PRE2016 3 Groep19

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Space Debris

Contents

Group members

  • Jeanpierre Balster - 0864027
  • Mike Beckers - 0943224
  • Elise Levert - 0883583
  • Joël Peeters - 0939193
  • Kady Schotman - 0958295
  • Elise Verhees - 0950109

Planning

A rough planning of the whole project containing the milestones in the process:

Week 1: Decide on the subject by brainstorming.

Week 2: Do some basic research about the chosen subject and make a presentation about it (including objectives and approach).

Week 3: Create a planning (inclusive a presentation about it), finalize definition deliverables, define milestones. Start working on specific tasks of the literature research.

Week 4: State-of-the-art literature research. Decide solution on Thursday.

Week 5: Do further more detailed literature. Begin Netlogo simulation.

Week 6: Refine solution based on literature. Continue on Netlogo simulation.

Week 7: Finalize NetLogo deliverable.


A Gantt chart of the planning is given below. LR is literature research. SR is solution research, so combining the knowledge from the literature research to find the best solutions. For more detailed explanation on the different tasks, see section 'Approach'.

File:GanttChartGroup19.jpg

We have switched from Netlogo to Freeflyer, but the planning did not change because of this. During the course, the planning has slightly been changed, but overall the planning has remained the same.

Case study

Group 19 will work on researching a space cleaning robot. This is currently a relevant problem; millions of pieces of space debris are orbiting the Earth. These pose a threat to satellites and spacecrafts which can collide with the debris (forming even more debris). Even if from now on nothing would be shot into space anymore, the amount of space debris would still increase, since pieces of debris can collide with each forming new pieces of debris. The problem needs to be solved using an Artificial Intelligent autonomously functioning device, since it is not possible to control the device from Earth, due to the time it would take to receive and send information from and to space (the debris would already be out of reach before the information is send back to Earth). The robot should be able to autonomously complete the following tasks: locate the debris, either collect and store the debris or push it in the right direction, return to Earth. Furthermore, research should be done on how to get rid of the debris. Will the robot burn the debris upon reentering of the Earth's atmosphere or will it bring he debris back to Earth (for the use of recycling).

Objectives

The project’s goal is to investigate the best solution to remove space debris from Low Earth Orbit (LEO) through literature research and a simulation. A set of criteria will be defined and for each aspect of the solution the best option will be chosen using these criteria.

Criteria:

  • Utility: Will it solve the problem? If the solution will not or only partially solve the problem, it cannot be seriously considered. The main objective of the project is to come up with a potentially working concept.
  • Safety: The space debris is a safety issue, thus the mission to lessen this should not be unsafe.
  • Environment: The solution will be less preferable if it pollutes the Earth’s atmosphere or contributes to space debris itself. The effects of the end of life phase of the mission on the environment should be taken into account as well as the way of removal of space debris.
  • Costs: If the costs are too high, the project will not be possible. For this reason, although costs may be difficult to estimate, the cheapest options will likely be preferable.
  • Technological feasibility: In order for the option to be realistic, the technology involved should be available or available in the near future. Future developments may provide many more opportunities but this will be impossible to predict and thus unreliable.
  • Reliability: In order for this technology to be used for a long time it should be reliable. This will also make it more cost effective.

For each of the aspects of the final solution these criteria are discussed in the corresponding paragraphs.

Approach

The approach is to realize the following deliverables:

1. A literature research on:

  • The current impact of the space debris on society
  • The current impact of the space debris on enterprises
  • The current ways of finding space debris
  • The current ways approaching the debris (to catch it)
  • The current ways of retrieving debris from space (catching and storing or pushing in the right direction)
  • The current ways if getting rid of the debris (burn up/bring back to Earth)
  • The current ways of returning the device to earth

2. A concept for the best solution / improving existing solutions

  • The impact of the solution on society
  • The impact on enterprises of the solution
  • The best way to find debris
  • The best way to approach it
  • The best way to retrieve the debris from space
  • The best way to get rid of it
  • The best way to return to earth

3. A Freeflyer simulation

  • Which will simulate the search path
  • Which will simulate transporting the debris

Kessler Syndrome

In 1978, American astrophysicist Kessler published a paper called “Collision Frequency of Artificial Satellites: The Creation of a Debris Belt.” In this paper, he predicted that by 2000 the density of space debris in Earth’s orbit would be so great that random collisions would definitely occur. The outcome of these collisions would be more debris and, thus, more collisions creating more debris. Due to the exponential growth, this will inevitably lead to an infinitely large amount of debris (La Vone).

Collisions occur because each piece of debris is subject to drift and decay, not due to the high speeds in space (22000 km/h). Therefore, the uncontrolled objects do not follow a straight path and, thus, are more likely to collide. Take, for instance, a highway. On a highway, all cars travel around the same speed and follow the same path; accidents rarely occur in this instance. Accidents on a highway often happen when cars switch lanes. The switching of lanes can be compared to the drift and decay of space junk. Therefore, it is the drift and decay of the space debris that leads to the accidents in space (La Vone).

Kessler even suggested a way to avoid the exponential growth of collisions: reduce the number of non-operational spacecraft left in orbit. The only way to get rid of those is to actively remove them. It is, thus, also important to prevent the creation of new debris by removing everything sent into the LEO before they break down (La Vone).

USE aspects of space debris

Society on earth

As long as humans have existed, objects from space have hit the earth’s surface. People even think that it was a meteorite that once killed most of the organisms on Earth, including dinosaurs (Choi). A well-known and recent example is the meteorite that hit Russia in 2013. In this occasion at least 1200 people were injured (Sample). In November 2016, in Myanmar, a mining facility became the crash site of a huge piece of space debris (Galeon, BBC.com). What is special about this particular crash is that the object that crashed was clearly not an object originating from space. Some smaller pieces of debris with Chinese markings on it even destroyed the roof of a house nearby.

