PRE2016 3 Groep19

HET KAN

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'.

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 objectives are answering the following research questions through literature research:

Furthermore, a NetLogo simulation will be made.

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 Netlogo simulation

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

USE aspects of space debris

Society on earth

Since as long as we, humans, have lived, objects from space have hit the earth’s surface. People even think that it was a meteorite with an impact so big, it killed most of the organisms, including dinosaurs, once (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 the fact that the object that crashed was clearly not an object originating from space. Some smaller piece of debris with Chinese markings on it even destroyed the roof of a house nearby. In fact, NASA estimated that the odds of being hit by space debris is 1 in 3200 (Mortillaro). In comparison, the odds of dying in a car accident is 5000 to 1, while winning the lottery is a chance of 175 million to 1 (Amadeo).

But the impact of space debris on the society on earth does not stop at debris turning into meteorites. The debris could hit working satellites, and since 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, since a collision happened between two of them, imagine all the possible collisions between satellites and space debris. These collisions damage the satellites we depend on so much. The BBC wrote an article about what would happen if all satellites would stop working at the same time. Now this is very unlikely, but the effect of it would be tremendous. From missing the tv-journal during breakfast, to not being able to reach your overseas colleagues at work immediately. GPS would stop working, so you would find yourself finding your way with the help of an old school map. These are small, inconvenient changes, but the missing of weather information makes things as air traffic a lot more difficult, and even more dangerous. And then we have not even talked about all the wars where bombs are dropped via drones, with which communications has become impossible(Hollingham). Clearly, the lives we live today are made possible by the satellites in space. Losing the information we gain from then would bring us back quite some years.

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(ISS). 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 manoeuvre to a safer orbit. It missed the ISS by 12 km (Weinberger). In May of 2016, Tim Peake (a British astronaut, who was at the ISS at the time) tweeted a picture of one of the ISS windows with a chip in it. It was caused by space debris of only thousands of a millimeter across(Placko). Currently the NASA worries about every piece that is larger than a baseball, however, there are more than 21000 pieces of manmade space debris with this size (Lewis). “Being hit by a ‘sugar-cube’ of space debris is the equivalent of standing next to an exploding hand-grenade” (Clark). But clearly, every piece of space debris poses a threat to life in space.

The impact on people all over the world after each of the space disaster in which people died has been enormous. But people know that the launch and return are dangerous parts of the travel to space. Losing a life within space could lead to an even bigger shock worldwide, and is something we desperately need to prevent.

Enterprises

Back in 2001 Dennis Tito became the first space tourist after paying 20 million pounds (Wall). Since then, 6 more people have made the trip, with the last one going in 2009. One of them event went twice (the Guardian). These days, companies are trying to launch affordable flights to space (Jee). 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, predict 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"). Combine this with the 700 tickets sold, as of 2014, by Virgin Galactic(Howell) it is important that space is as clean as possible, to prevent accidents. With 21 people losing their lives due to accidents (Venugopal), failures of technology, design and management are risks of space travel everyone is aware of. Awareness of space junk however is far away. It lead to British scientist and artist creating an online interactive project to raise awareness, about the 27000 pieces larger than 10cm, travelling at 8 kilometers a second (Carpeneti).

But money is not only involved with the cleaning up of space, and tourism. 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 them, makes it worth to invest into cleaning up space, even though this is very expensive. The Swiss Space Systems (S3) created the CleanSpace One, which is slated to launch in 2018. The costs of this? About 16 million dollars. Another method, named Laser Orbital Debris Removal (LODR) would cost 1 million dollar per object to be cleaned up (Markham). Astroscale, a Singapore-based company, has secured a 30 million dollar funding, in order to create two pieces of technology(Mckirby), 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. These are just a few of the many projects started to clean up space, making it obvious, that 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 Low Earth Orbit ( LEO). This is both due to explicit regulatory requirements as well as to customer demand. In this section, governments pressure on enterprises will be discussed.

Since the problem of space debris in LEO is increasing, governments like the US government have been introducing regulations to minimize further increment of debris. Elsewhere, in for example Europe, the European Union is doing something similar adopting a Code of Conduct including debris-mitigation measures. All these requirements can be combined to one widely applicable standard, which is expected to happen in the near future. This standard can then be enforced in laws or in 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 to care about space debris by governmental regulations.

