Active satellite collision risk increases as the number of objects in space grows. The high likelihood of collision renders an orbital regime potentially unusable once it becomes saturated with objects. This paper defines orbital saturation and calculates the number of objects required to reach that critical level. These calculations are based on research determining a satellite’s rate of conjunction within a debris field.[i] The role debris and other objects play in rendering an orbit unusable is like the historic role served by terrestrial mines on land and sea. This paper defines the cause and consequences of orbital saturation, provides a method for calculating it, and concludes with recommendations to mitigate orbital saturation.
There are more than 25,000 man-made objects in Earth’s orbit. That total had been increasing linearly since the former Soviet Union launched the first artificial satellite into orbit more than 64 years ago. The number of man-made objects in orbit has increased exponentially since 2019. As the number of objects in orbit continues to increase, collision has also become more likely, especially for satellites which may encounter the mega-constellations responsible for the most recent exponential increase. This trend toward mega-constellations has the potential for weaponization as mega-constellations can be employed by spacefaring nations to seize key orbital regimes in an anti-access/area-denial (A2/AD) strategy. This paper details the evolution of concern over orbital collisions and proposes a method for determining when an orbital regime might become inaccessible. In addition, the paper draws upon comparisons to Earth-bound mines to highlight some similarities and differences in employment as specific to the space domain.
Objects in Orbit
The U.S. Space Surveillance Network tracks and catalogs the total number of objects in Earth’s orbit bigger than 10 centimeters, which is approximately the size of a baseball. While the number of satellites continues to increase, debris has stayed relatively constant since 1997, save three events. China’s intentional destruction of the weather satellite Fengyun 1C during an anti-satellite (ASAT) test in 2007 created more than 3,500 pieces of fragmentation debris, as indicated by vertical line 1 in Figure 1. The unintentional collision between Iridium 33, a U.S. communications satellite, and Cosmos 2251, a Russian military communications satellite, caused the second jump in fragmentation debris in 2009. This event generated more than 2,300 pieces of debris. In November 2021, Russia destroyed their own Cosmos 1408 via an ASAT, creating the third spike in fragmentation debris, totaling more than 1,700 pieces.
See Figure 1: Total Number of Objects in Orbit over Time
While the level of fragmentation debris has stayed constant over the last thirty years, aside from the three examples cited above, the number of active spacecraft in orbit has been rising. By the next decade and for the first time in history, active spacecraft may outnumber fragmentation debris as the highest contributor to the total number of objects in orbit.
Shifting Concerns: Mega-constellations
Starlink and OneWeb are two examples of mega constellations that aim to provide global internet coverage. OneWeb has 426 satellites in orbit. In May 2020, OneWeb filed an application with the Federal Communications Commission (FCC) to increase the number of satellites in its constellation to almost 48,000. Starlink has 2,550 satellites in orbit. The FCC has authorized a total of 42,000. Just these two combined mega-constellations account for 41 percent of all active satellites in orbit.
The difference between OneWeb and Starlink is that OneWeb’s final satellite orbit altitude is spread between 1,200 and 1,220 kilometers (km), whereas the bulk of Starlink’s constellation will orbit between 546 and 548 km. The high concentration of orbital objects within such a tight altitude range increases the probability of collision (PoC) over time for other satellites at that same altitude. That is, the rate of conjunction warnings issued to an active satellite is proportional to the number of space objects at the same altitude.
Upon notification of a pending conjunction, a satellite’s operator decides whether to maneuver the satellite out of harm’s way. This decision is based on several criteria that involve the PoC and the satellite operator’s risk tolerance. For example, the entire fleet of NASA satellites in low Earth orbit (LEO) experiences about 700 conjunctions per month. Most of those conjunctions do not exceed NASA’s PoC threshold. However, those that exceed the PoC require plans for satellite maneuver. This level of maneuver planning and effort happens approximately 20 times per year for each satellite in NASA’s Earth Science constellation of 10 satellites. Of those 20 planning efforts every year per satellite, two of them are concerning enough that the satellite is directed to maneuver. The rest are waved off because of last-minute decreases in PoC.This amount of planning and maneuvering requires an entire team of skilled collision avoidance experts working around the clock.
