Guest Post: Sqn Ldr Tara Brown on “The Problem of Responding to Subthreshold Threats in Outer Space”

Today I am extremely pleased to introduce a new contributor to Lawfire®, Squadron Leader (Sqn Ldr) Tara Brown of the Royal Air Force.   Sgn Ldr Brown is currently working towards her PhD while teaching at the U.S. Naval War College where she specializes in space law.

Photo: US Air Force

In this post she examines the response options international law currently provides in the event of a hostile action against a satellite that stays below the level of force required to trigger the right to self-defence under the UN Charter.  Such “subthreshold” activities, even if they don’t amount to a “use of force” or an “armed attack” as those terms are used in international law, can still be very harmful to those who depend upon satellite services.

Sqn Ldr Brown warns that “there is a question as to whether the conventional means of responding to subthreshold threats, encompassing countermeasures, retorsion, and assurances and guarantees, etc., are adequate in a domain so fragile and upon which we are so dependent.”  She ultimately concludes that the “existing measures in international law to respond to subthreshold activities are not adequate for an environment upon which so much depends, and which is so fragile.” 

Given the vital importance of satellites and space activities these days, you’ll want to educate yourself to Sgn Ldr Brown’s analysis.  She writes with refreshing clarity and is able to explain complicated matters in a way everyone can understand.  What is more is that she is dead on with her conclusions.  

The Problem of Responding to Subthreshold Threats in Outer Space

Sqn Ldr Tara Brown

The importance and vulnerability of outer space

Space systems underpin many multi-industry services, such as global communication and navigation systems, and are essential to environmental monitoring, weather forecasting, and disaster relief. They are also a key enabler of military operations.

For example, satellite communications are critical to the use of early warning systems and launch systems. They also allow for greater mobility of armed forces by eliminating the requirement for ground-based infrastructure, and intelligence surveillance and reconnaissance satellites are critical to maintaining national security.

With dependence comes vulnerability, however, and outer space is congested, contested and competitive with the environment and adversaries posing challenges. Space weather is harsh and encompasses high levels of radiation and solar activity involving significant temperature shifts.

Moreover, the volume of space debris continues to climb because there are no affordable means of removal. Space debris is especially problematic because even a small piece of debris moving at 17,500 miles per hour can destroy or disable a space station or satellite.

Satellites are inherently vulnerable to potential adversaries that have developed advanced counterspace capabilities. Satellites follow a set trajectory so their movements are predictable, they cannot be camouflaged, and they ordinarily have passive defense systems.

The rise in subthreshold activity   

While Article 2(4) of the UN Charter requires all Member States to refrain in their international relations from the threat or use of force, most States consider that Article 51 of the Charter only permits States to lawfully exercise force in self-defense where an armed attack[i] has occurred. This leaves a “force gap” potentially open to exploitation as threats or uses of force falling below the threshold of an armed attack do not induce a right of self-defense.[ii]

Any activity falling below the level of an armed attack, regardless of whether it amounts to a use of force, is referred to as “subthreshold.” Thus, there is a question of whether the available response options for such activities are robust enough to deter or counter such threats.

Across all domains (air, land, maritime, cyber, and space) there has been an increasing frequency of subthreshold activity that has necessitated a substantial shift in military philosophy. For example, the United Kingdom (UK) launched its Integrated Operating Concept (IOpC) in August 2021 to deal with the issue of “rivals seeking to win without eliciting a warfighting response,” thus recognizing the evolving character of warfare in a time of great power competition.

What do subthreshold threats in space look like?

Broadly, “space threats” are activities that threaten the space environment and sustainability of space, space infrastructure (including supporting infrastructure such as ground stations), or the functionality of an asset (be it temporary or permanent).

Spoofing or meaconing are two such subthreshold activities that seek to mislead. Spoofing mimics a satellites normal communication system and transmits a false signal to the receiver. Whereas meaconing intercepts the real signal and transmits it with a subtle time delay. A spoofed navigation satellite will produce a false position and velocity, which could impair the operation of military aircraft, ships, and other transportation systems. The right to respond to these threats requires successfully attributing the action to another State, which is not always possible. 

