Active Space-Debris Removal an Inevitability (RCAF Journal - FALL 2014 - Volume 3, Issue 4)

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By Major Nathan Arleigh Burgess, MA

The box office hit Gravity, starring Sandra Bullock and George Clooney, provided the general public great insight into a very serious problem that is developing in outer space—namely, the accumulation of hazardous space debris in Earth orbit. The film portrays the devastating effect of the rapidly moving space debris on spacecraft, made even more horrifying when these spacecraft are occupied by astronauts. Some may wonder if the movie exaggerated the severity of the space-debris situation. In truth, the space-debris situation in Earth orbit has passed the “tipping point,” thus requiring active debris removal in order to ensure Earth orbits remain useable.

Space debris is defined as “manmade objects in space that are no longer in use.”[1] This debris has been accumulating since man first started operating in space in the late 1950s. “These objects [i.e., space debris] include non-operational spacecraft, derelict launch vehicle stages, mission-related debris, and fragmentation debris.”[2] Mission-related debris, which is created during the launch/deployment process, includes “items such as sensor and engine covers, straps, springs, and yo-yo despin weights.”[3] Fragmentation debris is comprised of fragments from defunct rocket bodies and spacecraft which are the product of explosions or collisions.[4] Fragmentation debris comprises the majority of the space-debris population. “According to NASA [National Aeronautics and Space Administration], 42 percent of total extant debris is fragmentation debris (resulting primarily from the break-up of satellites), 22 percent is non-functional spacecraft, 19 percent is mission related debris, and 17 percent is rocket bodies.”[5]

Space debris comes in varying sizes, ranging from diameters of less than 1 centimetre (cm) to full-size defunct spacecraft.[6] Space debris less than 1 cm in diameter is usually mitigated using shielding and orientation.[7] As a result of tremendous impact speeds, up to 15 kilometres per second (km/s) in low earth orbit (LEO), space debris 1 cm in diameter can potentially damage or even destroy a satellite, and debris 10 cm or larger will most likely have a catastrophic effect on spacecraft.[8] Furthermore:

debris as small as 10 cm in diameter carries the kinetic energy of a 35,000-kg [kilogram] truck travelling at up to 190 km [kilometres] per hour. While objects have lower velocities in Geostationary Earth Orbit (GEO), debris at this altitude is still moving as fast as a bullet—about 1,800 km per hour. No satellite can be reliably protected against this kind of destructive force.[9]

Debris 10 cm in diameter or larger is currently tracked by space surveillance; however, debris between 1 cm and 10 cm is not tracked and, thus, represents an undetectable threat to spacecraft. There are currently 21,000 pieces of tracked space debris 10 cm in diameter or greater. Additionally, there are an estimated 600,000 pieces of debris between 1 and 10 cm and 100,000,000 pieces between 0.1 cm and 1 cm in Earth orbit.[10] This debris can persist from a few days to indefinitely, depending on gravitational and non-gravitational factors such as solar activity, altitude and corresponding atmospheric drag.[11] At low altitudes, debris is eventually pulled into the Earth’s atmosphere where most of it is burned up upon re-entry. Debris 10 cm in diameter at an altitude of 200 km or less will only persist for a matter of days, whereas the same debris at an altitude between 200 km and 600 km can last for years. Debris can persist for hundreds of years at altitudes above 600 km and indefinitely at altitudes above 36,000 km (GEO).[12]

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In addition to debris persistence, there is the Kessler syndrome, whereby space debris collides with other space debris or objects (e.g., functional satellites), creating more space debris, thus increasing the likelihood of further collisions.[13] This leads to a self-propagating space-debris situation. The result of debris persistence combined with the Kessler syndrome is that LEO has become dangerously “polluted” with space debris.

