Space Traffic Management

Abstract

This chapter gives an overview of the evolution of the near-Earth space environment since the beginning of the space age, discusses the current situation, and projects how future developments such as the growing space debris population and active debris removal will affect that environment. Just as the growth in air travel led to air traffic management, assuring that future space systems will have minimal interference to their operations requires a system to warn operators of potential collisions and other hazards. The chapter discusses components of a space traffic management system and the corresponding legal and policy framework.

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1 Introduction

Space traffic management is defined here as an organized process that assures the long-term use of space and space assets without harmful interference. Space traffic management includes policies, regulations, services, and information that:

• Minimize the possibility of short- and long-term collisions, radio frequency, or other interference among orbiting objects, both operating satellites and debris

• Assure compliance with rules and regulations imposed by governments and with best practices adopted by launch and satellite operators

• Minimize interference with and by non-satellite operations such as ground-based telescopes and directed energy sources

• Provide warnings to minimize possibilities of loss of operations or other detrimental effects resulting from space weather and other predictable events

This chapter gives background on the rationale for space traffic management, discusses the various components and requirements, and provides a status and possible future directions.

2 Evolution of a Problem

The first human-made satellite was launched in 1957 by the Soviet Union. At that time, there was no issue with that object colliding with another human-made object (except the very remote possibly that hardware used to loft the satellite onto orbit or released during its deployment might pose a threat). Of course, early satellites were not the most reliable, and many simply died in their mission orbits. Accepted practice was simply to leave hardware in orbit; very little thought was given to the fact that some of these space objects would remain in orbit for hundreds or thousands of years and could one day cause damage to valuable space assets.

On occasion, some satellites and rocket stages exploded, and fragments from these events joined non-exploding, nonfunctional hardware as “space debris” in orbit. As the space age evolved, deployment systems used metal bands that were exploded to release satellites; lens covers were released and floated away; exploding bolts released clouds of small fragments as sensors deployed; experimenters released clouds of “needles” to test communication strategies; and blobs of liquid metal leaked from nuclear reactors that were left in high-altitude orbits for disposal. The “big sky theory” said that the chance of one of these objects colliding with another object or with an operating satellite was vanishingly small – and it was in the early years.

As Fig. 12.1 shows, the use of space increased as time progressed, and the number of operating satellites now accounts for about 1,230 of the tracked objects (objects larger than about 10 cm in size) currently in orbit. About 850 of these are in low Earth orbits (LEO), orbits within 2,000 km of Earth’s surface, and 320 are in geosynchronous equatorial orbits (GEOs), circular orbits with altitudes of approximately 35,786 km and inclinations near zero degrees. Smaller populations of objects exist in highly elliptical GEO transfer orbits, with apogees approaching geosynchronous altitude and perigees in the LEO region. There are also approximately 60 operating satellites in orbits other than LEO and GEO.

Fig. 12.1

Number of tracked objects (objects larger than about 10 cm) versus time

Fortunately, Earth’s atmosphere provides a mechanism for removing some objects from orbit. The very small drag force encountered by objects in or passing through the LEO regime gradually lowers their orbits and causes these objects to reenter and “burn up” as aerodynamic heating and loads increase. Note the periodic “dips” in the growth in the number of tracked objects shown in Fig. 12.1. These dips result from the small increase in the atmospheric drag during periods of high solar activity, which result in increased atmospheric density at higher altitudes and therefore earlier reentry. Of course, some fragments survived these reentries, but no injuries are known to have occurred, and probabilities of injuries are extremely low, but not zero – more on this later.

Figure 12.2 shows how the atmosphere affects the lifetime of objects. This figure was developed assuming an average atmosphere and a space object with a ratio of area to mass similar to that for the International Space Station. While the figure shows the orbit lifetime for objects in circular orbits, for objects in elliptical orbits with low-altitude perigees, the small aerodynamic forces during perigee passes will gradually lower the orbit’s apogee and cause a relative early reentry. As might be expected, the lifetimes of objects in higher circular orbits (and orbits with higher perigee altitudes) are substantially longer – debris objects at these altitudes will remain threats to operating satellites for very long times.

Fig. 12.2

Variation of satellite lifetime as a function of altitude (assumes circular orbits, average atmosphere, and satellite ballistic coefficient of 150 kg/m²)

As a result, our first 55 years in space have left our planet surrounded by millions of human-made objects with sizes ranging from tiny flecks of paint to satellites and rocket stages several meters in length. Given this population, the “big sky theory” no longer applies, particularly in the LEO regime, where models predict an increasing frequency of collisions as more objects are added. Events are occurring that support these predictions.

2.1 Space Object Collisions

The first confirmed accidental collision event was in 1991 when a nonoperational Russian Cosmos navigational satellite collided with debris from a sister Cosmos satellite. The first known collision involving an operational satellite was the collision of Ariane launch stage debris in July 1996 that severed a boom on the French scientific satellite Cerise. Cerise returned to limited service after the collision. Using the best data tracking available at the time, the probability of the Cerise collision was tiny – on the order of 2 in a million (Alby et al. 1997).

Collisions of satellites are bad for two main reasons. First, objects in orbits, particularly in LEO, have orbital velocities in the range of 7–8 km/s, and two approaching objects can be in orbits in different directions, causing the relative velocities at impact to be as high as 10–14 km/s. At these velocities, even a very small object such as a fleck of paint can pit a window (see Fig. 12.3), damage a critical sensor, or affect solar panel performance, and an object as small as 1 cm in size, the size of a pencil eraser, can terminate operations of a functioning satellite if it strikes a critical area. Secondly, collisions involving both operating satellites and debris release more fragments, increasing the population of orbiting objects and increasing the likelihood of future collisions. Many of these fragments will remain in orbit and pose threats to other satellites for years.

