April 2024
By Kelsey Davenport

European and U.S. officials threatened to pursue action against Iran at the next International Atomic Energy Agency (IAEA) Board of Governors meeting if Tehran does not meet its legally binding safeguards obligations.

The agency has been pressing Iran for years to account for the presence of nuclear materials at two sites that were never declared to the IAEA as part of Iran’s nuclear program. The agency assesses that one of the locations, Turquazabad, was used to store nuclear materials and equipment, and the other, Varamin, included a pilot plant for uranium milling and conversion.

In a Feb. 26 report, the IAEA said Iran did not provide the agency with “any information on the outstanding safeguards issues relevant to either of the two undeclared locations.” It added that the IAEA “will not be able to confirm the completeness and correctness” of Iran’s nuclear declaration until Tehran provides technically credible explanations for the presence of the uranium at the two locations and accounts for the current location of the nuclear materials.

France, Germany, and the United Kingdom, known as the E3, said in a March 7 statement to the IAEA board that action to “hold Iran accountable to its legal obligations is long overdue.” They made clear that they will pursue a resolution at the board’s quarterly meeting in June if there is no “decisive and substantive progress” on the safeguards investigation.

An official from one of the E3 countries told Arms Control Today in a March 12 email that several European countries favored pursuing a resolution censuring Iran for its failure to cooperate with the agency during the March board meeting, but the United States opposed the proposal.

The board last passed a resolution regarding the investigation in November 2022. That resolution said it is “essential and urgent” for Iran to clarify all outstanding safeguards issues. Following the passage of that resolution, Iran agreed in a March 2023 joint statement with the agency to “provide further information and access to address the outstanding safeguards issues.”

The E3 statement also said the board may need to consider “making a finding under Article 19 of Iran’s Safeguards Agreement,” which includes the option of reporting Iran to the UN Security Council if the agency cannot verify that all of Iran’s nuclear materials are being used for peaceful purposes.

The board reported Iran to the Security Council in 2006, a move that led to a series of council resolutions requiring Iran to halt certain nuclear activities and the imposition of sanctions when Tehran failed to implement those provisions.

Iran defended its cooperation with the IAEA in a March 5 note to the agency. The note said that Tehran has “done its utmost” to enable the IAEA to “effectively carry out verification activities.” It said that Iran has fulfilled all of its legal commitments, including under its safeguards agreement. The note repeated allegations that the IAEA assessment of the undeclared locations is “based on unreliable information and unauthentic documents.”

In a March 7 statement to the board, Laura Holgate, U.S. ambassador to the IAEA, also condemned Iran’s failure to cooperate with the IAEA investigation, but suggested that the board ask the agency to prepare a “comprehensive summary report” on Iran’s nuclear program and the “degree to which the agency is in position to verify that Iran’s program is exclusively peaceful.”

She said that if Iran continues to “delay and deflect” the agency’s inquiries, the board must consider “further action for the sake of demonstrating that no state can indefinitely thwart implementation of its…safeguards obligations [under the nuclear Nonproliferation Treaty] by obstructing” the IAEA.

If the board pursues a resolution censuring Iran for failing to cooperate with the agency, Tehran is likely to retaliate. The U.S. intelligence community, in its 2024 Worldwide Threat Assessment, released March 11, assessed that Iran “probably will consider installing more advanced centrifuges, further increasing its enriched uranium stockpile, or enriching uranium to 90 percent” uranium-235 in response to a censure, further sanctions, or an attack against the nuclear program.

The intelligence community also assessed that Iran “is not currently undertaking key nuclear weapons-development activities” but that the expansion of the country’s program “better position[s] it to produce a nuclear device, if it chooses to do so.”

According to the most recent IAEA report on Iran’s nuclear program, Iran’s overall stockpile of enriched uranium grew over the last quarter. But Tehran down-blended 32 kilograms of uranium enriched to 60 percent U-235 by mixing the material with low-enriched uranium. As a result, Iran’s stockpile of 60 percent U-235 material decreased slightly from 128 kilograms to 121 kilograms.

Although a slight decrease in the stockpile of 60 percent U-235 is positive because that material can be quickly enriched to weapons-grade levels, or 90 percent U-235, the down-blending has little impact on the immediate proliferation risk posed by Iran’s nuclear program.

If Iran made the decision to produce weapons-grade uranium, it could still enrich enough material for one bomb in about a week and enough for about six bombs in a month. After that, it would take Iran an estimated six months to one year to build a bomb. But those activities would take place at covert facilities, making the weaponization process more difficult to detect and disrupt.

Holgate told the IAEA board that the United States has “serious concerns” about the 60 percent U-235 stockpile. “Iran should down-blend all, not just some, of its 60 percent stockpile, and stop all production of uranium enriched to 60 percent entirely,” she said.

April 2024
By Xiaodon Liang

The U.S. intelligence community remains concerned that Russian President Vladimir Putin could resort to the use of nuclear weapons in Ukraine in response to Russia’s failure to achieve decisive battlefield successes, Director of National Intelligence Avril Haines told the Senate Select Committee on Intelligence on March 11.

After Putin made several veiled nuclear threats in the spring and fall of 2022, U.S. intelligence officials made public an assessment that senior Russian officials had discussed the use of tactical nuclear weapons in Ukraine that year. (See ACT, December 2022.)

Haines, presenting the 2024 edition of the worldwide threat assessment report, also said that the intelligence community worries that Russia will put at risk long-standing norms against the use of “asymmetric or strategically destabilizing weapons, including in space and the cyber domain.”

In February, U.S. officials accused Russia of developing a new anti-satellite (ASAT) weapons system that would violate the Outer Space Treaty. (See ACT, March 2024.) The accusation of a treaty violation strongly implies that the new ASAT system would carry a nuclear warhead in orbit.

The report said that Russian capabilities in space would remain competitive with those of the United States despite the imposition of sanctions on the Russian space industry in response to the invasion of Ukraine. In contrast, last year’s report speculated that sanctions, in concert with resource constraints and sectoral difficulties, might imperil Russia’s long-term space goals.

The intelligence community assesses that North Korea, having supplied Russia beginning last year with conventional arms and munitions to bolster the war effort in Ukraine (see ACT, November 2023), is probably seeking to leverage this assistance to secure acceptance as a nuclear power. Speaking to this concern, Haines said that Russia’s reliance on its few allies may lead to weakening of “long-held nonproliferation norms.” North Korea’s shipment of military goods to Russia constitutes a violation of UN Security Council sanctions prohibiting exports of arms from North Korea.

Although the threat assessment did not include new information on Chinese strategic systems, it did eliminate a finding present in last year’s report that China was not interested in agreements that could restrict its strategic forces. This comes after a November summit between U.S. President Joe Biden and Chinese President Xi Jinping, two subsequent military-to-military meetings over the winter, and a January meeting in Bangkok between U.S. National Security Advisor Jake Sullivan and Chinese Foreign Minister Wang Yi. (See ACT, March 2024.)

In its assessment of Iran’s nuclear program, the report said the intelligence community found that the country is not currently performing key weapons-related activities.

In a shift from last year’s threat assessment, the report placed special emphasis on chemical and biological threats.

It highlighted the growing risk of states using chemical weapons against their own general population and individual critics. In addition to outlining the threat posed by actors employing dual-use biotechnologies to design new pathogens and toxins, the report noted the success that China and Russia have had in undermining public trust in countermeasures.

April 2024
By Kelsey Davenport

In the absence of dialogue with North Korea, the United States will redouble its efforts alongside allies to deter Pyongyang, a top U.S. official said.

Washington still views negotiations with Pyongyang as the only viable pathway to peace on the Korean peninsula and remains focused on denuclearizing North Korea, Jung Pak, the U.S. senior official for North Korea, said March 5 at the Carnegie Endowment for International Peace.

But the United States assesses that North Korea is undergoing a long-term strategic shift, Pak said. North Korean leader Kim Jong Un no longer believes that he can achieve his primary goal, preservation of the regime, through negotiations with the United States or South Korea, she said. Kim is viewing the world through a “new Cold War lens” where he believes that North Korea will benefit from aligning more closely with Russia and China, she said.

