Abstract
Since 1957, the near-Earth population of trackable space objects has grown in number to over 36,000. Of these 36,000+ trackable objects now in low Earth orbit, just a few thousand are working spacecraft. The rest are Earth-orbiting objects which are no longer operational and are considered to be space junk. Because this junk can no longer receive maneuvering commands from its Earth-based owners, the survivability of other spacecraft traveling through or operating in Earth orbit can be jeopardized by the impacts of any number of pieces of this space junk, whose origins can usually be traced back to defunct satellites. As a result, a major design parameter for Earth-orbiting spacecraft is the possibility of such high-speed impacts and the damage they can cause. Furthermore, several private companies are now launching several thousand spacecraft into Earth orbit, many of which are satellites built for communication purposes. Other satellites have been launched to expand the reach of the World Wide Web and to provide better tools for disaster management. Two questions quickly become evident, namely, what is the beneficial purpose of these large satellite constellations, and what are some of the deleterious consequences of their proliferation? Numerous topics related to space debris will be discussed in this paper, including issues in space law that concern the growing problem of orbital debris. In the end, several areas of concern will be noted that are vital to the continuing presence of humans in near-Earth space and must be addressed as the near-Earth orbital environment becomes more congested and space traffic management becomes more difficult.
1. Introduction
Since 1957, there have been nearly 6800 launches of rockets intended to reach orbital altitudes or carrying payloads destined for Earth orbit. In this time, the population of detectable space objects in near-Earth space has increased from 1 (Sputnik) to over 36,000 [1]; detectable space objects are usually 10 cm in diameter or larger and are usually trackable by ground-based radar. Of these 36,000+ trackable objects, approx. 10,000 are operational spacecraft, mostly in large satellite constellations; the remainder are pieces of “space junk”. By “space junk”, we mean objects in space which either have outlived their usefulness or are no longer useful for other reasons, are the remnants of on-orbit explosions, or are pieces resulting from satellite or rocket booster break-ups for other reasons. Besides these detectable objects, there are hundreds of thousands of objects that are marble-sized and millions of objects the size of sand grains.
Since this space junk, which is also called ‘space debris’ or ‘orbital debris’, can no longer receive maneuvering commands, the survivability of other spacecraft traveling through or operating in Earth orbit can be jeopardized by the high-speed impacts of this debris. As a result, a major design parameter for Earth-orbiting spacecraft is the possibility of such high-speed impacts and the damage they can cause—even a marble-sized piece of debris can cause catastrophic damage or destroy a fully functional satellite.
A recent development since approx. 2019 is that several private companies have launched several thousand satellites into low- to middle-Earth-orbit. Although these satellites are geared primarily towards communications, they expand the World Wide Web into a space-based operation. While individual satellites provide only limited coverage areas, a system of satellites (also known as a “satellite constellation”) can provide a much wider area of coverage. According to publicly available plans (which are being frequently updated), we can expect upwards of 88,000 satellite launches in such satellite constellations within the next ten years [2]. Two questions quickly become evident, namely, what is the beneficial purpose of these satellite constellations, and what might be some of the deleterious consequences of their proliferation?
This paper addresses an important gap in the existing literature. While there are many technically oriented papers related to orbital debris and an increasing number of papers related to space law and its application to space debris, there are few, if any, papers that bridge the gap between these two areas by addressing both issues at the same time. This paper attempts to do exactly that. Namely, by providing an introduction to both areas herein, this paper creates a foundation that can lead to a bridge connecting these two distinct, but related, areas of space debris study.
In this paper, numerous topics related to space debris will be discussed, as well as issues in space law that concern the growing problem of orbital debris. In the end, several areas of concern will be noted that are vital to the continuing presence of humans in near-Earth space. These concerns must be addressed as the near-Earth orbital environment becomes more congested and space traffic management becomes exceedingly more difficult.
2. Space Debris—A Primer
A straightforward way of classifying all of the objects in space is in terms of their usefulness to someone or some organization—either they are useful, or they are not useful. The useful objects are, for example, all of the satellites that are still working; as described previously, the not useful objects are space junk. We will begin with a brief discussion of the useful objects, that is, the still-functioning satellites, and then move on to a discussion of space junk itself.