The impact of space debris on society is not limited to debris as projectiles. The debris could hit working satellites, and as our lives revolve around the information they provide, it is important to keep the working satellites working. In February 2009, two communication satellites collided accidentally (Dunbar). Satellites are put in orbits around the Earth in such a way to decrease the chances of them colliding. When a collision occurs between two of them, more debris is created. Imagine all the possible collisions between satellites and space debris that could occur at any moment. Now consider how any one collision multiplies the odds of another collision occurring. These collisions damage the satellites we depend so much on. The BBC wrote an article about what would happen if all satellites would stop working at the same time. Although this is very unlikely, the effect of it would be tremendous. From missing the morning news show at breakfast to not being able to reach your overseas colleagues at work immediately, satellites are an integral part of daily life. GPS would stop working, so you would also be limited to getting around using an old school map. These are small, inconvenient changes, but the missing weather information makes things such as air traffic a lot more difficult. In fact, air traffic could be entirely impossible due to the difficulties faced with limited communication. Satellites are also relevant when considering wars where drones are used to drop bombs; limited communication in this case could also have fatal consequenecs (Hollingham). Clearly, the lives we live today are made possible by the satellites in space. Space debris endangers these satellites and can consequently effect our daily lives.

Society in space

A crack in the window of the International Space Station.

Space debris also leads to danger in space itself. In June 2011, a piece of junk whistled 335 meters past the International Space Station. In March 2012, NASA ground control did spot another piece of space debris going near the station, but it was simply too late for the station to maneuver towards a safer orbit. It missed the ISS by 12 km (Weinberger). NASA worries about every piece that is larger than a baseball; however, there are more than 21000 pieces of man-made space debris with this size (Lewis). Small pieces of debris are also problematic: “being hit by a ‘sugar-cube’ of space debris is the equivalent of standing next to an exploding hand-grenade” (Clark).

Space disasters can be fatal to humans and the impact of such an event is felt worldwide. People are aware of the dangers when launching and returning to space, but the dangers within space are largely underestimated. Losing a life within space could lead to an even bigger shock worldwide, and this is something we desperately need to prevent.

Enterprises

Back in 2001, Dennis Tito became the first space tourist for 20 million pounds (Wall). Since then, six more people have made the trip, with the last one made in 2009. One of them even went twice (the Guardian). These days, companies are trying to launch affordable flights to space (Jee). As of 2014, 700 tickets have been sold by Virgin Galactic (Howell). With Virgin Galactic promising to start commercial flights in 2016 and SpaceX wanting to fly passengers on private trips around the moon in 2018, space tourism is really something of the near future (Cofield). Futron, an aerospace and technology consulting firm, predicts that space tourism could become a billion-dollar market within 20 years ("The Economic Impact of Commercial Space Transportation on the U.S. Economy in 2009"). As more people will be entering space, it is important that space is as clean and safe as possible. 21 people have lost their lives due to accidents in space (Venugopal); failures of technology, design, and management are risks of space travel everyone is aware of. Space junk, however, is a lesser known threat. In order to raise awareness, a British scientist and artist created an online interactive project about the 27000 pieces in space larger than 10 cm and travelling at 8 kilometers a second (Carpeneti).

The satellites getting damaged cost money too. Launching new satellites into space costs between 11.3 and 34.5 million dollars per ton (Smith). The costs of new satellites or repairing the broken satellites could make space debris removal a lucrative industry. Investing in this technology would lessen the risk of satellites being damaged.

The Swiss Space Systems (S3) is creating the CleanSpace One, which is slated to launch in 2018. This space debris removal project costs about 16 million dollars. Another method, Laser Orbital Debris Removal (LODR), would cost one million dollars per object to be cleaned up (Markham). A Singapore-based company named Astroscale has secured a 30 million dollar funding, in order to create two pieces of technology: one for finding debris smaller than a millimeter and the ADRAS1, an adhesive-smeared spacecraft designed to stick onto debris and move it out of harm’s way (Mckirby). These are just a few of the many projects started to clean up space. Clearly, cleaning up space is going to cost millions, if not billions.

Why enterprises are forced to care about space debris by governments

Enterprises will be forced to take into account space debris that orbits Earth in the LEO. This is due to explicit regulatory requirements as well as to customer demand. In this section, governments pressure on enterprises will be discussed (Schilling).

As the problem of space debris in LEO is increasing, governments like the US government have been introducing regulations to minimize further increment of debris. In Europe, the European Union is doing something similar: adopting a Code of Conduct including debris-mitigation measures. All these requirements can be combined into one widely applicable standard, which is expected to happen in the near future. This standard can then be enforced with laws or with a treaty. These requirements have to be followed by governmental organizations but also by non-governmental ones (enterprises). This way, as the title already states, enterprises are forced by governmental regulations to care about space debris (Schilling).

At the moment, there are not yet many requirements levied by law or regulation. The following list sums up the current requirements (Schilling):

  • Commercial space launch operations are not allowed to generate space debris on purpose.
  • Components of the launch system are not allowed to collide with each other (creating space debris).
  • Commercial space launch operations should passivate the launch vehicle. This means they should deplete propellants, pressurant gases, and batteries.
  • In some regulations, commercial satellites should have end-of-life disposal.


However, no requirements exist for the safe disposal of launch vehicle hardware by commercial launches. Such requirements will probably be included in future laws/regulations. If not, commercial launch providers will still have to consider this, as governmental customers will have their own internal requirements. Governmental customers are the main clients of launch providers, so this will put an increasing pressure on the providers to satisfy the requirements of the customers (Schilling).

Unfortunately for the enterprises, these requirements will cost the commercial launch providers money. This is in conflict with minimizing costs, which is of great importance in the commercial world. Research will, thus, have to be done on low-cost debris mitigation technologies (Schilling).

Retrieving Mechanisms

Electrodynamic tethers can be used to remove space debris. In this option, the tether attaches itself to a piece of debris and current is induced along the tether. A Lorentz force is created between the tether and Earth’s magnetic field, causing the space debris to accelerate. This can significantly decrease the time needed for the object to de-orbit, particularly for debris close to earth (Barbee).

A momentum exchange tether could also be used to change the path of debris. Here, the tether, moving at high speeds, will attach to a slower moving piece of debris. If the debris is released at its highest retrograde velocity, then it will come closer to the atmosphere (lower perigee) (Barbee).

Lasers for vaporization are rather unfeasible as they require high precision and power. The debris moves quickly and somewhat unpredictably, so the precision is a huge issue. Also, the power requirement is beyond our current capabilities. The object could also potentially explode if it contains some unspent propellent. A laser in space could even infringe upon UN regulations (Barbee).

Surface material could be sent into space and affect the travel path of all objects which hit it. The object would be at risk of breaking and creating more space debris (Barbee).

