Robert m pimpsner orbital debris mitigation - ens 100

Robert m pimpsner orbital debris mitigation - ens 100

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  Robert Pimpsner ENS100 Space Debris Mitigation in Earth Orbit Robert M. Pimpsner College of Staten Island Introduction to Engineering ENS 100 May 2021
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  Robert Pimpsner ENS100 As humans venture into space more frequently and our reliance on satellites grows, the problem of orbital debris also increases exponentially. Orbital debris, or space junk as it is commonly called, has existed since the formation of the planet, but while natural debris like micro meteors pose a potential danger to satellites, spacecrafts, and space stations it is the growth of man-made debris such as defunct satellites and discarded rocket bodies that pose an even greater risk. At the velocity needed to achieve orbit, even the smallest piece of debris can cause significant damage. The speed of an object in orbit depends on multiple factors including the mass of the satellite, the mass of the object it is orbiting and the radius of its orbit. It is expressed in the equation: 𝑣 = √ 𝐺 ∗ 𝑀𝑐𝑒𝑛𝑡𝑟𝑎𝑙 𝑅 1 Where 𝑀𝑐𝑒𝑛𝑡𝑟𝑎𝑙 is the mass of the object the satellite is orbiting, R is the radius of the orbit, and G represents the universal gravitational constant which is: 6.673 𝑥 10−11 𝑁∗𝑚2 𝑘𝑔2 . For objects in orbit of the Earth we know that the mass of the planet is 5.972 𝑥 1024 𝑘𝑔. At present the oldest piece of debris in orbit is VANGAURD-1 which was launched by the United States on March 17, 1958. The satellite’s mission ended in May 1964 and is not expected to enter Earth’s atmosphere until 2198. In addition to the satellite itself, there are two other pieces of debris from that 1958 launch remaining in orbit, the rocket body and a smaller piece of debris.2 There is no doubt 6that the public’s imagination has once again been captured by the romance of space flight thanks to a private space race of sorts being led by SpaceX, Blue Origin, Rocket Lab and the other 166 private rocket launch companies as well as the mainstays like Lockheed Martin and Boeing’s United Launch Alliance (ULA). In 2020 for example there were 114 successful launches with 10 failures.3 The United States led the pack with 44 total launches and SpaceX’s Falcon 9 being the most used launch vehicle of the year. This was nearly a 12% increase over 2019 and a 54% increase over 2010.4
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  Robert Pimpsner ENS100 Growth of man-made space debris in Earth Orbit5 The Problem With this ever-increasing launch schedule comes a problem. Space debris has been growing at an alarming rate with each launch adding more objects in orbit of the planet. Since 2007, the number of trackable space debris has more than doubled with close to 34,000 individual pieces of space debris that is larger than 10 centimeters and almost 129 million pieces of debris larger than 1 mm to 10 cm in size.6 This presents a threat to the human race’s ability to explore and provide service to those on-planet because a piece of debris with the diameter of a pin head traveling at 8 km/second can cause tremendous damage to operational satellites or orbiting laboratories like the International Space Station (ISS). Animated representation of orbital debris surrounding Earth in 20207
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  Robert Pimpsner ENS100 While the Space Station and most manned spacecrafts do have a Whipple Shield, a protective layer placed a small distance from the hull designed to slow down and fragment debris that hit it, they would not protect against the worst-case scenario, a collisional cascade in orbit called Kessler Syndrome. In his 1978 paper, NASA scientist Donald J. Kessler theorized that the density of objects in low Earth orbit (LEO) grows to the point where collisions occur in such a frequency that they cause additional collisions due to the growth of debris created by each collision in a chain reaction like dominos falling into each other. This would leave the orbit unusable due to the high amounts of debris.8 The damage that can be done even by a small piece of debris can be life threatening for humans inside of a spacecraft. During a workshop held at NASA’s Lyndon B. Johnson Space Center in 1982 to discuss the issue of orbital debris, scientists studied the potential danger of an orbital debris cloud to Apollo-era spacecraft such as Skylab and the Apollo Command Module. The findings concluded that the average force of natural-meteoroid impacts was .5 g/cc at a velocity of 20 km/second with man-made orbital debris impacting the hull with a force of 2.78 g/cc at a velocity of 10 km/sec. It went on to study the potential damage small-size debris (50 micrometer to 1.37 centimeter in diameter) can do to spacecraft in orbit when they impact at hypervelocity speeds.9 Test of a 1.5 mm sphere hitting a piece of 2024-T3 Aluminum at 7 KM/Second 10