The growing space-debris population has increased the likelihood of collisions with spacecraft. For example, the International Space Station (ISS) has had to make avoidance manoeuvres, to avoid catastrophic collisions with space debris, on average once per year.[14] On one occasion, a crew of six was ordered to evacuate the ISS and take refuge in two Russian Soyuz spacecraft until the threat passed; the debris was detected too late to perform an avoidance manoeuvre. “The two Americans, three Russians and one Japanese astronaut spent a nerve-racking half an hour before mission control gave them the all clear.”[15] Recent studies have indicated that the space-debris population in LEO has now reached a level whereby the population of debris will continue to increase even if no further debris is added.[16]

This has led most space experts, including the Inter-agency Space Debris Coordination Committee (IADC),[17] to determine that the only option is to intervene and prevent this cascading effect by removing space debris from LEO.[18] Within LEO, certain areas are particularly polluted with space debris; studies have determined that the LEO regions with the highest space-debris mass and collision probability are the 600-km, 800-km and 1000-km regions. These regions are mostly comprised of large defunct spacecraft and rocket bodies with a high probability of collision, which could further increase the debris population.[19] These represent a logical target for active debris removal.[20] NASA simulations estimate that the removal of two large objects (such as a defunct spacecraft or rocket body) per year, starting in 2020, would slow the debris population growth by 50 per cent. If five objects are removed per year, the debris population can be maintained at, or slightly above, current levels for the next 200 years.[21] Of course, if the objective is to ameliorate the situation, more than five objects per year would have to be removed. Given the uncertainty and possibility of future collision events, it would be wise to err on the side of caution and remove more than five objects per year.[22]

These particularly “polluted” regions of LEO are of great concern to the Royal Canadian Air Force (RCAF) and the Canadian Armed Forces (CAF). Many of Canada’s military space-based capabilities are provided by LEO satellites. For example, Canada’s first dedicated military satellite, Sapphire, resides in LEO. Sapphire provides space situational awareness (SA) to the CAF and United States (US) military, as a contributing sensor to the US’s space surveillance network (SSN). Sapphire tracks space debris and operational satellites in Earth orbit. This space SA data supports the RCAF’s North American Aerospace Defence Command (NORAD) responsibility to provide aerospace warning for North America.[23]

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Canada’s RADARSAT 2 satellite, which provides remote sensing capabilities in support of the CAF, also resides in LEO. RADARSAT 2 provides, inter alia, Arctic and maritime ship detection as well as environmental sensing in support of Canadian security operations. The RADARSAT Constellation Mission (RCM), the follow-on to RADARSAT 2, will also reside in LEO. Due to launch in 2018, RCM will provide maritime domain awareness, a ground-moving-target-indicator (GMTI) capability and an automated identification system (AIS)[24] capability for ship identification.

The CAF also uses commercial satellite imagery in support of military operations via the Joint Space Support Team (JSST). The JSST provides SA data to commanders through their Unclassified Remote Sensing Situational Awareness (URSA) system. Commercial imagery satellites also reside in LEO.[25]

Finally, Canadian search-and-rescue-satellite (SARSAT) payloads also reside in LEO. SARSATs can detect emergency beacons at sea, in remote forests and in the Arctic region. Canadian SARSAT capabilities have been used in thousands of search and rescue operations since 1982 and have been “instrumental in the rescue of over 24,000 lives worldwide.”[26] The loss of these military space capabilities as a result of an over-polluted LEO region would be detrimental to the CAF as well as Canadian safety and security.

Now that the debris population has reached a critical level in LEO (and most experts agree that active debris removal is required), space-faring nations and the international community must determine how to remove this debris from outer space, who will pay for it and how this will be regulated. There are a number of active debris removal techniques under exploration, ranging from space-based robotics systems to ground-based “contact-less” systems.[27] Space-based robotics systems require a chaser or servicing vehicle that can be used to: 1) manoeuvre orbitally; 2) rendezvous with space debris; 3) capture the space debris; and 4) transfer the captured space debris to a graveyard orbit or to a lower orbit for re-entry into the Earth’s atmosphere.[28]

Such a chaser vehicle could be designed using conventional spacecraft and propulsion technologies; however, some studies are examining alternatives, such as using modified upper stages of launch vehicles as chaser vehicles. The upper stages of launch vehicles are normally discarded as debris during the launch process, therefore, comprising a significant portion of large debris objects in Earth orbit. Modifying the upper stages of launch vehicles to become a chaser vehicle has the added benefit of preventing further debris build-up, while helping to remove extant space debris. This would allow the removal of one large piece of space debris after each launch and at a reduced cost when compared to conventional vehicles.[29]

