The Risk to Humans from Man-made Space Debris and Objects

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The Risk to Humans from Man-
made Space Debris and Objects
By Stuart McGowan
12 September 2016

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Abstract
This literature-based review will examine the threat to the global and space-faring population from man-
made orbital space debris and discuss:
 The current extent of the problem
 The risk to humans
 Responsibility for safety
 Mitigation and actions in the present
 Future proposals to reduce debris
Since the development of satellite technology, there has been an exponential increase in objects orbiting
Earth. As the problem has increased, there has been a number of impacts and fragmentations that have
increased the amount of debris in orbit. This has created a more hazardous environment for astronauts
and increased the risk of uncontrolled re-entries, where a number of impacts close to populated areas
have been recorded. This has resulted in the need to track debris, introduce procedures to mitigate
against dangerous collisions and build in shielding to protect vulnerable parts of spacecraft. International
agreements to tackle the problem have resulted in the creation of the Inter-Agency Debris coordination
Committee (IADC) to develop international cooperation to reduce the impact of space debris and
ultimately coordinate technical solutions to remove debris.
A number of software modelling solutions have been developed to assess the risk of re-entries and
predict the likely impact point. Procedures and mathematical models have also been developed to
assess the collision risk to spacecraft to assist decision making regarding potential avoidance
manoeuvres. Furthermore a number of proposals have been put forward to remove debris from orbit,
some are more viable than others, due to cost and technological availability.
It is increasingly important that debris removal begins as soon as possible as further debris creating
episodes are likely and will make future missions more hazardous. This will also make re-entries more
difficult to predict and impossible to control increasing the risk of damage to life and property on the
ground. (296 words)

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List of abbreviations
D4D – Design for Demise
DDMS – Department of Defence Manager for Space transportation system contingency
support operations
DoD – Department of Defence
ESA – European Space Agency
GEO – Geostationary Earth Orbit
GPW – Gridded Population of the World
IADC – Inter-Agency Debris coordination Committee
ISS – International Space Station
JSpOC – Joint Space Operations Centre
LEO – Low Earth Orbit
MEO – Medium Earth Orbit
NASA – National Aeronautics and Space Agency
ORIUNDO – On-ground Risk estimation for UNcontrolleD re-entries tOol
SCARAB – Spacecraft Atmospheric Re-entry and Aerothermal Breakup
TIRA – Tracking and Imaging RadAr
WG – Working Group

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Contents Page
Title page……………………………………………………………………………………………………. 1
Abstract……………………………………………………………………………………………………… 2
List of Abbreviations……………………………………………………………………………………….. 3
Contents…………………………………………………………………………………………………...... 4
1. Introduction………………………………………………………………………………………………. 5
2. The history of space debris…………………………………………………………………………….. 5
3. Current estimated debris, concentration and distribution…………………………………………… 6
4. The risk of injury from space debris
4.1 The global population……………………………………………………………………….......... 8
4.2 Astronauts………………………………………………………………………………………….. 9
5. Current mitigation procedures…………………………………………………………………………. 9
6. Responsibilities regarding safety
6.1 The global population……………………………………………………………………….......... 12
6.2 Astronauts………………………………………………………………………………………….. 13
7. Future proposals to remove space debris……………………………………………………………. 14
7.1 Debris capturing methods………………………………………………………………………… 14
7.1.1 Tentacles capturing………………………………………………………………………... 14
7.1.2 Electrostatic force field (tractor)…………………………………………………….......... 15
7.2 Debris removal methods………………………………………………………………………….. 15
7.2.1 Drag augmentation system……………………………………………………………….. 16
7.2.2 Laser system……………………………………………………………………………….. 16
8. Conclusion……………………………………………………………………………………………….. 17
References………………………………………………………………………………………………...... 18