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

• 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 gasses, 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. So at what time this hardware is left in Earth orbit. Such requirements will probably be included in laws/regulations in the future. If not, commercial launch providers will still have to deal with this, since government customers will have their own internal requirements. Government customers are the main clients of launch providers, so this will lay an increasing pressure on the providers to satisfy the requirements of the customers.

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. A lot of research will 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 our project

For our purposes, the vaporisation lasers, surface material, and streams of air are almost immediately out of consideration. We plan to create a software that will create a travel path to the debris, so these capture mechanisms would not be of use to us. A solar sail can also be put aside as the concerns for this mechanism seem to be too technical for us to investigate. The amount of drag produced by solar photons may be near impossible for us to learn and estimate in the time needed. This leaves the tethers, ion beams, and nets. The tether seems to be the most popular mechanism used in space debris missions. The electrodynamic tether may be much more efficient than the momentum exchange tether, as the momentum exchange tether requires more control. The net seems a bit difficult to execute, as the encompassing and closing of the object requires even more control than the tether. The ion beam may require less control, since it does not require any contact.

Research has been conducted by F. Zuiani and M. Vasile on the topic of debris removal missions by means of simplified models for low-thrust, many-revolution transfers using the Ion Beam Sheperd (IBS), which is a spacecraft developed both by JAXA and Bombardelli. In this paper it's stated that one could use a concentrated solar beam instead of an ion beam, however this method has various drawbacks compared to the ion beam. They referred to research carried out by M. Vasile, C. Maddock and C. Saunders on the topic of orbital debris removal with solar concentrators. From their research it can be concluded that the most effective way is to use a network of satellites which send a laser towards a reflective disk which concentrates these lasers on the target. This method however is a variation of the vaporisation laser method which we have already ruled out. The other method, which did not use laser satellites, would require a larger reflector and will use sunlight to cause the enhanced Yarkovsky effect. This method in itself is very fuel-efficient since it only uses direct sunlight to work.

Their paper states it will take 3 satellites with each a 5m diameter reflector 1 year to move a 1 ton rocket from GEO orbit to the graveyard orbit, which usually requires around 11 m/s delta-V. To compare, the IBS takes about one month to reduce the altitude from 1000 km to 500 km, assuming an Isp of 2700 s for both thrusters and a beam momentum transfer efficiency of 0.8. This is a significant difference, since this is a lot faster than using the reflectors. The concentrated solar beam is therefore not recommended for larger objects, both because these tend to have a lower AMR and also because they're heavier. Our focus will primarily be on larger objects, so the usage of an ion beam is therefore the best option.

Getting rid of space debris

Burning in atmosphere

Often space junk gets attracted by the Earth and starts to fall down, but most of it burns up in the atmosphere and will never reach the surface (Redd). This could also be used as a method to actively get rid of the space debris through burning the 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 thrash 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 since about 70% of the Earth’s surface is water (Redd), but when deliberately sending debris into the Earth’s atmosphere one wants to make sure that no large chunks start raining down and damage property. There are two important factors that influence the extent to which a piece of space debris will burn up. First of all, the size; obviously smaller pieces are more likely to burn up entirely then larger ones. Secondly, the speed with which the atmosphere is entered. 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 important, due to the high kinetic energy, the air in front of the debris will get compressed and compressing air means its temperature will increase. However, it is a bit more complicated, since size plays yet another role: smaller pieces slow down more quickly since 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’ve melted away. (“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 get burned up in the atmosphere. By controlling the entrance speed of 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 exactly the (chemical) impact on 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 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 global level. (David)

Still, more research needs to be done on the density of particles, types of particles, and how long 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. But what is done with ‘new’ space debris, meaning for example old space stations and satellites that are nearing their end 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. Which way to go 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 spacecraft in low orbit, will not entirely burn up before reaching the surface. To make sure such a space station does not crash down in for example a big city, space operators plan for their old satellites to crash down 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 then to get back. Therefore, these satellites will be send even farther away from Earth instead of sending them back to Earth. Similarly to the spacecraft cemetery, these satellites will be send into a graveyard orbit. This orbit is almost 200 miles higher than where the farthest away 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 therefore will not cause any more debris to form. But may be some time in the future, humans need to send some kind of space cleaning device to get rid of these satellites. ("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 a $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 at recycling space debris (old satellites that stopped working). They are looking into recycling parts of broken satellites 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) and attach these antennas to new smaller satellites (satlets) launched from Earth. This would save a lot of launch costs, since antennas are big and bulky and they therefore need a lot of fuel to be send to space. 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)

Spacecraft movement

Right now, once in space, the current method of maneuvering space crafts is very much consistent over almost all devices: steering rockets. However, there are still several other techniques that can be used to achieve the same goal. Below, the two methods most relevant to our case have been explained.