Fig. 12.3

4-mm-diameter crater on the windshield of the Space Shuttle orbiter caused by a fleck of white paint approximately 0.2 mm in diameter impacting at a relative velocity of 3–6 km/s (NASA photo)

In the late 1970s, concern began to grow that the population of orbiting debris objects would continue to grow to the point where the total population of objects in orbit would reach a tipping point – a point beyond which the population would continue to grow due to collisions among the existing objects even if no new satellites are launched (Kessler and Cour-Palais 1978). It was concluded that some form of space debris mitigation must be initiated before we reached that point.

2.2 Debris Mitigation Begins

These and other concerns led to the establishment of the Inter-Agency Space Debris Coordination Committee (IADC) in 1993 to coordinate international research into the problem and develop guidelines for space hardware design and operations that would slow the growth of the debris population. Working through the IADC, space agencies have identified the orbital regions shown in Fig. 12.4 as “protected regions,” regions that have unique mission-related characteristics and should be protected “with regard to the generation of space debris.” These are (1) the low Earth orbit (LEO) region, a spherical region including orbits below 2,000 km in altitude, and (2) the geosynchronous region, a portion of a spherical shell that includes the altitude of the geostationary Earth orbit, Z_GEO, 35,786 km.

Fig. 12.4

Protected regions (Z_GEO = 35,786 km)

In addition, the IADC developed the following space debris mitigation guidelines:

• 1. Limit debris released during normal operations by preventing the release of lens covers and other hardware associated with satellite deployment and operations.

• 2. Minimize the potential for on-orbit breakups, either planned or accidental, which will generate long-lived debris. This includes preventing accidental explosions and ruptures during normal operations or at end of mission, as well as planning or conducting intentional destructions that would generate long-lived debris. Spacecraft and orbital launch stages should be passivated (i.e., all sources of stored energy should be depleted) prior to end of life.

• 3. Dispose space hardware at the end of its mission. Typically, this means that satellites and orbital launch stages in or passing through the LEO protected region should either be deorbited into a safe ocean area (controlled reentry – the preferred approach) or, if the casualty expectation for an orbit decay reentry does not exceed a specified limit, moved to an orbit that will naturally decay leading to an uncontrolled reentry within a prescribed time frame (25 years is used in some countries). Note that some nations use a casualty expectation of 1 × 10⁻⁴ as the limit above which deorbit into a safe area is required. The casualty expectation is the number of injuries or deaths worldwide associated with an object’s reentry. Satellites operating in the GEO protected region should be maneuvered at end of mission such that their orbits remain above the GEO protected region for at least 100 years.

• 4. Prevent on-orbit collisions. Designers of spacecraft and orbital launch stages should estimate and limit the probability of collision during the space vehicle’s orbital lifetime, and if reliable data is available, operators must maneuver the space vehicle to avoid collisions with known objects. In addition, space vehicle designers should take steps to limit the possibility that impact of small untracked debris will cause loss of control and prevent end-of-mission disposal and passivation.

These guidelines have since been incorporated in government regulations and captured in best practices and international standards (e.g., ISO 24113: Space Systems – Space Debris Mitigation).

2.3 Collision Avoidance

As humans began visiting and spending significant amounts of time in LEO, safety concerns were raised which led to the development of tools to predict when collisions of other objects with crewed vehicles might be possible. Based on these predictions, the US Space Shuttle was moved several times to avoid collisions, the crew of the Mir space station was ordered to move to their escape pod during close approaches, and more recently the International Space Station (ISS) was maneuvered away from approaching threats. On several occasions, the ISS crew was ordered to move into the crew return vehicles until the collision threat passed.

These close approaches to crewed spacecraft did not go unnoticed by operators of other spacecraft, and in the mid to late 1990s through the early 2000s, two Federally Funded Research and Development Centers (FFRDCs) offered prototype satellite collision avoidance services to commercial and international satellite operators. MIT Lincoln Laboratory provided high-precision services to several operators via Cooperative Research and Development Agreements (CRADAs), and The Aerospace Corporation offered a service combining operator-provided data with publicly available data on tracked objects. The goal of both organizations was to gather information on operator needs for collision avoidance services and to develop recommendations for how the US government might provide such services in the future.

The seriousness of the growing population of orbiting objects was highlighted again in 2007 when China tested an antisatellite capability by striking the Chinese Fengyun-1C polar-orbiting weather satellite operating in an 855 km orbit with an interceptor launched from Earth. This collision added over 3,000 tracked objects to the debris catalog (see the jump in the number of tracked objects at that time in Fig. 12.1) and many of these objects remain in orbit today. Of course, the collision also resulted in a larger number of objects that are hazardous but cannot be tracked due to their small size.

2.4 First Major Collision

In 2009, a second event occurred that was a game changer: a dead Russian satellite, Cosmos 2251, collided with the operating Iridium 33 satellite at a relative velocity exceeding 11 km/s. The collision ended the life of the Iridium 33 satellite and added over 2,000 tracked debris fragments to the catalog of tracked objects. The event likely resulted in the addition of a much larger cloud of smaller objects to the orbiting population. In this case, pre-collision tracking data noted the close approach, but using the best-available tracking data, the probability of the two objects actually colliding was estimated to be in the one in 100,000 range – a risk level where an avoidance maneuver was judged not warranted.