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1. Introduction
Since the development of space flight, each mission into orbit has produced debris from the size of paint
chips to rocket booster stages and decommissioned satellites, and this debris has accumulated at
varying rates over the last 55 years, culminating in over 100 million pieces of debris (Hall, 2014). This
has led to an increased hazard when undertaking space flight for astronauts as well as those currently
occupying the International Space Station (ISS), and although the risk is significantly smaller, there are
safety concerns for the Earth-based population.
Although the chances of a fatality from space debris is relatively small for civilians and astronauts alike,
the ever increasing debris coupled with a continuous presence in orbit and increasing global population
mean that these chances are increasing. Most debris that makes it through the atmosphere impacts the
oceans, but there have been incidents where large objects have impacted land. In June 1979 Skylab re-
entered the Earth’s atmosphere scattering over the Indian Ocean and sparsely populated areas of the
Australian outback (NASM, 2014). Various other large objects have been recorded to have impacted
areas in Texas in 1997, South Africa in 2000, Saudi Arabia in 2001 and Mongolia in 2010 (Ailor, 2012)
and in Oklahoma in 1997 Lottie Williams is reported to have possibly been struck on the shoulder by
debris, although unharmed (Gini, 2011).
The aim of this report is to investigate the current orbital debris and its potential impact on the welfare
and safety of the population. This will be done by:
 Estimating the current number, distribution and residence time of space debris.
 The mitigation procedures used to avoid debris for launch vehicles and the ISS.
 The preventative measures that are in place to reduce the further accumulation of debris.
 Discuss the implications and ethics of leaving space debris to burn up in the atmosphere.
 The likely actions to be taken in the event of a potential terrestrial impact, the decision making
process and how this will be communicated.
Future methods for reducing orbital debris will also be examined which range from contact to contactless
methods that are either space or non-space based designs (Shan et al, 2015).
This report has been researched using Open University library searches, databases such as science
direct, as well as broader internet searches which have concentrated on peer-review papers for subjects
ranging from the analysis of population damage by space debris (Lee et al, 2016), and the current
progress on active debris removal (Bonnal et al, 2013). Technical papers covering the current ESA
(European Space Agency) debris mitigation guidelines (ESA, 2015) and of NASA (National Aeronautics
and Space Administration) (APPEL, 2015), as well as conference papers such as the statistical analysis
of re-entry hazards (Matney, 2011) will be used in combination to create a homogenous report that
attempts to define and understand the issues regarding the human risk from orbital space debris, and
highlight areas that may be lacking detailed study or understanding.
2. The history of space debris
In October 1957 the USSR launched the first artificial satellite into orbit, Sputnik and its rocket stages
burned up on re-entry three months later. By March 1958 the USA launched the Vanguard I satellite
followed by two more in 1959, unlike Sputnik they still remain in Medium Earth Orbit (MEO) (between an
altitude of 2,000 – 35,786 km) to this day, meaning their non-operational status marks them as space
debris (Hall, 2014). Since then there has been an exponential growth in the space industry, from military
applications, scientific experiments, and increasingly, from the commercial sector largely establishing
communication networks. Currently there are around 900 operational satellites in orbit (Lee et al, 2016).
However, the general increase in objects has been caused by the fragmentation of ancillary items, which
has scattered debris all around the Low Earth Orbit (LEO) creating a more hazardous working
environment. The blue line in figure 2.1 displays the steady and increasing growth of spacecraft in orbit,
many of which are non-operational.

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Figure 2.1 Graph displaying the increase in orbital debris more than 10 cm in diameter since 1957. The graph clearly shows that the majority
of the debris has been caused by fragmentation episodes (APPEL, 2012)
Although there has been targeting of satellites in orbit by the USA and the USSR during military
exercises, the destruction of the Fengyun-1C by China was a notable incident in the increase of debris;
as was the collision between the American Iridium 33 communications satellite and the non-operational
Russian Cosmos 2251 (see chapter 3). The exponential increase in debris is becoming an ever
increasing threat to future manned space missions and to a lesser extent the Earth-bound population
and addressing this threat is becoming increasingly more urgent.
3. Current estimated debris, concentration and distribution
As of 2015 there are more than 500,000 pieces of space debris orbiting the Earth, travelling at speeds of
up to 17,500 mph (Garcia, 2015), of these 21,000 pieces are at least 10 cm in diameter or more. The US
Figure 3.1 (a) Graphical representation of the range and distribution of orbital debris and (b) the concentration of LEO debris
(NASA, 2009b).