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  Robert Pimpsner ENS100 As I stated above, the danger of debris impact is not to just manned spacecraft but also to operational satellites. With the exponential increase in satellite deployment over the last decade comes a greater risk of two satellites, whether they be active or inactive, colliding in orbit and creating even more debris. NASA scientist James W. McCarter examined the probability of orbital collisions in 1972 in his technical memorandum dated June 8 for the George C. Marshall Space Flight Center. These calculations were computed for a proposed modular space station with the goal of identifying the probability of the station being hit by the 2,588 pieces of debris that was in orbit at the time.11 The idea of two man-made satellites colliding was theoretical in 1972 and became a reality in 1996 when the French military satellite CERISE collided with a discarded piece of an Ariane rocket that was launched in 1986.12 Luckily, the collision only created one additional piece of debris in orbit. This was not a case for the active commercial communications IRIDIUM 33 and the inactive Russian military satellite COSMOS 2251 collision in 2009 which created over 2,000 individual pieces of debris in various orbits.13 Evolution of the debris cloud created by the IRIDIUM 33/COSMOS 2251 collision in 200914 Collisions are not the only way satellites can create multiple pieces of space debris. Since the 1960s there have been roughly 250 cases where satellites have broken up in orbit and created space debris. Many of these incidents were that of satellites colliding with small, untracked pieces of debris such as micrometeoroids. This was the case in 2014 when the ESA’s SENTINEL 1A satellite collided with a small piece of a debris, likely a micrometeoroid. The satellite did not destroy the satellite but did create eight pieces of trackable small debris.15 The
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  Robert Pimpsner ENS100 breakup of FENGYUN 1C in 2007 is regarded as the worst contamination of LEO in space flight history with the creation of 3,442 pieces of the debris from the test of a Chinese anti-satellite weapon.16 The 1996 breakup of a Pegasus Hydrazine Auxiliary Propulsion Stage ruptured and created 754 pieces of debris.17 Over the next few years this problem will be compounded as the number of satellite constellations grow. Currently, SpaceX has 1,300 Starlink satellites in LEO , representing 39% of the total number of active satellites in orbit. Starlink will grow to over 30,000 satellites over the next decade and will soon be joined by Amazon’s Project Kuiper. This is in addition to already existing constellation networks like OneWeb, Telesat, Iridium, and more. 18 While satellites are now required to have a plan for end-of-life (EOL) service there are many providers looking into backup plans. For example: SpaceX’s Starlink satellites use Hall- effect thrusters to achieve their final orbit, station keeping, and collision avoidance. They recently received approval from the FCC to operate the first batch of satellites at an altitude between 540-570km. This change from the initial 1,100-1,300km altitude that was initially approved allows Starlink to offer lower latency and better speeds but also provides a backup in case of a failure in the satellite’s thrusters.19 If a satellite fails at the new altitude, it will naturally deorbit and burn up in the atmosphere in as little as a week due to atmospheric expansion. OneWeb has also worked with the European Space Agency and the Japanese company Astroscale to develop a backup for the 600 satellites planned for their constellation which will operate at an altitude of 1,200km. Modern life is heavily reliant on the use of satellites for our society to work. From our financial system and communications to the Global Positioning System we use this technology every day to work, travel, and have fun. As this reliance on satellites increases the threat of Kessler syndrome is even more prevalent. Roughly 43% of satellites in orbit are communications satellites, which we use for cell phone service, internet access, television access, and the financial system. If by some chance these satellites are unreachable it would have lasting impact across multiple industries and the cascading effect on Earth can be devastating. This would be true if we lost contact with the Global Positioning Satellites that are in geosynchronous orbit (GEO).20 Connectivity loss is just one potential disaster that could occur from space debris. The other major issue is the increasing number of large debris that is falling back to Earth