1. Rocket engines or thrusters

Rocket engines work by bringing a gas to high pressure and temperatures, and then burning it in a combustion chamber with a small exhaust nozzle, allowing for a rapid escape of the gas, and causing the vehicle to be thrusted the other way. Because of the high speed build-up, rocket engines can work very efficiently, peaking at around 60 percent.

2. Ion beams

Ion thrusters send large electric currents through ions, allowing them to reach large speeds. The thrust these ions create while being hurled away allows for the movement of the vehicle These types of engines are very effective, requiring op to ten times less fuel than the standard rocket engine, and generally weigh much less. NASA has recently uncovered a system called NEXT, which manages these engines with great success.

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). Which is called “space fence” in common tongue. It consists of a network of very high frequency radar network located at multiple locations of 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 is regularly updated by the SSN. In the year 2001 the number of cataloged objects was about 20.000.

Improved tracking systems:

The current big state-of-the-art project for detection of space debris is the Lockheed Martin’s Space Fence Program. The program is currently still focussing 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 that 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 at the low-earth orbit where the ISS performs operations for example. 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 system types uses the traditional Gallium Arsenide(GaN) and have approximately 50% less range as the GaH has. GaH is a 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

References

Amadeo. "What Are the Odds of Winning the Lottery." thebalance.com. 22 Sep. 2016. Web. 12 Mar. 2017. https://www.thebalance.com/what-are-the-odds-of-winning-the-lottery-3306232

Barbee, B. W., Alfano, S., Gold, K., Gaylor, D., Pinon, E., "Design of Spacecraft Missions to Remove Multiple Orbital Debris Objects," Advances in the Astronautical Sciences, Vol. 144, Univelt, Inc., San Diego, CA, 2012, pp. 93-110, also AAS/AIAA Paper AAS 12-017, 35th Annual AAS Guidance and Control Conference, Breckenridge, CO, February 3-8, 2012

Bischof, Visentin, Starke, Guenther, Foth, Kerstein, Oesterlin, Ebert, Macaire, Wegener, Krag, Oswald, Lampariello, Agrawal, Nimelman, Ilzkovitz, Ashford, and Yoshida. "ROGER Executive Summary." European Space Agency. EADS, 10 June 2003. Web. 5 Mar. 2017. https://gsp.esa.int/documents/10192/43064675/C15706ExS.pdf/18bb5154-fa12-44f0-a240-d84672ac49d5

Carpineti. "UK Artists and Scientists Launch Space Junk Awareness Project." 18 Nov. 2016. Web. 2 Mar. 2017. http://www.iflscience.com/space/uk-artists-and-scientists-launch-space-junk-awareness-project/

"Charles Simonyi buys return ticket to space for $35m." The Guardian. 24 Mar. 2009. Web. 2 Mar. 2017. https://www.theguardian.com/travel/2009/mar/24/charles-simonyi-space-tourist-microsoft

Choi. "Asteroid Impact That Killed the Dinosaurs: New Evidence." 7 Feb. 2013. Web. 20 Feb. 2017. http://www.livescience.com/26933-chicxulub-cosmic-impact-dinosaurs.html

Clark. "How dangerous is space debris?" 4 Oct. 2012. Web. 20 Feb. 2017. https://www.theguardian.com/science/across-the-universe/2012/oct/04/astronomy-space

Cofield. "SpaceX to Fly Passengers On Private Trip Around the Moon in 2018." 27 Feb 2017. Web. 2 Mar. 2017. http://www.space.com/35844-elon-musk-spacex-announcement-today.html

"e.Deorbit." European Space Agency. European Space Agency, 12 Apr. 2016. Web. 12 Feb. 2017. http://www.esa.int/Our_Activities/Space_Engineering_Technology/Clean_Space/e.Deorbit

David, Leonard . "How Huffing and Puffing Could Remove Space Junk." Space.com. N.p., 5 Apr. 2012. Web. 05 Mar. 2017. http://www.space.com/15178-space-junk-removal-spade.html

David, Leonard. "Space Littering Can Impact Earth's Atmosphere." Space.com. N.p., 19 May 2009. Web. 02 Mar. 2017. <http://www.space.com/6720-space-littering-impact-earths-atmosphere.html>.