2.1. The Useful Objects
Since 1957, every few years, a new country has launched its own satellite for the first time and then has usually continued to do so until the present. Table 1 below shows a summary of first satellite launches by country (using its own rocket).
Table 1.
History of first satellite launches by country.
The ensuing launch rates of space-faring nations were relatively stable and consistent up until a few years ago—by that time, there were “only” several thousand rocket launches by a few dozen launching agencies. Around ten years ago, however, a few companies decided that it was in everyone’s best interests to launch several thousand (or several tens of thousand) of satellites into low Earth orbit. Current plans call for ~88,000 satellites to be launched over the next few years—so the near-Earth region of space is going to get a lot fuller very soon. Table 2 below summarizes the planned launches of satellites by various companies that are intended to form some of the larger “satellite constellations” [2].
Table 2.
Planned satellite constellation launches (approximate).
What has been the purpose or function of all these satellites that have been launched over the past 65+ years? Approximately two-thirds of them are for communication, whether it be personal, business, government, or military. Table 3 below shows the most common uses of satellites currently in Earth orbit and the percentage of such satellites for each of these uses [3].
Table 3.
Most common uses of satellites currently in Earth orbit.
Satellite constellations are needed in today’s techno-centric society, and there are several reasons why they should replace our present-day, aging satellite networks. For example, today’s satellites typically provide limited coverage areas—these coverage areas are either narrow bands (if they are in low Earth orbit) or small circles (if in geostationary orbit) on the Earth’s surface. Alternatively, a system or constellation of satellites can provide a coverage area that encompasses almost the entire globe. Our demand for faster and faster broadband internet connection is pushing the limits of today’s satellites. Streaming video content, streaming video games, HDTV, etc., are “bandwidth hogs”, and current satellites simply cannot keep up.
Furthermore, traditional $X00M satellites are very expensive and have a very lengthy design, manufacture, and launch timeline—so, by necessity, they are designed with a really long operational lifespan. The problem is, as a result of the very long lead time ahead of their launch, traditional satellites are frequently obsolete before they are launched and reach orbit! Furthermore, if one of these “old school” satellites is lost, you are really out of luck—your investment is gone. As an alternative to this situation, constellation satellites are much cheaper, can be built a lot quicker, and if you lose one, the loss is not as devastating [4].
Because of these and other considerations, satellite constellations are expected to result in, for example, improved internet access and communications in places that now have either slow internet or no internet at all (access will no longer be limited by fiber capacity), better assistance with disaster management (disaster mitigation managers require satellites with sensors that collect many different kinds of data), and assistance for astronomy-related studies without atmospheric interference (e.g., the BRITE/CANX-3 satellite-based research program, where on-orbit observation precision is more than 10X better than ground-based observations [5]).
With specific regard to disaster management, in order to be able to effectively deal with the many different disaster scenarios that can occur and their causes, disaster mitigation managers require data that has been collected over a wide spectrum of wavebands. Agricultural droughts, which frequently occur in the absence of atmospheric moisture (also known as clouds), for example, require optical and near-infrared data; however, tracking a hurricane and monitoring flooded areas beneath clouds require microwave sensors. Landslide studies, on the other hand, require accurate high-resolution digital terrain models that are created using stereo-viewing optical sensors, interferometric synthetic aperture radars, and light detection instruments, while the study of fires and volcanoes requires thermal imaging sensors. Clearly, then, to be as effective and efficient as possible, disaster mitigation managers and their teams need satellites that are built with sensors with the ability to collect data across their entire electromagnetic spectrum. This can only be accomplished by a satellite system or constellation with a suite of sensors [6].
From this discussion, it is evident that satellite constellations can surely benefit society in many ways, but the hope is that the companies who are launching all these satellites to achieve all these lofty goals have employed scientists and engineers that are really good at math and physics. They must, for example, ensure that their satellites not only stay out of each other’s way, but also do not interfere with other already resident and fully functional space objects.
As a result, a potentially seriously deleterious effect of launching all these satellites is that there will be a tremendous increase in the number of close approaches (CAs) that NORAD will have to monitor and report back to satellite owners. Using current threshold values, more than 25,000 warnings are expected to be issued each day, which must all be reviewed and adjudicated by the satellite operators that receive them [7]. This, in turn, could lead to an overall five-fold increase in the probability of an on-orbit collision [8]. Complicating matters is the effect of space weather on NORAD’s ability to accurately determine satellite and debris trajectories, which makes conjunction analysis exceedingly difficult. The launching of such “mega-constellations” will only complicate matters, of course, and has made clear the need to include the effects of space weather in satellite and debris trajectory modeling and CA calculations [9,10].