Jamming of satellite communications is another activity that is ordinarily viewed as subthreshold. Article 41 of the UN Charter supports this conclusion, as does State Practice, which shows a high tolerance to extensive jamming activities. For example, North Korea jamming activity impacted the signals of more than 1000 aircraft and 700 ships but the United States (US) did not call it out as a use of force, nor did Norway during Russian jamming during NATO exercises.

Cyber-attacks are a further concern, with the North Korea cyberwarfare unit being considered a substantial threat. The Center for Strategic and International Studies assessed that cyberattacks in space could be the preferred course of action to compensate for the imbalance of capabilities in other domains.

As for jamming, there has been a reluctance by States to categorize cyber-attacks as an unlawful use of force. For example, the Colonial Pipeline hack led to the declaration of a state of emergency, but it was not called out as a violation of the use of force prohibition.

Lasers may be employed to blind a satellites sensor, thereby causing heat damage to the detector or high-power microwaves (HPM) to overwhelm the satellites electronics, resulting in the corruption of data. While HPM attacks require a high level of pulse, this may become easier if there is a possibility of converting solar energy into microwave power, as is currently being tested by the US.

Of wider concern are reckless launches, debris-creating events such as ASAT tests (previously conducted by China, Russia, the US, and India), and Rendezvous Proximity Operations (as happened in 2015 when a Russian satellite maneuvered within 5-10 km of a US satellite, which was alarming as satellites in geostationary orbit (GEO) travel at 3 km per second).  

The response problem with subthreshold threats

The Outer Space Treaty (OST) entered into force in 1967 and was formed at a time when the focus was on the exploration of outer space, scientific research, and a race between the Soviet Union and the United States to be the first State to land on the Moon. The utility of outer space in support of military operations was not recognized until later. Therefore, there is a question as to whether the OST provides adequate tools to robustly counter subthreshold threats.

Additionally, while Article III of the OST confirms the applicability of international law, there is a question as to whether the conventional means of responding to subthreshold threats, encompassing countermeasures, retorsion, and assurances and guarantees, etc., are adequate in a domain so fragile and upon which we are so dependent.

The OST  

As to the first issue, the author’s view is that the due regard principle in Article IX of the OST is a useful tool but one that States have yet to properly utilize. A breach of Article IX would permit the victim State to respond with countermeasures. Article IX of the OST contains three separate obligations.

First, States must avoid harmful contamination of outer space and adverse changes in the environment of the Earth. While this could be of relevance for debris-creating events and reckless launches, the utility of this provision is limited, as the duty extends only to avoiding harmful contamination and does not prohibit it.

Likewise, there is an obligation to carry out international consultations where an activity might cause potentially harmful interference. However, the obligation does not extend to a positive prohibition against such activity.

The provision with the most “bite” is the obligation to conduct activities with due regard to the corresponding interests of other States. However, States have shown a huge tolerance to subthreshold threats, such as jamming, presumably because they conduct similar activity. Anti-satellite weapon (ASAT) tests have also provided ample opportunity to call out a violation of the due regard principle.

While Article IX is yet to be invoked, States have started to show a waning tolerance to irresponsible behavior and heavily criticized the 2021 Russian ASAT test. Further, ongoing UN work on responsible space behaviors, as well as steps taken by the US, such as the unilateral commitment to not conduct ASAT testing, are contributing to the development of soft law (non-binding norms of behavior).

It is worth noting that there is currently no mechanism for States to respond to concerning Rendezvous and Proximity Operations (RPO’s), unless they amount to a use of force, because Article II of the OST contains a non-appropriation principle. An RPO against a military satellite linked to national security is of the highest concern but, as the law currently stands, RPOs are unfriendly but lawful actions.

Therefore, retorsion is the only response option that merely allows for the severance of consular relations, expulsion of diplomats, and the withdrawal of economic ties. The use of zones, as used in other domains, could offer some assurance in this regard.