Reflective Solar Sails are another alternative. They could attach onto debris, and as solar photons strike the sail, the object will in turn accelerate. The issue with solar sails is that it may not significantly alter the acceleration of the orbiting bodies unless it is acting on the body for months. At low altitudes this technology couldn’t be used due to corrosion (Barbee).

Another general concept is to produce streams of air from within the atmosphere that will be directed towards debris to change its travel path. Methods of producing these air streams vary from balloons to high altitude planes. This approach could affect multiple pieces of space debris in one attempt and is at no risk of creating more space debris if it fails (David).

The use of ion beams is also considered to help move debris; one such example is the Ion Beam Shepherd. The concept is to create an ion beam which will produce a force that can propel the debris forward. This also forces the mission to move in the opposite direction with the same force; thus, two beams are necessary in order to move the debris forward and keep the mission in the same position relative to the debris (Zuiani).

A net mechanism could be used. This would consist of four mass which will be shot out with a spring. The masses will pull the net out and surround the debris. The net size can be rather easily adjusted. This net is attached to a tether which is controllable using a reel and a motor. The net, once encompassing the debris, will close behind the target and tighten slightly. The object, now captured, will be slowed down and sent on a new travel path (Bischof).

Considerations for the project

Summary of the results of the considerations. The T stands for tie, this means that they rank equally.

Utility: When looking to the utility of possible solutions, not all of them are equally preferable. The tethers, both electrodynamic and momentum exchange, will work, as well as the streams of air, and ion beams. For the laser evaporation, surface material, solar sails, and net mechanism, utility is lower. Lasers for evaporation will not work, as both the power and precision needed are too high. The surface material is not useful because the risk of creating even more smaller debris is too high. Solar sails are inoperable in the LEO. With the main focus of the project being to clean up the LEO, it is clear that the solar sails are not an option. The net mechanism is a potentially effective, but due to the precision needed, it is too hard to use.

It is not needed to discuss the other criteria on the solutions that are out of consideration based on the utility criteria, as the most important criteria is that the solution has to actually work. Therefore, for the remaining other criteria only the tethers, streams of air, and ion beams will be discussed.

Safety: If the tethers would be used, the debris would be sent to Earth. Although most of it will burn up in the atmosphere, some will make its way to the Earth’s surface. This means that the tethers will continue to pose a threat to society. This is also the case for the ion beams. Yet, with the Earth’s atmosphere existing for 70% of water, the odds of it really hitting people or big cities is really small, and thus a risk worthy to take. Moreover, the path of debris deorbited from LEO can be controlled very precisely. With the streams of air, the same effect can achieved plus the risk of creating more smaller (less manageable) debris about zero. Thus, the streams of air are the safest.

Environment: One might think that burning space debris in the Earth’s atmosphere would have a big environmental impact on the atmosphere. This is not the case. The burning of the debris has a tiny impact where it reaches the atmosphere, but on a global level this is negligible. Furthermore, all three solutions will need energy, but there are windmills, solar cells, and other environmentally friendly ways of getting energy. These can be used to charge the batteries needed for the cleaning mission. The three solutions, therefore, rank equally.

Costs: For all of the remaining solutions, the exact costs are (publicly) unknown. However, it can be safely said that streams of air is the cheapest option. Comparing the tethers and the ion beam, it is most likely that the ion beam is cheaper as the accuracy needed for this solution is lower.

Technological Feasibility: The use of tethers has already been applied in the Kounotori mission (Japanese H-II Transfer Vehicle Kounotori 6 fails to deploy magnetic tether to clear junk in earth orbit), and is thus technologically feasible. Streams of air will probably not be that hard to make either. With the ion beams already being used in the medical field, it can be concluded that these are possible to effectively create as well. However, it has not yet been used in a space application. Therefore, the ion beam will probably hardest to make.

Reliability: For the tethers, it is known that they will always work. For the streams of air, this is not the case, because not yet a lot is known about this solution. The ion beams are relatively easy to control (easier than tethers), thus it can be said that they are reliable. This means that the ion beam is the best solution when it comes to reliability and streams of air the worst.

Conclusion: In summary, the only possible solutions are the tethers, both electrodynamic and momentum exchange tethers, the streams of air, and the ion beams. Based on the other criteria, it can be concluded that the ion beams fulfills these criteria the best. Therefore, it is best to use the ion beam in order to clean up space.

Getting rid of space debris

Burning in atmosphere

Often, space junk is attracted by the Earth's gravitational field and begins to fall down; most of it burns up in the atmosphere and will never reach the Earth's surface (Redd). This could be used as a method to actively get rid of space debris: through burning debris in the Earth’s atmosphere on purpose. However, how will this affect the Earth’s atmosphere and to which dimensions of space debris would this physically be possible?

When pieces of debris fall down, nearly all those bigger than 10 cm will not entirely burn up but instead be fragmented into smaller pieces. These pieces will fall into water most of the time, as about 70% of the Earth’s surface is water (Redd). When deliberately sending debris into the Earth’s atmosphere, one wants to ensure that no large chunks return to the ground and hurt people or damage property. There are two important factors that influence the extent to which a piece of space debris will burn up. First, the size; obviously, smaller pieces are more likely to burn up entirely than larger ones. Secondly, the speed with which the debris enters the atmosphere. The Earth’s atmosphere is full of matter, which means that a lot of friction is created when space junk speeds through it with high velocity. Friction creates heat. Heat, when reaching the boiling point of the debris, vaporizes the space debris layer by layer ("How big does a meteor have to be to make it to the ground?"). But even more significant is the air in front of the debris that will compress due to the high kinetic energy of the object, increasing the temperature. However, it is a bit more complicated, as size plays yet another role: smaller pieces slow down more quickly because the friction is very large compared to their mass. Eventually, they might just start drifting down. This means that, sometimes, very small dust grains will not burn up whereas a bigger piece of debris would have melted away entirely (“Why burn up on entering Earth's atmosphere”).

It is, therefore, not possible to define one maximum size for a piece of space debris to be burned up in the atmosphere. By controlling the entrance speed into the Earth’s atmosphere, it is possible to ‘change’ this maximum size. However, this will only work up to a certain size, so the very large chunks of space debris will never be able to entirely burn up in Earth’s atmosphere.