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  Robert Pimpsner ENS100 uncontrollably and not burning up completely during re-entry. This issue has occurred twice so far during 2021, the first in March when a Falcon 9 second stage failed to re-light for a re-entry burn and broke apart over the Northwestern United States.21 Debris from this stage including a pressure vessel that housed helium landed on private property was found throughout the area. The second example of this was in May 2021 when the Chinese Long March 5 rocket that launched the Tianhe module of their new space station in LEO. The Long March 5 rocket does not have engine re-light capability which means that its re-entry is uncontrolled and unpredictable. While the 2021 re-entry of the Long March 5 narrowly missed hitting the Maldives, the 2020 re-entry of the same rocket showered an African village with debris.22 Major Satellite Constellations Name Operator # Use Global Positioning System (GPS) USSF 24 Navigation GLONASS Roscosmos 24 Navigation Galileo ESA 24 Navigation Globalstar Globalstar 48 Internet Access Iridium NEXT Iridium 66 Internet Access OneWeb OneWeb 74 (600*) Internet Access Starlink SpaceX 1558 (42,000*) Internet Access IntelSat IntelSat 54 Broadcast SIRUS/XM Radio SIrus XM 10 Broadcast Telesat Telesat 117-512* Internet Project Kuper Amazon 3236* Internet Access *Planned Satellite constellation size The Solutions Finding a solution to the ever-growing problem of space debris in Earth orbit has proven to be extremely difficult. One of the first major hurdles needed to fix the issue is regulatory. Presently, according to the Outer Space Treaty a country cannot interfere with another country’s satellite even if that satellite is past the end of its life. Only the country of origin for the satellite can service or retrieve the satellite, leading to several private companies working to solve the
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  Robert Pimpsner ENS100 problem, but none of the companies have gotten past the testing stage. Several different methods for capturing debris, as well as deorbiting technologies have been tested with the first expected active debris removal (ADR) mission to launch in 2025. In addition to removal of debris, there has also been work on technology to extend the life of a satellite and create fully reusable rockets which will further reduce debris in orbit. Using a combination of ADR, in-orbit reservicing, and fully reusable rockets we will be able to clean up the mess of debris currently in orbit of Earth. This will be a great advantage over the current system of leaving LEO satellites in place and moving GEO satellites to a graveyard orbit. Active Systems Active ADR systems are satellites designed to capture and actively de-orbit a target. These systems are currently in active testing with several prototypes currently in LEO. These technologies are important for the future cleanup of space debris and the prevention of Kessler Syndrome. Astroscale ELSA – Magnetic Capture Technology The most recent company to launch a satellite to test ADR was Astroscale, which launched on March 22 onboard a Soyuz rocket. The End-of-Life-Services by Astroscale Demonstration (ELSA-d) mission uses two spacecraft. One is a 17kg client satellite which will act as the debris for the test, and the other is a 175kg servicer satellite.23 Mission plan for Astroscale ELSA-OW24
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  Robert Pimpsner ENS100 The Astroscale plan includes launching the servicing satellite into orbit at approximately 500 km. The servicer will be raised to a rendezvous orbit with its target. For the OneWeb application that will be approximately 1,200 km. It will then examine the satellite before capturing it to bring it down to a lower orbit for a natural re-entry. The servicing craft will then re-orbit for a later mission. Astroscale’s solution is unique as it uses a special magnetic docking plate. The docking plate is designed not only to be used as a capture system for the satellite but also as a control surface for the satellite’s guidance and navigation system. The magnetic capture system is designed to allow the servicer satellite to capture a client’s object that is stable and also tumbling. The downside is that for Astroscale’s ELSA servicer to be used the satellite will have to have the magnetic docking plate pre-installed before launch. This means that their solution will not be useful in clearing debris that either does not have the docking plate, or debris that is the result of a breakup.25 RemoveDEBRIS – Harpoon and Net Capture The first attempt to demonstrate in-orbit capture and de-orbit technology is the RemoveDEBRIS project from the University of Surrey. It was launched to the International Space Station on April 2, 2018 onboard a Falcon-9. On June 20, 2018 it became the largest satellite to ever be deployed from the ISS as it got into position to complete its mission. This mission included two CubeSats that were designed to represent pieces of space debris in orbit as well as several capture experiments including using a Net and a harpoon.26 RemoveDEBRIS satellite cross section27