On October 29, 2020, NASA wrote a letter to the FCC regarding AST & Science’s proposal to launch a mega-constellation.According to NASA’s letter, the satellites in this mega-constellation might not be able to sufficiently maneuver and, given enough time, will experience a collision. This collision may result in more debris generation, which would threaten NASA’s Earth-observing satellite constellation and “render its orbit regime unnavigable.”
The word - unnavigable - indicates that there exists a point at which an orbit regime becomes so saturated with debris or other objects as to render that orbit unusable. No entity—government, commercial, or otherwise—would be able to operate within that orbit regime, or even safely exist within it.
Part of the planning process for collision avoidance maneuvering involves ensuring that the threatened satellite does not maneuver itself into an equally dangerous situation; or as NASA puts it, maneuver planning “includes choosing a maneuver that will remediate the main conjunction (without introducing any new conjunctions of concern).”This “orbital saturation” state occurs where a collision avoidance maneuver cannot be executed without introducing an equally threatening situation that would then occur within the same time period as the first. In other words, that orbital regime has become unnavigable.
For example, consider a hypothetical scenario where a maneuver team detects a conjunction between an active satellite and a piece of debris with a PoC of 10 to the negative fourth power, or 1 in 10,000, and is expected to occur within 24 hours. The maneuver planning team then examines all possible maneuvers that their satellite could execute and realizes that every possible maneuver induces a conjunction with another object, each of which has a PoC of at least 10 to the negative fourth power and would occur within 24 hours. At that point, the active satellite is in checkmate; no remaining moves are available, and the operator of the satellite must rely on chance to determine whether the upcoming conjunction will result in a collision. By definition, this checkmate scenario will occur when an orbit regime is saturated.
Among the satellites in LEO, 80 percent have less than a 30 km difference between perigee and apogee, meaning most are in circular orbits. For example, most of the satellites in the Starlink constellation have a perigee of 546 km and an apogee of 548 km.Thus, the orbit regime of the Starlink constellation exists within a 2-km altitude band. The challenge for other satellites is that, as mega-constellations continue to proliferate, more altitudes may become saturated and possibly even preclude sharing of that orbital regime.
The rate of conjunctions is also proportional to the amount of time a satellite spends in the saturated band. A satellite operating only partially within this altitude band, in an elliptical orbit for example, would not experience as many close approaches attributable to the saturation. Also, a launch vehicle passing through would only spend a portion of its time in the saturated band and would not likely be blocked from higher altitudes. Operators would have more collision avoidance maneuvers available to them. However, there is a possibility that larger altitude bands could become saturated as the number of orbital objects continue to grow.
Saturation would occur in any given altitude band where a certain number of objects is reached in that altitude band. That number of objects is dependent on the allowable miss distance and the amount of time until closest approach (TCA) between any two objects. Relying on these two values means that saturation could occur at varying levels of each. This means that an altitude band might be saturated such that a satellite fully within that band has an upcoming conjunction with another object at a miss distance of 1 km with a mean reoccurrence rate of every 24 hours; but that same altitude band might not yet be saturated at a miss distance of 0.5 km with a reoccurrence rate of every 3 hours. A different number of objects would be required to saturate that same altitude band at those two different levels.
Predicting the Number of Objects Required to Saturate an Orbit
Satellite altitude, number of debris pieces, and miss distance between two objects can be represented in an equation to determine mean orbits between conjunctions (MOBC). MOBC describes the average number of orbits that a satellite would complete before experiencing a conjunction. MOBC can be multiplied by the satellite’s orbital period to find the mean time between conjunctions (MTBC). This number is a mean, not a maximum, so a maneuver under saturated conditions may not result in another conjunction at the same time as the first, but on average the conjunctions would occur around the same time frequency. The equation to find the number of objects, n, required to saturate an altitude band can be depicted as follows.
The variable a is simply the semi-major axis of the center of the altitude band. MOBC is in units of revolutions, or the number of times a satellite would orbit the Earth before a conjunction occurs. The NASA Conjunction Assessment (CA) Team typically chooses to conduct a mitigating maneuver roughly 24 hours prior to the TCA between the two objects. For example, a satellite would complete approximately 15 revolutions in 24 hours at the altitude of the Starlink constellation. Some satellite operators might choose to conduct a maneuver as close as 2 revolutions prior to the TCA. MOBC is set to upper and lower boundaries of 2 and 15 for the purposes of this paper.