But what is the (chemical) impact on the Earth’s atmosphere of burning up all this space debris in it? In 1994, a study team commissioned by the Environmental Management Division of the Space and Missile Systems Center looked into the impact of burning space debris in the atmosphere on stratospheric ozone. Their findings were that the ozone was affected. When space debris travels through the Earth’s atmosphere with high speed, a shock wave is created. This shock wave produces nitric oxide, causing a decrease in stratospheric ozone (known as ozone depletion). However, the impact is not significant on a global level. Still, more research needs to be done on the density of particles, types of particles, and length of time they are suspended in the atmosphere to determine the long term effect of actively pushing large amounts of space debris into the Earth’s atmosphere (David).

Big space junk – burning or pushing away

The section above mainly focused on the smaller debris that is already orbiting the Earth for some time. Now, ‘new’ space debris will be investigated. 'New' space debris refers to older space stations and satellites that are nearing their ends and have not yet been broken down into smaller pieces. These objects will likely be too large to entirely burn up in the Earth’s atmosphere.

At the moment, there are two ways of getting rid of these satellites. The method chosen depends on how high the satellite is orbiting Earth.

Satellites closer to Earth will use their last bit of fuel to slow down. They will then fall out of orbit and burn up in the Earth’s atmosphere. The smaller satellites might burn up to such an extent that they can do no harm anymore. However, the bigger ones, like space stations and larger spacecrafts in low orbit, will not entirely burn up before reaching the surface. To ensure such a space station does not crash down in, for example, a big city, space operators plan to crash their old satellites in the so-called spacecraft cemetery. This spacecraft cemetery is situated in the Pacific Ocean, as far away from any human civilization as possible.

Spacecraft cemetery in the South Pacific Ocean ("Where do old satellites go when they die?")

Satellites that are further away from Earth would need more fuel to slow down than those closer to Earth. Most of these high orbiting satellites need less fuel to get farther away from Earth than to get back. Therefore, these satellites will be sent even farther away from Earth instead of sending them back to Earth. Similar to the spacecraft cemetery, these satellites will be sent into a graveyard orbit. This orbit is almost 200 miles higher than where the farthest active satellites are orbiting. Here, they will continue orbiting, some of them for a very long time. For now, they will not be able to bump into the intact satellites and cause them any harm. But, it may be necessary in the future to send some kind of space cleaning device to get rid of these satellites in the graveyard orbit ("Where do old satellites go when they die?").

Recycling of space debris

Destroying space debris is one way of getting rid of it. Another option could be collecting reusable junk and recycling it. There is over $300 billion worth of dead satellites drifting through space (Vijayaraghavan). The United States Department of Defense (DAPRA) is already working on a program, called The Phoenix program, that aims to recycle broken satellites. They are looking into recycling the parts that can still be used to incorporate into new space systems. According to DAPRA director Regina Dugan, "if this program is successful, space debris becomes space resource." The program wants to use a robot to reclaim still-working antennas from dead satellites which orbit the Earth in the graveyard orbit (see section about big space junk). These antennas will then be attached to new smaller satellites (satlets) launched from Earth. This would save a lot in launch costs, as antennas are big and bulky, which costs more fuel. Launching the satlets without antennas is, thus, much cheaper (SPACE.com staff).

However, recycling space debris is not as simple as recycling plastic bottles. According to DARPA program manager David Bernhart, “satellites in orbit are not designed to be disassembled or repaired, so it’s not a matter of simply removing some nuts and bolts. This requires new remote imaging and robotics technology and special tools to grip, cut and modify complex systems” (Pultarova).

The robot that DAPRA is working on will have grasping arms and remote vision systems. The Phoenix program can make use of some existing ‘Earth technology’ to start with. These technologies include surgery systems, that make it possible for doctors to do the surgery from thousands of miles away, and remote imaging systems, used by oil drillers to view as far as the ocean floor thousands of feet underwater. However, these systems need to be adapted to work in outer space where there will be no gravity, vacuum, and harsh-radiation (SPACE.com staff).

Considerations for the project

Utility:

All solutions will solve the current problem. One might argue that sending the debris into a graveyard orbit further away from Earth is only a temporary solution; thus, burning up the debris is preferred. The same goes for dumping the debris into the spacecraft cemetery. However, when combining the graveyard orbit with the possibility of future recycling, this solution will increase in utility, because it solves two problems at the same time.

Safety:

Safety is very important in this project. Sending (small) pieces of debris into the Earth’s atmosphere to burn up is safe, as can be concluded from the literature research done above.

Planning for the (larger) debris to crash into the spacecraft cemetery is also safe, but has a slightly bigger risk. Operators can plan for their spacecraft to crash in a certain area, but this depends on many factors. The most important factors are the velocity when entering the Earth’s atmosphere and the entry angle. The values of these factors must be within certain limits. For the circumstances of the project, the velocity should not pose a problem to the safety. It will mainly affect the extent to which a spacecraft will burn upon reentering. This would be very important for manned spacecraft but not for the space debris this project is about. Moreover, if entry is from LEO, the velocity can be determined very accurately (Scuka).

The entry angle is of bigger importance regarding the safety criterion of sending the debris to the spacecraft cemetery. If the angle is too steep, the debris will decelerate too much, and it might break up. This would result in uncontrollable smaller pieces of debris. A steeper angle will also result into a higher heat flux (the thermal energy absorbed by the debris), but again this does not really matter. If the angle is too shallow, the deceleration becomes too low, meaning that the debris will not slow down enough and that it will miss its target. It might then crash into an urban area. If the entrance angle is really low, the deviation could be even worse. Due to the very shallow entry angle, the debris will not enter the denser atmosphere layers. This means that there will be less braking as friction will be lower and then the debris will just continue in its orbit. The debris’ orbit is not entirely circular but a little elliptic. This causes the debris to gain altitude again. It will leave the Earth’s atmosphere, and then re-enter as it gets attracted by the Earth’s gravitational field again. However, it will now crash into an entirely different location than was planned for, possible very far away from the spacecraft cemetery (Scuka).

It might sound like a lot can go wrong when intentionally sending debris into the Earth’s atmosphere, which is true. However, the chance of something going badly wrong is very small. The chance of an object re-entering Earth’s atmosphere crashing onto a human is smaller than 0.0001 according to NASA’s Orbital Debris Office (Stirone).

Sending debris to a graveyard orbit seems safe, as this way it is out of the LEO where all intact satellites still orbit the Earth. It can therefore no longer collide with still working satellites causing even more space debris. However, some predict that as more and more debris accumulates in the graveyard orbits, eventually no spacecraft could pass through it anymore. This would mean that Earth would be locked out of space, and is known as Kessler syndrome (Hill).