The Iridium-Cosmos collision and the ASAT test alerted space-faring nations that, in addition to creating clouds of debris, collisions could have economic consequences and satellite operators increased requests for collision predictions that were “actionable” – that were accurate enough for threatened satellites to take evasive maneuvers.

2.5 Effects of Debris on Cost of Operations

A recent study (Ailor 2010; Ailor et al. 2010) describes how the evolving debris environment would affect the costs of space operations over the next 50 years. The study assumed that constellations of satellites are placed in circular orbits at 850 km altitude – the region where, as Figs. 12.5 and 12.6 show, the density of tracked objects (hence the probability that a collision will occur) is the highest. Each constellation was maintained for 20 years after beginning initial operations in 2010, 2020, and 2030. The study looked at the cost of replacing satellites damaged by small debris that would “sandblast” solar panels and reduce their power output and by larger debris that might strike a critical component or destroy an entire satellite. The study utilized debris models that projected the current debris environment into the future with satellite launches and collision rates consistent with current predictions.

Fig. 12.5

Number density of tracked objects as a function of altitude. Peaks are in the LEO, Medium Earth Orbit (MEO), and GEO regions

Fig. 12.6

Number density of tracked objects in the LEO protected region as a function of altitude

As might be expected, frequent strikes by small debris led to replacement of the most satellites; collisions with larger tracked objects were much less frequent. As a result, the greatest costs increases, up to 18 % higher than operating in a no-debris environment for satellites launched in 2030, resulted from replacing satellites due to damaged solar panels. Increasing the robustness of solar panels reduced the cost increase to 10 % or less. The study also found that a collision avoidance service would lower the overall cost increases due to the debris environment by approximately 10 % (the service would prevent collisions of satellites in the constellations with other larger tracked objects, but not the smaller untracked objects).

The results noted above show that limiting the growth in the population of very small objects will be important to minimize costs, but operating in the debris environment is manageable for the next half century or so, assuming that the current launch rate continues and that proper end-of-mission disposal becomes accepted practice. But some possible future developments may exacerbate the current situation. For example, small satellites such as nano-satellites or Nanosats (1–10-kg mass) and pico-satellites or Picosats (0.1–1-kg mass) (see Fig. 12.7a, b) are being developed by university and other researchers and carried to orbit as secondary payloads (Janson 2011), hitching rides on launches delivering larger payloads to orbit. While not debris by definition, these objects are relatively inexpensive to build, many can be carried on a single launch, they may not be maneuverable, and most are at the lower bound of our current tracking capability. As a result, they could be a significant collision hazard to other operating satellites.

Fig. 12.7

(a) A picosat (b) Another picosat (Photos courtesy The Aerospace Corporation)

Figures 12.8a, b show the number of these satellites being launched versus time. There are also proposals (Bekey 2006; Iida and Pelton 2003) for future radar and communications satellite systems in GEOs with arrays of hundreds to a hundred thousand of very small, tethered or untethered Picosats flying in fixed formations that extend 50 km or more. Clearly, proper disposal of these small satellites at end of mission will be critical over the long term, and entities monitoring space traffic and providing space traffic management and collision avoidance services must evolve their tools and capabilities as these types of systems emerge.

Fig. 12.8

(a) Number of Nanosats launched versus time (b) Number of Picosats launched versus time

2.6 Other Threats to Normal Operations

Collisions are not the only threats to normal operations of operating satellites. Other threats are:

• Close approaches of satellites are known to have confused sensors that fix satellite orientations, causing loss of communications with ground stations. Since satellites transmit critical data and provide coverage of important events, loss of communications can be a serious problem and can be avoided with sufficient information.

• Radio-frequency interference (RFI) caused by an uncontrolled but broadcasting “zombie” Intelsat Galaxy 15 satellite has caused satellite operators to take evasive measures. Similar to loss of communications, RFI can seriously affect normal operations.

• Impingement of ground-based laser energy can damage sensors (and satellites passing in front of ground-based telescopes can damage telescope sensors, as well).

• Constellations of satellites are planned without regard for the physical presence of other operating satellites in the same vicinity. (There are controls related to limiting the radio-frequency interference of satellites; these controls do not prevent two satellites using non-interfering frequencies from occupying the same physical space.)

Finally, natural space hazard events can also affect normal operations of space systems, and operators can mitigate these effects if provided sufficient, timely information. As an example, in 1999, an event occurred that could have affected many satellites: a yearly passage of Earth through the trail of debris shed in previous passages of comet Tempel-Tuttle. The 1999 passage was predicted to possibly cause a meteor storm on Earth and shower satellites with tiny, fast-moving (70 km/s) comet “dust.” During this event, some operators took precautions, including “feathering” solar panels (orienting solar panels parallel to the oncoming meteoroids) and disabling attitude control to prevent activation by electric impulses that might result from impact of high-speed particles. These last actions were in response to a mechanism that might have caused the loss of the European Olympus satellite during the Perseid meteor shower event in 1993, where impact of a micrometeoroid on the satellite’s solar panel was suspected of creating charged plasma and a resulting current spike that activated the spacecraft’s attitude control jets (Caswell et al. 1995). This action depleted propellant for the attitude control system, making the satellite uncontrollable.