These modified upper stages of launch vehicles could be equipped with electro-dynamic tether (EDT) propulsion systems rather than conventional propulsion technologies. An EDT is a long conducting wire that is attached to a spacecraft; the wire “can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy.”[30] In other words, this is a propellant-less propulsion system that uses the Lorentz force which results from the interaction between the Earth’s magnetic field and the conductive tether. “The main advantage of the EDT for space debris removal is that it does not require propellant. This reduces cost and improves reliability of in-space propulsion and operations.”[31]

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In addition to determining an appropriate chaser vehicle and propulsion system, there are various capturing-technique options. These capturing techniques can be categorized as pulling, pushing and contact-less.[32] Pulling techniques use items such as throw nets and harpoons. Pushing techniques utilize robotic arms with clamping mechanisms. Contact-less techniques use technologies such as ion beams, dust, lasers, foam or air bursts to exert a force on space debris.

“One of the most promising techniques for actively controlling debris during re-entry or re-orbiting is to attach a tether to it and pull it.”[33] Using a tether allows for capturing options that can handle target space debris of any shape, attitude or spin rate. This removes the very difficult requirement to dock with a rapidly moving/spinning target. In 2012, the European Space Agency (ESA) studied the use of throw nets to capture space debris.[34] “The idea is simple, a net ejector mechanism ejects a net from a canister. The net is pulled open by the inertia of a number of corner masses that have a high mass relative to that of the net as well as a radial velocity.”[35] The masses at each corner of the net[36] would be ejected outward at an angle (e.g., 30 degrees) to the centre of the net, thus forcing the net to open. Their simulations demonstrated that the net would passively wrap around the debris, fully securing the target. “However, while the simulations show that a fully passive net closure is likely to be sufficient, it is also possible to implement a simple closing mechanism consisting of winches in two of the corner masses and a thread between them.”[37] Once the target debris is secured with the net, the chaser vehicle would pull the debris to a graveyard orbit or back towards the Earth for re-entry. ESA has determined that throw nets are “a very promising capture mechanism that could work … [for] a large range of target sizes, shapes, attitudes and spin rates.”[38]

Another tethered option is using a harpoon to capture space debris. Harpoons have many of the same advantages as a throw net; however, there are a few potential concerns. These concerns include the ability of the harpoon to penetrate satellite material, the anchor strength once penetration is achieved, and the potential creation of more debris through fragmentation. Initial testing has demonstrated promising results, and none of these initial concerns were identified as “show-stoppers.”[39]

One drawback of pulling techniques is that the thruster plume is directed towards the tether and net, thus requiring the use of a heat-resistant material, such as Zylon, on the portion of the tether that is exposed to the thruster plume. “Thermal failure of the Zylon tether is predicted [upon re-entry] at 74 km altitude.”[40] Zylon is tough enough to withstand the thruster plume but weak enough to burn up upon re-entry into the atmosphere, therefore posing no risk to humans.

Pushing technologies include the use of robotic arms or tentacles with some form of grappling device at the end to grab the space debris.[41] Pushing techniques are inherently difficult during the rendezvous and capture phases of the mission, since they require great precision, especially when dealing with a rapidly moving, tumbling object. Inaccurate sensors could lead to fragmentation of the debris or to a collision between the chaser vehicle and the debris. Once the debris is fully secured, the chaser vehicle and attached debris would propel (i.e., push) forward up to a graveyard orbit or towards the Earth for re-entry.

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An issue that is common to both pushing and pulling techniques is the risk of the explosion of debris energy stores.[42] Some nations, such as France, have recently developed regulations to ensure the passivation of all future spacecraft at end of life.[43] Passivation is the depletion of all on-board sources of stored energy in a spacecraft. Energy is stored in batteries, pressurized tanks and propellant tanks. Unfortunately, extant space debris was not subject to regulations requiring passivation; therefore, there is a risk of explosion when attempting to capture this debris. Fortunately, “propellant tanks are located inside the central cylinder and are therefore well protected from e.g., [sic] clamping mechanisms.”[44] Harpoons, on the other hand, which penetrate into the body of the debris, must be carefully targeted so as not to strike a tank. There is also the risk that a throw net or grappling mechanism could break off a thruster, causing propellant leakage. Fortunately, the propellant valve is located upstream of the nozzle and flange, meaning that the valve would remain uncompromised even if the thruster, nozzle and flange broke away. Since the valve controls the flow of propellant, the propellant would remain in the tanks. From a passivation perspective, the throw net represents the lowest risk for an explosion.