To conclude, for larger space debris which cannot be entirely burned up in the Earth’s atmosphere, deorbiting would be safest in the long run.

Environment:

As stated before, the burning of debris into Earth’s atmosphere has no significant influence on the environment.

The spacecraft cemetery affects the environment of the ocean, since space debris is basically dumped into it. However, most of the debris is burnt up in the Earth’s atmosphere, so the ocean’s environment is not significantly harmed (Progress space ship 'buried' in Pacific Spacecraft Cemetery).

The Earth’s environment will not get polluted if debris is sent to a graveyard orbit. However, the graveyard orbits themselves will get fuller, possibly to the extent that nothing can pass through them anymore (Kessler syndrome).

So none of the solutions will dramatically damage the environment. However, sending debris to the graveyard orbits might eventually overpopulate the graveyard orbits.

Costs:

For costs the two main alternatives will be compared: deorbiting debris vs. pushing debris to a graveyard orbit.

Option 1 – debris in high (geostationary) orbit: A lot more force is needed to shove the debris back into the Earth’s atmosphere than to push it away to a graveyard orbit. To be more precise, it takes about 140 times more velocity to send it back to Earth. This means much more fuel would be needed, and thus much more money. For satellites in geostationary orbit it is too expensive to deorbit (Hill).

Option 2 – debris in LEO: In LEO it is easier to bring back debris into the Earth’s atmosphere. The debris can be slowed down, and then the Earth’s gravity will pull it down into the atmosphere. This does need to be done very carefully; control is everything, otherwise it is not safe (Hill).

Technological feasibility:

All discussed options for getting rid of space debris are technologically feasible. However, sending the space debris into a graveyard orbit and combining this with recycling parts is still up to future developments. Various projects concerning recycling space debris are running, but none have really been successfully executed yet.

Reliability:

All solutions are reliable.

Conclusion:

To conclude, deorbiting of space debris is the best solution, regarding the above criteria and the fact that this project is focused on the space debris in LEO.

Spacecraft movement

Spacecrafts rely on propulsion systems to navigate through space. A commonly used propulsion system is an engine, although these can vary greatly in size. Smaller engines used for this purpose are called thrusters. Larger engines can also be appropriate for transporting a spacecraft through space; the engine used to launch a spacecraft into space can also be used for navigation within space. A newer alternative to an engine is an ion propulsion system. These options will be discussed in the following sections (“How does solar electric propulsion (ion propulsion) work?”).

Engines: A conventional rocket engine provides a large amount of thrust. It propels a spacecraft forward by burning fuel and creating superheated gases which stream out the backside of the spacecraft. This stream produces a forward thrust. Conventional rockets are, thus, reliant on fuel, as well as an oxidizer that will allow for the burning of the fuel. A chemical engine needs a large amount of fuel and oxidizer to work, making its capacity to carry these reactants also its engine limit. Once the fuel or oxidizer has run out, the engine will no longer be of use. Generally, rocket engines will be best suited for shorter trips, which don’t require much fuel or oxidizer, or larger, heavier spacecrafts, capable of carrying large amounts of fuel and oxidizer (“How does solar electric propulsion (ion propulsion) work?”).

Ion propulsion: In order for ion propulsion to work, a spacecraft needs to generate electricity (potentially through the use of solar panels) to positively charge atoms within the chamber. A magnet in the rear of the spacecraft pulls these atoms towards it until the atoms are repelled and shoot out the rear. This creates a thrust which can move the spacecraft forward. Ion propulsion requires only a small amount of gas as it propulses this gas at a very high speed, as opposed to the rocket engine which propulses larger amounts of gas at lower speeds. Ion propulsion, thus, requires less fuel and is not limited to its fuel capacity: it is limited by energy. The ion engine needs to generate electricity; the energy required for this can either be carried on the spacecraft or collected using solar panels. Ion propulsion is a great option for long journeys, as energy capacity will not affect the mass of the spacecraft as much as fuel capacity. Also, this energy can be generated in space, allowing the spacecraft to always have the potential to move. Ion propulsion does, however, generate much less thrust than a rocket engine. The DS1, a technology-testing mission, uses an ion propulsion system which produces 92 mN of thrust. To put this into perspective, it is the same amount of force that gravity pushes on a couple of drops of water. Within the realm of navigating through space, this is not necessarily an issue because ion propulsion has a high specific impulse (the gases leaving the rocket have a high velocity). Still, this technology will never be able to bring a spacecraft into space, and it takes a long time to propulse a spacecraft, within space, up to speed. In the case of the DS1 mission, the ion propulsion system will take 15 months to accelerate a spacecraft in the LEO beginning at 17,000 mph up to 25,000 mph. In conclusion, ion propulsion can be used for long journeys without requiring a massive spacecraft, but it does not generate much thrust and may take longer accelerating (“How does solar electric propulsion (ion propulsion) work?”).

Considerations for the project

Utility: Both options will be able to move the spacecraft within space.

Safety: Movement within space does not cause any safety concerns to humans.

Environment: The environmental effects of engines in space are unknown. As there is no atmosphere to be damaged by the fumes, it will be assumed that the effects are negligible.

Costs: Although the ion beam could lessen the mass of the fuel needed to bring into space, at this time it is a newer technology. For this reason, the rocket engine seems to be the cheaper option. It has been developed and perfected for a much longer time, and assuming the spacecraft must enter space itself, the rocket engine will already be present on the spacecraft. The ion beam could only pose an additional cost.

Technological feasibility: The rocket engine is a much more commonly chosen method for navigation through space. The ion propulsion technology is new and less developed.

Reliability: Although the rocket engine is more developed, in principle the ion propulsion system is more reliable. Once the rocket engine runs out of fuel or oxidizer, it is entirely stuck. There is no alternative method to moving it. The ion propulsion system, however, can have an endless supply of energy, if it makes use of solar panels. This system can run longer and is less likely to be entirely stuck in space.

Conclusion: In conclusion, a combination of an ion beam and an engine is ideal as each method compensates for the other's short comings. The ion beam alone may not be strong enough and the engine alone requires too much fuel. If these technologies are combined, the mission will be much more reliable than one method alone.

Finding Space Debris

SPACETRACK is the current program for worldwide Space Surveillance Network (SSN). It consists of multiple, dedicated, electro-optical, passive, radio frequency and radar sensors. The purpose of the SSN is not only space debris cataloging and identification but also satellite attack warning and space treaty monitoring. In total the SSN tracked 39,000 space objects.