2.7 The Future

2.7.1 Active Debris Removal

A recent study predicted that “the debris environment in low Earth orbit (LEO, defined as the region up to 2,000-km altitude) has reached a point where the debris populations will continue to increase even if all future launches are suspended” (Liou et al. 2010). This study shows that active debris removal (ADR) of at least five objects per year combined with implementation of the space debris mitigation measures for new space systems recommended by the IADC will stabilize the LEO environment in the next 200 years. The study suggests that “an effective removal strategy can be developed using a selection criterion based on the mass and collision probability of each object.” ADR would likely focus on dead satellites and orbital launch stages in LEO that would be the sources of many additional debris objects in the event of a collision. The study notes that “the fastest debris growth region is between 800 and 1,000 km altitudes, where massive payloads and rocket bodies currently reside, and higher collision probabilities are expected. Even without specifying altitude, the removal criterion based on mass and collision probability effectively reduces the population growth in that critical altitude regime.”

ADR designs being proposed include some that might use the force generated by the interaction of Earth’s magnetic field with current flowing along long tethers to slowly decrease the altitude of attached debris objects. Other proposals would use lasers to illuminate small debris in LEO to gradually lower their orbits toward reentry or would expel clouds of “dust” or “mist” to intercept and gradually lower small debris objects. At GEO, space tugs are proposed as a means to refuel satellites and to move dead or dying satellites away from the GEO protected region. As is evident, development of these services must be accompanied by the development of policies and regulations that limit the creation of additional debris during grappling and other operations of such devices, plus services that minimize the possibility of interference with operating satellites and collisions with other objects.

Some of these ADR activities, most of which are in very early research and development stages, will challenge services designed to assure that operating satellites avoid collisions with other objects. For example, collision avoidance service providers will need tools and capabilities to predict collisions with tether systems that may be kilometers long and whose orbits will be gradually changing. Predictions will require implementation of new tools plus the ability to incorporate frequent updates on the positions and orbits of the tether systems and on orbiting objects that might be threatened. Long tethers will also be susceptible to breakage if intercepted by space debris, so predicting such interference for tracked debris objects and providing timely and accurate warnings to tether system operators will also be required.

2.7.2 Disposal Hazards

One of the first ADR activities may be to move large objects to lower altitudes where the atmosphere will cause their orbits to decay and they will reenter randomly, never to be seen again. Unfortunately, current estimates are that anywhere between 10 % and 40 % of an object’s dry mass (i.e., not including the mass of propellant or pressurized gases) will survive to impact the ground.

Figure 12.9 shows an example of hardware that can survive a reentry. In this case, a 250-kg stainless steel propellant tank that had been part of an orbital launch stage landed 45 m from a Texas farmer’s house after 9 months in orbit. No one was injured in this reentry, and in fact, no injuries or deaths have been reported resulting from space hardware reentries since the beginning of the space age. Should there be an injury or death from such an event, the launching state would be liable according to international space treaties from the 1960s and 1970s. However, if the object had been touched by an ADR entity, the legal community would certainly question whether the ADR entity had the right to interfere with such an object, and the ADR entity might be liable if it interacts with the object – more on this in Sect. 12.4.

Fig. 12.9

Reentered debris (Photo courtesy NASA)

As is evident, there are substantial nontechnical issues associated with ownership and liability that must be resolved as ADR concepts move forward.

2.7.3 Expanding Services

Future activities that might also emerge and challenge future space traffic management activities include:

• Space-based services such as space tourism and hotels, debris salvage and removal services, asteroid mining services, and possibly orbital “factories” that would use the gravity-free environment for the manufacture of new materials and medicines and shuttle products to and from Earth using small, autonomous vehicles

• Space elevators using cables extending from the Earth’s surface to GEO altitudes and beyond

• Tether systems that operate in LEO and GEO regimes

• Constellations of very small satellites for communications and other missions (see Sect. 12.2.5)

• Orbiting objects preserved for historical purposes and as museums

3 Components of Space Traffic Management

As is evident, human use of space is evolving in similar fashion to human use of resources on Earth: a new resource is discovered, new technologies are developed, there are consequences that were not anticipated, and subsequent management and controls are implemented to assure continued safe use of the resource for future generations. Typically, careful management requires increased focus on the safety of users and the public, cooperative actions among users, and development of appropriate mitigation services, rules, and regulations to minimize hazards. Near-Earth space is such a resource. Over the last 50-plus years, capabilities have evolved that provide the basic foundation for space traffic management services. These basic requirements are discussed below.

3.1 Knowledge of the Orbital Environment

Similar to information that helps airline pilots avoid adverse weather conditions, providing satellite operators with information that can help them minimize possibilities that space weather (e.g., micrometeroid storms, solar storms) might cause the loss of a satellite or the loss of a satellite’s mission is an important part of an overall space traffic management (STM) system. The STM system would alert satellite operators of natural events that could threaten normal operations.

As has been discussed, human-made objects are becoming a significant part of the orbital environment. Minimizing interference and collisions requires knowledge of where objects are at any given time, where they will be in the future, and some basic characteristics of each object (e.g., is the object an active satellite or space debris). Fortunately, such information exists, with the catalog of space objects maintained by the United States being the most complete. Data from the US Space Surveillance Network is used by the Joint Space Operations Center (JSpOC) located at Vandenberg Air Force Base in California to maintain a catalog of resident space objects. This catalog includes orbits and basic information on objects as small as 10 cm in size in LEO (upgrades are planned that will lower the size limit) and 1 m in size in GEO. A version of this catalog is available to the public via the Space Track website (http://www.space-track.org).