A shared advantage of pulling and pushing techniques is the ability to control the re-entry. This is particularly important when dealing with large objects that are likely to survive re-entry into the Earth’s atmosphere, thus endangering human lives and property on the surface of the Earth. Overall, studies involving pulling and pushing techniques show promise.[45]

In addition to pulling and pushing techniques, there are numerous contact-less techniques (ion beams, dust, lasers, foam or air bursts) for space-debris removal, “even though the term is not entirely accurate.”[46] One study undertaken by the ESA and the Universidad Politécnica de Madrid is the ion-beam shepherd (IBS) concept. The concept involves directing the plasma from an ion-beam thruster towards a piece of space debris, thus exerting a force.[47]

During a typical mission profile the IBS would rendezvous with the target debris and, while co-orbiting at constant distance have one of its ion beams constantly pointed at its surface to produce a small continuous drag force while the other ion beam would point at the opposite direction to keep the relative distance constant. This way the IBS can be used to remotely manoeuvre space debris without physical contact (docking), and can be repeated for multiple targets.[48]

It is highly advantageous that this technique can be used for multiple targets. Studies show that this technique is a very promising option with no show-stoppers. A similar concept using chemical thrusters instead of ion-beam thrusters is also under consideration.

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Another concept under investigation is the use of expanding foam. Similar to the IBS concept, the chaser vehicle would have to rendezvous in close proximity to the space debris prior to spraying it. The foam would subsequently expand and remain stuck to the debris so as not to create further debris. “The underlying principle of the method is to increase the area-to-mass ratio of debris in sufficiently low earth orbiting debris in order to increase their natural atmospheric drag and thus substantially decrease their natural orbital lifetime leading to their natural re-entry.”[49] An alternative delivery method is to embed the foam into future spacecraft as a preventative measure. Of course, this foam technique would only work in lower altitude orbits where there is sufficient atmospheric drag to allow for re-entry within a reasonable amount of time. Studies indicate that a one-ton (907-kg) piece of space debris could be de-orbited from a 900-km altitude within 25 years, demonstrating great promise.[50]

The use of air-burst vortex rings offers another possibility.[51] “The concept uses an air burst mechanism to place air molecules in the path of space debris, inducing a drag force which lowers orbital trajectory until intersection with the atmosphere.”[52] One possible implementation would use a ground-based system to fire an air vortex into space. “LEO debris encountering this air burst would undergo a drag force from the increased ambient density, perturbing its orbit to the point of intersection with the atmosphere. Drag forces would then continue negative acceleration of the material until deorbit is achieved.”[53] This ground-based implementation would require a lot of energy; however, it would be logistically simple.

Alternatively, a rocket could be used to carry an air-burst payload into orbit. This would be logistically complex relative to the ground-based option; however, it would reduce “energy costs associated with the initial burst and propagation.”[54] The use of air bursts is considered very safe in terms of the risk of accidentally creating more debris. Even if a failure occurred, “the systems themselves cannot produce more debris as they never reach orbit.”[55]

The United States Naval Research Laboratory is studying the use of dust to remove space debris. A rocket would be used to carry a dust payload into the path of the target debris.

The debris population is engulfed by the dust cloud, experiencing enhanced drag and results in the loss of debris altitude. The debris population descends to an altitude … [at] which Earth’s natural drag is sufficient to force reentry within a desired time. The dust cloud also descends under gravity and re-enters the atmosphere.[56]

Tungsten dust is a prime candidate due to its high density, abundance and relatively low cost.

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A ground-based laser also has potential as an active debris-removal element. Lasers could be used to remove both large and small pieces of debris. “LODR [laser orbital debris removal] uses the impulse generated by laser ablation of the debris surface by a focused, pulsed ground-based laser to change the debris orbit and cause it to re-enter the atmosphere. We use a telescope to focus the laser down to a 30 cm diameter circle on a target 1000 km away.”[57] The technique does not create more debris because “only a few nanometres of surface are vaporized and the object is not melted or fragmented by the gentle ablation pulse.”[58] The lasers and telescopes required for such a mission now exist, making this a feasible option.

Space debris in Earth orbit is a problem necessitating intervention in the form of active debris removal. Potential active debris removal techniques—including pulling, pushing and contact-less—are being explored. While further analysis is required, initial studies at various institutions around the globe indicate very promising results. The technical feasibility of active debris removal, however, is only part of the challenge that lies ahead. Perhaps of equal or greater difficulty are the regulatory, economic, political and legal challenges related to active debris removal. These areas require further examination as part of a broader effort to understand global space-governance requirements.