Then, there is the Air Force Space Surveillance System (AFSSS), also called Space Fence. It consists of a very high frequency radar network located in various places in South America. The system became operational in 1961 and tracked about 10,000 objects in space.

The United States Department of Defense (DoD) also maintains a space catalog. These catalogs are stored on multiple satellites and are regularly updated by the SSN. In 2001, the number of cataloged objects was about 20,000.

Improved tracking systems:

The current state-of-the-art project for detection of space debris is Lockheed Martin’s Space Fence Program. The program is currently still focusing on getting the system operational and the initial operational capacity is scheduled for 2018. The machine is constructed on Kwajalein Atoll in the Marshall Islands. There is already a second site planned to go online in 2021. The primary tasks regarding space debris of the system are the following:

  1. Detection
  2. Tracking
  3. Measurement
  4. Catalog

These action are performed for space debris more than 1.5 million times a day to predict and prevent collisions between debris and satellites or space stations. The system will primarily trace space debris in the low-earth orbit where the ISS performs operations. The space fence system predicts to simultaneously detect, track, and characterize around 200,000 objects anywhere in its field of view. This would be a ten-fold increase in comparison to the current systems.

The improved performance comes from the combination of S-Band radars and Gallium High power amplifiers (GaH HPAs). Older radar systems use the traditional Gallium Arsenide (GaN) and have approximately 50% less range as the GaH has. GaH is an efficient semiconductor material and outperforms the traditional GaH in the following aspects:

  • Higher Power Density: Increased range and detection sensitivity
  • Efficiency: Less energy consumption and less cooling
  • Operates at higher temperatures: More robust and so applicable in more situations

Considerations for the project

Utility: Using a database to find space debris will be effective. This will solve the problem of finding it, as the robot will have access to a large, up-to-date database of potential debris.

Safety: Using a database does not compromise safety; however, as the robot does not need to detect space debris itself, the robot must be able to avoid oncoming debris with some sensor.

Environment: A database will have no direct effect on environmental factors.

Costs: This database is already in existance, so it will be much cheaper than having the robot detect debris itself with precise, expensive sensors.

Technological feasibility: These databases already exist, so sending them to the robot in space is very feasible. Information is constantly sent to and from space.

Reliability: The database is currently quite reliable and the Lockheed Martin Space Fence in development should only enhance the information and reliability of this method.

Conclusion: The database will be used to find space debris. It is not absolutely complete, but it is complete enough to be effective and much cheaper and more reliable than finding debris using sensors.

The ion beam

The ion beam shepherd (IBS) is the current state of the art ion beam technology. It is a satellite that can emit two beams of accelerated quasi-neutral plasma; one beam will be directed towards a target and the second beam is used to keep the satellite within range of the target (typically 5 to 10 meters at least). A controller could be developed to ensure that the target and IBS remain within distance of each other as they move through space. This is especially an issue if the orbit is conical and not circular. In theory, the IBS would be able to hold a following distance in a perfectly circular orbit, but if an orbit is at all elliptical, the following pattern would change. Most orbits in the low earth orbit is quasi-circular, so this area will be most dependable for this technology (Bombardelli).

An IBS sends high velocity ions to its target where the ions lose energy and momentum through ionizing collisions up until they abruptly stop nanometers away from the target’s surface. At this point, the surface material rejects the ion from its lattice, although occasionally surface material detaches due to a collision (sputtering). This effect can, however, be neglected. One main concern of the ion beam shepherd is the beam divergence. If this angle is minimal, the ion beam will be able to control the movement of its target from further distances. A higher specific impulse and system mass would allow the beam divergence to lessen, but clearly, this is a trade-off. Although the beam can be assumed to be conical for simplicity, the jet of plasma is not perfectly conical in reality (consider the effects of the magnetic field in space) (Bombardelli).

The time needed for the IBS to move a target from a 1000 km orbit to a 300 km orbit using constant tangential thrust and a specific impulse of 2500 seconds is shown for various masses in the figure. For the 5 ton target, this goal is reached within a year. There are two relevant methods of advanced electric propulsion: ion engines and Hall effect thrusters. Although Hall thrusters are more compact and lighter, the ion engines are preferred, largely due to its lower divergence angle. A comparison of specific ion engines is included in these tables (Bombardelli).

In order to move on with a representative ion engine, Bombardelli created a list of properties for a standard ion engine: initial radius of 0.1 m, Xenon as a propellant, electron temperature of 5 eV, initial mean plasma density of 2.6 · 1016 m-3, initial ion axial velocity of 38000 m/s, ion kinetic energy of 1 keV, mass flow rate of 6.85 mg/s, ion current of 5 A, initial plasma Mach number of 20, initial beam divergence parameter of 0.2, and a thrust force of 100 mN (Bombardelli).

Time needed to remove space debris of various masses (Bombardelli) Specifications of various ion engines (Bombardelli)

Combining the ion beam with Electrodynamic Tethers

In this subsection, using electrodynamic tethers (EDTs) in combination with the ion beam will be discussed. But these tethers are not yet conventionally used for space missions, so why should electrodynamic tethers be considered in the first place? The answer: EDTs can supply power and/or create thrust without using a propellant, which could be hugely cost-beneficial. Instead of having to launch large quantities of propellant into space, an EDT can generate power and/or thrust using the magnetic field around a planet, in this case Earth (Bombardelli, "Electrodynamic Tethers.").

An electrodynamic tether is made up of two masses connected by a long conducting wire extended from a spacecraft and moving through the magnetic field of the Earth. The workings of EDTs is based on the following: when a conductive wire moves through a magnetic field, current is produced and the field exerts a force, called a Lorentz-force, on the current (Christensen, "Electrodynamic Tethers.").

So, the two main applications of EDTs in space missions are:

  • Propulsion for LEO spacecraft without using a propellant: an EDT system in passive mode needs nothing more than its conducting wire. If the tether is long enough, high enough currents can be obtained to not need any other active power supplies. The EDT can then generate thrust through Lorentz-force interactions with the Earth’s magnetic field only (Bombardelli, "Electrodynamic Tethers.").
  • Power generation in LEO: an EDT can also be used to convert orbital energy into electrical power. However, this has a downside: when converting orbital energy into electrical power, this will lower the orbit of the spacecraft ("Electrodynamic Tethers.").

