Why There Should Not Be a Norm for “Minimum Safe Distance” Between Satellites
Outer space is infinite, but the parts of space most important to life on Earth are becoming increasingly congested. Satellites vital for infrastructure, economy, and security are filling up the thousands of miles above the Earth’s surface. With this crowding comes an increase in close approaches between satellites, placing communications, navigation, weather monitoring, missile warning, and other satellites at risk.
There are many reasons to fear close approaches — also known as conjunctions — in space, even if they are accidental. From a safety perspective, accidental satellite collisions could produce thousands of pieces of long-lived debris, as happened when an Iridium satellite and a defunct Russian satellite collided in 2009. Satellites that get too close to each other could also, intentionally or unintentionally, cause radio frequency interference that jams communications. If the approach is intentional, a threatening satellite could grab on to, physically destroy, electromagnetically interfere with, or collect intelligence for a future strike on a victim satellite.
Due to both safety and security concerns, international strategists and diplomats have raised two proposals related to the concept of “How close is too close?” in space. One safety-focused proposal is the concept of “minimum safe distance,” which would constitute a spherical safety zone in the three-dimensional space environment. On the security side, the concept appears as spherical “keep-out zones,” which would prohibit satellites from operating within a certain distance of another satellite without consent. For example, one set of authors proposed 700 kilometer (430 mile) radius keep-out zones for military communications and missile warning satellites in geostationary orbit.
At first glance, the ideas for minimum safe distance and spherical keep-out zones make sense as options to reduce the risk of accidental or purposeful collision. However, both proposals conflict with the basic principles of how objects move in space, and as safety or security norms of behavior they would create more problems than they could solve. Instead, norm development efforts should focus on increasing situational awareness, improving collision probability and risk assessment, and establishing lines of communication between satellite operators.
Why Spherical Keep-Out Zones or Minimum Safe Distance Won’t Work in Space
Satellite close approaches have become the source of numerous public complaints and U.N. discussions on space behavior. In February 2020, the first U.S. Space Force Chief of Space Operations, Gen. John Raymond, made a public statement that a Russian satellite had been actively maneuvering in the vicinity of a U.S. government satellite in a way that appeared to be threatening. China has complained that close passes of Starlink satellites to their crewed space station posed enough of a collision risk that the Chinese station had to maneuver out of the way. Meanwhile, U.S. military leaders have raised concern that China’s claimed debris removal satellite could pose a threat if it was used to approach or grab on to the satellites of other countries.
Spherical keep-out zones work best in environments where actors are free to approach an object from any direction. This is not the case for satellites in space. Despite the vastness and relative emptiness of space, satellite motion in Earth orbit is tightly constrained. Whenever a satellite makes the slightest change in speed or direction, it must change either its altitude or its orbit orientation. This means that, theoretically, two satellites perched in the exact same orbit should never collide even if they are relatively close to each other. Furthermore, a satellite positioned either higher or lower in altitude than a nearby satellite will drift away from the other naturally.
As a result, large keep-out zones for satellites in the same orbit are unnecessary because physics automatically minimizes the risk posed in these directions. It also naturally constrains the behavior of operators planning a threatening approach. Given these maneuver constraints, waiting for a potentially threatening satellite to enter a spherical keep-out zone could also mean that the threatened satellite does not have enough time to maneuver away. Yet, expanding the keep-out sphere to provide more time or warning would likely result in a sphere so large that it renders significant parts of an orbit unusable.
Another major concern with applying the concept of spherical keep-out zones is that it may negatively impact important space norms and behaviors, especially if the keep-out spheres are large. For instance, all nations who operate satellites in geostationary orbit must register with the International Telecommunication Union for a “slot,” a roughly 75-kilometer (45-mile)-wide region where they can operate their spacecraft safely without having their communications link interfered with. A single 700-kilometer-wide keep-out zone would eliminate 10 of these slots. Additionally, geostationary satellite operators almost universally dispose their satellites in a graveyard orbit 300–500 kilometers (180–310 miles) above geostationary orbit when the useful life of the spacecraft is nearly over. Enforcing large keep-out zones would force the graveyard orbit higher, which will either reduce spacecraft lifespans due to the need for more propellant to reach the new graveyard or incentivize operators to ignore disposal best practices, all without improving the safety of geostationary spacecraft.
For low Earth orbit spacecraft, spherical keep-out zones make even less sense. Unlike in geostationary orbit, where most satellites lie in roughly the same orbital plane — in a ring aligned with the equator — low Earth orbit satellites occupy many different orbits at varying altitudes and orientations. Many of these orbits intersect, which may or may not lead to potential impact points. Depending on the timing of the conjunction, a satellite on the other side of Earth may pose a greater and more immediate risk of collision than a satellite just a couple of miles away. Applying a single spherical keep-out zone to every satellite, especially in the crowded regions where large low Earth orbit constellations reside, will lead to either keep-out zones that are too small to be meaningful or so large as to be impossible to enforce.