General international law

International law provides no mechanism for responding forcibly to subthreshold activities except in the very narrow circumstances in which a plea of necessity, enshrined in Article 25 of the International Law Commission’s Articles of State Responsibility, is justified (see also the International Court of Justice case of Gabcikovo-Nagymaros).

A plea of necessity allows a State to take action to safeguard an “essential interest” against grave and imminent peril and requires consideration of the nature of the protected interest, the gravity of harm posed, and the need for immediate action – with the key takeaway being that all criteria must be interpreted restrictively.

It is potentially useful as a response option as the action it addresses does not necessarily need to constitute an armed attack, nor does the state have to demonstrate attribution. However, as it only arises in very narrow circumstances, and as a last resort, most subthreshold threats will not invoke the right.

A breach of the use of force prohibition, not rising to the level of an armed attack, will give rise to the right to respond with countermeasures. The right to invoke countermeasures will also apply where there have been other internationally wrongful acts – for example, a violation of the non-intervention principle, or of the due regard principle.

While there is no internationally agreed definition of countermeasures, Professor Mike Schmitt helpfully sums them up in the Tallinn Manual as “State actions, or omissions, directed at another State that would otherwise violate an obligation owed to that State and that are conducted by the former in order to compel or convince the latter to desist in its own internationally wrongful acts or omissions.”

To exercise the right of countermeasures, an activity must be attributable to a State (for a discussion on the nature and scope of Article VI of the OST whereby States bear international responsibility for national activities, i.e. for US nationals, see here). Attribution is problematic from a practical standpoint for certain space activity, such as spoofing.

Even where these hurdles are overcome, the important takeaway for countermeasures is that they are non-forcible and taken in response to a previous wrongful act. Therefore, they must cease once the offending state resumes compliance.

While their scope is consistent with the de-escalatory nature of international law, it heavily limits their utility as an effective response in the space domain where so much is at stake and where the environment is so fragile because, of all the domains, it has the least ability to repair itself.


While the OST was crafted over half a century ago, the due regard principle does have potential to offer a route for States to call out irresponsible behavior, but there has been a complete failure by States to rely on this provision as a tool. In addition, the existing measures in international law to respond to subthreshold activities are not adequate for an environment upon which so much depends, and which is so fragile.

As the saying goes, people in glass houses should not throw stones. As to the solution, the hard law (treaties and conventions) versus soft law (non-binding resolutions and guidelines) debate is another topic of itself.

While Transparency and Confidence Building Measures (TCBM’s) coming out of the United Nations’ work on norms of responsible behavior is a step in the right direction, the value of the work is of limited utility until consensus is reached with major players such as Russia and China.


[i] It should be noted that while most of the international community accept the International Court of Justice (ICJ) judgement in Nicaragua, that framed armed attacks as the most grave uses of force and held that distinguishing them from lesser uses of force is a question of scale and effects, the U.S. position ( as outlined in the DoD Manual) is that the inherent right of self-defense potentially applies to any illegal use of force.

[ii] Except in the very limited circumstances in which a plea of necessity could be invoked – discussed in the response section above.

About the author

Squadron Leader Tara Brown is a commissioned officer in the Royal Air Force and currently serves as a military professor at the Stockton Center for International Law at the U.S. Naval War College, where she co-teaches a course on Air, Space and Cyber Law. She is studying for her PhD in space law and has been involved in the State Consultation process for the Woomera Manual on the International Law of Military Space Activities and Operations.  She has served in various assignments including prosecuting and defending at court martial, a deployment to the Combined Air Operations Center and most recently advising a wide range of policy clients within U.K. Headquarters Air Command.


The views expressed in this essay are those of the author alone and do not necessarily reflect the official policy or position of the Naval War College, the US government, or the U.K. government.

The views expressed by guest authors do not necessarily reflect my views or those of the Center on Law, Ethics and National Security, or Duke University.  See also here.

Remember what we like to say on Lawfire®: gather the facts, examine the law, evaluate the arguments – and then decide for yourself!



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