Figure 1 below depicts a satellite-filled sky that is now a reality and getting more and more crowded every day. This image adds together exposures taken over 30 min on a single night at a 51° N latitude. The field of view is about 100° by 75° (see https://www.instagram.com/amazingskyguy/p/C75atf0pmpu/ for additional infomation and details regarding the image. Accessed 2 January 2025.).
Figure 1.
The crowded night sky (Image © 2024 Alan Dyer/AmazingSky.com; used by permission).
At present, NORAD does not insist on any action (e.g., an orbit change, an avoidance maneuver, or just roll the dice and hope that nothing happens) by the owners of two or more satellites that might have a close approach—that is a decision to be made by the satellite owners themselves. Of course, the wrong decision could mean that the satellites expected to have a CA could collide and create several thousands of pieces of space debris, which leads to the discussion of this topic in the following section.
2.2. The Not Useful Objects
As can be imagined from the discussion in the preceding paragraphs, satellite collisions are one source of the debris circling the Earth. This debris is then a problem of our own creation, whether intended or not. So far, we have been lucky—we have not had an explosive growth in the amount of debris up in orbit; the growth has been steady, but not explosive. Nations that launch missions are aware of the debris problem and the need to limit the creation of additional debris. The steps taken to avoid the unnecessary creation of space debris, as well as the effectiveness of these mitigation strategies, vary from nation to nation.
For most spacecraft launches, a significant component of mission planning has always been a review of the entire mission and its orbits from the perspective of possible encounters with larger pieces of orbital debris. We did have a wake-up call, though, in 2009, when a spent Cosmos booster hit a (very much alive and functioning) Iridium satellite, destroying the satellite and creating approx. 2000 large (i.e., 10 cm or larger) pieces of space debris. A replacement was soon moved into position, but this event effectively served as a notice to the space community that such an event could happen again.
Figure 2 below shows that the debris population has been, overall, growing steadily, and that any time there is some artificial perturbation of the environment (as a result of an anti-satellite test, or ASAT, for example), the number of larger objects in Earth orbit leaps up by several thousand. For example, in Figure 2, the labeled increases in the number of LEO objects are sudden increases due to (1) the 2007 Fengyun ASAT test by China, (2) the accidental collision between Iridium 33 and Cosmos 2251 in 2009, and (3) an ASAT test conducted by Russia in late 2021. In early 2019, India performed an ASAT test as well, but the debris created by this test quickly decayed and fell back to Earth. By late summer, fewer than 50 trackable debris particles remained in orbit out of the several hundred originally created [11]. Also in Figure 2, the frequent minor perturbations in the plotted curves are likely due to debris fragments resulting primarily from debris-on-debris collisions in Earth orbit. Figure 2 also shows a recent steady increase in the number of LEO objects due to the steady launching of mega-constellation satellites since 2019.
Figure 2.
Change in the number of objects in Earth orbit. (Image credit: Orbital Debris Quarterly News, July 2024 Issue.)
2.3. The Hazards Posed by Space Debris
The extent of the damage caused by the impact of a piece of space debris on a functioning spacecraft or satellite depends on how big it is and what it is made of. Depending on the size of the impacting debris particle, it can either cause minor damage like a small hole (if it is the size of a grain of sand, for example) or destroy an entire satellite or spacecraft (if it is the size of a cricket ball, for example). But, of course, catastrophic damage can also come from small collisions—imagine the mirror of the Hubble Space Telescope being struck by something, even something as small as a sand grain, traveling at orbital velocity. The telescope as a whole might survive, but its mirror could be destroyed if the impact were to occur in a critical location. So, in general, while larger pieces cause more damage, the impact of a tiny piece of debris can also render a functioning spacecraft useless, depending on where the impact occurs.
Why are impacts by pieces of space debris such a problem? One reason is that orbital debris travels at speeds in the order of 8 km/s, so the closing velocity between a piece of debris and a satellite can be as high as 16 km/s. This energy has to be absorbed by whatever the debris particle hits, which can cause catastrophic failure.