While that catalog is the most complete in the world, it has limitations for certain aspects of STM. In particular, the catalog provides orbit information based on tracking data collected periodically by radar and optical sensors and does not contain position and maneuver information provided by satellite operators (operators have best and most current information on where their satellites are and where they will be in the near future). Thus, satellite maneuvers after the last tracking data were collected are not reflected in the catalog. This information is critical when predicting possible close approaches involving operating satellites. Secondly, the catalog available to the public is not the most accurate, and the software required to propagate the most accurate catalog is also not available. Finally, the catalog available to the public does not contain the total number of tracked objects. Thousands of objects that have not yet been correlated to an owner or event are not included (this is the reason for statements that there are over 20,000 tracked objects, but the number of objects in the public catalog is nearly 16,000, as Fig. 12.1 demonstrates). As a result, even though discovered and tracked, debris from collisions and explosions may not appear in the catalog for weeks or months after the event that created the debris. Needless to say, any service using the publicly available data has limited ability to provide collision avoidance or other interference warning, particularly for objects in the LEO regime where the risk of collision is highest.

In addition to the US Space Surveillance Network (SSN), catalogs have been developed by other nations, by private individuals, and by partnerships that combine data to focus on specific orbital regions. Foremost in this latter group is the International Scientific Optical Network (ISON), which is being organized by the Keldysh Institute of Applied Mathematics (KIAM) of the Russian Academy of Sciences. ISON has established partnership agreements linking 23 observatories in 11 countries operating more than 30 optical instruments (Molotov 2011). Since 2004, ISON has “concentrated on developing and operating the international network of optical instruments capable to search and track faint space debris objects on higher geocentric orbits. The aim is improving our knowledge about pollution of unique regions of the near-Earth space (first of all, GEO) due to launches, on-orbit operations, explosions, deterioration of the spacecraft outer surfaces in time, etc.” The network includes subsystems to study bright objects in LEO, HEO, and GEO and maintains a database of space objects.

3.2 Standard Formats for Data Exchange

A major consideration for developing catalogs of tracked objects is to be able to compare data in each catalog with data for the same object in other catalogs. It also means that data that is shared among satellite operators and with collision avoidance service providers should utilize standard formats. Data is currently exchanged among operators and service providers, and international standards for exchanging some information have been developed (e.g., ISO 26900:2012 (CCSDS 502.0-B-2) CCSDS Orbit Data Messages). Standards will be required that capture rules-of-the-road for satellite maneuvers for avoiding interference, for propagating operational and disposal orbits, for techniques predicting close approaches, and for other features of STM.

3.3 Best Practices

While services defined above will provide information to operators on possible collisions or other interference, best practices and rules-of-the-road are required to assist operators in deciding who will move, when they will move, and how will they move given a predicted interference involving two operating satellites. In addition, some satellite operations may dictate periodic close approaches or interference with other operators (e.g., overlapping assigned slots at GEO). In these cases, operators may wish to develop cooperative strategies to minimize propellant consumption while satisfying other requirements. Such strategies exist and are in use.

3.4 Ownership and Operating Characteristics of Spacecraft

The tracking services that develop catalogs of tracked objects, services that predict possible collisions, and governments that want to verify that operators are adhering to operational and disposal requirements need to know who is operating and who is responsible for each satellite, orbital launch stage, and other mission-related hardware in orbit. The object’s size and operating characteristics (e.g., station keeping requirements, maneuver and operating characteristics such as ongoing drag compensation and low-thrust orbit transfers, and current status such as operational vs. nonoperational) are important for screening for close approaches. Accurate and current owner and operator information is important for knowing whom to contact in case a problem is detected.

These requirements point to a need for an internationally agreed upon catalog of space objects, possibly maintained by space traffic control service providers, that includes owner, operator, and mission-specific information. Since concerns are increasing about the possibility that a launching satellite might interfere with or impact debris, an operating satellite, or a human-occupied vehicle, there is also a need to know when a launch is planned (specific launch dates and launch windows) and the most current launch profile to be followed. This information, similar to that for an aircraft takeoff and flight plan, can be fed to space traffic control services to assure that interference possibilities with objects already in orbit are minimized. Launch collision avoidance is currently conducted for some high-priority launches.

It should be noted that the International Telecommunication Union (ITU), a United Nations agency that allocates global radio spectrum and satellite orbits, assigns slots in geosynchronous orbits where satellites must operate. Satellite operators are required to register the orbit to be used and specify the broadcast frequency and purpose of the satellite. While these factors limit the number of optimal control slots available in geostationary orbits, the ITU’s concern is preventing radio-frequency interference. However, physical interference is still a possibility. The Cosmic Study on Space Traffic Management notes that, in general, “the choice of orbits utilized is not regulated by any entity, neither national nor international. The orbits of choice—and the choice of orbits—are determined by the proponents/operators of the satellite system(s).” The reference goes on to state that “Similarly, the number of satellites comprised in any satellite system is the prerogative of the proponents of new systems, and operators of satellite systems already in orbit. Their decisions are based on technical, economic, and political factors, and rarely on legal questions, since there is no world agency that has the authority to limit the number of satellites launched, or the orbits used.” Over the long term, foreknowledge of satellite and constellations locations may assist planners in designing orbital systems that minimize interference possibilities.