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  Robert Pimpsner ENS100 The net capture technology was successful tested on September 16, 2018. The test saw DebrisSat-1 (DS-1) deploy from the RemoveDEBRIS satellite. DS-1 measured 100 x 100 x 227 mm and included the power systems and avionics for the test which included an inflatable balloon which was designed to give the net a larger target area for the experiment. The net was ejected from RemoveDEBRIS at a distance of 7m from the target, once it connects with DS-1 it encloses it so that the satellite is not able to escape. The satellite is then deorbited at an accelerated rate due to the larger surface area of the balloon. Harpoon capture was successfully demonstrated on February 8, 2019 when RemoveDEBRIS extended a simulated target on a 1.5 m boom and the harpoon was fired at the speed of 20 m/s. The force of the impact knocked the target off the boom, but it remained connected to the satellite via the harpoon. ClearSpace-1 – Tentacle Capture The first active debris removal mission will happen sometime in 2025 after the ESA contracted ClearSpace to remove the Vega Secondary Payload Adapter (VESPA) left by the 2013 launch of a Vega rocket. This will be the first removal of a piece of debris. ClearSpace’s technology includes a satellite capable of grabbing the 112kg object using robotic tentacles and deorbit it from an altitude of 660 km into a lower orbit so it can disintegrate in Earth’s atmosphere.28 Laser Satellites A 2012 paper from NASA Langley Research Center examined the possibility of using laser satellites to remove space debris by either vaporization or to de-orbit the object to burn up in the atmosphere. The paper laid out the case of a short wavelength laser stationed in orbit, powered by solar panels. They used a theoretical 10 cm cube made from aluminum with a mass of approximately 2700 gm which would require 87,160 kJ of energy for vaporization and ionization. They determined that it would require a three-minute continuous laser beam of at least 5.38 MW of power if the aluminum absorbs 9% of the beam. It further noted that in order to produce a laser beam of 5.38 MW they will need to generate 108 MW of power generation. In order to generate that amount of power through the use of solar power would require approximately 202,500 square meters of solar cells. The amount of power required for vaporization and ionization of a piece of space debris was determined to be outside of our capabilities with current technologies.29
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  Robert Pimpsner ENS100 Within our technological capabilities is the ability to use a laser satellite to de-orbit a piece of debris through Laser Ablation. The power required for this approach is drastically less than that of vaporization. The method uses short pulses from a laser to create plasma plumes to generate thrust. This will theoretically slow down the object to below orbital velocity and fall into the Earth’s atmosphere. Using a laser to slow the speed of a piece of debris in Earth orbit. 29 In the paper, they used the same theoretical piece of a 2.7 kg aluminum as the vaporization research and traveling at a velocity of 10 km/s. They determined the amount of energy needed to slow a piece of debris by 30% using 2600 laser pulses is 100 J, a far cry from the 87,160 kJ of energy required for vaporization. Passive Systems Passive ADR systems are technologies built into satellites that require no active control from the ground or artificial intelligence to bring a body into the Earth’s atmosphere for disintegration. Draft devices such as the drag sail that was to be tested on RemoveDEBRIS remain one of the most common technologies first being used a decade ago. Drag Sail While the drag sail on RemoveDEBRIS did not properly deploy, the technology has flight heritage, with the first version being flown in 2010. The technology has matured since the
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  Robert Pimpsner ENS100 NanoSail-D2 mission that launched in 2010 and was deployed in 2011. The CanX-7 satellite, launched in 2016 deployed its drag sail in 2017 and successfully lowered its orbit altitude from an altitude of 688 km to 686.5 km within a month and a half as shown below. The results were impressive, the deployment of the drag sail increased the orbital decay rate from approximately 0.5km/year to 20 km/year.30 Altitude Change of the CanX-7 satellite before and after drag sail deployment. 30 In addition to the drastic increase in decay rate that occurred after the deployment of the drag sail, the ballistic coefficient decreased from an average of 42 𝑘𝑔 𝑚2 ⁄ to an average of 0.88 𝑘𝑔 𝑚2 ⁄ . This would slow the object down during re-entry and mean it will be less likely that it will hit the ground, if it survives re-entry, at supersonic speeds. As seen in the chart below, the sail reduced the estimated time for the CanX-7 satellite to deorbit from 178 years to 2.9 years. Predicted Long-Term Deorbit Performance for CanX-7 30