Most operators, including NASA, take mitigating actions when the PoC between their satellite and another object exceeds 10 to the negative fourth power, or 1 in 10,000. PoC is calculated using ellipsoidal covariance volumes around each object, with the longest axis typically in the direction of the velocity vector of each satellite. The intersection of these two cigar-shaped volumes determines the PoC. The active satellite would also have a sphere of radius r around it, such that any object entering this sphere would violate its safety constraints. The NASA CA Team uses a 1 km radius sphere to form a “watch volume” around each of their satellites. Any object predicted to enter this watch volume triggers further analysis and potential concern. Therefore, r will be set to 1 km.
The number of objects required to saturate a 2-km altitude band - using the values described above - is depicted in Figure 2. The histogram at the bottom shows the number of satellites by altitude in LEO in 2-km bins. As depicted in Figure 2, 547 km is already saturated at a MOBC of 15 revolutions, due to Starlink.A satellite in a 547-km circular orbit would have its 1-km standoff radius violated once per day. The upper line indicates that it would take approximately 3,000 objects in a 2-km altitude band to violate the 1-km standoff radius of a satellite every other revolution. While 3,000 objects in a single orbit seems excessive, it should be remembered that there are already nearly 1,500 objects at 547 km.
See Figure 2: Number of Objects Required to Saturate a 2-km Altitude Band
The number of objects required to saturate an altitude would increase where the watch volume’s radius, r, decreases. Improving accuracy of known object positions in orbit, automated avoidance maneuvers, and improved communications could enable an operator to reduce the watch volume around a satellite. However, given existing technology, spacepower practitioners can use 3,000 co-altitude objects as a benchmark of concern.
Any given 2-km altitude band will become saturated and unusable for circular orbits as the band edges closer and closer to 3,000 objects. In essence, Starlink is the primary occupant of the orbital altitude of 547 km. An orbital mission planner, deciding which altitude to place a new satellite, would have to consider excluding 547 km from the decision-making process because of being saturated with active satellites.
This also has implications for space security. A spacefaring nation could make a similar orbital grab through the intentional mining of an orbit regime as part of an anti-access/area denial (A2/AD) strategy. There are existing historical examples of how land and sea mining were employed during periods of conflict.
Comparison to Land and Sea Mining
Land and sea mining have existed for centuries. In modern times, allied forces mined the waters in the North Sea between Scotland and Norway, laying over 50,000 mines in a matter of months during World War I. This North Sea mine barrage sunk or damaged 21 U-boats and prevented German warships from raiding Allied ships carrying iron ore from Norway to England.
This same concept can be applied in space. A nation-state could mine space to deny access to strategic orbits. Space mines would not be employed like traditional mines containing an explosive substance. At orbital velocities, even a small piece of debris has enough power to destroy, or at least render inoperative, anything with which it collides. Some strategic orbits with relatively thin altitude bands include geostationary orbit and LEO with ground tracks that repeat at regular intervals. Rendering any of these orbits unusable would hinder the activities of spacefaring nations and the global space community.
Key differences exist between terrestrial mines and space mines. Land and sea mines prevent passage through a location, in addition to preventing operations within an area. Space mines would not prevent passage through an altitude, unless that altitude were saturated at a much higher rate than described above. While rockets could still carry their payloads safely through the mined altitude especially if a lower altitude was completely mined, a satellite could not operate solely within a mined altitude band.
Land-based mines can be removed, albeit with great difficulty, with the previously mined territory reverting to its original state. However, there is currently no approved technology and no collective willpower to remove space debris from orbit, although this may be changing. The U.S. Space Force, for example, does not consider orbital debris removal to be one of its missions. Space Force, however, has started to invest in studies of debris removal technology, but those studies are in their nascency and effective technologies are years away. The existing international legal regime also makes space debris removal challenging, because the Outer Space Treaty vests exclusive jurisdiction and control over all spacecraft (and their parts) with the launching state.
