Over the years, nuclear explosions have been proposed for a variety of peaceful, nonmilitary purposes. More than 150 peaceful nuclear explosions have been conducted, mainly by the Soviet Union. They were used for creating reservoirs, blasting away earth to make canals, and facilitating gas and oil extraction, among other purposes. The last of these occurred in 1988.

Nuclear devices aren’t required to build infrastructure or extract resources. These activities can readily be done with non-nuclear technologies, hence we do not hear the construction and extraction sectors pushing for access to nuclear devices. But there is one sector in which nuclear explosions appear to be the best option available: protection from asteroids and comets, collectively known as near-Earth objects, or NEOs. The impact of an NEO hitting Earth could be catastrophic to humanity.

A 2007 NASA study evaluated a range of options for deflecting NEOs, including conventional and nuclear explosives; kinetic impact (hitting an NEO with a non-explosive object); solar focusing (which would reflect sunlight to heat and “boil off” parts of the NEO); lasers; mass drivers (involving landing on the NEO and shooting pieces off into space); gravity tractors (objects with sufficient gravitational pull to move the NEO); tugs (which could attach to a NEO and pull it); and the “enhanced Yarkovsky” (essentially painting the NEO so that sunlight pushes it). Of all these options, the study found that “[n]uclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives analyzed in this study.” In 2014, a US Government Accountability Office report indicated that the National Nuclear Security Administration is retaining some nuclear device components “for potential use in planetary defense against earthbound asteroids.”

A new article in the Bulletin’s subscription journal, “The dilemma of nuclear energy in space,” explores this theme further. The article is written by John L. Remo, a research associate at the Harvard-Smithsonian Center for Astrophysics and the departments of Astronomy and Earth and Planetary Sciences at Harvard University. Remo notes that retaining nuclear devices for NEO protection is at odds with the goal of nuclear disarmament. He also points out that nuclear explosions in space would be prohibited by the Comprehensive Nuclear Test Ban Treaty, if it comes into force, and the Outer Space Treaty. The former is unambiguous in prohibiting all nuclear explosions. The latter prohibits nuclear “weapons;” one could debate whether this includes nuclear devices used for non-weapon, non-military purposes. Remo calls for rethinking these treaties in light of the NEO threat. He argues for dedicating a small stockpile of nuclear devices (no more than 10 or 20) to NEO protection.

Ten or 20 nuclear devices would be insignificant compared to the thousands now held in military arsenals. While there are campaigns to gradually reduce the number of nuclear weapons in the world, it will be many years before the issue of whether to maintain an anti-NEO nuclear stockpile becomes critical. However, it is worth thinking through the dilemma now in order to be prepared when the time comes.

Today the International Campaign to Abolish Nuclear Weapons is calling for a new treaty to ban them, and some people argue that the weapons are categorically immoral. The idea of an anti-NEO nuclear stockpile is fundamentally incompatible with both the ban effort and the moral stance—unless a distinction is made between nuclear weapons and peaceful nuclear devices.

I don’t believe that nuclear devices, which include nuclear weapons, are categorically immoral. The problem is not the devices themselves but their potential consequences, which include massive humanitarian and environmental damage, though possibly also fewer wars (as in deterrence). Politically, it might be easier to simply ban all nuclear devices instead of attempting to retain some for peaceful purposes, such as NEO protection, or even beneficial military purposes, such as deterrence. In the decision over whether a small stockpile should be maintained for protection against asteroids and comets, the question is whether the reduced risk that one might crash into Earth is worth whatever harm the stockpile might cause to the political process of nuclear disarmament.

The risk posed by NEOs is not zero, but it is small relative to the risk posed by nuclear weapons. A large NEO impact and a nuclear war would have similar consequences: massive initial explosions followed by severe global cooling. But NEO collisions large enough to threaten all of human civilization occur approximately once every 100,000 years. (Smaller NEO impacts are more common and less damaging.) The probability of nuclear war is harder to estimate but clearly much larger; based on current arsenals, it is reasonable to assume a rate of between once per 100 years and once per 1,000 years. So if retaining an anti-NEO nuclear stockpile would halt progress on nuclear disarmament, then we shouldn’t try to keep one. Nuclear war is too much larger of a threat.

However, it may be possible to maintain a small stockpile of nuclear devices—as distinct from weapons—while continuing with disarmament. The key is to assure all stakeholders that the anti-NEO stockpile will not be used for military purposes. If any government believes otherwise, it might decide to maintain its own nuclear deterrent. If it is clear that the retained stockpile is strictly for protection from asteroids and comets, disarmament could proceed all the way down to zero.

The international community could build confidence that the anti-NEO stockpile wouldn’t be used for the wrong reasons by splitting control across several major non-allied countries. For example, the devices, their delivery systems, and their launch codes could be divided among the United Nations Security Council members, including both the permanent and rotating members, such that using one would require unanimous agreement.

Achieving consensus likely would be easy if a NEO threat were detected. If there is one thing that every state agrees on, it is that collision with an asteroid or comet would be bad. (The apocalyptic Islamic State is a possible exception; they should be given no role in NEO protection.) An NEO flying towards earth would present a clear global threat with no bad guys to muck up the politics. That said, achieving unanimous agreement can take time. It will thus be important to create as much time as possible to respond to the threats, for example by upgrading detection systems. Existing ones work well mainly for certain large NEOs, but there is a lot of room for improvement.

At least at first glance, it does not seem that a stockpile of nuclear devices dedicated exclusively to NEO protection should interfere with the tasks of nuclear weapons control and disarmament. Given that asteroids and comets pose a significant risk of a low-probability extreme catastrophe, and given that the alternative means of deflecting them do not work as well, an anti-NEO nuclear stockpile merits serious consideration.