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  Robert Pimpsner ENS100 Electrodynamic Tethers (EDT) Electrodynamic tethers use a conduct tether to generate an electromagnetic force by converting their kinetic energy into electrical energy using the magnetic field of the planet it is passing. The magnetic field of the tether interacts with the magnetic field of the planet and the direction of the current determines the direction the object moves. The technology has also been studied as a way to re-boost the International Space Station and save on the use of chemical propellants. Tethers also have potential to generate power required for operations of a satellite. An EDT system will include several components including the tether, an electron collector, an emitter, a reel and a deployment mechanism. 31 The Japan Aerospace Exploration Agency (JAXA) has studied the use of electrodynamic tethers as a means to de-orbit space debris. EDT has the advantage that it does not require a power source, chemical propellants, and can be attached easily to satellites as well have a higher specific impulse. 31 In their paper, JAXA scientists proposed a braided tether or a net tether because of the possibility that a single line tether would be susceptible to being severed by a small piece of debris. In the simulation they examined what it would take to de-orbit a 3400 kg satellite in a sun-synchronous orbit (SSO) and a 1400 kg rocket body at an altitude between 900-1000 km. It was determined that an EDT with the length of 10 km will be able to de-orbit the 2400 kg SSO satellite in 180 days and the 1400 kg rocket body at 1000 km within 260 days. 31 De-orbit time of a 3400 kg satellite in SSO 31 De-orbit time of a 1400 kg rocket body 31
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  Robert Pimpsner ENS100 Reuse and In-Orbit Servicing While most of the ideas to deal with the abundance of orbital debris has dealt with de- orbiting satellites to burn up in Earth’s atmosphere, there are also attempts planned to service satellites, so they remain usable after their expected life span or in case of damage. Technical information on these projects is not as readily available as the technology used to de-orbit satellites, however they are an important step forward in bringing the number of non-functioning satellites down in the future. OSAM-1 NASA’s On-orbit Servicing, Assembly, and Manufacturing (OSAM)-1 mission is scheduled to launch at some point in the mid-2020s is designed to test the ability to extend the life span of a satellite in LEO that was not originally designed to be serviced in orbit. The demonstration will rendezvous the Landsat 7 satellite and attempt to refuel and boost it to extend the life. If successful, it will give satellite owners the ability to repair and refuel satellites in orbit which will theoretically be cheaper than launching a new satellite in orbit.32 MEV-1, MEV-2, MRV OSAM-1 is not the first demonstration of in-orbit servicing. Northrop Grumman launched MEV-1 in 2019 and docked with INTELSAT 901 in 2020 in a graveyard orbit. Within two months it was able to bring the satellite back to life and reposition it in a new orbit for operational service. The mission plan is to keep the satellite in operation for five years before moving to another satellite. MEV-2 launched in August 2020 and docked with INTELSAT 10-02 on April 12, 2021. The mission for MEV-2 is also slated to be five years before it moves the satellite to a graveyard orbit.33 The success of the MEV satellites has earned Northrop a Defense Advanced Research Projects Agency (DARPA) contract to develop the Mission Robotic Vehicle (MRV) which is being designed to carry out repairs as well as refueling and relocating satellites. SpaceX Starship/Super Heavy Building off the success of the Falcon-9 rocket and the dream of settling Mars, SpaceX has been rapidly developing their Starship/Super heavy system which will be the first fully
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  Robert Pimpsner ENS100 reusable rocket platform. Currently, only the Flacon-9’s first stage is reused to lower costs and decrease the amount of debris in LEO. A full reusable rocket platform will be crucial in the future as according to the chart from the ESA, roughly 15% of space debris is from the rocket body used to launch satellites into orbit. The oldest being the rocket that launched VANGUARD-1 in 1958. 5 Conclusion While Kessler Syndrome is unlikely in the near-term, the active expansion of satellite constellations in LEO has brought the fear of the cascade back in the forefront. In addition, the fear of large orbital debris re-entering the Earth’s atmosphere was once again brought to light by the uncontrolled re-entry of the Falcon-9 upper stage and Long March 5b rocket in 2021. Both situations have helped grow the public interest in solving the problem of orbital debris. In any case, the regulatory issues will also need to be addressed so that either countries or private business will be able to remove inactive satellites and fragments that currently belong to other states. In many ways the politics of space debris removal is even more difficult and complex than the actual engineering required to achieve the feet. It is one that many lawyers and world leaders are currently working on behind the scenes while the technical solutions are perfected. With the amount of orbital debris tripling over the last decade it is imperative we come up with a solution or multiple solutions to prevent the problem from getting to the point where we render whole orbits unusable. It will take an international effort and both public and private interest to clean up space around Earth. While we do not currently have an active ADR technology, there are plenty of intriguing options examined in this paper and more that were not touched on.