Furthermore, many potential threats to satellites do not require physical proximity. Instead, for threats like interference with or jamming of satellite signals, the object causing interference only needs to be between the target satellite and the device that satellite is trying to communicate with. Other threats, such as cyber attacks, might target ground stations instead of satellites and their communications links. So, norms that categorize threats solely based on how physically close they are to a satellite may be more of a placebo than a means of effectively identifying aggressive behaviors before they go too far.
Finally, spherical keep-out zones would do nothing to reduce the problem of orbital debris or non-maneuverable satellites. For both types of objects, there is no operator making the decision to move out of the way if they enter a keep-out zone. And thanks to the high speeds and difficulty of maneuvering in orbit, waiting until a piece of debris or non-maneuvering satellite enters a spherical keep-out zone could make it too late to avoid a collision. The infamous 2009 crash between a defunct Russian Kosmos satellite and an Iridium communications satellite occurred at a right angle with a relative speed of over 22,000 miles per hour: No keep-out sphere or minimum safe distance would have provided warning in time because the satellites were continents away from each other minutes earlier. Debris always has the right of way in space, so the options when on the collision path with a piece of debris are to move or get hit. Spherical keep-out zones do not add any information or tools to make that equation easier.
What Should Be Done Instead?
What, then, can be done to prevent collisions and mitigate threats in space if there is no minimum safe distance or spherical keep-out zone concept that can be applied as a norm? One option would be to expand communication and coordination of the methods that satellite operators already employ during a conjunction. Operators judge how close is too close not by the literal distance, but by the probability of collisions, calculated by predicting where and when the paths of two objects will intersect combined with and accounting for the uncertainty of that prediction. Operators do use spherical areas around a satellite to estimate the probability of collision at the closest point of a pass between objects, but the decisions revolve around the probability, not where the satellites are relative to each other prior to the potential collision.
This is how, hours or days in advance, operators can move their satellites long before they get close to one another. A common problem is that operators are using different data, different analytic tools, or different thresholds of what they consider risky. Communicating and reconciling those differences would be far more effective than creating a normative bubble around each satellite. Even in the international space security discussions, norm proposals have formed much more around identifying types of irresponsible or threatening behavior and proposing national lines of communication to resolve disputes instead of trying to set any specific dimensions for keep-out spheres.
One need only look to the maritime domain for examples of how “rules of the road” can help to set parameters for behaviors and reduce the risk of collision without resorting to a specifically defined minimum safe distance. Despite only having two dimensions of movement to work out and none of the forced relationship between speed and position, maritime “rules of the road” did not set a minimum safe distance. The Convention on the International Regulations for Preventing Collisions at Sea, or COLREGs, sets up a series of criteria for operators to decide what was too fast or too close, such as the visibility conditions and the maneuverability of the ships. COLREGs even accounts for the responsibility of operators to get out of the way of non-maneuverable ships such as those in the middle of fishing, a problem reflected in space because not all satellites can maneuver without disrupting their missions and therefore could have a “right-of-way” over maneuverable spacecraft. Other attempts to prevent collisions at sea and minimize threatening behavior looked for alternatives to a minimum safe distance between ships. When American and Soviet naval officers went to negotiate an agreement to minimize escalatory incidents between their respective fleets, the U.S. diplomats specifically rejected the notion of minimum safe distance and focused on unsafe or unprofessional behaviors instead.
Even without establishing spherical (or circular) keep-out zones at sea, most naval operators have a sense of what they believe is too close, and some are more sensitive or more risk averse than others for various reasons. This can be the same in space, where satellite operators have different thresholds for what they consider too close (or too high of a risk of collision). Satellite operators can and should call out potentially threatening close approaches, as U.S. leaders have done. The problem is that there is no “sphere” that would work well as a broadly applied norm for space safety or security. Because operators have a wide range of factors for what distance would give them concern, and because the physics of motion in space don’t lead to spherical metrics for measuring risk, a spherical keep-out zone would simply not be effective as a universal metric for judging unsafe or threatening behavior.
Ultimately, there are many possible paths to create norms, rules, and tools for mitigating threats and hazards in space. It is not yet entirely clear what the best ones are, but it is quite obvious what one of the wrong ones is. The safety- and security-driven desire for keep-out zones for satellites is commendable. We should be finding ways to improve the safe operation of spacecraft and to proactively identify threatening behaviors. However, as we do so, we should consider the unique physics-based constraints under which satellites operate. Techniques that work on Earth do not directly apply to space, and failure to take this into account could significantly degrade the usefulness of our space-based infrastructure.
Robin Dickey is a space policy analyst at The Aerospace Corporation’s Center for Space Policy and Strategy, where she has written extensively on the subject of norms of behavior in the space domain. Her experience prior to Aerospace includes risk analysis, legislative affairs, and international development and she has a bachelor’s and a master’s degree from Johns Hopkins University in international studies.
James Wilson is an engineering manager in the Astrodynamics Department at the Aerospace Corporation. He specializes in orbit analysis, constellation design, and space operations planning. He has a master’s degree in mechanical and aerospace engineering.
Image: NASA