Once again, agreements at the international level are needed to define what information is appropriate and to specify how, when, and to whom and in what form such information should be provided. Current practices for aircraft flight plan sharing among nations might be appropriate models.

3.5 Space Traffic Control Service

3.5.1 Service Requirements

A service is needed that combines data from multiple sources to develop accurate estimates of where all tracked objects will be at future times, that provides warnings to operators of close approaches and other interference events involving close physical approaches, and that can refine the predictions with additional tracking and operator-supplied data. Two services are currently available that provide related services to the general space operations community, but neither is complete and all inclusive in meeting operator needs.

The first is provided by the Joint Space Operations Center (JSpOC). JSpOC services combine US SSN tracking data on all orbiting objects larger than 10 cm in LEO and 1 m in GEO to develop the most complete and accurate catalog currently available. The JSpOC utilizes that catalog and corresponding orbit propagation software to predict where objects will be in the future and develop information on coming close approaches and can task the SSN to collect updated information to refine close approach predictions. As noted earlier, that catalog does not contain information on satellite positions and planned maneuvers provided by satellite operators. The JSpOC offers a free service for subscribers that does incorporate subscriber inputs to verify satellite maneuvers designed for collision avoidance purposes. When a possible collision is predicted, the JSpOC provides free warnings to any satellite operator, not just to subscribers (Bird 2010).

The JSpOC warning messages are in the form of a Conjunction Summary Message (CSM) that includes detailed information on a coming close approach, including the full details on the close approach, which an operator can then use to estimate the probability of collision. Operators use the probability of collision as a measure of the overall quality of the data being used for the estimate, and some have set thresholds for probability levels where they would request additional data or would begin maneuver planning. Once a maneuver plan is developed, the plan can be sent to the JSpOC for assessment of its effectiveness in reducing the probability of collision and assurance that the maneuver does not significantly increase the probability of colliding with another object.

The JSpOC service is the most complete currently available, but the fact that it does not include operator-supplied data on current location and latest maneuvers for its general collision predictions can lead to inaccurate collision predictions and false alarms. This is one factor that led several commercial operators of satellites in geosynchronous orbits to establish the Space Data Association (SDA), a nonprofit organization headquartered on the Isle of Man. Founded by commercial operators Intelsat, Inmarsat, and SES, the SDA’s Space Data Center (SDC) combines operator-supplied data on satellite position and maneuver plans with publicly available data from the JSpOC (not the most accurate catalog) to develop forecasts of close approaches. At present, the SDA/SDC does not have its own sensor network that can be tasked to refine data on debris objects or on satellites operated by entities that are not customers for its service. The SDA/SDC currently provides service to approximately a dozen operators responsible for 237 GEO satellites and 100 satellites in LEO and other orbits. NASA has signed an agreement with the SDA to use the SDA’s services.

As stated, current capabilities limit the size of tracked objects (objects with known orbits that can be verified periodically with new tracking data) to approximately 10 cm for LEO objects and 1 m for GEO objects. Catalogs used for collision predictions currently contain on the order of 20,000 of these objects. Once capabilities are available to track and catalog objects smaller than 10 cm, the catalogs could include 100,000 objects or more. Conducting ongoing and timely all-on-all conjunction assessments for this number of objects will require substantial upgrades in computers, in software, and eventually in sensor systems. In addition, many more close approaches of debris objects to operating satellites are likely to be predicted. More predictions mean increasing the load on the JSpOC and other services for detailed assessments of each predicted close approach and point to the need for more accurate initial predictions to avoid unnecessary satellite maneuvers and avoid overloading these services.

Since this service would be in frequent contact with satellite operators, it might also coordinate with appropriate agencies to provide alerts and warnings of other events, including:

• Possible radio-frequency interference due to satellites that may be passing within an operator’s sphere of influence

• Existing or planned physical presence of other satellites in nearby orbital regions that may pose threats to normal operations during the lifetime of the operator’s satellite or constellation

• Space weather events that may require operator actions to prevent anomalies

• Satellites with periodic close approaches, enabling operators to develop cooperative approaches for minimizing interference

The service might also maintain contact information on other operators to enable development of cooperative maneuver strategies that minimize interference possibilities over the long term.

3.5.2 Government Information Requirements

While satellite operators clearly need STM services, governments are also major players in space and have information requirements for the following reasons:

• Governments operate lots of satellites, some of which, based on national security concerns, are not included in publicly available catalogs and may not be available to STM service providers. Thus, protecting these satellites and all satellites from interference requires that governments have access to as much information as possible on other operating satellites, including the best information on satellite positions and planned maneuvers from satellite operators. This is an important consideration as STM services evolve.

• Governments (in the United States via government agencies such as the Federal Aviation Administration and the Federal Communications Commission) and organizations such as the ITU impose requirements on locations where and how satellites may operate and on operations and disposal to limit generation of space debris and require information to be sure satellites are operated in accordance with those regulations and agreements.

Lastly, there will likely be increased presence of humans in space, both as passengers on space transportation systems traveling to and from space and possibly as guests at orbital hotels. Demands for protection of these individuals from hazards during launch and reentry, as well as in space, will increase pressure for an STM system that is closely coordinated with the existing air traffic management system.

All of these factors must be considered when defining and developing a comprehensive STM system.