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Space command regularly tracks the larger pieces using a network of ground-based radars and optical
systems (Ailor, 2012), smaller pieces are not routinely tracked but are estimated using the same systems
and by inspecting the surfaces of returned satellites and space vehicles, which allows estimates on the
number, density and location of these pieces (Garcia, 2015). Much of this debris is made up of stray nuts
and bolts, decommissioned satellites or discarded booster stages. Unfortunately this problem has been
made worse by various factors, such as military experiments targeting old decommissioned satellites
with ballistic missiles, such as the tests carried out by China in 2007 which targeted the non-operational
Fengyun-1C satellite, or the spontaneous fragmentation of or collision of satellites. However, 73% of this
debris has been created by discarded booster rocket stage explosions, which is caused by solar heating
increasing the internal pressure of the booster, igniting the unspent fuel inside (Hall, 2014). One of the
main issues regarding this debris is that it is concentrated in LEO where many astronauts, satellites and
the International Space Station (ISS) operate. Around 66% of the man-made space debris exists in this
LEO which is below an altitude of 2,000 km, but generally resides at an altitude of 600-1200 km and
largely concentrated in a Polar orbit (VideoFromSpace, 2013), which is, according to the European
Space Agency (ESA), largely made up of commercial communications satellites (ESA, 2005).
It is this continuing increase in the satellite population which led to a collision between an operational
USA Iridium 33 communications satellite and a non-operational Russian Cosmos 2251 communications
satellite in the congested polar LEO in February 2009. The resulting collision created 1,700 pieces of
debris larger than 10 cm and around 200,000 pieces larger than 1 cm. Figure 3.2 below shows how this
debris redistributed itself over the course of a year.
Figure 3.2 Gradual distribution of debris over a year resulting from the 2009 collision between the Iridium 33 and Cosmos 2251
satellites, tracked by NASA and the Department of Defence (DoD) using a space surveillance network, which includes
Goldstone and haystack radar as well as optical systems. Green represents the Iridium 33 fragments, and red represents the
Cosmos 2251 fragments (Johnson, 2009).
Such collisions and explosions clearly increase the hazard for manned missions into orbit as well as
those currently occupying the ISS, and increases the risk of uncontrolled re-entries into Earth’s
atmosphere.

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4. The risk of injury from space debris
4.1 The global population
In global terms the risk of a fatality to an individual due to the re-entry of space debris is extremely low,
largely due to the fact that around 70% of the Earth is covered by water and of the remaining 30% of
land, large swathes are either unpopulated or sparsely populated as illustrated in figure 4.1.
Figure 4.1 Map showing the density and distribution of the human population (NASA, 2009a).
Also for consideration is the survivability and point of impact of the re-entering object which can be
determined by using modelling programs such as ESA’s SCARAB (Spacecraft Atmospheric Re-entry
and Aerothermal breakup) (Lee et al, 2015).
When considering the re-entry of an object calculations need to be made that take numerous factors into
consideration, such as the flight path angle, altitude, Earth’s rotation and eccentricity, but also provide
corrections to astronomical factors, such as eccentricity anomalies, and gravitational perturbations
caused by the Sun and the Moon. On entering the atmosphere, atmospheric friction, wind direction and
strength, and the aerodynamic effects affecting an objects trajectory need to be factored in when
calculating a potential impact point; this is further complicated by the nature of the materials used to
construct the object, its area-to-mass ratio, velocity and shape. Although this many variables makes
precise predictions difficult, the mathematical models used make adjustments for these issues and go
some way in reducing the margin of error (Klinkrad, 2006). Table 4.1 displays the common materials
used for spacecraft, where higher melting points increase the survivability of a material.
Material
ρ Cp ϵ Tm hm
[g/cm³] [J/kg K] [-] [K] [J/g]
Titanium (high strength grade 5 alloy) 4.420 750.0 0.302 1900 400
Stainless steel 8.030 611.5 0.350 1650 274
Inconel (nickel alloy) 8.190 417.1 0.122 1570 309
Zerodur (aluminosilicate ceramic glass) 2.530 1265.6 0.622 1424 250
Copper 8.960 434.1 0.216* 1356 243
CFRP (reinforced carbon fibre polymer) 1.700 1100.0 0.850 1160 6650
Aluminium (zinc alloy) 2.800 751.1 0.141 870 385
* ϵ for an oxidised surface (ϵ = 0.012 for a polished surface)
Table 4.1 Properties of typical materials which are relevant for the survivability analysis of re-entry objects (density ρ, specific heat cp,
radiation emission coefficient ϵ, melting temperature Tm, and melting enthalpy hm) (Klinkrad, 2006).