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  Robert Pimpsner ENS100 1 Mathematics of Satellite Motion. [Online] Available at: https://www.physicsclassroom.com/class/circles/Lesson-4/Mathematics-of- Satellite-Motion 2 Stuff in Space, Rogue Space Systems [Online] Available at: https://sky.rogue.space/ 3 Orbital Launches of 2020. [Online] Available at: https://space.skyrocket.de/doc_chr/lau2020.htm 4 Orbital Launches of 2010. [Online] Available at: https://space.skyrocket.de/doc_chr/lau2010.htm 5 About Space Debris. [Online] Available at: https://www.esa.int/Safety_Security/Space_Debris/About_space_debris 6 Space Debris by the Nunbers. [Online] Available at: https://www.esa.int/Safety_Security/Space_Debris/Space_debris_by_the_numbers 7 Recognising sustainable behaviour. [Online] Available at: http://www.esa.int/Safety_Security/Space_Debris/Recognising_sustainable_behaviour 8 Donald J. Kessler, B. G. C.-P., 1978. Collision Frequency of Artificial Satellites: The creation of a debris belt. Journal of Geophysical Research, pp. 3637-2646. 9 Parcs Small Satellite Test, Potential Hazards of Debris Clouds. Houston, Texas, NASA Lyndon B. Johnson Space Center, pp. 246-271. 10 Parcs Small Satellite Test, Potential Hazards of Debris Clouds. Houston, Texas, NASA Lyndon B. Johnson Space Center, pp. 268. 11 James W. McCarter, NASA TM X-64671 – Probability of Satellite Collision. Huntsville, Alabama, NASA George C. Marshall Spae Flight Center 12 Phillip D. Anz-Meador, John N. Opiela, Debra Shoots, J.-C. Liou, History of On-Orbit Satellite Fragmentations 15th Edition. Houston, Texas, NASA Lyndon B. Johnson Space Center, pp. 400.