Galeon. "The Space Debris Problem: Dual Impact in Myanmar Shows What’s to Come." 14 Nov. 2016. Web. 20 Feb. 2017. https://futurism.com/the-space-debris-problem-dual-impact-in-myanmar-shows-whats-to-come/

Hollingham. "What would happen if all satellites stopped working?" 10 June 2013. Web. 2 Mar. 2017. http://www.bbc.com/future/story/20130609-the-day-without-satellites

"How big does a meteor have to be to make it to the ground?" HowStuffWorks Science. HowStuffWorks, 10 Oct. 2000. Web. 02 Mar. 2017. <http://science.howstuffworks.com/question486.htm>

Howell. "Virgin Galactic: Richard Branson's Space Tourism Company." 17 Feb. 2016. Web. 2 Mar. 2017. http://www.space.com/18993-virgin-galactic.html

"Japanese H-II Transfer Vehicle Kounotori 6 fails to deploy magnetic tether to clear junk in earth orbit" Tech2. TechFirstpost, 07 Feb. 2017. Web. 12 Feb. 2017. http://tech.firstpost.com/news-analysis/japanese-h-ii-transfer-vehicle-kounotori-6-fails-to-deploy-magnetic-tether-to-clear-junk-in-earth-orbit-361268.html

"JAXA is going to test removing orbital debris in collaboration with a company that makes fishing nets." Tech2. TechFirstpost, 05 Dec. 2016. Web. 12 Feb. 2017. http://tech.firstpost.com/news-analysis/jaxa-is-going-to-test-removing-orbital-debris-in-collaboration-with-a-company-that-makes-fishing-nets-351294.html

Jee. "Commercial space travel: A decade of broken promises | Space X, Virgin Galactic and space tourism." 28 Feb. 2017. Web. 2 Mar. 2017. http://www.techworld.com/picture-gallery/personal-tech/space-tourism-decade-of-broken-promises-3630327/

Lewis. "Trouble in orbit: the growing problem of space junk." 5 Aug. 2015. Web. 20 Feb. 2017. http://www.bbc.com/news/science-environment-33782943

Markham. "9 concepts for cleaning up space junk." 20 Apr. 2012. Web. 2 Mar. 2017. http://www.treehugger.com/clean-technology/9-concepts-cleaning-space-junk.html

Mckirdy. "Company aiming to clear space junk wins funding." n.d. Web. 2 Mar. 2017. http://www.japantimes.co.jp/news/2016/03/01/national/science-health/company-aiming-clear-space-junk-wins-funding/#.WL5vylXhCrR

Mortillaro. "Space debris: how dangerous is it to people on earth?" globalnews.ca. Shaw Media. 14 Jan. 2016. Web. 12 Mar. 2017. http://globalnews.ca/news/2453752/space-debris-how-dangerous-is-it-to-people-on-earth/

"Myanmar debris: 'Mystery object lands at jade mine'." 11 Nov. 2016. Web. 20 Feb. 2017. http://www.bbc.com/news/world-asia-37946718

"Orbital Debris Remediation." NASA. NASA, n.d. Web. 12 Feb. 2017. https://www.orbitaldebris.jsc.nasa.gov/remediation/

Pultarova, Tereza. "DARPA Phoenix Program to Recycle Space Debris." Space Safety Magazine. N.p., 19 May 2014. Web. 02 Mar. 2017. <http://www.spacesafetymagazine.com/space-debris/debris-removal/darpa-plans-recycle-space-debris/>.

Redd, Nola Taylor. "Space Junk: Tracking & Removing Orbital Debris." Space.com. Purch, 8 Mar. 2013. Web. 02 Mar. 2017. <http://www.space.com/16518-space-junk.html>.

"RemoveDebris: Experiments." Surrey Space Center. University of Surrey, n.d. Web. 21 Feb. 2017. http://www.surrey.ac.uk/ssc/research/space_vehicle_control/removedebris/experiments/

Sample. "Scientists reveal the full power of the Chelyabinsk meteor explosion." 7 Nov. 2013. Web. 20 Feb. 2017. https://www.theguardian.com/science/2013/nov/06/chelyabinsk-meteor-russia

Schilling, John H. Orbital debris mitigation for commercial launch vehicles. Tech. Long Beach, California: American Institute of Aeronautics and Astronautics Inc, AIAA, 2016. Scopus. Web. 12 Mar. 2017.

