For those who attended the 113th annual meeting of the American Society of International Law in Washington, D.C. at the end of March, the session entitled International Law and Interstellar Events: Who Speaks for Earth? offered a welcome respite from sessions devoted to climate change, trade wars, and Brexit. Instead of dwelling on an actual, real-life crisis, the panel was asked to speculate on how international law might guide or constrain nation states in responding to threats to global security that are merely theoretical in nature, namely those that might arise from outer space. In addressing this issue, members of the panel were asked to analyze two hypothetical scenarios. The first scenario was defensive in nature: If scientists determine that an asteroid is on course to strike the Earth and cause catastrophic damage, what individual or collective responsibilities do states have to try to prevent this from happening? The second scenario was more affirmative in nature: Does international law provide any guidance as to whether individual states, groups of states, or private actors, may broadcast Earth-identifying signals into outer space or respond to signals that are detected from an alien source?
The panel consisted of Chris Borgen, Professor of Law and Co-Director of the Center for International and Comparative Law at St. John’s University; Robin Frank, who recently retired after serving as an Associate General Counsel at NASA; Andrea Harrington, an Associate Professor at the U.S. Air Force Command and Staff College; and Lindley Johnson, NASA’s Planetary Defense Officer. (Yes, that really is his job title!) Shane Harris, a staff writer at The Washington Post covering intelligence and national security matters, served as the moderator.
Harris initiated the discussion by asking Johnson how we would know that an asteroid large enough to cause significant damage is on course to collide with the Earth, and how much time we might have to respond. As Johnson explained, Congress enacted legislation in 2005 mandating that NASA develop a program for identifying and tracking Near-Earth Objects (NEOs) in order to assess their threat potential. NEOs are defined as asteroids and comets at least 100 meters in diameter whose orbits bring them within relatively close proximity to the Earth. In 2013, at the recommendation of the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS), the International Asteroid Warning Network (IAWN) was established to enable government and private sector entities operating at the national and regional levels to cooperate in tracking NEOs, share information about them, and develop responsive measures. As a result of these initiatives, researchers using ground-based telescopes already have identified and catalogued more than 25,000 NEOs. This figure includes more than 90 percent of NEOs with diameters in excess of 1,000 meters, which are large enough to cause catastrophic damage, approaching the level of disruption unleashed by the asteroid strike that led to the extinction of the dinosaurs.
How much time would we have to respond to a potentially devastating asteroid strike? According to Johnson, there should be ample warning time to prepare for a strike by a known NEO whose trajectory is being actively tracked. Most likely, the window for developing a response would be measured in years, if not decades. Of greater concern are newly discovered NEOs, whose trajectories may not be known until less than a year before impact. As in the past, some asteroids will continue strike the earth without being detected in advance. The good news is that the vast majority of undiscovered asteroids will be small enough to disintegrate upon contact with the Earth’s atmosphere and cause minimal, if any, damage upon impact. That is why NASA and other members of the IAWN prioritize identifying NEOs that are large enough to cause more significant damage.
If scientists determine that an asteroid is going to collide with the Earth, how might we respond? Much will depend on its size and trajectory. If the asteroid is relatively small and is projected to land in a rural, sparsely populated area, the best option may be simply to evacuate the impact zone. What if the asteroid is projected to land in a more densely populated area and cause significant damage and loss of life? According to Johnson, there are at least two plausible options available. One option would be to deflect the asteroid from its current path to prevent it from striking the Earth. This could be accomplished by launching a spacecraft to fly alongside the asteroid in close proximity for an extended period of time – a year or more, if there is sufficient warning. Eventually, the gravitational pull of the spacecraft would alter the trajectory of the asteroid and cause it to veer away from the Earth. Alternatively, if the response time were more limited, the spacecraft could be launched to collide with the asteroid and knock it off its current path. The other, more drastic option would be to launch a nuclear device into space and detonate it in close proximity to the asteroid, causing it to disintegrate before it can reach the Earth. The viability of these options would depend, in large part, on the amount of time available, but also on the composition of the asteroid. An asteroid comprised of loosely held-together rocks and space debris would be relatively easy to break apart, whereas an asteroid comprised of solid metal or rock might be a better candidate for deflection.
