Impact of an asteroid on Earth: the need for legal clarifications

Impact of an asteroid on Earth: the need for legal clarifications

Scientists believe that the fall of a destructive asteroid on Earth is not entirely unlikely. If so, what would be the means of defence available to States in order to comply with international law?

The potential situation of an asteroid falling on Earth would put the international legal regime under pressure. Would a State have the right to use a ‘weapon’ in outer space to repel the threat? If so, what would be the legal consequences if the defence mission failed? The recent report of the ad hoc working group on legal issues of the Space Mission Planning Advisory Group (SMPAG) provides some initial answers.

Historical background

On 15 February 2013, an asteroid of approximately 17 to 20 metres in size crashed in the Chelyabinsk region of Russia. Although the asteroid was limited in size, the blast wave injured some 1600 people and damaged thousands of buildings. This event served as a reminder of the need for international cooperation to prevent the fall of larger near-earth objects (NEO) on Earth that could cause widespread loss of life.

In the aftermath of this event, the Scientific and Technical Subcommittee of the UNCOPUOS recommended the establishment of an advisory group whose main purpose would be to plan an international response to a NEO threat. The group was created in 2014, under the name Space Mission Planning Advisory Group (SMPAG). In 2016, the SMPAG set up an ad hoc working group to address legal issues related to its planetary defence methods, in particular those related to the use of ‘weapons’. But what exactly came out of it?

Legality of the use of ‘weapons’ in outer space

A State would potentially incur international liability through the use of impulsive methods such as conventional explosives or nuclear explosive weapons. Indeed, the Outer Space Treaty (OST), which regulates the activities of States in outer space, provides that outer space shall be used exclusively for peaceful purposes while prohibiting the testing of any type of weapon on the Moon and celestial bodies (Article IV). Even if the use of weapons would be solely for the purpose of defending the planet, the fact remains that a ‘weapon’ is a ‘weapon’, whether it is used for peaceful purposes or not.

With particular regard to the potential use of nuclear weapons to change the trajectory of an asteroid, the widely ratified Partial Test Ban Treaty, prohibits nuclear explosions in outer space. Furthermore, the Treaty on the Prohibition of Nuclear Weapons (TNPNW) requires States Parties to never ‘under any circumstances’ use nuclear explosive devices. Although not yet in force, the TPNW will create an additional obstacle to the use of nuclear weapons in outer space when it enters into force.

The need to save the planet as a justification

What if the only manner to avoid a destructive asteroid impact would be the use of a (prohibited) nuclear weapon?

The International Law Commission’s Articles on State Responsibility (ARSIWA), applicable in outer space through Article III OST, provide that a State becomes internationally responsible for the violation of an international legal obligation (wrongful act). The SMPAG noted that the articles in ARSIWA provide for certain situations which could preclude the wrongfulness of the use of a nuclear weapon for planetary defence. When a State gives its ‘valid consent’ to another State for an unlawful method of defence, ‘no other reasonable means’ is possible to save lives, or the act is necessary to defend an essential interest against a ‘grave and imminent’ peril.

Nevertheless, the SMPAG indicated that the application of the above-mentioned provisions would be very complicated in practice. The terms ‘grave and imminent’ or ‘no other reasonable means’ are subject to interpretation based on a subjective analysis of the situation, which may differ from State to State. Also, the consent of a State would not preclude the wrongfulness of the act in relation to third States. More generally, the systematic application of these provisions may endanger the effectiveness and legality of international law.

The additional issue of potential liability

Even if the wrongfulness were to be ruled out by the provisions of ARSIWA, what would happen if the defence mission failed, for instance if the asteroid were to strike State X instead of State Y because of a failed intervention?

The Liability Convention, which may be considered as a lex specialis with respect to ARSIWA for space matters liability, provides that a State that launches a space object into space is liable for damage caused to another State by said object. For damage caused on the Earth’s surface, the liability is absolute, which means that no fault is required. The absolute liability would mean that even if the State that launches the weapon takes all measures to avoid the error but the error still occurs, it would be held liable. This risk could severely limit any planetary defence initiative by some States.

What next?

International law limits the option of using ‘weapons’ to prevent the fall of an asteroid on Earth. However, in an emergency situation related to a threat of NEO impact, prompt action will be required. In this context, the SMPAG emphasised the need to establish specific rules on planetary defence as of now. These rules will have to establish a specific mandate for each State in the event of a threat, modalities for cooperation between participants in the mission, and procedures common to each mission. Furthermore, liability issues would have to be clarified in order to protect victim States while not discouraging any planetary protection initiative by some States.

The need for additional rules on planetary defence is indisputable, but the form and scope of such rules remains unclear. Only time will tell.


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