A common misconception is that space is big and empty, so why do we even worry about being hit by anything in that vast emptiness? The reality is that debris particle impacts happen all the time. Luckily, nearly all of these impacts are by very small particles, so they are hardly noticed. As a result, the question is not IF a satellite will get hit by a piece of debris, but HOW BAD will the damage be WHEN it does get hit. Spacesuits, for example, can be punctured by a projectile 1 mm in diameter [12]—this makes EVAs a very tricky business!
Figure 3 below shows a portion of the International Space Station (ISS), along with several impacts (quite a few, actually) on one of its radiators.
Figure 3.
Multiple impact sites (red circles) on the International Space Station. (Image Credit: NASA.)
So, spacecraft designers work very hard to protect the spacecraft we send into LEO, especially the ones with humans on board, like the ISS. Debris travels slower in GEO (in the order of only several hundred m/sec), so satellites in GEO typically do not have a lot of protection. Satellite protection in LEO is often in the form of shielding that is placed on the exterior of a spacecraft, which is basically there to disperse the energy of an impact when it happens. On occasion, a threatened satellite or spacecraft, such as the ISS, is able to move out of the way of a piece of debris that is thought to be on a collision course with it. This “collision avoidance” technique is one type of “active protection” of our space assets (shielding is referred to as “passive protection”, since, like a car bumper, it is there “just in case” you need it).
Figure 4 below shows a summary of the number of times that the ISS has had to implement a collision avoidance maneuver since the mid-1990s—an average of 2–3 times a year. The blue circles indicate the number of potential orbital-path-crossing objects that have come to NASA’s attention over that same time period—a number which, overall, is seen in Figure 4 to be steadily increasing. Needless to say, this is a very dangerous trend!
Figure 4.
Number of ISS-orbit-crossing objects tracked by the ISS (blue circles), the Solar F10.7 daily flux (black dots), and the history of ISS collision avoidance maneuvers (orange histograms) (Image Credit: Orbital Debris Quarterly News, August 2020 Issue).
Of course, shields protect the working satellites in Earth orbit, but they do nothing to keep defunct objects from falling back down to Earth, which they do from time to time. As a result, space debris is not just a problem for satellites and astronauts—it is also a problem for the people living on Earth. In addition to physical harm to animals and people, hazards posed by debris that falls to Earth include pollution of the atmosphere, airline flight delays or cancellations, and toxic or radioactive debris on the ground, among others. Unfortunately, we cannot predict exactly where on Earth the falling debris will land—the best that we can do is estimate where it might land and then use population density and the probability of impact distribution to calculate the on-ground casualty risk. Figure 5 below illustrates, in a general sense, the re-entry of a non-functioning satellite and highlights some of the phases during that re-entry, along with the uncertainties involved with predicting its (or its pieces) landfall.
Figure 5.
Re-entry of a spent satellite, including landfall uncertainties (Image Credit: The Aerospace Corporation).
2.4. What Can We Do About This Problem?
When confronted with the task of cleaning up the near-orbit environment of all the debris that is in it, there are a number of options and factors to consider. These factors might include, for example, the feasibility of the effort (e.g., cleaning up debris in GEO is a different challenge than cleaning up debris in LEO) and time criticality (e.g., a large and object that will re-enter shortly and has been designed to disintegrate upon re-entry versus a small one with an exceedingly long orbital lifetime versus one that poses a ground risk because it will likely not disintegrate upon re-entry).
An early discussion [13] of various debris removal technologies and their effectiveness shows that when choosing between (1) cleaning up small debris particles or (2) cleaning up large spent rocket boosters and large satellites, while the former might be easier than the latter, the right thing to do is, in fact, the latter and not the former. Simply put, to effectively reduce the debris population, we need to bring down all the big objects that are up there first, then, as time and resources allow, address the smaller objects. After all, if you spend all your time and money chasing down and cleaning up millions of little pieces of junk, all it takes is a single collision between two large boosters to immediately put all those millions of small pieces back in orbit. Ref. [14] provides an overview of various more recent approaches and technologies under consideration and actually developed by a number of space agencies and private industries to clean up the space environment.