Pak said that North Korea currently is not interested in engagement, but the United States continues to reiterate its willingness to engage in talks “at any level” and on “any topic” without preconditions. If there is an opening for diplomacy, denuclearization will not happen “overnight” given the “scope of [North Korea’s] weapons activities and its proliferation,” she said, adding that denuclearization will require “interim steps.”

In the absence of dialogue, Pak said the United States will “redouble” its efforts to deter North Korean aggression.

Pak’s comments came as the United States and South Korea commenced a military exercise, called Freedom Shield, that North Korea described as an “undisguised” military threat that “can never be called defensive.”

During the exercises, the South Korean military conducted drills simulating a strike on North Korean ballistic missile launches and practiced intercepting cruise missiles. North Korea accelerated testing of what it claims are nuclear-capable cruise missiles in recent months. Cruise missiles, which are maneuverable during flight, are more difficult to intercept than ballistic missiles.

The drills also included simulating a response to a North Korean invasion. South Korean Defense Minister Shin Won-sik said that the exercises included field training for special operations forces, which must be “capable of swiftly eliminating the enemy leadership should Kim Jong Un wage war.”

Gen. Paul J. La Camera, head of U.S. forces stationed in South Korea, told The Wall Street Journal in a March 11 interview that the exercises are designed to respond to an array of threats posed by North Korea. Kim must be assured that “positive [actions] will be met with positive actions, and negative will be met with negative,” he said.

As the Freedom Shield exercises wrapped up, North Korea conducted military exercises that included paratroopers simulating an infiltration into South Korea and attacking a South Korean guard post. Kim observed parts of the exercise.

In addition to expanding its missile capabilities, North Korea appears to be working to meet Kim’s goal of expanding the country’s nuclear arsenal.

In a March 4 statement, International Atomic Energy Agency (IAEA) Director-General Rafael Mariano Grossi said the agency is continuing to observe activities indicative of the commissioning of the light-water reactor (LWR) at the Yongbyon nuclear complex.

He said that the “continuation and further development” of North Korea’s nuclear program, including the commissioning of the LWR, “are clear violations of relevant UN Security Council resolutions and deeply regrettable.”

Grossi called on North Korea to “cooperate promptly” with the IAEA and effectively implement its safeguards agreement.

Laura Holgate, U.S. ambassador to the IAEA, told the agency’s Board of Governors in a March 6 statement that North Korea’s “dangerous, irresponsible, and escalatory nuclear rhetoric, and its unprecedented number of ballistic missile launches…threaten international peace and security and undermine the global nonproliferation regime.”

Holgate said that North Korea’s “rejection of diplomacy and dialogue underscores” that Pyongyang alone is responsible for “continued provocations.”

North Korea has not responded to U.S. offers for dialogue, but Kim’s sister, Kim Yo Jong, suggested that the country might be open to engagement with Japan.

She said that if Japan “makes a political decision to open up a new way of mending the relations,” the two countries “can open up a new future together.” In addition, if Tokyo “drops its bad habit” of criticizing Pyongyang “over its legitimate right to self-defense” and the issue of Japanese citizens abducted by North Korea, there “will be no reason for the two countries not to become close,” she said.

She appeared to be responding to a statement by Japanese Prime Minister Kishida Fumio that called for “boldly” changing the country’s relationship with North Korea.

April 2024
By Daryl G. Kimball

A special UN Security Council meeting on nuclear disarmament issues convened by Japan in March underscored agreement among all 15 members that the risk of nuclear war and arms racing is higher than at any point since the end of the Cold War. But it also highlighted chronic differences among the nuclear-armed states about how to reduce the danger. As the Japanese foreign minister warned ahead of the meeting, “The world now stands on the cusp of reversing decades of declines in nuclear stockpiles.”

To address such challenges, UN Secretary-General António Guterres outlined several commonsense, achievable steps that could begin to move the world away from the nuclear precipice if pursued by China, Russia, the United States, and others.

Noting that “states possessing nuclear weapons are absent from the table of dialogue,” Guterres said they “must reengage” to reduce nuclear stockpiles, prevent nuclear use, negotiate a joint no-first-use agreement, stop nuclear saber-rattling, and reaffirm support for the 1996 Comprehensive Test Ban Treaty.

He put specific emphasis on countries with the largest arsenals, Russia and the United States, and said that they “must find a way back to the negotiating table to fully implement the [New Strategic Arms Reduction] Treaty [New START] and agree on its successor.”

Last month, more than two dozen members of Congress introduced an important resolution calling for stronger U.S. efforts to engage Russia and China in arms control talks. Moving the nuclear-armed states in the right direction will, however, require much stronger and sustained pressure from civil society, legislators, and the international community.

At the Security Council meeting on March 18, U.S. Ambassador Linda Thomas-Greenfield criticized Russia’s nuclear rhetoric and reiterated the 2023 U.S. offer to engage in bilateral talks with Moscow on a post-New START nuclear arms control framework. Unsurprisingly, Russia’s delegate renewed the Kremlin’s rejection of the U.S. offer, claiming that there is no basis for such work if Western countries refuse to “respect [Russia’s] vital interests.”

In reality, maintaining limits on their strategic nuclear arsenals is in the vital interest of both countries. Yet, New START, the last remaining bilateral arms control treaty, is due to expire in fewer than 675 days. Moreover, Russia and the United States are obligated under Article VI of the nuclear Nonproliferation Treaty (NPT) to engage in negotiations to halt the arms race and move toward disarmament.

NPT member states should make it their highest priority at the NPT preparatory committee meeting in July to press Moscow and Washington to observe the New START limits on deployed warheads until a more permanent, comprehensive nuclear arms control arrangement is concluded.

Thomas-Greenfield also called out China’s nuclear buildup and said that, despite a round of bilateral talks in November, China “remained unwilling to engage in substantive talks on nuclear risk reduction and arms control.”

China’s delegate agreed that “the risk of a nuclear arms race and a nuclear conflict is rising,” but insisted that U.S. criticisms of China “don’t hold water.” He invited other nuclear-armed states to explore the possibility of a no-first-use agreement.

China’s proposal is designed, of course, to highlight its long-standing no-first-use posture and divert attention from its nuclear buildup. Nevertheless, such an agreement would help reduce nuclear risk. As U.S. President Joe Biden said in October 2022 about Russia’s threat of a potential first use of nuclear weapons in Ukraine, “I don’t think there’s any such thing as the ability to easily [use] a tactical nuclear weapon and not end up with Armageddon.” The same logic applies to U.S. or Chinese first use.

If the United States wants more substantive dialogue with China, the White House should agree to seriously discuss China’s proposal and say the United States does not plan to threaten nuclear coercion against China. Such a shift could reduce tensions and lead to more concrete measures designed to prevent a Chinese-U.S. nuclear arms race.

Guterres also called for reforms at the Conference on Disarmament (CD) to open the way for long-delayed talks on a fissile material cutoff treaty (FMCT) and on legally binding negative security assurances against nuclear attack for non-nuclear-weapon states, a priority for most nations.

To advance progress at the CD, the United States indicated in February that it would drop its opposition to talks on legally binding assurances against nuclear attack for non-nuclear states in good standing with their NPT commitments if other states, including China and Pakistan, drop their objections to long-delayed talks on an FMCT. Such a quid pro quo, if accepted by Beijing, could jump-start CD activity and lead to tangible results that reduce nuclear risks and guard against unconstrained arms buildups.

The world has faced grave nuclear dangers before. Then as now, it will take strong domestic and international pressure, smart diplomacy, and some luck to prevent disaster.

April 2024
By Xiaodon Liang

The Biden administration’s $850 billion defense budget request for fiscal year 2025 would increase spending for Defense Department nuclear weapons programs by 31 percent over the current year and projects sharply rising future costs for some key nuclear modernization programs.