But how can the electrodynamic tethers be used in combination with the ion beam? The earlier discussed Ion Beam Shepherd / IBS program has looked into this with detail, and their findings will be summarized below.

In the IBS design, it is possible to replace the secondary repulsion system with an EDT. This would save power, propellant, and thus costs. In this case, the IBS module has to be located at the center of mass of the system. Otherwise, it would not be possible to coorbit space debris at constant distance. This means that two tether arms from each side of the IBS have to be used. However, there is one critical aspect: the lateral mode of oscillatorion of the IBS with respect to the end masses of the tether arms, called the “butterfly mode.” The amplitude of this mode’s oscillations have been evaluated for the IBS project using an EDT numerical model. The values for the amplitude produced by the model were too high, posing serious collision avoidance issues. So, this IBS-EDT combination is not technically feasible (Bombardelli).

The above hybrid IBS/EDT concept for space debris removes appears too complicated. But since the propellantless nature of the EDTs is so beneficial, there are other ways to implement EDTs in an IBS-based space debris removal mission that should be subject to future studies. For example (Bombardelli):

  • Using EDTs to reorbit an IBS system, this is done in active mode. Through this, the IBS system is taken to the next piece of debris to start another deorbiting maneuver.
  • Deorbiting space debris using only the EDT. This can be done whenever it is possible to do a docking maneuver. It will only be a possibility if the IBS can be used for detumbling and attitude stabilization.

USE aspects of the ion beam

Society

When deciding on which the solution to use, safety and environment have been taken into account. These criteria are in line with the impact on society; thus, the ion beam is a solution society could very well live with.

Enterprises

As of right now, not much is publicly known about the costs of the ion beam. The money invested in certain other space cleaning projects are known and to be found in the general enterprise aspect. It can be expected that the costs for the ion beam are not much different, therefore millions, but because ion beams are also used for cancer treatment (Preuss), creating high quality ion beams for relatively low costs should be possible in the (near) future. Yet, even with minimum costs for the ion beam, the entire operation costs money, as more research is needed. Also, the ion beam needs to be sent to space as well, which can also be costly. For enterprises, the ion beam may, thus, not be the greatest solution as it does not cost any less than the other solutions.

NASA currently spends almost 7 million dollars a year tracking space junk (Adams). This number should drop once the bigger pieces of junk are cleaned up, as the NASA is currently only tracking the bigger pieces of junk. As these have the greatest impact, these should be cleaned up first.

With the rules set by governments that anything sent into the LEO must leave it, the LEO will be cleaned up, in the long run. Once the LEO is cleaned up, space tourism will be more attractive to a lot of people because it will be safer. It, therefore, seems like a good idea to have enterprises such as Virgin Galactic and SpaceX invest into the cleaning of space; in the end, the money they invest can be earned back due to the increase of space tourism.

Life-cycle management

To determine the best solution, the entire life-cycle of the product needs to be considered. This includes everything from building the device until the end of its life. For this project's purpose, apart from developing the best solution concept for the space debris problem, the main focus will lie on looking into what happens after the mission is completed, called the “End of life (EOL) phase”. (Product Lifecycle Management)

End of life (EOL) phase

It is very important to decide on what will happen to the space cleaning robot at the end of its life because this will influence the choice of material and design. A few option will be discussed below.

1. Suicide at end of life

One option is to have the device, at the end of its life, send itself to either the graveyard belt in space or the spacecraft cemetery on Earth. So basically, after it has done its duty cleaning debris from space, it will clean itself from space. This way, there will be no new debris created in the LEO. However, one could argue that this is a bit of a waste of the material investment.

2. Recycling

Another option could, therefore, be to recycle (parts of) the device. This can be done in space, so for example, by sending incomplete satellites into space and then finishing them in space. An example of this approach is the DAPRA project (see section “Getting rid of space debris”, subsection “Recycling of space debris”).

3. A space docking and repair station

The last option is having a space docking and repair station. This is a bit of an ‘in- between’ option, as eventually the device is just not repairable anymore. At this point, one of the two options above needs to be combined with the repair station.

Considerations for the project

Utility: All concepts will potentially solve the problem. However, option 1 (suicide at end of life) is easiest to execute, as both option 2 (recycling) and option 3 (a space docking and repair station) will require more research.

Safety: All options are safe. The safety of option 1 is similar to the safety discussed in the section about getting rid of space debris.

Environment: Recycling and repairing are preferable over suicide at end of life.

Costs: Option 1 is cheapest. For the other options, more research is still needed, which costs money.

Technological feasibility: The technology involved in option 1 is already available. The other two options still need more research but are most likely available in the near future. Research is already in progress, for example the DAPRA project discussed in the section about getting rid of space debris.

Reliability: In all concepts things can go wrong, but at the moment, option 1 has the highest reliability.

Conclusion: Option 1 currently seems to be the best solution. However, more research needs to be done regarding the other options since these will have certain advantages over option 1, like being less wasteful. Due to the limited period of time and resources of the project, these options cannot be extensively investigated and tested.

Conclusion for the final design

Summary of the design choices based on criteria

Based on all of the considerations, a final design has been outlined. The use of an existing database does not interfere with the objectives set in the beginning of the project, thus this can be used to used by the debris cleaning spacecraft to track down the space debris. In order to reach the space debris, a combination of thrusters and ion beam propulsion will be used. The ion beam will also be used to change the movement of the space debris. The debris will then be sent towards the Earth, so it can (partly) burn up in the atmosphere. The remainder will crash into the spacecraft cemetery. At the end of its mission, the ion beam will be used to send the spacecraft itself towards the atmosphere.

Dimensions

To get an impression of the dimensions and weight of the debris cleaning spacecraft, some research was done on current rockets. The cleaning robot will be sent into space with another space mission to save costs. However, the dimensions should therefore be suitable for this. Comparing dimensions of the eDeorbit (e.Deorbit), Kounotori6 (Richardson), RemoveDebris University of Surrey (RemoveDebris Mission Confirms Launch in 2017 using the ISS), and ROGER mission (RObotic GEostationary orbit Restorer (ROGER)) some estimations were made. The following indications for weight and dimensions apply for the space cleaning device:

Weight = +- 2000 [kg]

Dimensions = max. 3x3x3 [m^3]

The dimensions mentioned above should suffice to have the debris cleaning spacecraft work as it should. It will be possible to use the number of thrusters needed, the ion beam, and it can commit suicide at the end of the mission. Therefore, all the solutions based on the criteria set at the beginning of the project can in theory be realized.