If debris population growth is unchecked or not sufficiently controlled, there is the very real possibility of the Kessler Syndrome. In this scenario, a runaway or chain reaction of impacts between debris pieces and satellites or other debris pieces in earth orbit would create a permanent and opaque cloud of debris dust [15]. This would, in turn, render some regions of Earth orbit unusable, thereby threatening the sustainability of space activities for many generations to come. This possibility underscores the need for all countries to engage in coordinated activities of space traffic management and reduction in debris generation to enable continuing space operations in Earth orbit.
3. Some Thoughts on Legal Issues Related to Space Debris
Satellites will get hit by pieces of space junk. As mentioned previously, this is not a matter of IF, but rather of WHEN and HOW BAD the damage will be. Some of these impacts will result in catastrophic failure or the destruction of that satellite. Since the building and launching of a satellite (or satellite constellation) can run into the USD 100 millions, it is only natural to attempt to recoup that loss, and either try again or try something else. Legal issues related to ownership of the “satellite-killer” junk do come up, as well as to where that junk came from, how it came to be junk, and who was responsible for that. Answers to simple questions like why? and how? quickly become a tangled mess of cause and effect that is often impossible to unravel.
For better or worse, there appears to be a lot of leeway in what countries can or cannot do in outer space, despite the existence of quite a few treaties that have attempted to define the boundaries of acceptable behavior. Some of the key international treaties covering issues to space and its use are listed as follows:
- (1)
- Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (aka The Outer Space Treaty, or more simply, as “the OST”), 1967.
- (2)
- Agreement on the Rescue of Astronauts, the Return of Astronauts, and the Return of Objects Launched Into Outer Space (aka The Rescue Agreement), 1968.
- (3)
- Convention on International Liability for Damages Caused by Space Objects (aka The Liability Convention), 1972.
- (4)
- Convention on Registration of Objects Launched into Outer Space (aka The Registration Convention), 1974.
There are also various organizations that deal with space debris (some based in specific countries of origin, some international in scope), and they have their own guidelines and/or requirements. These organizations are, to name a few, as follows:
- (1)
- Inter-Agency Space Debris Mitigation Coordination Committee (IADC)
- (2)
- Committee on the Peaceful Use of Outer Space (COPUOS)
- (3)
- United Nations Office for Outer Space Affairs (UNOOSA)
- (4)
- International Law Commission (ILC)
- (5)
- International Academy of Astronautics (IAA)
These organizations have, on their own, also developed numerous policy statements regarding the appropriate and shared use of space. With regard to space debris, these policies include the following:
- (1)
- Space Debris Mitigation Guidelines, IADC, 2007 [16].
- (2)
- Space Debris Mitigation Guidelines of the Committee on the Peaceful Uses of Outer Space, COPUOS, 2010 [17].
- (3)
- Space Debris Mitigation Requirements Standard, ISO, 2023 [18].
- (4)
- Guidelines for the Long-term Sustainability of Outer Space Activities, COPUOS, 2021 [19].
The reader is referred to the paper entitled, “Space Law and Hazardous Space Debris”, authored by Dr. Martha Mejía-Kaiser from the International Institute of Space Law [20]. This paper is a very comprehensive treatment of current issues related to space debris from a legal perspective. The following are some thoughts, comments, and questions that arise from a review of the information in that paper. Issues related to those discussed herein are presented in some detail in Ref. [21]. Additionally, discussions regarding the legal issues related to space debris, including ownership, liability, and clean-up, now appear more frequently in the proceedings of a number of conferences, including the quadrennial European Space Debris Conference (sponsored and organized by ESA) and the quadrennial International Orbital Debris Conference (sponsored and organized by NASA, Washington, DC, USA).
To begin, once a space object is considered to be “hazardous”, common sense might dictate that the “launching state” for that space object should be responsible for reducing or removing that hazard. However, the question then arises regarding how is it decided that an object is “hazardous” and by whom? Complicating matters is the realization that there is currently no internationally agreed definition of what “hazardous” really means!
Interestingly enough, space treaties (those mentioned above, as well as others) neither define space debris, nor do they expressly and specifically address it by name. In this case, what organization has the internationally recognized legal authority to say how states should manage the different sorts of hazards posed by space debris, when it is not even called out or given formal standing? Of course, next come the questions regarding who will enforce those specified debris management approaches? How?