The request for National Nuclear Security Administration (NNSA) weapons-related activities is 4 percent higher than appropriated by Congress for fiscal year 2024. In all, the budget request, unveiled on March 11, calls for $69 billion for nuclear weapons operations, sustainment, and modernization, including $49 billion for Pentagon programs and the rest for the NNSA. The combined budgets would be 22 percent higher than last year.

Three key nuclear rearmament programs are driving increasing costs. The funding request for the new Sentinel intercontinental ballistic missile (ICBM) system foresees lifetime research and development (R&D) and procurement costs that are 44 percent higher than anticipated in the 2024 budget request. The Columbia-class nuclear-powered ballistic missile submarine program will consume 30 percent of the Navy’s $32 billion shipbuilding budget under the administration’s spending plan for 2025, up from 17 percent in the budget authorized by Congress for 2024.

Meanwhile, the cost of producing plutonium pits at the 80-unit-per-year rate mandated by Congress is projected to rise to more than $4 billion per year from fiscal years 2027 to 2029.

The administration released the new budget request before Congress completed work on the appropriations bills that actually fund the government for the current fiscal year. Congressional negotiators finalized the fiscal 2024 appropriation figures for the Defense Department in late March.

In line with the Air Force’s disclosure in January that the Sentinel ICBM program likely would exceed baseline unit costs by 37 percent and its entry into service would be delayed by two years, the president’s request substantially raised projected R&D spending associated with the program. (See ACT, March 2024.) Last year, the R&D costs for fiscal years 2025 to 2028 were estimated at $11 billion, and now that projection is $14 billion.

Speaking at an industry conference March 7, Air Force Secretary Frank Kendall acknowledged the budgetary squeeze created by the cost overruns. “We see very big problems dealing with [fiscal] ‘26. We're looking at a number of things which are increasing. Sentinel is one of them,” he said.

The Air Force requested $539 million in advance-year procurement money on the Sentinel program in 2024, but later asked congressional appropriators to shift that money to R&D. There is no further procurement request in the 2025 budget. In 2020 the Pentagon estimated that the total cost of the next-generation Sentinel program, including decades of operations and support, could be as high as $264 billion. (See ACT, March 2021.) Taking the new increases into account, the total cost of the program over its planned 50-year life cycle could be as high as $300 billion, plus another $15 billion to produce the new W87-1 warhead for the missiles. (See ACT, March 2024.)

The cost overruns put the Sentinel program in “critical” breach of the Nunn-McCurdy Act, triggering a mandatory investigation into the root causes of the unanticipated cost increases. By mid-April, the Defense Department is required to give Congress an explanation of the cost increase, changes in the projected cost, changes in performance or schedule, and action taken or proposed to control growth.

The Sentinel program is in “deep trouble,” Rep. John Garamendi (D-Calif.) of the House Armed Services Committee and Sen. Elizabeth Warren (D-Mass.) of the Senate Armed Services Committee wrote in a March 14 letter to Kristyn Jones, the acting undersecretary of the Air Force. The lawmakers called for a thorough assessment of alternatives to the Sentinel program, including possibly extending the life of the Minuteman III ICBM to 2030, 2040, or 2050.

Funding for the W87-1 warhead associated with the Sentinel ICBM would stay flat at $1.1 billion in 2025 under the administration’s budget proposal.

The request calls for $8 billion for R&D and procurement of the new long-range B-21 strategic bomber, slightly less than the 2024 appropriation. The Air Force would receive less for the Long-Range Standoff (LRSO) weapons system, a new nuclear-armed, air-launched cruise missile, with funding falling from the $950 million appropriated in 2024 to $833 million for 2025. Spending on the W80-4 warhead for the LRSO system would increase from $1 billion to $1.2 billion.

Spending on the Columbia-class submarine would increase sharply from $6.1 billion in 2024 to $9.8 billion in 2025. Several media outlets, citing unnamed sources, reported March 11 that the first ship would not launch until 2028, a year later than planned.

To address production challenges and delays affecting the Columbia-class submarine and the Virginia-class attack submarine programs, the administration asked for $3.3 billion in 2024 supplemental funding to invest in the submarine industrial base. Speaking in support of the supplementary request March 11, Senate Armed Services Committee Chairman Jack Reed (D-R.I.) called on contractors to “do better” and “get their personnel situation straightened out,” according to National Defense.

The budget also seeks $743 million for development of a new W93 submarine-launched ballistic missile warhead and its aeroshell, an increase above the $516 million that was appropriated by Congress in fiscal 2024.

The administration’s request did not include funding for the nuclear-capable sea-launched cruise missile despite the mandate in the 2024 National Defense Authorization Act that the administration establish a program of record for the system. Congress appropriated $90 million for the missile and $70 million for its warhead in the 2024 budget. (See ACT, January/February 2024.)

In the NNSA request, funding for plutonium-pit modernization and production at the Savannah River Site would increase from the $1.1 billion enacted by Congress in 2024 to $1.3 billion, while funding for the same activities at Los Alamos National Laboratory would decline from $1.8 billion to $1.5 billion. The NNSA significantly raised its projections for plutonium production and modernization costs for the 2025-2028 time period from $12.3 billion to $14.8 billion.

In a January 2023 report, the Government Accountability Office (GAO) assessed that the NNSA had not developed a comprehensive schedule or cost estimate for the plutonium modernization program that met GAO best practices. The GAO found activities and milestones missing from the NNSA schedule and flagged a likelihood of disruption and delay.

Meanwhile, spending on NNSA arms control and nonproliferation programs would increase from $212 million appropriated by Congress for 2024 to $225 million. The administration request for the Defense Department Cooperative Threat Reduction program would remain unchanged at $350 million.

Following testing setbacks and delays, the administration has eliminated funding for procuring the Navy’s hypersonic Conventional Prompt Strike system while requesting R&D spending of roughly $900 million. The Army variant of the system, the Long-Range Hypersonic Weapon, would receive $538 million in R&D funding and an additional $744 million for procurement under the proposed budget.

Two months after Congress eliminated funding for the Air Force’s Air-Launched Rapid Response Weapon (see ACT, January/February 2024), the Biden administration increased its R&D request for the service’s other hypersonic program, the Hypersonic Attack Cruise Missile. That program would receive $517 million in 2025, according to the budget proposal, up from $343 million appropriated by Congress for 2024.

Spending on missile defense programs would decline under the administration request, with total costs for the Aegis ballistic missile defense system and purchases of Standard Missile-3 Block IB and IIA interceptor missiles declining from the $1.7 billion appropriated last year to $1.3 billion.

Likewise, spending on design and development of the Missile Defense Agency’s Next Generation Interceptor, a new component of the Ground-Based Midcourse Defense system, would be reduced from the $2.1 billion appropriated in 2024 to $1.7 billion.

April 2024   
By John Burroughs

Contradicting the widespread and complacent post-Cold War belief that the risks of the nuclear age are declining, threats of the possible use of nuclear weapons are on the rise.

In the summer and autumn of 2017, the United States and North Korea exchanged incendiary warnings of nuclear destruction.¹ In September 2019, Pakistan referred to possible nuclear war in connection to its dispute with India over Kashmir.² In recent months, North Korean leader Kim Jong Un on several occasions reiterated his country’s readiness to resort to nuclear arms to defend its fundamental interests.³ Most alarmingly, the Russian government on numerous occasions, beginning with President Vladimir Putin’s speech on February 24, 2022,⁴ and up to the present, has raised the possibility of resorting to nuclear weapons should the United States and its NATO allies intervene to defend Ukraine against the full-scale Russian invasion.

Such threats are utterly unacceptable, above all because they greatly increase the risks of a humanitarian and environmental catastrophe resulting from use of nuclear weapons. The position adopted by the Group of 20 (G-20) states, an intergovernmental forum that includes the world’s major powers, in a declaration in Bali in November 2022 is striking in this regard. The declaration states in part, “It is essential to uphold international law and the multilateral system that safeguards peace and stability. This includes defending all the Purposes and Principles enshrined in the Charter of the United Nations and adhering to international humanitarian law, including the protection of civilians and infrastructure in armed conflicts. The use or threat of use of nuclear weapons is inadmissible.”⁵

Although that position clearly was occasioned by Russian nuclear threats, by its terms, it is not limited to that circumstance.⁶ It was repeated in another G-20 declaration in New Delhi in September 2023.