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Although the material type is an important factor, so is the size and shape of the object as well as a
materials ability to store heat (Cp) and re-radiate this heat (ϵ). This means that materials with high
melting temperatures and good re-radiation capability, such as titanium and steel are more likely to
survive re-entry. Similarly, disk shaped objects are more likely to survive re-entry due to a large
area/mass ratio, followed by spheres and then cylinders. Therefore, knowledge of an objects dimensions
and construction materials is equally important when calculating the survival probabilities of a re-entry
and impact point.
If an object is likely to have large segments survive re-entry, tools such as ESA’s Oriundo (On-ground
RIsk estimation for UNcontrolleD re-entries tOol) can be used to determine the potential of an object to
cause fatalities on the ground. It accomplishes this by combining NASA’s Gridded Population of the
World (GPW) software which resolves the density and distribution of the population down to a resolution
of 1 km at the equator and the UN’s World Population Prospects data set, which estimates and makes
projections on population based on socioeconomic factors. The tool then uses the Monte Carlo method
which carries out repeated random sampling to obtain the casualty probability and the casualty cross
section threshold. This then provides agencies with possible casualties, although in reality the risk of a
fatality from a space object re-entry is around 1 in 1.2x1011
, putting this in context, being hit by lightning
has a probability of 1 in 2x106
(Klinkrad, 2006).
4.2 Astronauts
Orbital debris provides a more hazardous environment for astronauts, where up until 2009 it is believed
there has been around 173 in-orbit fragmentation events since 1956 (Johnson, 2009). The US space
command tracks objects larger than 10 cm, but smaller objects are not routinely tracked (Mehrholz et al,
2002). Although there has been no incidents or fatalities involving astronauts and orbital debris, strikes
on space vehicles and the ISS are not uncommon, which are of concern due to the relative velocities
involved, which can be up to 22,000 mph. In the first 63 flights of the space shuttle orbiter vehicle it
registered 177 impacts on the windows alone and 70 windows needed to be replaced between 1981 and
1998 (Hall, 2014).
Figure 4.2 Impact on the shuttle window during 1983 STS-7 mission, where a 0.2 mm paint chip hit the space shuttle Challenger creating a
0.4 mm pit (Hall, 2014)
5. Current mitigation procedures
An exponential rise in debris means detection and tracking of debris is vital in the mission planning stage
up to the launch in order to reduce the risk of impact with larger debris. This is accomplished by using

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tracking radars such as the Goldstone antenna in California, which is capable of detecting debris as
small as 2 mm (APPEL, 2012) and the combined tracking and imaging radar (TIRA) system in
Wachtberg, Germany, who now collaborate with ESA to detect and track space debris (ESA, 2013a).
Responsibility for tracking space debris for NASA assets falls to the DoD’s Joint Space Operations
Centre (JSpOC) who scan for debris every eight hours for manned spaceflight vehicles and daily
(Monday to Friday) for unmanned vehicles. Subsequently NASA is warned when debris is a significant
risk to an asset and further tracking is carried out to a greater resolution. NASA then computes the
probability of a collision based upon the provided data. NASA then makes a decision if the risk is great
enough to require a collision avoidance manoeuvre. If a manoeuvre is required NASA liaises back to
JSpOC to determine whether a manoeuvre will place the vehicle in more peril in the future (Garcia,
2015). Debris that is 1 mm or smaller poses no significant risk to launch vehicles, and objects that are
around 1 cm or smaller are mitigated for by shielding and the orientation of the launch.
The ISS has had to make numerous manoeuvres to avoid debris since it came into service but when
manoeuvres are not possible (which has happened on three occasions) then the crew are instructed to
shelter in the Soyuz escape module. However, the ISS is shielded in vulnerable areas by Whipple
shielding, which has a sacrificial layer of aluminium, slowing the projectile sufficiently to prevent it
penetrating the module (see figure 5.1).
Figure 5.1 Most vulnerable areas on the ISS to debris flux (Klinkrad, 2006).
Whipple shielding is simple, yet effective, and works on the principle of two layers of aluminium with a 10
cm air gap in between. As a small projectile penetrates the first layer, it breaks into smaller fragments
and fans out, reducing its velocity and combined mass. This then impacts the thicker inner wall creating
at worst a small dent or small pitted marks (see figure 5.2 & 5.3) and as an impact hole is the only
outcome, the subsequent impact creates little additional debris in orbit. This design can be improved
upon by using additional bumper layers, thereby slowing the projectile quicker, or by using different outer
materials such as CFRP (see table 4.1), Kevlar, or other metal composites. For inner bumper layers
woven ceramic fabrics may be used (Klinkrad, 2006).