Regarding debris that falls down to earth, the reality is that most of the debris pieces that re-enter the atmosphere burn up during re-entry. This is especially true of structures and structural materials with relatively low melting points (e.g., aluminum). However, large pieces or small ones made of heat-resistant materials, such as silicon carbide or glass reinforced composites, can land on Earth relatively intact [22]. Because these re-entries, especially for the latter parts of the re-entry process, cannot be controlled, that is, one cannot control the entry flight trajectory, it is not possible to know precisely where such a re-entering piece of debris will make landfall. The (potentially highly contentious) point of interest here is that countries are not obligated to inform each other about imminent atmospheric re-entries. Launching countries are only *recommended* to notify other countries that they may be in danger of the atmospheric re-entry of a space object only if that object contains radioactive materials.
Compensation for damage caused by space objects (either falling or in orbit) is discussed in the Liability Convention. There, damage is defined as “loss of life, personal injury or other impairment of health, or loss of or damage to property of states, or persons, or property of international inter-governmental organizations”. However, no mention is made of what legal and financial framework is to be put in place to assess that damage, how that compensation is to be sought, and how (and by whom) it is to be enforced.
Unfortunately (at least, from the perspective of someone who has worked 35+ years developing systems to protect spacecraft and astronauts from the hazards posed by space debris), the OST falls short of a legally binding obligation for countries that launch satellites or other spacecraft into earth orbit to avoid the generation of additional space debris, to minimize the associated risks of orbital space debris, or to reduce the hazard posed by space debris to functional space objects and to humans who live, work, or travel in space.
However, the OST does state that countries that launch a space object have “jurisdiction and control” over that object. The question naturally arises regarding how long this “jurisdiction and control” lasts and what are its ramifications when it comes time to remove a non-functioning satellite or other large piece of space debris from orbit. A number of issues and concerns arise when considering the removal of defunct space objects, especially when the launching country and the removing country are not the same country. Two such issues/questions are noted as follows:
- (1)
- What if a space debris removal mission results in an uncontrollable atmospheric re-entry of the removed object, and then object remnants cause damage to land or property in another country or to aircraft in flight?
- (2)
- What if a failed removal operation results in collisions in outer space that, in turn, result in the creation of even more space debris objects?
In other words, by trying to make things better, things could actually get worse. If that were to happen, which country should be the one to pay compensation for the ensuing damages or worsening situation? The country that tried to make it better by cleaning things up, or the country whose junk it was that the other country was attempting to remove or clean up? Drilling down a bit more, the country whose junk was being removed could argue that its defunct or derelict satellite was rendered as such by the impact of some object from a third country. NOW which country is liable as the responsible party?
In the end, it would appear that even if a space object is considered by a country (other than the country that launched the object) to be “junk”, it seems like it cannot be (or in the least, should not be) captured, removed, or even moved to another orbit, without the express permission of the country having “jurisdiction and control” over that object. States which do not wish to cooperate in the removal of their hazardous space object therefore present a serious challenge. In such a situation, what is to be done?
4. Conclusions
The problem of orbital debris is, indeed, a multi-faceted problem with technical, political, social, and legal implications. Of the 36,000+ trackable objects in low Earth orbit, just a few thousand are working spacecraft. The rest are Earth-orbiting objects which are no longer operational and are considered to be space junk. Because this junk can no longer receive maneuvering commands from their Earth-based owners, the survivability of other spacecraft traveling through or operating in Earth orbit can be jeopardized by the impacts of any number of pieces of this space junk whose origins can be traced back to these defunct satellites. As a result, a major design parameter for Earth-orbiting spacecraft is the possibility of such high-speed impacts and the damage they can cause. Furthermore, several private companies are now launching several thousand spacecraft into Earth orbit, many of which are satellites built for communication purposes. Other satellites have been launched to expand the reach of the World Wide Web, provide better tools for disaster management, and to increase the availability of opportunities for additional collaborations among astronomers. Several topics related to space debris have been discussed in this paper, including issues in space law that concern the growing problem of orbital debris. Several key areas of concern have been noted, involving key legal questions in areas related to space traffic management that have been created by the increased congestion of Earth’s orbital environments.
Funding
This research received no external funding.
Data Availability Statement
No new data were created or analyzed in this study. Data sharing is not applicable to this article.
Conflicts of Interest
The author declares no conflicts of interest.
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