The Legal Dimension

Is a declaration of the “inadmissibility” of the threat and the use of nuclear arms an articulation of a political and moral norm, or does it also have a legal dimension? After all, the reference to inadmissibility does have a legal flavor; in a trial, evidence is found to be admissible or inadmissible. Further, there is a strong case to be made that threats of nuclear weapons use are not only unacceptable and illegitimate but contrary to international law. That is true under law governing when the resort to force is lawful (jus ad bellum) and under law governing the conduct of conflict (jus in bello), the law of armed conflict or international humanitarian law.

Article 2(4) of the UN Charter provides that “[a]ll Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.” If a use of force would violate Article 2(4), a threat to engage in such force violates that article. As the International Court of Justice (ICJ) stated broadly in its 1996 nuclear weapons advisory opinion, “The notions of ‘threat’ and ‘use’ of force under Article 2, paragraph 4, of the Charter stand together in the sense that if the use of force itself in a given case is illegal—for whatever reason—the threat to use such force will likewise be illegal.”⁷

It follows that a threat to use nuclear weapons as part of an aggressive attack is illegal. That certainly applies to the nuclear threats made in support of Russia’s invasion of Ukraine.

What is the law, though, when a state threatens to use nuclear arms not as part of an aggressive attack as in the Russian case? As the ICJ explained, the use or threat of force in self-defense pursuant to Article 51 of the Charter must be necessary and proportional.⁸ A defensive threat to use nuclear weapons that does not meet those criteria would be illegal under jus ad bellum. In this context,⁹ proportionality requires that the defensive use of force have a reasonable relationship to the aggressive act responded to and a reasonable relationship to the lawful goals of the defensive use of force, for example, expelling troops from the attacked state’s territory. In many or all circumstances, a defensive first use of nuclear weapons would not be necessary or proportionate as a matter of jus ad bellum. Notably, the ICJ observed that the risk of escalation must be taken into account in assessing proportionality.¹⁰ Recent North Korean nuclear threats fail to meet the requirement of proportionality.

The court also found that any threat to use nuclear weapons, whether aggressive or defensive, must be of a use that would comply with international humanitarian law or jus in bello. In general, the ICJ said that “[i]f an envisaged use of weapons would not meet the requirements of humanitarian law, a threat to engage in such use would also be contrary to that law.”¹¹ Under that principle, if a use of nuclear arms is illegal, the threat of their use is illegal.

Putting aside marginal cases, nuclear use in typical scenarios, even if defensive, would be illegal under international humanitarian law.¹² The ICJ went a long way toward accepting this conclusion, finding that “the threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict, and in particular the principles and rules of humanitarian law.”¹³ The court did not reach a conclusion, one way or the other, however, regarding an extreme circumstance of self-defense in which the very survival of a state is at stake.¹⁴ Nearly three decades after the court issued its opinion, the global community should move beyond the court’s uncertainty in that circumstance.

The ICJ Analysis of Threat

The ICJ analysis of UN Charter requirements barring aggressive or disproportionate threats of force is unexceptionable; it flows from the Charter and the well-established rule that a defensive use of force must be necessary and proportional. In contrast, the basis for the court’s finding that a threat to use weapons in violation of international humanitarian law is illegal is not clear from the advisory opinion, nor is such a principle readily ascertainable in treaty law or ICJ case law.

Yet, the proposition that threats of illegal force are themselves illegal is rooted in the most important modern treaty, the UN Charter. Although the Charter does not address directly the issue of whether the illegality of threats of illegal force extends to violations of international humanitarian law, it does imply that the legality of threat of force and the legality of use of force should be analyzed together. The court’s reference under Article 2(4) to the illegality of threats of uses of force when the latter are illegal “for whatever reason” is consistent with that implication.¹⁵ Moreover, the Article 2(4) prohibition of the threat of force inconsistent with the purposes of the UN provides some support for an analysis going beyond the prohibitions of aggressive or disproportionate threats. Purposes set out in Article 1 include the maintenance of peace and security and cooperation in solving problems of a humanitarian character and in promoting respect for human rights.

Beyond those Charter considerations, it appears that the court enunciated the principle that a threat to use weapons in violation of international humanitarian law is illegal on its own authority. The ICJ is the highest court in the world on general questions of international law, and its judges include eminent international lawyers. It is not unusual for the highest court in a judicial system to develop the law or, put another way, to make visible already existing principles.

Sometimes, there is a distinction made in international law between conduct preparatory to a wrongful act and the wrongful act itself, which is illegal or criminal. This tendency is visible in the crime of aggression included in the Rome Statute of the International Criminal Court, which provides that an individual may be convicted of “planning” or “prepar[ing]” for aggression, but only if a state “act of aggression” is actually committed. If this approach were taken in the realm of state responsibility, a threat would be illegal only if contrary to a treaty, as it is in the case of threats of aggression under the Charter.¹⁶

A threat to use weapons in violation of international humanitarian law, certainly a specific and credible threat, is different from preparatory conduct such as acquiring military capabilities enabling an aggressive attack. A thought exercise regarding biological and chemical weapons illustrates the soundness of the court’s finding. The use of biological weapons and the use of chemical weapons would be violations of international humanitarian law prohibitions of attacks with indiscriminate and uncontrollable effects.¹⁷ Further, a nearly universal convention, the Chemical Weapons Convention, prohibits the possession and use of chemical arms; another one, the Biological Weapons Convention, prohibits possession of biological arms, reinforcing an existing ban on their use. Should a specific and credible threat to use such arms be considered lawful? Going beyond the sphere of weapons, under the Genocide Convention, incitement, conspiracy, and attempt to commit genocide are prohibited but not threats. Yet, it seems highly doubtful that a specific and credible threat to commit genocide should be considered lawful.

There are partial international prohibitions of threat under international humanitarian law.¹⁸ A key instrument, Additional Protocol I to the 1949 Geneva Conventions, includes a provision prohibiting “acts or threats of violence the primary purpose of which is to spread terror among the civilian population.”¹⁹ Another provision prohibits threatening that there shall be no survivors.²⁰

Such prohibitions can be viewed in two ways. One is that their partial character demonstrates the lack of a comprehensive prohibition of threats to use weapons in violation of international humanitarian law. The second is that they are expressions of an underlying principle, namely, that it is prohibited to threaten to carry out a prohibited act.²¹

The Evolution of International Law

The latter view is more consonant with the evolution of international law, recently illustrated by the 2017 negotiation of the Treaty on the Prohibition of Nuclear Weapons (TPNW) and the UN Human Rights Committee’s 2018 General Comment on the right to life. In that comment, the committee found that “[t]he threat or use of weapons of mass destruction, in particular nuclear weapons…is incompatible with respect for the right to life and may amount to a crime under international law.”²²

The TPNW is the latest manifestation of the view of a majority of the world’s states that the threat or use of nuclear arms is illegal. Previously, such views were expressed in arguments to the ICJ and, regarding use, in numerous UN General Assembly resolutions going back to 1961.²³ Moreover, beginning with the 1967 Treaty of Tlatelolco, non-nuclear-weapon states have negotiated regional nuclear-weapon-free-zone treaties with protocols that, when ratified, obligate the five nuclear-weapon states acknowledged by the nuclear Nonproliferation Treaty not to use or threaten to use nuclear arms against members of the regional zones.24

In June 2022, five months prior to the adoption of the G-20 declaration in Bali, the first meeting of TPNW states-parties adopted the Vienna Declaration. In it, states-parties “stress that any use or threat of use of nuclear weapons is a violation of international law”; and they “condemn unequivocally any and all nuclear threats, whether they be explicit or implicit and irrespective of the circumstances.” The second meeting of states-parties, in New York City in late 2023, reiterated those points. The TPNW itself obligates states-parties “never under any circumstances” to “use or threaten to use nuclear weapons.” TPNW states-parties do not include any nuclear-armed states.