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Debris avoidance manoeuvres are based on the probability of an impact and are discussed in further
detail in chapter 6.2. Such manoeuvres are small adjustments and need to be made in a timely manner,
several hours before in the case of a launch vehicle and 30 hours for the ISS (Garcia, 2015).
Attempts are being made to coordinate cooperation from numerous space agencies including the USA,
Europe, Russia and China, so the Inter-Agency Debris coordination Committee (IADC) has been created
in acknowledgement of an increasingly congested LEO. The primary goal of the IADC is to exchange
debris research, facilitate and review cooperative activities and identify debris mitigation options (IADC,
2015), and in February 2016 the IADC proposed guidelines for future missions and satellite
deployments. It was proposed that measures be taken to prevent debris from spacecraft and orbital
stages, minimise the potential for break-ups and that systems should be designed to prevent accidental
explosions; which entailed recommendations for depleting or making safe volatile items such as fuel and
batteries. Future recommendations for spacecraft design and operating procedures have been made
which entail: designs that reduce accidental break-ups, spacecraft and orbital stages to be periodically
monitored for malfunctions and if such an event occurs, it be recovered, disposed of or made passive. It
also makes recommendations that intentional collisions or destruction be avoided and at the end of their
operational life, if in LEO, they should be de-orbited or retrieved if possible. Therefore, future satellites
Figure 5.2 Images displaying the principles of Whipple shielding (NASA, 2008).
Figure 5.3 Single layer Whipple shielding showing the impact and dispersal of a projectile (Klinkrad, 2006)

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are to be designed to de-orbit within 25 years from the end of their functional life. Finally, it states that
any de-orbiting craft or satellite that may survive re-entry should not be a risk to lives or property (IADC,
2016a).
Unfortunately the IADC has no direct power to enforce any recommendations to member agencies and
so is there primarily to coordinate the management of space debris between agencies and provide
advice and collate research. However, membership does require that each agency should actively
undertake space debris research and contribute to improving the understanding of space debris issues.
This organisation is split into four working groups (WG) which independently address:
 Measurements (WG1)
 Environment and Data Base (WG2)
 Protection (WG3)
 Mitigation (WG4)
WG4 is tasked with studying and investigating measures and designs that will reduce the creation of,
future space debris, reduce collision hazards, prevent the creation of space debris, remove debris, and
produce guidelines for debris mitigation. Removal of debris is the ultimate endgame for the IADC, but
they exist mainly to promote and improve debris research and reach coordinated agreements by
members of the IADC, but not to produce any primary research themselves. The weakness of the IADC
is that there appears to be no set goal or time limit for tackling the problem and is reliant on the work of
other agencies and aerospace manufacturers to provide data and develop physical methods for reducing
the size and amount of debris. Progress is monitored by the IADC through regular meetings between
agencies, but purely exists as a forum to exchange information and research between member space
agencies (IADC, 2016b).
As it stands the current mitigation procedures are based largely on detection and avoidance, as well as
shielding.
6. Responsibilities regarding safety
6.1 The global population
On the initial launch of a space vehicle, safety is the responsibility of the agency in charge of the launch
in conjunction with the other emergency services. When the space shuttle Challenger exploded after
take-off on the 28 January 1986, the recovery operation initially fell to three NASA booster recovery
ships and then the US navy were tasked by the Department of Defence Manager for Space
Transportation System Contingency Support Operations (DDMS) to search for debris (La Vone, 2014).
Likewise, the space shuttle Columbia disaster presented a different challenge as it broke up over Texas
on the 1 February 2003. It required the response of 16,500 wildland firefighters and 21 federal Incident
Management Teams who had a search area of 25,000 square miles to cover. This clearly presented a
significant risk to the population as debris rained down from the atmosphere, although there was no
reported damage to people or property. However, there was the challenge of hazardous material, of
which 50 such materials were eventually recovered (Keller, 2011). This presents another aspect of risk
to the population in the form of radioactive and other hazardous materials within space vehicles and
satellites which could be a threat to human health. As this material would require specialist handling, it
may be some time before teams will be deployed to remove such materials and decontaminate land.
This means that as the 25 year rule is enforced more orbital debris will re-enter the Earth’s atmosphere
and therefore increase the risk of injury, damage or exposure to contamination. Small fragments pose
little risk, but larger fragments (above 10 cm in diameter) and complete spacecraft do. Large items such
as stages from rocket boosters and satellites have occasionally impacted over land and in populated
areas too, such as a Delta II rocket propellant tank which fell next to a farmhouse in Texas in 1997
(APPEL, 2012).