 


 

Major Nathan Arleigh Burgess is a communications and electronics engineering officer in the RCAF. He has held positions within various RCAF and CAF organizations, including the Directorate Aerospace Equipment Program Management (Radar & Communication Systems), 42 Radar Squadron and 4 Wing Telecommunications & Information Services Squadron at 4 Wing Cold Lake, Canadian Operational Support Command, and the Canadian Forces School of Communications & Electronics. Major Burgess recently completed a Master of Arts in War Studies with an aerospace-power concentration at the Royal Military College of Canada. He is currently employed as the Space Warfare Officer at the Canadian Forces Aerospace Warfare Centre in Trenton.

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Abbreviations

CAF―Canadian Armed Forces
cm―centimetre
EDT―electro-dynamic tether
ESA―European Space Agency
GEO―geostationary Earth orbit
IAASS―International Association for Advancement of Space Safety
IADC―Inter-agency Space Debris Coordination Committee
IBS―ion-beam shepherd
ISS―International Space Station
kg―kilogram
km―kilometre
LEO―low earth orbit
NASA―National Aeronautics and Space Administration
NORAD―North American Aerospace Defence Command
RCAF―Royal Canadian Air Force
RCM―RADARSAT Constellation Mission
SA―situational awareness
SARSAT―search and rescue satellite
SSN―space surveillance network
US―United States

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Notes

1. Tobias Evers, The EU, Space Security and a European Global Strategy (Stockholm: The Swedish Institute of International Affairs, 2013), 16. See also, Nodir Adilov, Peter J. Alexander, and Brendan Michael Cunningham, “Earth Orbit Debris: An Economic Model,” Social Sciences Research Network (May 14, 2013), accessed September 18, 2014, http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2264915, 4; “Orbital Debris Management & Risk Mitigation” (Washington: NASA, Academy of Program/Project & Engineering Leadership), 6, accessed September 18, 2014, http://appel.nasa.gov/knowledge-sharing/publications/appel-releases-ibook-html/; and Farhad Aghili, “Active Orbital Debris Removal Using Space Robotics,” ESA, 1, accessed September 18, 2014, http://robotics.estec.esa.int/i-SAIRAS/isairas2012/Papers/Session%208B/08B_01_aghili.pdf‎.   (return)

2. “Orbital Debris Management,” 6.  (return)

3. “Orbital Debris Management”.  (return)

4. “Orbital Debris Management”. See also, T. P. Brito, C. C. Celestino, and R. V. Moraes, “A Brief Scenario about the ‘Space Pollution’ around the Earth,” Journal of Physics: Conference Series 465 (2013): 1. (return

5. Adilov, Alexander, and Cunningham, “Earth Orbit Debris,” 58. (return)

6. Adilov, Alexander, and Cunningham, “Earth Orbit Debris,” 4. (return)

7. “Orbital Debris Management,” 16. (return)

8. Ibid.; C. Pardini and L. Anselmo, “Assessing the Risk of Orbital Debris Impact,” Space Debris 1, no. 1(1999): 59–60; Cesar Jaramillo, Space Security Index 2012 (Waterloo, ON: Project Ploughshares, 2012), 7, 27–28; and K. Wormnes et al., “ESA Technologies for Space Debris Remediation,” Proceedings of the 6th IAASS Conference: Safety Is Not an Option, (Montreal: European Space Agency, 2013), 1. (return)

9. Jaramillo, Space Security Index 2012, 27–28. (return)

10. Adilov, Alexander, and Cunningham, “Earth Orbit Debris,” 5. (return)

11. Adilov, Alexander, and Cunningham, “Earth Orbit Debris”; N. Johnson, “Increasing Solar Activity Aids Orbital Debris Environment,” Orbital Debris Quarterly News 16, no. 1 (2012): 4; and J. C. Liou, “A Note on Active Debris Removal,” Orbital Debris Quarterly News 15, no. 3 (2011): 8. (return)