These developments further support the soundness of the court’s findings: threats to use weapons that violate international humanitarian law and therefore, at least as a general matter, threats to use nuclear weapons are illegal. It bears repeating as well that threats to use disproportionate force in self-defense are illegal, and the risk of escalation, obviously an acute concern when it comes to nuclear arms, must be taken into account in assessing proportionality.

Without attempting a definition of “threat” as a matter of international law, it can safely be stated that a specific and credible governmental statement making demands qualifies. Take a concrete context where the stakes are high, such as an armed conflict involving a nuclear-armed state; and the message is, if you do not refrain from doing X or if you do Y, we will resort to nuclear arms. That undoubtedly is a legally cognizable threat. It certainly describes Putin’s threat at the outset of the invasion of Ukraine, in which he expressed a readiness to resort to nuclear force should NATO states “interfere” in Russian military operations in Ukraine.

What about standing policies declaring a state’s readiness to resort to nuclear weapons when subjected to a nuclear attack and more generally when vital interests are stake? One could argue that specific threats are not involved in those policies and therefore they are not illegal. It is true that references to vital interests and similar formulations are vague. Yet, doctrinal signals that a nuclear attack may or will be met with a responsive or preemptive nuclear attack are focused and credible, even if not issued in an actual circumstance of potential use.

More broadly, if specific threats are illegal, that points toward the illegality and certainly the illegitimacy of the machinery and doctrines of nuclear deterrence. In its advisory opinion, the ICJ stated that it “does not intend to pronounce here upon the practice known as the ‘policy of deterrence.’”²⁵ Yet, as the United States observed in its oral argument to the court, “it is impossible to separate the policy of deterrence from the legality of the use of the means of deterrence.”26

It is highly disturbing that nuclear threats are on the rise, in the Russian-Ukrainian conflict and elsewhere. Even so, there are positive trends, although their importance should not be overstated. It is encouraging that there has been a counterassertion of a norm against nuclear threats in G-20 summits, as well as in the TPNW meetings. It is also encouraging that the United States and other NATO states for the most part have refrained from any public nuclear threats in response to those made by Russia.

The norm against threatening the use of nuclear weapons has a firm legal foundation. It is imperative that governments and civil society strive to entrench it even more deeply in international and national understanding and practice.

ENDNOTES   
 

1. John Burroughs and Andrew Lichterman, “Trump’s Threat of Total Destruction Is Unlawful and Extremely Dangerous,” InterPress Service, September 25, 2017.

2. “Pakistan’s Khan Warns of All-Out Conflict Amid Rising Tensions Over Kashmir; Demands India Lift ‘Inhuman’ Curfew,” UN News, September 27, 2019, https://news.un.org/en/story/2019/09/1047952. Earlier in the year, Indian Prime Minister Narendra Modi referred to India’s nuclear weapons capability in relation to tensions with Pakistan. Pallak Nandi and Vimal Bhatia, “Our nukes not for Diwali: PM Narendra Modi on Pakistan's N-threat,” The Times of India, April 22, 2019, https://timesofindia.indiatimes.com/elections/news/our-nukes-not-for-diwali-pm-on-pakistans-n-threat/articleshow/68982495.cms

3. Josh Smith, “Explainer: How Could North Korea Use Its Nuclear Weapons?” Reuters, December 20, 2023.

4. From the transcript provided by the Kremlin: “I would now like to say something very important for those who may be tempted to interfere in these developments from the outside. No matter who tries to stand in our way or all the more so create threats for our country and our people, they must know that Russia will respond immediately, and the consequences will be such as you have never seen in your entire history,” http://www.en.kremlin.ru/events/president/transcripts/67843.

5.  “G20 Bali Leaders’ Declaration,” November 15-16, 2022, para. 4, http://www.g20.utoronto   
.ca/2022/G20%20Bali%20Leaders-%20Declaration,%2015-16%20November%202022,%20incl%20Annex.pdf.

6. A February 24, 2024, declaration by the Group of Seven states (Canada, France, Germany, Italy, Japan, the United Kingdom, and the United States) is limited in its scope, stating, “Threats by Russia of nuclear weapon use, let alone any use of nuclear weapons by Russia, in the context of its war of aggression against Ukraine are inadmissible.” The White House, “G7 Leaders’ Statement,” February 24, 2024, para. 2, https://www.whitehouse.gov/briefing-room/statements-releases/2024/02/24/g7-leaders-statement-7/.

7.  Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1996 I.C.J. 226, para. 47 (July 8), https://www.icj-cij.org/sites/default/files/case-related/95/095-19960708-ADV-01-00-EN.pdf.

8. Ibid., para. 41.

9. The jus ad bellum requirement of proportionality differs from the international humanitarian law principle of proportionality, which requires that the collateral damage and injury resulting from an attack not be excessive in relation to the anticipated military advantage.

10. Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. at para. 43.

11. Ibid., para. 78.

12. See “End the War, Stop the War Crimes,” Lawyers Committee on Nuclear Policy, April 21, 2022, pp. 5-6, https://www.lcnp.org/s/4-21-22-russia-ukraine_lcnpstatement2.pdf; Charles J. Moxley Jr., John Burroughs, and Jonathan Granoff, “Nuclear Weapons and Compliance With International Humanitarian Law and the Nuclear Non-Proliferation Treaty,” Fordham International Law Journal, Vol. 34, No. 4   
(2011): 595.

13. Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. at para. 105(2)E.

14. Ibid.

15. Ibid., para. 47.

16. The International Law Commission has taken this position, stating, “Some rules specifically prohibit threats of conduct, incitement or attempt, in which case the threat, incitement or attempt is itself a wrongful act.” Yearbook of the International Law Commission, 2001, Vol. 2, Part 2, A/CN.4/SER.A/2001/Add.1 (Part 2), 2007, p. 61.

17. In addition to general rules of international humanitarian law, an amendment to the Rome Statute, so far ratified by only a small number of states, specifically makes use of biological weapons a war crime. Article 8(2)(b)(xxvii). Use of chemical weapons arguably is specifically criminal under a provision of the Rome Statute setting forth the Geneva Gas Protocol prohibition of use of “asphyxiating, poisonous or other gases, and all analogous liquids, materials or devices.” Article 8(2)(b)(xviii).

18. Principles embedded in national legal systems are also relevant to determination of general principles of international law. U.S. jurisdictions, for example, prohibit threat or menacing in certain contexts. Thus a Colorado statute provides that “[a] person commits the crime of menacing if, by any threat or physical action, he or she knowingly places or attempts to place another person in fear of imminent serious bodily injury.” Colo. Rev. Stat. § 18-3-206. Under federal law, transmitting “in interstate or foreign commerce any communication containing any threat to kidnap any person or any threat to injure the person of another” is a crime. 18 U.S.C. § 875(c).

19. Protocol Additional to the Geneva Conventions of 12 August 1949, and Relating to the Protection of Victims of International Armed Conflicts (Protocol I), June 8, 1977, 1125 U.N.T.S. 3, art. 51(2). The U.S. 2022 Nuclear Posture Review states that “longstanding U.S. policy is to not purposely threaten civilian populations and objects.” U.S. Department of Defense, “2022 Nuclear Posture Review,” October 27, 2022, p. 8, https://media.defense.gov/2022/Oct/27/2003103845/-1/-1/1/2022-NATIONAL-DEFENSE-STRATEGY-NPR-MDR.PDF#page=44. Taken on its face and separated from the realities of targeting military objectives, that statement is consistent with the prohibition.

20. Protocol Additional to the Geneva Conventions of 12 August 1949, art. 40.

21. Jean-Marie Henckaerts and Louise Doswald-Beck, Customary International Humanitarian Law, Vol. I (Cambridge: Cambridge University Press, 2005), p. 162.

22. UN Human Rights Committee, “General Comment No. 36 (2018) on Article 6 of the International Covenant on Civil and Political Rights, on the Right to Life,” CCPR/C/GC/36, October 30, 2018, para. 66.