3.6 Focus on Safety

3.6.1 Reentry Events

A space traffic control service would help assure that humans in space are protected from approaching debris, but reentering debris can injure humans on the ground, in ships, and in aircraft. As noted earlier, current guidelines dictate that space systems that reenter randomly must have a casualty expectation for people on the ground less than some prescribed value (1 × 10⁻⁴ in the United States), and adherence to this guideline will help minimize this risk. But what is the risk for people in aircraft? The annual worldwide risk of a commercial aircraft being struck with a piece of reentering space debris is on the order of 3 × 10⁻⁴ (Patera 2008). While this risk is very low, the risk to aircraft from a random reentry is “above the long term acceptable risk for a flight exposed to such a risk [italics used in the reference], but below the short term acceptable risk based on risk acceptability guidelines used by the FAA for other types of threats” (Ailor and Wilde 2008). The associated mean time between occurrences of a worldwide accident is about 3,300 years, and to date, there have been no proven cases where an aircraft has been struck by a fragment from a reentering object. For comparison, the likelihood of a meteor striking an aircraft is between 1.3 × 10⁻⁵ and 1.7 × 10⁻⁵ (Patera 2008).

The goal is for all hardware with debris that would survive a reentry and pose a serious threat to humans to be purposefully deorbited into safe ocean areas. Notices could then direct ships and aircraft away from these areas when the hazard was expected. Hardware from past launches will continue to reenter and it is possible that some ADR services will be unable to control the reentry location of debris, so there will continue to be large objects entering each year. Ideally, an STM service would be able to warn aircraft away from areas where debris is falling from uncontrolled reentries of large debris objects.

3.6.2 Air/Space Boundary

While previous discussions have centered on the vulnerabilities in the vacuum of space, with increasing frequency emerging space transportation systems using reusable vehicles, some with humans aboard will cross the air/space boundary and utilize spaceports for landings and takeoffs. For a portion of their flights, these new systems will share airspace with aircraft, whose flight rules have evolved based on a different set of flight characteristics.

Near the end of World War II, the International Civil Aviation Organization (ICAO) developed the ground rules for regulating air traffic and established the basic principle that each nation controls its own airspace. No such basic principle is possible for space, since a single spacecraft passes over many countries in a single orbit and may pass over many different countries in the next orbit or during reentry. In addition, while an aircraft accident might have primary effects only within single country’s geographic area, a space accident such as a collision or explosion affects essentially all operating satellites in a similar orbit regime. Thus, it is clear that “safe” space operations are a global issue.

An object in orbit will pass over all areas within the latitude bounds of the object’s orbit. This means that just as each nation is responsible for objects launched within its boundaries and traversing its airspace on its way to space, so might each nation be responsible for vehicles returning from space and landing within its borders. A complication is that, similar to the US Space Shuttle, reentry trajectories from orbit can cover thousands of kilometers, and for some nations, significant portions of each reentry trajectory would be within, or perhaps above, another nation’s airspace. During these periods, an accident similar to that experienced by the Space Shuttle Columbia would rain debris over another nation’s airspace and potentially cause damage to aircraft and injuries on the ground.

It is clear that new rules, regulations, and tools will be required to assure safety of flight for these new systems and for aircraft as these new capabilities evolve. Potentially, the air traffic control system may need to have regular updates from the STM service on when and where a space vehicle will launch or reenter and also add tools to predict hazard areas and issue warnings to aircraft below should there be an accident during reentry.

4 Legal and Policy Framework

Much of this chapter has discussed the technical background for the current situation in space, but as is evident, there are important nontechnical steps that are required to build an effective STM. Some of these are:

• 1. Notification system and data exchange standards. Similar to protocols for air traffic control, organizations responsible for tracking and minimizing interference involving orbital objects must be notified of planned launches, satellite orbits and maneuvers, reentries (both planned and random), and other activities that might cause interference both during the planning stages and during satellite operations and disposal. Internationally accepted standards specifying formats and protocols for data exchange should be utilized where available or developed as needed.

• 2. Framework for handling and protecting resident space object data. As has been stated, accurate data on where satellites are and will be over the next few days, plus data on planned launches and deorbits of hardware, are essential elements of an effective STM. The best data are owned by governments, satellite operators, and launch service providers; each must be satisfied that its data will be protected and used only in ways it prescribes. These entities would also need to protect space objects for which they are not willing to share data.

  This suggests that an internationally accepted “clearinghouse” might be needed that can accept data from satellite operators and both government and private tracking services. Participating entities will need to agree on and have oversight of how the data are protected, shared, and utilized. By agreement of the data providers, the clearinghouse could be authorized to provide the complete catalog of tracked objects, or subsets of that catalog, to approved entities (one of these entities might be a service that provides ongoing collision and interference warnings to satellite operators). A legal and policy framework that encourages operators and governments to share appropriate data (utilizing accepted formats as noted earlier) is essential for the evolution of space situational awareness services.

• 3. Addressing liability. “We don’t want to tell a satellite operator to move,” a statement heard in discussions of conjunction services, reflects the concern that if the operator takes action (or does not take action) given information by the service provider and for whatever reason there is a collision, the service provider will be judged to be at least partially at fault. This discussion is made more difficult given the uncertainties associated with the data used. Collision predictions are probabilistic in nature, and it is virtually impossible to say with complete certainty that a close approach will or will not lead to a collision. It is likely that at least in some cases, satellite moves will be sized to lower the probability of collision to below an acceptable level, but perhaps not to zero. Thus, a small probability of impact will remain. Ideally, a service provider will be established that is sanctioned and protected from liability by governments, able to enter into contractual agreements with space operators and launch providers to protect data, and operate in a manner in which all parties are satisfied.