12. Brito, Celestino, and Moraes, “A Brief Scenario,” 1. (return)

13. Stavros Georgakas, “Sweeping Away Space Debris with Dust,” Space Safety Magazine (July 1, 2012): 1, accessed September 18, 2014, http://www.spacesafetymagazine.com/2012/07/01/dust-actively-mitigate-space-debris-problem/; and J. C. Liou, “An Update on LEO Environment Remediation with Active Debris Removal,” Orbital Debris Quarterly News 15, no. 2, (2011): 4. (return)

14. NASA, “International Space Station Avoids Debris from Old NASA Satellite,” Orbital Debris Quarterly 15, no. 1 (2011): 1; and NASA, “Another Debris Avoidance Maneuver for the ISS,” Orbital Debris Quarterly News 17, no. 1 (2013): 3. (return)

15. Andrew Osborn, “International Space Station Evacuated after Debris Threatens Craft,” The Telegraph, June 28, 2011, 1. (return)

16. Wormnes et al., “ESA Technologies for Space,” 1; Ali S. Nasseri, Matteo Emanuelli, Siddharth Raval, and Andrea Turconi, “Active Debris Removal Using Modified Launch Vehicle Upper Stages,” Proceedings of the 6th IAASS Conference, 1–2; Georgakas, “Sweeping Away Space Debris,” 1; Liou, “An Update on LEO,” 4; Aghili, “Active Orbital Debris Removal,” 1; Brito, Celestino, and Moraes, “A Brief Scenario,” 4; Liou, “A Note on Active,” 7–8; and Adilov, Alexander, and Cunningham, “Earth Orbit Debris,” 5, 19. (return)

17. The primary purpose of the IADC is to exchange information on space-debris research activities between member space agencies, to facilitate opportunities for cooperation in space-debris research, to review the progress of ongoing cooperative activities and to identify debris-mitigation options. See “Terms of Reference for the Inter-Agency Space Debris Coordination Committee (IDAC),” Inter-Agency Space Debris Coordination Committee, accessed September 18, 2014, http://www.iadc-online.org/index.cgi?item=torp_pdf. (return)

18. Aghili, “Active Orbital Debris Removal,” 1; Georgakas, “Sweeping Away Space Debris,” 1; and Liou, “An Update on LEO,” 4. (return)

19. Liou, “A Note on Active,” 4.  (return)

20. Wormnes et al., “ESA Technologies for Space,” 1; and NASA, “Orbital Debris Remediation,” NASA Orbital Debris Program Office, accessed September 18, 2014, http://orbitaldebris.jsc.nasa.gov/remediation/remediation.html. (return)

21. Liou, “A Note on Active,” 4. (return)

22. Wormnes et al., “ESA Technologies for Space,” 1. (return)

23. Director General Space, “Sapphire Shines as FOC Approaches,” Apogee: Newsletter of the Canadian Defence Space Programme 2, no. 2 (November, 2013): 3; and “Agreement Between the Government of Canada and the Government of the United States of America on the North American Aerospace Defense Command”, 28 April 2006, Article 1, accessed September 18, 2014, http://www.treaty-accord.gc.ca/text-texte.aspx?id=105060. (return)

24. This is akin to identification friend or foe (IFF) in the aerospace environment. (return)

25. Director General Space, “Space-Based Capabilites,” Apogee: Newsletter of the Canadian Defence Space Programme 2, no. 2 (November, 2013): 4; and Director General Space, “Joint Space Support Team at JOINTEX,” Apogee: Newsletter of the Canadian Defence Space Programme 2, no. 2 (November, 2013): 8. (return)

26. “Search and Rescue Satellite Aided Tracking System (SARSAT),” Major Jason Terry, Canada, Department of National Defence, Chief of Force Development, accessed September 18, 2014, http://cfd.mil.ca/sites/intranet-eng.aspx?page=5939 (Defence Wide Area Network [DWAN] Intranet page).  (return)

27. Matthew A. Noyes, Peetak Mitra, and Antariksh Dicholkar, “Propagation of Surface-to-Low Earth Orbit Vortex Rings for Orbital Debris Management,” Proceedings of the 6th IAASS Conference, 1; and Aghili, “Active Orbital Debris Removal,” 1. (return)

28. Aghili, “Active Orbital Debris Removal,” 1. (return)

29. Nasseri, Emanuelli, Raval, and Turconi, “Active Debris Removal Using,” 1–3. (return)

30. Nasseri, Emanuelli, Raval, and Turconi, “Active Debris Removal Using”. (return)

31.Nasseri, Emanuelli, Raval, and Turconi, “Active Debris Removal Using,” 3. (return)