23. See UN General Assembly, “Declaration on the Prohibition of the Use of Nuclear and Thermo-Nuclear Weapons,” Res. 1653 (XVI), November 24, 1961.

24. Nuclear-weapon states’ ratifications of the protocols are far from complete and in some cases are heavily qualified. Also, within and outside the nuclear Nonproliferation Treaty, nuclear-armed states have provided negative security assurances, qualified in most cases, that they will not use and in some cases not threaten to use nuclear arms against non-nuclear-weapon states. The International Court of Justice unsurprisingly found that nuclear-weapon-free-zone treaties and their protocols and negative security assurances do not establish a comprehensive prohibition of the threat or use of nuclear weapons. Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. at para. 63. See generally Anna Hood and Monique Cormier, “Nuclear Threats Under International Law Part 1: The Legal Framework,” NAPSNet Special Reports, March 1, 2024.

25. Legality of the Threat or Use of Nuclear Weapons, 1996 I.C.J. at para. 67.

26. Verbatim record, CR 95/34, November 15, 1995, p. 63, https://www.icj-cij.org/sites/default/files/case-related/95/095-19951115-ORA-01-00-BI.pdf.

April 2024
By David A. Koplow

What should be done if it is discovered some day that a large asteroid is on an imminent collision course with Earth? In particular, should a nuclear weapon be employed to divert or destroy such a threat if no other remedies seem sufficient?

This nightmare scenario is not now on the horizon; no one has detected any imminent massive inbound celestial body. Yet, humanity cannot take too much comfort because even the best astronomy is unable to detect all significant asteroids in adequate time to prevent their impact and because the world currently possesses only a very limited capacity for doing anything effective about these dangers.

Any potential deployment of a nuclear device in orbit or beyond and the prospect a nuclear explosion in space would face enormous political, legal, and technical obstacles. Even if the unprecedented activity were undertaken entirely for the peaceful purpose of saving humanity from a sudden extraterrestrial threat, it would be deservedly controversial and complicated. Long-standing international legal restrictions, established to preserve nuclear stability and restraint on Earth, would be severely challenged by any nuclear anti-asteroid program.

There are four main components of this scientific, political, and legal dilemma: the scope of the potential asteroid problem, the currently inadequate array of technically feasible responses, the legal and policy impediments that would apply against any possible use of a nuclear explosive device for anti-asteroid protection, and a proposed reconciliation via adaptations in existing international law.

Dodging Asteroids

There are many millions of asteroids in the solar system, of widely varying size, composition, location, and trajectory. Most of them stay safely in the main asteroid belt between the orbits of Mars and Jupiter, but as gravity, random collisions, and space weather exert chaotic influences, some asteroids adopt more widely erratic orbits, several of which could bring them uncomfortably close to Earth. An asteroid that approaches within 45 million kilometers of our planet’s orbit is categorized as a “near Earth object”; if it flies closer than 7.5 million kilometers and is larger than 140 meters in size, it is labeled a “potentially hazardous object.”¹

Distressingly, a great many of these fast-flying, portentous objects remain totally undetected. NASA and its companion space agencies in other countries are engaged assiduously in the search for these celestial bodies, and hundreds of new specimens are identified each year. Additional resources, including telescopes on the ground and in space, are being deployed to enhance “space situational awareness.” Yet, asteroids are relatively small and dark on the cosmic scale, and they can be especially difficult to detect if they approach from the general direction of the sun, which obscures the efforts of Earth-based astronomers. NASA’s Jet Propulsion Laboratory maintains a vivid Asteroid Watch Dashboard, which displays tracking information about upcoming near approaches, usually several per month, identifying each asteroid’s approximate diameter and date of closest Earth proximity.²

Generally, an asteroid larger than approximately 140 meters in diameter would be regarded as capable of inflicting regional damage, afflicting thousands of square kilometers, in a collision on Earth. There are an estimated 25,000 objects of this magnitude that could come near this planet, and only about 43 percent of them have been detected and tracked. A larger asteroid, measuring a kilometer or more across, would generate catastrophic global effects, comparable perhaps to those envisioned as the abrupt “nuclear winter” climatic alterations that could be triggered by a nuclear war. Some 854 of those giants have been detected, and dozens more are suspected.³

Throughout history, substantial asteroid impacts have been a recurrent phenomenon for this planet. Most famously, during the Chicxulub event some 66 million years ago, an asteroid 10-15 kilometers in diameter crashed into what is now the Gulf of Mexico, off the Yucatan Peninsula. The catastrophic geophysical effects led to the extinction of the non-avian dinosaurs and perhaps 75 percent of all other animal and plant species living then.⁴

More recent episodes, involving much smaller asteroids, also have proven momentous. For example, in 1908, an asteroid perhaps 50 meters in diameter exploded above the Russian region of Tunguska. There were no documented human witnesses in that remote Siberian locale, but the force of the detonation has been estimated as equivalent to 10-50 megatons of TNT. It flattened 80 million trees over a 2,000-square-kilometer area.⁵

In 2013, near the Russian city of Chelyabinsk, a roughly 18-meter-wide asteroid exploded at an altitude of 30 kilometers with a yield estimated at 400-500 kilotons, damaging 7,200 buildings and injuring 1,500 people.⁶ No one had seen this asteroid coming, and as it streaked across the morning sky, it could have been misinterpreted as a U.S. missile.

Of course, smaller particles penetrate Earth’s atmosphere all the time, including the spectacular meteor showers that regularly illuminate the night sky. The passage of larger objects, a meter or more in size, dazzles observers somewhere on the planet three or four times per month. More ominous are the very close approaches by still-larger bodies that observatories continuously track. Although none are projected to impact Earth in the foreseeable future, these calculations remain largely a cosmic game of uncertainties and probabilities, given the difficulty of accurately projecting the future course of a distant asteroid or comet. Even if a potential threat is discerned sufficiently early, it may take many months and carefully repeated observations to determine how likely an Earth impact really is, where on the planet it might occur, and how large and solid and therefore how consequential the asteroid would be.

As one vivid example, the asteroid denominated as 99942 Apophis has a diameter of approximately 340 meters. Initial observations in 2004 indicated a 2.7 percent probability that it would collide with Earth in 2029, which would be extraordinarily damaging. Subsequent observations eliminated that possibility, but also assessed that if the 2029 flyby happened to pass within a narrow region in space characterized as a precise “keyhole,” then gravitational effects would imply that the asteroid’s next close approach to Earth, projected in 2036, would create such an impact. Subsequent additional observations have eliminated the likelihood of a collision; the 2029 conjunction will pass very near Earth, but the 2036 rendezvous will miss our planet by millions of miles, and there is now no prospect of a collision for at least the next 100 years.⁷

Nonetheless, other significant asteroids have replaced Apophis atop the International Astronomical Union’s Torino Scale, which categorizes asteroid impact dangers.⁸ In fact, between 2027 and 2029, five other, even larger asteroids will approach Earth to within only four times the distance to the moon. Ominously, astronomers warn that a doomsday collision is statistically a question of when, not whether, because large asteroids will surely collide with Earth at some point as they have in the past. They just do not know whether the next Chicxulub-scale impactor will arrive within a few decades or not for another 60 million years.

Planetary Defense

In 2016, NASA established a Planetary Defense Coordination Office, charged with leading the development of capabilities to anticipate and respond to potential asteroid perils. Several candidate technologies have been pursued, and one, the kinetic interceptor, has been operationally tested. This concept entails launching a physical mass to ram into the targeted asteroid at high speed, nudging it off its trajectory or shattering the asteroid as a last resort, much as a missile defense interceptor might attempt to do to a hostile attacker much closer to Earth.