Simulation

FreeFlyer

FreeFlyer is a commercial software application that is created for space mission design, analysis and operations. It is a very powerful tool that can solve various types of astrodynamics problems. Well known organisations such as NASA and the United States Air Force (USAF) use this software for spacecraft simulations. For this project, FreeFlyer is very useful because it is not possible to build or launch an actual spacecraft to space. Using the software, a simulation can be made to test the robot in space. The FreeFlyer software homepage can be found here: https://ai-solutions.com/freeflyer/freeflyer/.

In FreeFlyer, the spacecraft object is central and it is used for many purposes. Every object that has some sort of motion is modeled as a spacecraft. The space cleaning robot is also modeled as a spacecraft in FreeFlyer. By doing this, it is possible to bring the robot into an orbit around the Earth. The spacecraft has multiple parameters such as mass, thrusters, tanks and sensors that can programmed.


What is done with the FreeFlyer simulation?

The purpose of the FreeFlyer model will be to answer the question if the design of the space debris cleaning robot works and if the benefits outweigh the costs of the robot. The model can calculate how much fuel would be needed to bring on the spacecraft to clean up “X” space debris parts. It can also give insights into how much time the spacecraft would need to clean up “X” debris parts. The simulation starts at the moment that the robot is in a stable orbit around the Earth and ends when there is no more fuel in the spacecraft to support another mission. The end of life scenario for the spacecraft can be modelled. The spacecraft will deorbit into the atmosphere as a suicide mission to prevent the spacecraft becoming space debris itself. The actual launch of the spacecraft into orbit is not modeled by FreeFlyer.

Roughly the model can be divided into three parts that individually need to be solved. First, the actual space debris will be modelled in an orbit around the Earth. Second, the spacecraft has to be modeled with all of its properties and hardware. Finally, the spacecraft has to change its attitude and navigate from one piece of space debris to the next. When all three parts are solved individually, everything can be tested together, and the results retrieved.

For gaining accurate results, the model must be able to implement real time space debris. Freeflyer can do this with the “.tle” file format. This file contains the orbital information expressed in two lines for each object. The construction of the .tle file can be found here: “https://en.wikipedia.org/wiki/Two-line_element_set”. A small example of how the information is stored in a .tle file:

ISS (ZARYA)

1 25544U 98067A 08264.51782528 -.00002182 00000-0 -11606-4 0 2927

2 25544 51.6416 247.4627 0006703 130.5360 325.0288 15.72125391563537

For this example, 51.6416 represents the inclination and 0006703 represents the eccentricity.


The properties of the orbit of the spacecraft can be changed instantly without the use of any propulsion; however, this would not be an accurate simulation of the spacecraft’s performance. Therefore, all maneuvers are done using Freeflyer’s maneuver function, which uses the rockets and fuel of the spacecraft. The orbit is changed with use of low-thrust-many-revolution

Modelling space debris

As stated before, in order for the model to be accurate, it must be able to take into account actual space debris. In order to do this, a NASA Database containing information about large quantities of debris can be used. This database stores the data of space debris moving around the earth in lower orbit in the form of a .tle file. This database was found on the website of www.space-track.org, and contains all information regarding the orbit of each object. This means that these objects still need to be given a size, and their own weight, which is done inside Freeflyer. In the code below these properties are determined. In the loop starting on line 41 for instance, LEO space debris is given a movement step size, and weight, one piece of space debris for every time the loop is run. The weight is determined more specifically in line 45 and alters between particles with weights up to 5 kg.

Space debris modeling code in Freeflyer

Modeling Spacecraft

To increase the realism of the model itself, the spacecraft needs to be properly modelled as well. Freeflyer has a system that allows the introduction of new spacecraft ­using several menus. First of all, the original course of the spacecraft needs to be determined:

Original orbital properties of a spacecraft

By only entering a preferred height, the spacecraft will automatically presume a path around the earth equator. The program then allows for the addition of several spacecraft components. The components that matter for the existing project are fuel sources and thrusters. For modelling these thrusters, there is once again a menu. The part of this menu shown here allows for changing the direction in which the output force of the thruster is applied. The total system of thrusters has a heavy impact on the manoeuvrability of the spacecraft. The spacecraft in the simulation is equipped with 6 equally powerful ion thrusters. One of these is in the front centre of the vehicle and will be used for moving around debris. The first one is in the back and will be the primary source for acceleration. The other four thrusters have been placed around the sides of the vehicle, pointed partially backwards, so they can be used both for changing the speed of the vehicle as well as the inclination and the position and direction compared to the earth’s surface.

Thruster properties in Freeflyer

Maneuvering and targeting

In order for the vehicle to move the space debris, it needs to reach this debris first. In order to do this, the spacecraft needs to use thrusters to alter the orbit parameters. In order to reach the same orbit as any piece of space debris, several parameters need to be altered: The lowest and highest point of the spacecraft orbit and the angle the spacecraft makes with the equator. Freeflyer can calculate what thrusters the spacecraft has to use to get to these parameters using the Target function. In the simulation, the spacecraft can target a chosen piece of space debris each time. This way, when the simulation is run, the spacecraft’s course will alter towards the course of a chosen debris particle. This, however, can result in a burn which takes a long time and jumps right to the moment when the burn ends. This can result in the spacecraft jumping from one side of the Earth to the other. Because of this we make a loop which is very similar, except for one thing: instead of changing the length of the burn it repeats a very short burn until the target requirements are fulfilled. This results in a system where we have the same level of control while still having the possibility of taking separate actions in between.

The Freeflyer targeting menu

Our Model

In our model the spacecraft changes its orbit in order to match that of a set number of targets. Firstly it changes its inclination close to the initial ascending node or descending node, which is are locations where the orbital plane of the debris intersects that of the satellite. The direction of the burn depends on the velocity vector of the target, since from this vector we can deduce if it's the ascending or descending node.

After the inclination change the apogee gets raised by burning prograde when the spacecraft approaches the perigee. We do this until the apogee approaches that of the target. The apogee of the spacecraft is a tiny bit lower, so when we have matched the perigee we have an orbit such that just by waiting we catch up to the target.

After the target orbit has been achieved we stay in this orbit for a set amount of time before repeating this procedure for the other targets.

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