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  Robert Pimpsner ENS100 13 Phillip D. Anz-Meador, John N. Opiela, Debra Shoots, J.-C. Liou, History of On-Orbit Satellite Fragmentations 15th Edition. Houston, Texas, NASA Lyndon B. Johnson Space Center, pp. 410. 14 Brian Weeden, 2009 Iridium-Cosmos Collision Fact Sheet. Secure World Foundtion 15 Phillip D. Anz-Meador, John N. Opiela, Debra Shoots, J.-C. Liou, History of On-Orbit Satellite Fragmentations 15th Edition. Houston, Texas, NASA Lyndon B. Johnson Space Center, pp. 506. 16 Phillip D. Anz-Meador, John N. Opiela, Debra Shoots, J.-C. Liou, History of On-Orbit Satellite Fragmentations 15th Edition. Houston, Texas, NASA Lyndon B. Johnson Space Center, pp. 422. 17 Phillip D. Anz-Meador, John N. Opiela, Debra Shoots, J.-C. Liou, History of On-Orbit Satellite Fragmentations 15th Edition. Houston, Texas, NASA Lyndon B. Johnson Space Center, pp. 386. 18 Dave Mosher, SpaceX may want to launch 42,000 internet satellites — about 5 times more spacecraft than humanity has ever flown [Online] Available at: https://www.businessinsider.com/spacex-starlink-internet-satellites-itc-filing- 30000-additional-42000-total-2019-10 19 Jeff Faust, FCC approves Starlink license modification [Online] Available at: https://spacenews.com/fcc-approves-starlink-license-modification/ 20 Aerospace Industries Associaton, SKYFAIL: WHAT HAPPENS IF AMERICA’S SATELLITES SUDDENLY GO DOWN? [Online] Available at: https://www.aia-aerospace.org/skyfail-americas-satellites-suddenly-go-down/ 21 Dominic Gates, After fireballs streaked across sky, space-junk sleuths got busy — and hit the jackpot in Washington [Online] Available at: https://www.aia-aerospace.org/skyfail-americas-satellites-suddenly-go-down/ 22 Browne, E., 2021. Chinese Rocket Reportedly Rained Metal on Ivory Coast Last Time One Fell to Earth. [Online]
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  Robert Pimpsner ENS100 Available at: https://www.newsweek.com/chinese-rocket-rained-metal-ivory-coast-earth-orbit- 1588865 23 Chris Blackerby, Akira Okamoto, Seita Iizuka, Yusuke Kobayashi, Kohei Fujimoto, Yuki Seto, Sho Fujita, Takashi Iwai, Nobu Okada. The ELSA-d End-of-life Debris Removal Mission: Preparing for Launch [Online] Available at: https://astroscale.com/wp-content/uploads/2020/02/ELSA-IV-Conference-IAC- 2019-v1.1.pdf 24 Chris Blackerby, Akira Okamoto, Seita Iizuka, Yusuke Kobayashi, Kohei Fujimoto, Yuki Seto, Sho Fujita, Takashi Iwai, Nobu Okada. The ELSA-d End-of-life Debris Removal Mission: Preparing for Launch [Online] Available at: https://astroscale.com/wp-content/uploads/2020/02/ELSA-IV-Conference-IAC- 2019-v1.1.pdf 25 Astroscale ELSA-d Press Kit [Online] Available at: https://astroscale.com/wp-content/uploads/2021/03/ELSA-d-Launch-Press-Kit- 2021-ENG_0322.pdf 26 RemoveDEBRIS Mission – University of Surrey [Online] Available at: https://www.surrey.ac.uk/surrey-space-centre/missions/removedebris 27 Jason L. Forshaw, Guglielmo S. Aglietti, Nimal Navarthinam, Haval Kadhem, , RemoveDEBRIS: An In-Orbit Active Debris Removal Demonstration Mission. 28 FROM E.DEORBIT TO CLEARSPACE-1: A DREAM COME TRUE, ESA [Online] Available at: https://blogs.esa.int/cleanspace/2021/01/20/from-e-deorbit-to-clearspace-1-a- dream-come- true/#:~:text=In%202025%2C%20ClearSpace%2D1%20will,on%20Earth%20and%20in%20spa ce%E2%80%9D. 29 Sang H. Choi and Richard S. Pappa , Assessment Study of Small Space Debris Removal by Laser Satellites, NASA Langley Research Center, Hampton, VA. 30 Brad Cotton, Ian Bennett, and Robert E. Zee, On-Orbit Results from the CanX-7 Drag Sail Deorbit Mission, UTIAS Space Flight Laboratory, Toronto, On. Canada
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  Robert Pimpsner ENS100 31 Satomi Kawamoto, Yasushi Ohkawa, Shoji Kitamura, and Shin-ichiro Nishida, Strategy for Active Debris Removal Using Electrodynamic Tether, Aerospace Research and Development Directorate, JAXA, Tokyo Japan 32 On-Orbit Servicing, Assembly and Manufacturing-1, NASA [Online] Available at: https://nexis.gsfc.nasa.gov/OSAM-1.html 33 Northrop Grumman Space Logistics [Online] Available at: https://www.northropgrumman.com/space/space-logistics-services/