In 2021-2022, NASA undertook the Double Asteroid Redirection Test mission, propelling a small spacecraft 11 million kilometers into the 160-meter-wide asteroid Dimorphos, which poses no threat to Earth. The program was a marvelous success, validating the ability of the craft’s autonomous guidance system to direct it unerringly to the impact point at 22,000 kilometers per hour. The asteroid’s original trajectory was indeed altered appreciably, even more than most prior calculations had anticipated.⁹

Nevertheless, the kinetic impactor concept has important limitations. It could succeed only against a relatively small target, a few hundred meters in size at most, and only if there was plenty of advance warning so that the small variance in the asteroid’s path could accumulate over time, causing the asteroid to miss Earth. Moreover, the scenario requires that the asteroid be a relatively solid, intact body rather than a mass of space rubble loosely held together by gravity, as some asteroids are. Additionally, the target asteroid could not be too fragile lest the impact shatter it rather than displace its course through space.

Other potential planetary defense concepts have been hypothesized and sketched out, but none has proceeded into testable hardware. One notion, the gravity tractor, would send a spacecraft to intercept an asteroid, but instead of crashing into it, the spacecraft would adopt a closely parallel flight path. As the microgravity between the two objects pulled them closer together, the spacecraft would incrementally power away, and the continuing gravity would bend the asteroid to follow, bit by bit departing from its original trajectory.¹⁰

Other even less-developed concepts would seek to install some type of engine on the asteroid or to paint it to alter the natural color and light reflectivity of one surface on the asteroid so it would differentially absorb solar energy as it spins, which could translate into slowly changing its course.

If the threatening asteroid was large, however, and if the available time frame was compressed, a more powerful response could be necessary. The attention of scientists and other experts therefore inevitably has been drawn to nuclear explosions, which offer the most efficient mechanism for transferring a large amount of energy to a distant target and represent a relatively mature technology, having been operationally tested 2,056 times, including several times in space.¹¹

There has never been anything like a test nuclear explosion against a celestial body, but scientists have begun to contemplate some plausible methodologies. The current leading concept would not undertake to fracture the asteroid, despite several Hollywood movies luridly featuring that scenario. Doing that would probably leave most of the asteroid’s mass, which after the explosion would be highly radioactive, on a trajectory to collide with Earth. Moreover, fracturing the asteroid could create several distinct impact points on Earth, which would be comparable to a nuclear weapon with multiple independently targetable reentry vehicles and might expand the area of damage on the planet, inflicting even more harm than a single, larger crash.

Instead, a nuclear planetary defense mission likely would rely on a detonation at some short standoff distance from the asteroid and would employ the nuclear energy to vaporize volatile material on the surface of the asteroid. As those molecules rapidly evaporate or sublimate away, the resulting vapor would exert a tiny equal and opposite force, nudging the asteroid off track. Again, nothing of this sort has ever been tested, and there are no plans to undertake such an experiment.

Two relatively new international entities have emerged to study and respond to the asteroid threat. The Space Mission Planning Advisory Group is a collection of leading national space agencies, with an ambitious collective agenda to share information about asteroid exploratory missions and to collaborate to evaluate and recommend possible responses to asteroid scenarios.¹² The companion International Asteroid Warning Network is an affinity group for dozens of astronomers and observatories to identify and characterize the population of potential asteroid and comet impact dangers and to serve as a clearinghouse for sharing their findings with the broader community.¹³

Legal Impediments

Any consideration of using nuclear detonations in space immediately inspires profound political and legal opposition.¹⁴ Three treaties or groups of treaties are implicated, each of which was created long ago for reasons having nothing to do with potential asteroid impacts. The treaties have enjoyed wide adherence and are recognized as foundational to terrestrial peace and security, but they nonetheless pose significant challenges for any nuclear planetary defense scheme.

First, under the 1963 Limited Test Ban Treaty, each party legally undertakes “to prohibit, to prevent, and not to carry out any nuclear weapon test explosion, or any other nuclear explosion, at any place under its jurisdiction or control…in the atmosphere [and] beyond its limits, including outer space.” This constitutes a direct, categorical prohibition against the contemplated nuclear planetary defense mission.

The 1996 Comprehensive Test Ban Treaty would reinforce the 1963 treaty and extend a comparable proscription to any environment, including underground, as well as in space. Although this treaty has been signed and ratified by 178 countries, it is not in force because several required countries, including the United States, have not ratified it. Its signatories are nonetheless obligated by customary international law, as reflected in Article 18 of the Vienna Convention on the Law of Treaties, “to refrain from acts which would defeat the object and purpose” of the treaty. In 2016 the UN Security Council authoritatively proclaimed that any nuclear explosion by any state would constitute such a defeat and therefore is already prohibited.¹⁵

Notably, these test ban treaties apply to nuclear weapons tests and to “any other nuclear explosion.” This explicit language ensures that even if a nuclear planetary defense operation might be characterized as a so-called peaceful nuclear explosion or “nuclear explosion for peaceful purposes,” there is no escape hatch from the legal prohibition.

Second, the 1967 Outer Space Treaty, the foundational instrument regulating human space activities, imposes the obligation “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.”

Although this treaty is sometimes cited as a comprehensive “no nukes in space” commitment, its actual coverage is narrower. It proscribes only three specified activities regarding nuclear weapons: their placement in Earth orbit, installation on a celestial body, and stationing in space. Notably, the treaty does not bar the transit of a nuclear weapon through space. Although the key terms are not defined in the treaty or in the subsequent practice by states interpreting the language, it might be possible for a nuclear planetary defense mission to adopt an operational profile that dodged these constraints. For example, the nuclear explosive device might not have to orbit the Earth even once before being directed toward its target; it might not have to be installed on the asteroid if it were detonated at a distance; and its flight plan might not be regarded as having it stationed in space.

In addition, the Outer Space Treaty language applies to nuclear weapons, and some might seek to characterize a nuclear explosive device being employed for a peaceful, planet-saving operation as not constituting a weapon. That term might be reserved most appropriately for instruments used for a hostile, warlike, or criminal purpose. Of course, many items are dual use; a knife would be regarded as a weapon if it is wielded to threaten or stab someone but not when it is used to slice an apple. The literature on this point reflects an unresolved international debate regarding the treaty’s applicability to a nuclear planetary defense mission.

Finally, the several treaties dealing with nuclear nonproliferation also could be implicated, especially if the nuclear planetary defense mission were undertaken by a coalition of countries that included some legally authorized to possess nuclear weapons and some not. The 1968 nuclear Nonproliferation Treaty (NPT), for example, prohibits parties that possess nuclear weapons from transferring to others a nuclear weapon or other nuclear explosive device or control over such a weapon, and it reciprocally prohibits non-nuclear-weapon states from receiving such a weapon or device or control over it. So, the respective roles of the various states collectively participating in a nuclear planetary defense mission would have to be precisely defined, to prohibit control passing to an unauthorized state.

Many regional treaties establishing nuclear-weapon-free zones go a significant step further in pursuit of nonproliferation, requiring their parties, in the language of the treaty applicable to Latin America, to “refrain from engaging in, encouraging or authorizing, directly or indirectly, or in any way participating in the testing, use, manufacture, production, possession, or control of any nuclear weapon.” This broad mandate would seem not only to bar a non-nuclear-weapon state from actively joining in a nuclear planetary defense mission, but also from requesting that other states undertake such a rescue or expressing political support for it. Although a nuclear planetary defense mission would have to be led by one or a handful of technologically advanced countries, it surely would be beneficial to develop a broad global consensus endorsing such an operation even if that required circumventing this legal restriction.

A similar effect could result from the ambitious scope of the 2021 Treaty on the Prohibition of Nuclear Weapons, which comprehensively bars its states-parties from possessing, testing, using, or conducting other operations regarding nuclear weapons and includes an undertaking never to “assist, encourage or induce, in any way, anyone to engage in any activity prohibited to a State Party under this Treaty.” That mandate also could inhibit any effort to generate international support for or participation in a multinational nuclear planetary defense operation.

Additionally, any future international agreement that comprehensively sought a global, permanent abolition of nuclear weapons would be challenged by the asteroid scenario. Already, opponents of the concept of creating a world free of nuclear weapons cite the possible planetary defense application as a rationale against progress toward zero.

The Search for a Solution

What should be done if a threatening asteroid appears on the horizon and the potential array of non-nuclear planetary defense options seems inadequate to the task? Is there any way to maintain international fidelity to the treaties and collective adherence to the rule of law while acting efficaciously to protect the planet?