After Johnson had sketched out the technological challenges and time frame for responding to anticipated asteroid strike, Harris asked the other panel members to discuss the legal aspects. Professor Borgen addressed the question of whether individual states should respond unilaterally to the threat or whether multilateral action is preferable. Under international law, a state has the inherent right to defend itself. As a practical matter, of course, only a handful of states currently have the technological capacity to attempt to deflect or destroy an asteroid. The case for unilateral action would be stronger if the threat posed by the asteroid is not worldwide, but limited to a particular country or region. Unilateral action also might be justified if the threat is not discovered in time to reach a consensus on a multilateral response. In such a scenario, only an individual state could act quickly and decisively enough to respond to threat in a timely manner.
Nevertheless, there are serious drawbacks associated with unilateral action. Foremost among them is the issue of liability in the event that the unilateral action fails to remove the threat or unintentionally exacerbates it. What if, for example, the unilateral action does not deflect the asteroid away from the Earth, but instead redirects the impact to another country or region? Or what if the unilateral action doesn’t destroy the asteroid, but instead causes it to break up into smaller pieces that are still large enough to cause significant damage over a much wider area? Article VI of the Outer Space Treaty provides that state parties to the treaty shall bear responsibility for national activities in outer space, whether they are carried out by governmental agencies or by non-governmental entities. Article VII of the Outer Space Treaty further provides that each state party to the treaty that launches an object into outer space, and each state party from whose territory such an object is launched, shall be internationally liable for damages caused by the object to any other state party or to any natural or juridical persons of any other state party.
Given the wide scope of liability imposed by the Outer Space Treaty, Professor Borgen believes that a multilateral approach is essential to properly assess the level of risk posed by the asteroid and to formulate an appropriate response that takes into account the interests of all states that may be adversely affected. Only if there were insufficient time to reach a multilateral consensus would unilateral action be warranted. The other members of the panel concurred with this analysis. The most plausible institution for forging such a multilateral consensus is the U.N. Security Council. A response authorized by the Security Council under Chapter VII of the U.N. Charter, which is implemented by multiple states acting cooperatively, wouldn’t necessarily absolve those states of liability if something went wrong, but it would at least prevent a single state from shouldering all of the blame and perhaps provide the legal basis for establishing an international indemnity fund.
Professor Harrington then addressed the question of what constraints international law might place on the nuclear option for preventing a potentially catastrophic asteroid strike. She acknowledged that Article IV of the Outer Space Treaty expressly prohibits state parties to the treaty from placing nuclear weapons and other weapons of mass destruction in orbit around the Earth, from installing such weapons on the Moon or on other celestial bodies, and from stationing such weapons in outer space in any other manner. Nevertheless, she believes that detonating a nuclear device in outer space for the purpose of preventing an asteroid from colliding with the Earth would not constitute a violation of Article IV because the device would not be used as a weapon. Frank disputed Harrington’s interpretation of Article IV and insisted that it does pose a significant obstacle to pursuing the nuclear option. Professor Borgen noted that the drafters of the Outer Space Treaty never anticipated the use of a nuclear device to prevent a potentially catastrophic asteroid strike. Their intent, which is apparent from the drafting history, was to ensure that outer space would be used exclusively for peaceful, non-aggressive purposes. It would be a stretch to interpret the language of Article IV loosely enough to justify the detonation of a nuclear device in outer space for the purpose of destroying a threatening asteroid. Johnson agreed with this assessment.
Another potential constraint on using a nuclear device to prevent a potentially devastating asteroid strike is the Limited Nuclear Test Ban Treaty. Article I of the treaty expressly prohibits state parties from carrying out “any nuclear weapon test explosion, or any other nuclear explosion, at any place under its jurisdiction or control” or “in the atmosphere, beyond its limits, including outer space.” While this might seem like an iron-clad prohibition, Professor Harrington noted that the doctrine of necessity, a longstanding principle of customary international law, offers a potential escape clause. Under the necessity doctrine, a state facing a grave and imminent peril may justifiably breach its obligations under international law in order to safeguard its essential interests, provided that two conditions are met. First, the state’s response to the grave and imminent peril must not seriously impair the interests of other states. Second, the state must not have caused or contributed to the situation that gave rise to the grave and imminent peril. If confronted with the prospect of an asteroid causing immense damage to its territory, the political leadership of a state party to the Limited Test Ban Treaty that has the capacity to launch a nuclear device into space likely would come under immense public pressure to deploy such a device in the hope of deflecting or destroying the asteroid. Panel members agreed that the state party’s political leaders almost certainly would invoke the necessity doctrine if they were to conclude that the nuclear option is the best or the only method of addressing the threat posed by the asteroid.
