Nuclear Proliferation in Space The use of nuclear power in space is nothing new. Radioisotope thermoelectric generators (RTGs) have powered spacecraft for decades, including ones designed for military use, such as the Navy’s Transit navigation satellite. However, RTGs use plutonium-238, which is non-fissile, for their power, meaning they cannot be used as nuclear weapons. It is the use of nuclear fission that has seen renewed interest among spacefaring nations, and which is the focus of this article. Russia’s anti-satellite weapon is certainly the most alarming example of how fission technology might be employed, but not all the capabilities currently under development would necessarily be for military use. The United States, China, and Russia are all researching the civil use of nuclear propulsion in space. Last year, Lockheed Martin was awarded a contract to begin development on a nuclear thermal rocket (NTR) for a Mars missions, with an initial flight demonstration scheduled for 2027. In March, China claimed to have made progress in the development of their own nuclear reactor for use aboard spacecraft. Around the same time, Bloomberg reported that Russian President Vladimir Putin has been looking to fund the development of a nuclear-powered spacecraft as part of a wider effort to breathe new life into Russia’s space program. NTRs could significantly improve fuel efficiency in rockets and shave months off of their current travel times. However, along with their promises, NTRs have the potential to add new complexities to nuclear dynamics in space, particularly if they are applied to military use. There has already been some discussion on the use of nuclear propulsion for military space mobility and logistics. Senior national security space leaders have conveyed the need for a shift from “positional” to “dynamic” space operations (DSO) in which, “spaceborne combat forces are no longer static and predictable,” but instead enabled by sustained maneuver. Industry leaders have stated that the nuclear propulsion capabilities currently under development for civil use in space could transform the speed and pace at which military spacecraft move, in turn enabling DSO. Taken together, the trends described above point to three possible nuclear futures in space: Future 1: Space Becomes Nuclearized, but Not Militarized or Weaponized Under this scenario, states limit the use of nuclear fission technology in space to civil and commercial purposes. In the near future, NTRs may become the most widespread means of space travel and exploration, while in the further future, nuclear reactors could power bases on the moon and Mars. This future is, of course, the most “ideal,” but, in all likelihood, it will not come to pass without an explicit and concerted effort by the world’s nuclear powers to make it so. Russia has thus far shown no willingness to halt their development of a nuclear-armed ASAT weapon, having recently vetoed a U.N. resolution affirming provisions in the Outer Space Treaty that ban the proliferation of nuclear weapons in space. As actualizing this future will depend heavily on cooperation, it is likely to be the least unstable of the three scenarios explored in this article. If arms control and nonproliferation measures can meaningfully curtail the development of nuclear power for military purposes, then this future could become a reality. Future 2: Space Becomes Nuclearized and Militarized, but Not Weaponized Under this scenario, nuclear fission technologies, including those designed for military use, are proliferated in space, but they are not used as weapons. As with the first scenario, civil agencies make use of nuclear thermal propulsion to power spacecraft, but so do military forces for missions such as mobility and logistics. While this is not the most dangerous scenario, it is the one most fraught with uncertainty. AUKUS has demonstrated that the non-weaponized military use of nuclear power can increase nuclear proliferation concerns, even when concerns are unfounded. The military use of space nuclear propulsion could lead to similar outcomes, in turn heightening the risk of inadvertently entering into the third future (described below), or at the very least a more unstable military environment in space. This is not to say that space nuclear propulsion should not be employed for military use. If done properly, the adoption of such technology could very well lead to a more stable environment by clarifying uncertain lines regarding nuclear proliferation in space. However, as with the first future, the latter outcome within this second future is likely only achievable if deliberately aimed for. Future 3: Space Becomes Nuclearized, Militarized, and Weaponized Under this scenario, the gamut of nuclear fission technologies, including those designed to be used as weapons, are proliferated in space. This future would be the most dangerous, and pose the most questions about U.S. extended nuclear deterrence. The most important question to answer is how the United States would respond to a nuclear attack on the space assets of both itself and its allies. If the U.S. developed its own space nuclear weapons in response to Russian (or other adversary) proliferation, a balance of power similar to that of the current doctrine of mutually assured destruction could emerge in space. Analysis has already suggested that non-nuclear anti-satellite weapons could cause enough damage by the debris field that they create to constitute a certain kind of “mutually assured destruction.” This would only be exacerbated by the destructive potential of a nuclear-armed ASAT weapon. While not ideal, such a state of affairs would, at the very least, be peaceful, and in line with what has become the reality of a world with nuclear weapons. If, however, the U.S. does not pursue the development of such weapons, a more destabilizing paradigm could result. Without satellites, the range of conventional options available to the president in the aftermath of the detonation of such a weapon by an adversary would be extremely limited. This would significantly undermine flexible response, and greatly increase the prospect of a land-based nuclear exchange. Choosing the Future The future of nuclear power in space is currently at a crossroads. While arms control efforts seeking to arrest the development of space nuclear weapons should be pursued, the U.S. must also prepare for the possibility that these efforts fail, as well as for the myriad of other futures that could arise as nuclear technology in space sees more widespread use. Speaking at a recent event at CSIS, Acting Assistant Secretary of Defense for Space Policy Dr. Vipin Narang characterized the present environment as a “new nuclear age.” This age will inevitably include some type of nuclear future for space. The question is what such a future will look like.