One potential recourse would be to alter relevant treaty obligations. Each of these agreements can be amended; each is also subject to withdrawal if a party’s “supreme interests” are jeopardized. These mechanisms are somewhat cumbersome, however, and can be time-consuming; in some instances, effective amendment requires a supermajority or even unanimity among the parties. Such changes also would constitute a permanent alteration in the treaty legal obligations rather than just a temporary exception. There are also valid excuses for nonperformance of international legal obligations, known as “circumstances that preclude the wrongfulness” of the state’s action, such as duress, necessity, and fundamentally changed circumstances. Again, none of these recognized rationales seems quite applicable to the situation under consideration here.¹⁶

Another alternative, favored by an ad hoc international lawyers working group established in 2017 by NASA and the space agencies of other cooperating countries, would turn to the UN Security Council.¹⁷ Under Chapter VII of the UN Charter, the Security Council possesses an extraordinary power in the event of a declared “threat to the peace” to create new law, including the even more extraordinary power effectively to supersede the obligations of prior treaties. The council could exercise that power to authorize a particular state or combination of states to undertake a nuclear planetary defense mission, notwithstanding the preexisting obligations of the Limited Test Ban Treaty, the Outer Space Treaty, the NPT, and other instruments noted above.

In so doing, the Security Council could immunize the acting states from any adverse legal consequences that would otherwise flow from their intentional, material departure from the implicated treaties. Once sufficient facts were known, the council could tightly restrict the authority it was delegating to certain states, limiting the number and type of nuclear weapons to be used and the way in which they would be applied, the identities of the states that could be involved in the most delicate parts of the operation, the time frame, the transparency of the operation, and other parameters.

Of course, the Security Council could adopt such a resolution only pursuant to unanimity or abstention among its five veto-wielding permanent members (China, France, Russia, the United Kingdom, and the United States). If those powers cannot see eye to eye regarding this peril, then the planet truly would be jeopardized.

Other international institutions could play a supporting role in legitimizing a nuclear planetary defense mission. The UN General Assembly and its subordinate Committee on the Peaceful Uses of Outer Space, as well as the Conference on Disarmament, do not possess the inherent lawmaking powers of the Security Council. Even so, a robust debate and endorsement by those bodies could manifest a high degree of global consensus on the appropriate course of action, helping politically to excuse what could otherwise constitute material breaches of legal obligations.

The danger of a catastrophic asteroid impact has only begun to attract the type and scale of attention and resources it demands. The problem requires technical finesse and large, sustained expenditures; it is grounded in a probabilistic assessment of potential hazards rather than a finite certainty; and it exposes a time frame that stretches far beyond any politician’s term of office—all factors that create a sure formula for shoving the issue to a back burner.

The use of a nuclear weapon would not be anyone’s first choice for a planetary defense mission. The financial costs and environmental consequences would be severe, the international politics could be corrosive, the legal jeopardy to important arms control treaties would be substantial, and the breaching of a decades-long taboo against nuclear weapons use would be dreadful.

Still, this option belongs on the list of possible responses, should the worst imaginable scenario emerge. Even the most substantial international legal commitments should never pose an obstacle to action that would be truly necessary to protect the planet from Armageddon, especially when there are mechanisms that could allow the world to stave off an incoming asteroid while demonstrating fidelity to the law.
 

ENDNOTES

1. Center for Near Earth Object Studies (CNEOS), NASA Jet Propulsion Laboratory, “Frequently Asked Questions,” n.d., https://cneos.jpl.nasa.gov/faq/ (accessed December 16, 2023).

2. NASA Jet Propulsion Laboratory, “Asteroid Watch,” n.d., https://www.jpl.nasa.gov/asteroid-watch (accessed December 16, 2023).

3. NASA, “Near-Earth Asteroids as of November 2023,” November 2023, https://science.nasa.gov/science-research/planetary-science/planetary-defense/near-earth-asteroids/.

4. ChicxulubCrater.org, “About the Chicxulub Crater,” n.d., http://www.chicxulubcrater.org/ (accessed December 16, 2023).

5. Natalia A. Artemieva and Valery V. Shuvalov, “From Tunguska to Chelyabinsk via Jupiter,” Annual Review of Earth and Planetary Sciences, Vol. 44 (2016), p. 37, https://doi.org/10.1146/annurev-earth-060115-012218.

6. Don Yeomans and Paul Chodas, “Additional Details on the Large Fireball Event Over Russia on Feb. 15, 2013,” Center for Near Earth Object Studies, NASA Jet Propulsion Laboratory, March 1, 2013, https://cneos.jpl.nasa.gov/news/fireball_130301.html.

7. NASA, “Apophis,” n.d., https://science.nasa.gov/solar-system/asteroids/apophis/ (accessed December 16, 2023).

8. CNEOS, NASA Jet Propulsion Laboratory, “Torino Impact Hazard Scale,” n.d., https://cneos.jpl.nasa.gov/sentry/torino_scale.html (accessed December 16, 2023).

9. Johns Hopkins University Applied Physics Laboratory, “Double Asteroid Redirection Test,” n.d., https://dart.jhuapl.edu/ (accessed December 16, 2023).

10. Committee to Review Near-Earth-Object Surveys and Hazard Mitigation Strategies Space Studies Board, “Defending Planet Earth: Near-Earth-Object Surveys and Hazard Mitigation Strategies,” National Research Council, 2010, pp. 66-88.

11. Arms Control Association, “Nuclear Testing Tally,” August 2023, https://www.armscontrol.org/factsheets/nucleartesttally.

12. European Space Agency, “Space Mission Planning Advisory Group,” n.d., https://www.cosmos.esa.int/web/smpag (accessed December 16, 2023).

13. See International Asteroid Warning Network (website), December 4, 2023, https://iawn.net/.

14. See Lt. Col. John C. Kunich, “Planetary Defense: The Legality of Global Survival,” Air Force Law Review, Vol. 41 (1997), p. 119; Jan Osburg et al., “Nuclear Devices for Planetary Defense,” n.d., https://ntrs.nasa.gov/api/citations/20205008370/downloads/Nuclear_Devices_for_Planetary_Defense_ASCEND_2020
_FINAL_2020-10-02.pdf; Bryce G. Poole, “Against the Nuclear Option: Planetary Defence Under International Space Law,” Air & Space Law, Vol. 45, No. 1 (March 1, 2020): 55.

15. On September 15, 2016, the permanent members of the UN Security Council (China, France, Russia, the United Kingdom, and the United States) collectively declared that a nuclear test by any country would defeat the object and purpose of the Comprehensive Test Ban Treaty. Office of the Spokesperson, U.S. Department of State, “Joint Statement on the Comprehensive Nuclear-Test-Ban Treaty by the Nuclear Nonproliferation Treaty Nuclear-Weapon States,” September 15, 2016, https://2009-2017.state.gov/r/pa/prs/ps/2016/09/261993.htm. This determination was subsequently endorsed by the full Security Council in Resolution 2310. UN Security Council, S/RES/2310, September 23, 2016.

16. “Yearbook of the International Law Commission, 2001,” Vol. II, Pt. 2, A/CN.4/SER.A/2001/Add.1 (Part 2), 2007, pp. 71-86.

17. Space Mission Planning Advisory Group (SMPAG) Ad-Hoc Working Group on Legal Issues, “Planetary Defence Legal Overview and Assessment,” SMPAG-RP-004, April 8, 2020, https://www.cosmos.esa.int/documents/336356/336472/SMPAG-RP-004_1_0_SMPAG_legal_report_2020-04-08+%281%29.pdf/60df8a3a-b081-4533-6008-5b6da5ee2a98?t
=1586443949723. See also David A. Koplow, “Exoatmospheric Plowshares: Using a Nuclear Explosive Device for Planetary Defense Against an Incoming Asteroid,” UCLA Journal of International Law and Foreign Affairs, Vol. 23, No. 1 (Spring 2019): 76.