32. Robin Biesbrock, Tiago Soares, Jacob Hüsing, and Luisa Innocenti, “The e.Deorbit CDF Study: A Design Study for the Safe Removal of a Large Space Debris,” Proceedings of the 6th IAASS Conference, 3; and Wormnes et al., “ESA Technologies for Space,” 1–2, 4, 5. (return)

33. Wormnes et al., “ESA Technologies for Space,” 2. Re-orbiting would involve moving the debris to a “graveyard” orbit where it would no longer pose a danger.  (return)

34.Wormnes et al., “ESA Technologies for Space,” 2–3. (return)

35. Wormnes et al., “ESA Technologies for Space,” 3. (return)

36. The ESA study used a 16-metre by 16-metre net with a mesh size of approximately 20 cm; see Ibid. Other studies assume a fish-net size of about 25 metres by 25 metres weighing approximately 25 kg. See, Guillermo Ortega Hernando et al., “Guidance, Navigation, and Control Techniques and Technologies for Active Satellite Removal,” Proceedings of the 6th IAASS Conference, 2. (return)

37. Wormnes et al., “ESA Technologies for Space,” 3. (return)

38. Wormnes et al., “ESA Technologies for Space,” 3–4. (return)

39. Wormnes et al., “ESA Technologies for Space,” 4. (return)

40. Biesbrock, Soares, Hüsing, and Innocenti, “The e.Deorbit CDF Study,” 5. (return)

41. Ibid., 6; Wormnes et al., “ESA Technologies for Space,” 4–5; and Hernando et al., “Guidance, Navigation, and Control Techniques,” 3. (return)

42. Wormnes et al., “ESA Technologies for Space,” 2. (return)

43. F. Bonnet, C. Cazaux, and N. Pelletier, “Passivation Techniques for Future Spacecraft to Comply with French Space Operations Act,” Proceedings of the 6th IAASS Conference, 1. (return)

44. Biesbrock, Soares, Hüsing, and Innocenti, “The e.Deorbit CDF Study,” 2. (return)

45. Wormnes et al., “ESA Technologies for Space,” 2–5. (return)

46. Wormnes et al., “ESA Technologies for Space,” 5. (return)

47. “Plasma is an electrically neutral gas in which all positive and negative charges—from neutral atoms, negatively charged electrons, and positively charged ions—add up to zero. Plasma exists everywhere in nature; it is designated as the fourth state of matter (the others are solid, liquid, and gas).” See, “Fact Sheet: Ion Propulsion,” NASA, Glenn Research Center, accessed September 18, 2014, http://www.nasa.gov/centers/glenn/about/fs21grc.html. (return)

48. Wormnes et al., “ESA Technologies for Space,” 5–6. (return)

49. Wormnes et al., “ESA Technologies for Space,” 7. (return)

50. Wormnes et al., “ESA Technologies for Space”. (return)

51. A vortex ring is a spinning donut-shaped mass of air (e.g., a smoke ring from a cigarette). (return)

52. Noyes, Mitra, and Dicholkar, “Propagation of Surface-to-Low Earth,” 1.  (return)

53. Noyes, Mitra, and Dicholkar, “Propagation of Surface-to-Low Earth,” 1–2. (return)

54. Noyes, Mitra, and Dicholkar, “Propagation of Surface-to-Low Earth,” 1. (return)

55.Noyes, Mitra, and Dicholkar, “Propagation of Surface-to-Low Earth” . (return)

56. “NRL Scientists Propose Mitigation Concept of LEO Debris,” Daniel Parry, United States Naval Research Lab, accessed September 18, 2014, http://www.nrl.navy.mil/media/news-releases/2012/nrl-scientists-propose-mitigation-concept-of-leo-debris. (return)

57. “Clearing Space Debris with Lasers,” Claude Phipps, SPIE, accessed September 18, 2014, https://spie.org/x84761.xml. Laser ablation is a process whereby the surface of a solid is removed (e.g., vaporized) through irradiation from a laser. (return)

58. “Clearing Space Debris with Lasers,” Claude Phipps, SPIE, accessed September 18, 2014, https://spie.org/x84761.xml. (return)

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