There is yet another difference between terrestrial and space mines. With land and sea mines, the entity that laid the mines knew their location and could move freely within the mine fields, whereas adversaries had to first detect the mines before being able to move freely. With space mines, neither the entity that laid the mines nor the adversary would have free maneuverability within the location.
An orbital grab provides a possible exception to the idea that the mine-laying entity and its adversary would be equally affected by space mines. A mine laying entity could employ a large constellation of co-altitude autonomously maneuvering satellites, like Starlink or OneWeb, to assure its own access, because a constellation could be designed to allow the owner to safely maneuver within that field and prevent the activities other satellite operators. This hypothetical constellation’s ability to raise or lower its altitude in unison, could even allow it to “push out” or threaten satellites at a variety of altitudes.
While the comparison between terrestrial mines and space mines is not exact, there are enough similarities to warrant heeding the historic examples set by land and sea mines. A concerned satellite operator could design its mission to function at a variety of altitudes, reducing the possibility that its mission would be affected by its original orbit being mined. However, there are strategic orbits that would cripple certain space missions if they were to become saturated. Saturation occurs when a satellite cannot reduce its collision risk by performing a co-altitude maneuver and may occur in LEO with 3,000 co-altitude orbital objects. As the number of objects in orbit continues to grow, orbital saturation becomes a real possibility and may occur sooner than anticipated.
The 2017 U.S. National Security Strategy was the first national document to state: “The United States must maintain our leadership and freedom of action in space.” In line with this national sentiment, and like one of the U.S. Navy’s missions to “keep the seas open and free,” the U.S. Space Force should adopt a mission to keep space open and free. Orbital debris should not be viewed simply as natural debris or manmade trash whose removal is necessary to protect spacecraft and space derived capabilities. Debris is better viewed as lethal objects comparable to mines. Further, active satellites may be just as dangerous, because they could be used to seize key altitudes and orbital regimes if deployed in sufficiently large quantities.
Nicholas Rotunda is a space systems engineer, who recently retired from the U.S. Navy. He has extensive experience in the fields of orbit determination and satellite collision avoidance. This paper represents solely the author’s views and does not necessarily represent the official policy or position of any Department or Agency of the U.S. Government. If you have a different perspective, we’d like to hear from you.
[i] A “conjunction” is a likely future event where a satellite may collide with another object. A “collision” is an actual meeting of two objects, with catastrophic damage to any satellite that experiences it.
 National Aeronautics and Space Administration, Orbital Debris Quarterly News, Volume 26, Issue 1, March 2022. Figure 1 shows the total number of objects in orbit as a maroon solid line. The end of the line is above 25,000 objects.
 Ibid. With the exception of the two vertical jumps in 2007 and 2009, the maroon solid line in Figure 1 can be seen to increase roughly steadily and linearly until approximately 2019.
 Ibid. Beginning at approximately 2019, the maroon solid line in Figure 1 can be seen entering an exponentially increasing curve.
 Unless otherwise specified, all data on the number of objects in orbit is derived from analysis of primary sources, particularly the satellite catalog published by the Combined Space Operations Center at www.space-track.org.
 National Aeronautics and Space Administration, “Space Debris and Human Spacecraft,” May 26, 2021, https://www.nasa.gov/mission_pages/station/news/orbital_debris.html (retrieved July 21, 2022).
 Edward Chatters and Brian Crothers, “Space Surveillance Network,” AU-18 Space Primer, Air University Press, 01 Sep 2009, 249.
 Shirley Kan, China’s Anti-Satellite Weapon Test, CRS Report for Congress RS22652 (Washington, DC: Congressional Research Service, 23 April 2007); Space-Track.org, “Satellite Catalog,” www.space-track.org (retrieved July 21, 2022).
 National Aeronautics and Space Administration (NASA), The Collision of Iridium 33 and Cosmos 2251: The Shape of Things to Come, 60th International Astronautical Congress (Daejeon, Republic of Korea: NASA, 16 October 2009).
 Jeff Foust, “Russia destroys satellite in ASAT test,” SpaceNews, November 15, 2021, https://spacenews.com/russia-destroys-satellite-in-asat-test/; Space-Track.org, “Satellite Catalog,” www.space-track.org (retrieved July 21, 2022).