  Liability will also be an issue for ADR services. The 1967 Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (Outer Space Treaty), provides that a space object (operating satellite, launch stage, or debris) remains the property of the launching authority for as long as it is registered with/by the launching authority. The launching authority is responsible for damages caused by the space object. As a result, the launching authority would need to give permission (or transfer ownership) and transfer liability before a space object such as a dead satellite or rocket stage could be touched or otherwise interfered with by an entity wishing to remove the object from a protected region in space. Formal agreements could be developed that deal with these issues and also incentivize the testing and operations of systems designed to actively remove space debris. A way forward might be for one nation to identify a set of representative debris objects (e.g., a launch stage and dead satellites with and without extended solar panels) for which it is the launching state and develop the legal framework to support ADR proposals for these objects as a way to encourage private industry to develop and test disposal options.

• 4. Internationally accepted space traffic control service provider. The goal is to have a service to warn satellite operators of coming hazards that is available to all satellite operators, that uses the best data available, that is reliable, that provides timely warnings, that is responsive to operator needs, and that protects sensitive data. An AIAA workshop in 1999 concluded that there is a need for “an internationally recognized entity” to provide these services.

  A government agency such as the JSpOC discussed earlier could take this responsibility, but this approach may not be optimal from the perspective of international operators (Ailor 2002). For example, given that effective space traffic control is a global issue, one option is for all satellite operators, government and nongovernment, to provide satellite maneuver plans and other sensitive or proprietary information to an agency of the United States or other government, and the agency would be required to guarantee data protection and a level of service no matter the political climate. It is not clear that operators and governments will be willing to participate in such a service if they have no direct control over the service itself.

  A second option is that a for-profit company might offer a service of this type. For-profit companies are in business to make money, have obligations to boards of directors and stockholders, and can be bought by other companies. Some of these features might again make it unattractive for satellite operators to provide the sensitive, proprietary data required for these services or to become dependent on a single company with a profit motive. There is also the potential risk of lawsuits should there be an unpredicted collision or other event, which might make for-profit companies reluctant to step into this area.

  As a third option, a nonprofit organization might be a solution. According to Wikipedia (http://en.wikipedia.org/wiki/Non-profit_organization ), “a non-profit organization (abbreviated “NPO”, also “not-for-profit”) is a legally constituted organization whose objective is to support or engage in activities of public or private interest without any commercial or monetary profit.” Continuing, “The extent to which [a NPO] can generate income may be constrained in amount, methods or both, and the use of those profits may be restricted not only in purpose but in proportions regarding self-maintenance and achievement of purpose. NPOs therefore are typically funded by donations from the private, public sector, or both, as well as from program service fees.” Since governments and satellite operators are key stakeholders, the NPO could be run by a board of directors made up of representatives of governments as well as private satellite operators (Ailor 2008). The board would assure that the services offered, data provided to governments, prices for services, data privacy and security, etc., meet the needs of both satellite operators and governments. Board members from governments could also work to bring appropriate tracking data to the nonprofit and assure that the data is protected. The nonprofit could also define relationships with satellite operators by contract, customizing services to operator needs and guaranteeing a level of service. Potential liability could be limited by contract or other means.

  As noted earlier, a nonprofit organization that meets some of these requirements has been established. The Space Data Association (SDA) was established by three commercial satellite operators – Intelsat, SES, and Inmarsat – and incorporated on the Isle of Man as a nonprofit corporation. At present, no government has direct oversight of this organization. While SDA’s Space Data Center (SDC) does receive position and maneuver data from operators of the satellites for which it provides services, it does not have access to the best tracking data available on other objects, including space debris. As stated earlier, the most complete tracking data is maintained by government organizations.

  Policy questions that must be addressed include the following: Will a service provided by an agency of a single government be accepted as the preferred service provider for international satellite operators for the long term? Is there confidence that such a service would be open to all nations, friend, and foe and that the service would be available no matter the world’s political situation? Will a nongovernment nonprofit service, such as the SDA/SDC, be accepted as the long-term provider of these services? Will an outside service have access to a sensor network or be able to request a government to task sensors to refine predictions for specific events? Are there other possibilities? Decisions and agreements need to be made at the international level to build a structure and organization for the long term.

5 Conclusions

An effective space traffic management system is necessary for the long-term sustainability of space activities and to assure the safety of both humans in space, in aircraft, and on the ground. Such a system should include:

• Collection and utilization of the most accurate data available on satellites, their orbits, and their maneuver plans

• An internationally accepted space traffic control service available to all users of space that warns satellite operators and launch service providers of collisions and other predictable hazards that might affect their operations

• A space traffic management service that provides governments and regulatory agencies information they need to assure that satellites are operated and disposed in accordance with approved plans

• Integration into the air traffic management system of some information on space systems that will be transiting above and reentering airspace

• A service that maintains accurate and current information on satellite owners and operators, future launches and launch profiles, and emerging plans for new satellite systems and constellations

• Policies and procedures that protect the safety of humans in space, on the ground, and in the air

• Development of best practices and international standards for hardware and space operations that mitigate hazards and minimize the creation of new space debris

• Development of a legal and policy framework and implementation infrastructure that encourages exchange of necessary data, addresses ownership and liability related to space debris objects, and incentivizes the evolution of active debris removal technologies