Smith. "How Much Does It Cost to Launch a Satellite?" 24 June 2016. Web. 2 MAr. 2017. https://www.fool.com/investing/2016/06/24/how-much-does-it-cost-to-launch-a-satellite.aspx

SPACE.com staff. "DARPA Wants to Recycle Space Junk Into New Satellites." Space.com. Purch, 20 Oct. 2011. Web. 02 Mar. 2017. <http://www.space.com/13339-darpa-space-junk-recycling-phoenix-satellites.html>.

Stillman, May. "What is a satellite?" 12 Feb. 2014. Web. 2 Mar. 2017. https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-satellite-58.html

"The Economic Impact of Commercial Space Transportation on the U.S. Economy in 2009." September 2010. Federal Aviation Administration, U.S. Department of Transportation. https://www.faa.gov/news/updates/media/Economic%20Impact%20Study%20September%202010_20101026_PS.pdf

Venugopal. "Space Disasters." n.d. Web. 2 Mar. 2017. http://www.spacesafetymagazine.com/space-disasters/

Vijayaraghavan, Akhila. "Plan to Recycle $300 Billion Worth of Space Debris." Triple Pundit: People, Planet, Profit. Triple Pundit, 05 Nov. 2011. Web. 02 Mar. 2017. <http://www.triplepundit.com/2011/11/plan-recycle-300-billion-worth-space-debris/>.

Wall. "First Space Tourist: How a U.S. Millionaire Bought a Ticket to Orbit." 27 Apr. 2011. Web. 2 Mar. 2017. http://www.space.com/11492-space-tourism-pioneer-dennis-tito.html

Weinberger. "Space junk: Why it is time to clean up the skies." 18 May 2012. Web. 20 Feb. 2017. http://www.bbc.com/future/story/20120518-danger-space-junk-alert

"Where do old satellites go when they die?" NASA. NASA, 14 May 2015. Web. 02 Mar. 2017. <https://spaceplace.nasa.gov/spacecraft-graveyard/en/>.

"Why burn up on entering Earth's atmosphere?" The Naked Scientists. N.p., 10 April 2011. Web. 02 Mar. 2017. <https://www.thenakedscientists.com/articles/questions/why-burn-entering-earths-atmosphere>

Zuiani F., Vasile M. Preliminary Design of Debris Removal Missions by Means of Simplified Models for Low-Thrust, Many-Revolution Transfers. International Journal of Aerospace Engineering, Volume 2012 (2012), Article ID 836250

Sutton, George P., and Oscar Biblarz. Rocket propulsion elements. 7th ed. Hoboken, NJ: John Wiley & Sons Inc., 2017. Web. 7 Mar. 2017. https://www.ewp.rpi.edu/hartford/~ernesto/S2013/EP/MaterialsforStudents/Lee/Sutton-Biblarz-Rocket_Propulsion_Elements.pdf

Patterson, Michael J. : Ion Propulsion thruster including a plurality of optic electrode pairs. The United States of America as Represented by the Administrator of National Aeronautics and Space Administration, assignee. Patent US 9,494,143 B1. 15 Nov. 2016. Web. 7 Mar. 2017. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160014457.pdf

Herman, Daniel A., Walter Santiago, Hani Kamhawi, James E. Polk, John S. Snyder, Richard R. Hofer, and Michael J. Sekerak. "The Ion Propulsion System for the Asteroid Redirect Robotic Mission." 52nd AIAA/SAE/ASEE Joint Propulsion Conference (2016): n. pag. Web.

"New Dawn for Electric Rockets - alfven.princeton.edu." N.p., 2009. Web. 9 Mar. 2017. http://alfven.princeton.edu/publications/choueiri-sciam-2009 .

Griffith, Anthony D., Sr., Rajiv Kohli, Susan H. Burns, Stephen J. Damico, David J. Gruber, Christopher J. Hickey, David E. Lee, Travis M. Robinson, Jason T. Smith, Peter T. Spehar, David S. Adlis, and Brian M. Kent. SYSTEM, APPARATUS, AND METHOD FOR ACTIVE DEBRIS REMOVAL. The United States of America as represented by the Administrator of the National Aeronautics and Space Administration, Washington, DC (US); The Aerospace Corporation, El Segundo, CA (US), assignee. Patent US 9,555,905 B2. 31 Jan. 2017. Print.