Turning to the second hypothetical, Harris asked Prof. Borgen what guidance, if any, international law would provide in the event that scientists detect a signal or message sent by intelligent beings from another solar system. In particular, what entity would be authorized to acknowledge receipt of the message on behalf of humankind and issue a response? According to Borgen, international law is silent on this question and does not impose any specific obligations or prohibitions on states or non-state actors. Prof. Harrington agreed with Borgen’s analysis, adding that the only tangentially relevant legal norm in this context is the obligation imposed on the member states of the International Telecommunications Union, a specialized agency of the United Nations, not to interfere with radio signals or the use of the radio spectrum. Apart from that general obligation, international law offers no guidance.
In the absence of any governing law, whichever entity or organization first detected the alien signal likely would turn to soft law principles and best practices within the scientific community for direction. Johnson emphasized the need to timely disclose and share information about the discovery with other scientists, with national and regional space agencies, and with relevant inter-governmental organizations. Information sharing is vital, not only for purposes of verification, but also for assessing risks and reaching a consensus on the appropriate response. Thanks to the ongoing work of the United Nations Office for Outer Space Affairs (UNOOSA) and its affiliated entities, well-established networks already are in place to facilitate information exchange and collaboration within the relevant scientific, legal, and policy-making communities.
If scientists interpret the alien signal as being potentially hostile or threatening, rather than benign, it would be imperative to seek input from an even wider range of actors, both public sector and private sector. As in the case of a potentially deadly asteroid strike, the U.N. Security Council would be the most likely forum in which to attempt to forge a unified, multilateral response. Panel members agreed that while input from private institutions and individuals with specialized knowledge and expertise should be solicited and taken into account, the question of how to respond to an alien signal, whether friendly or hostile, ultimately is a matter of public policy that should be determined by institutions that are publicly accountable. Prof. Borgen also emphasized the time factor. Whatever its origins, the alien signal most likely would have taken many years to have reached the Earth. It would be foolish to respond too quickly without taking adequate time to deliberate and think through all of the possible implications, unless there were extenuating circumstances that justified a more rapid response.
Before opening the floor to question from the audience, Harris added a final twist to the second hypothetical. What if, instead of an alien signal from outer space, an alien spacecraft were to land in international waters off the coast of California? Should the United States, the jurisdiction most immediately impacted by the landing, take the initiative in responding, or should it wait for the international community to act? If the aliens operating the spacecraft were behaving in a threatening manner, the United States would be justified, under international law, in taking measures to defend itself from the aliens. If the circumstances warranted, the U.S. almost certainly would invoke the necessity doctrine in order to take extraordinary measures that might otherwise violate its international legal obligations.
Even if the United States were disproportionately impacted by the landing of an alien spacecraft, panel members stressed that it would still be advisable for the U.S. to pursue multilateral consultation and cooperation in order to ensure that risks are properly assessed and that the interests of other states are taken into account. Prof. Harrington noted the inherent tension between law (what is legally permissible under the circumstances) and policy (what should be done to optimally address the threat at hand). She emphasized that when the law is largely silent as to what actions are permissible, policy-makers must look beyond the panic generated by sensationalist media coverage of an alien spacecraft, likely to be magnified by social media echo chambers, and do their best to avoid taking actions that will have unintended consequences. Several panelists raised the danger of inter-planetary cross-contamination – the transfer of micro-organisms from the alien spacecraft to the Earth, and Earth-based micro-organisms to the alien spacecraft – as one of the most likely unintended consequences. The panelists agreed that a multilateral approach, one that seeks input from a wide range of state parties and non-state actors, is more likely to avoid such unintended consequences than a unilateral response.
 See Douglas Preston, The Day the Dinosaurs Died, The New Yorker, March 29, 2019, for a fascinating (and nightmare-inducing) overview of the latest research into the massive asteroid, estimated to have been six miles wide, that struck what is now the Yucatán Peninsula sixty-six million years ago, causing a severe disruption to the Earth’s climate that led to the extinction of most forms of life.
 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies, Jan. 27, 1967, 18 U.S.T. 2410, 610 U.N.T.S. 205.
 Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space, and Under Water, Aug. 5, 1963, 14 U.S.T. 1313, 480 U.N.T.S. 43.