 National Aeronautics and Space Administration, Orbital Debris Quarterly News, Volume 26, Issue 1, March 2022.
 Ibid. The number of spacecraft is indicated by the blue dotted line, which increases linearly from its beginning until sometime between 2015 and 2020, when it begins an exponential curve.
 Federal Communications Commission, “Application for Modification,” File No. SAT-MPL-20200526-00062, May 26, 2020.
 Caleb Henry, “SpaceX submits paperwork for 30,000 more Starlink satellites,” SpaceNews, October 15, 2019.
 This percentage was calculated from the combined totals of the Starlink and OneWeb constellations currently in orbit, as reported on space-track.org; and all satellites listed as “Active” in the stkSatDb database at https://support.agi.com/satdb/default.aspx (retrieved July 21, 2022).
 NASA, “Report No. SAT-01501 Space Station Applications Accepted For Filing, AST & Science, LLC (SAT-PDR-20200413-00034),” Letter in Response to FCC Public Notice of October 2, 2020 Application for Fixed Satellite Service Mobile Satellite Service by AST & Science, October 29, 2020, 2-4.
 Ibid, 2.
 Ibid, 1. (“With the increase in large constellation proposals to the FCC, NASA has concerns over the possibility of a significant increase in frequency of conjunction events”).
 Ibid, 3.
 Nicholas Rotunda, “Space Debris: An Anti-Satellite Deterrent,” Air Command and Staff College, Graduate Thesis, December 2019, 55.
 Laurie Newman, “The NASA robotic conjunction assessment process: Overview and operational experiences,” Acta Astronautica, Volume 66, Issues 7-8, April-May 2010, Pages 1253-1261.
 Saika Aida, “Conjunction Risk Assessment and Avoidance Maneuver Planning Tools,” DLR German Space Operations Center (GSOC).
 Laurie Newman, “The NASA robotic conjunction assessment process.”
 Ibid, Table 1.
 The equation presented in this paper does not take PoC into account, but rather only the spherical standoff watch volume. Thus, MOBC represents the frequency of conjunction warnings based off a spherical volume and not an ellipsoidal covariance volume. The thesis upon which this equation is based proposes a value, k, which represents the relationship between spherical and ellipsoidal volumes.
 The length of the altitude band is twice the standoff radius r.
 Smithsonian Magazine, “The Historic Innovation of the Land Mines – And Why We’ve Struggled to Get Rid of Them,” https://www.smithsonianmag.com/innovation/historic-innovation-land-minesand-why-weve-struggled-get-rid-them-180962276/ (retrieved 7 October 2022); International Campaign to Ban Landmines, “A History of Landmines,” http://www.icbl.org/en-gb/problem/a-history-of-landmines.aspx#:~:text=Precursors%20of%20the%20weapon%20are,and%20the%20first%20Gulf%20War (retrieved 7 October 2022).
 American Battle Monuments Commission, “The Allied North Sea Mine Barrage of World War I,” June 11, 2018, https://www.abmc.gov/news-events/news/allied-north-sea-mine-barrage-world-war-i.
 NASA, “Space Debris and Human Spacecraft,” https://www.nasa.gov/mission_pages/station/news/orbital_debris.html (accessed 7 October 2022).
 Sandra Erwin, “Space Force eager to invest in debris removal projects,” SpaceNews, February 10, 2022, https://spacenews.com/space-force-eager-to-invest-in-debris-removal-projects/; UK Space Agency, “UK builds leadership in space debris removal and in-orbit manufacturing with national mission and funding boost,” Gov.UK, 26 September 2022, https://www.gov.uk/government/news/uk-builds-leadership-in-space-debris-removal-and-in-orbit-manufacturing-with-national-mission-and-funding-boost.
 The Outer Space Treaty, Article VIII, https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/outerspacetreaty.html.
 National Security Strategy of the United States of America, Dec 2017, 31. nssarchive.us/wp-content/ uploads/2020/04/2017.pdf.
We are non-governmental and self-owned.
Official Website. All Rights Reserved.