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In this case, controlled re-entries should have no more than a 1 in 10,000 risk of casualties, but the
larger the object, the greater the risk. This can be mitigated for by controlling the re-entry, so that the
spacecraft impacts in the ocean, where such re-entries can be calculated using modelling software such
as SCARAB, or move it to a higher orbit and out of the congested LEO. Redesigning future spacecraft
with “design for demise” (D4D) in mind, can be achieved by making spacecraft with a lower mass, using
materials with lower melting points with a build that promotes a complete breakup on re-entry, or one
that keeps all survivable components in one container minimising the spread of debris (APPEL, 2012).
This is ultimately the goal for space agencies such as NASA, but will still need to incorporate a controlled
re-entry, impacting in unpopulated areas, such as the ocean.
6.2 Astronauts
As stated in chapter 5, a network of radars and telescopes track debris in orbit where the assessment of
risk is primarily the responsibility of the space agency involved. The probability of a collision risk between
a spacecraft and debris is determined using two linear equations: eq 6.1 is used if the orbital debris flux
(number of impacts m-2
y-1
expected on a randomly orientated planar surface of an orbiting structure) is
known (NASA, 2008):
(𝑜𝑟𝑏𝑖𝑡𝑎𝑙 𝑑𝑒𝑏𝑟𝑖𝑠 𝑓𝑙𝑢𝑥) 𝑥 (𝑐𝑟𝑜𝑠𝑠 − 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎) 𝑥 (𝑡𝑖𝑚𝑒) (𝑒𝑞 6.1)
If the spatial density (altitude and latitudinal concentration of debris) is known instead (ESA, 2013b) then
eq 6.2 will achieve the same result:
(𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑐𝑜𝑙𝑙𝑖𝑠𝑖𝑜𝑛 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦) 𝑥 (𝑐𝑟𝑜𝑠𝑠 − 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎) 𝑥 (𝑠𝑝𝑎𝑡𝑖𝑎𝑙 𝑑𝑒𝑛𝑠𝑖𝑡𝑦) 𝑥 (𝑡𝑖𝑚𝑒) (𝑒𝑞 6.2)
If the risk of a collision is 1 in 10,000 a collision avoidance manoeuvre is performed, if it is greater than 1
in 100,000, a manoeuvre is only performed if it does not compromise the mission objectives, for robotic
vehicles the threshold is higher 1 in 1,000 (APPEL, 2012). To reduce the likelihood of damage during
launch, impact reduction procedures are used, such as keeping the spacecraft’s cylindrical length axis
orientated perpendicular to the orbital plane therefore reducing the area that can be impacted by debris.
The flight attitude should also point vulnerable surfaces aft or towards Earth, empty tanks should be
depressurised once depleted and momentum storage devices, such as flywheel and gyros kept inactive
until required (NASA, 2008).
Figure 6.1 A Delta II rocket propellant tank impacts next to a farm building in Texas 1997 (APPEL, 2012)

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7 Future proposals to remove space debris
Unless urgent measures are made to reduce space debris it could jeopardise the safety of future space
missions, as well as increase the occurrence of uncontrolled re-entries which will increase the risk of
damage to life and property (Tewari, 2013). Modelling simulations carried out by NASA and other space
agencies, including the IADC suggests that five large objects per year will need to be removed from
densely populated orbits to keep the environment stable (Bonnal et al, 2012). There are numerous
proposals for removing debris from LEO with a vast array of designs and methods from capturing large
pieces that can be contact or contactless and debris removing solutions that can be space or non-space
based. Each proposal carries its own benefits and drawbacks and the use of one system over the other
will be largely dependent on the nature of the debris.
7.1 Debris capturing methods
This method is largely aimed at capturing large pieces of debris, such as rocket booster stages and
intact non-operational satellites and has numerous devices from contact to contactless (see figure 7.1).
There are too many to be discussed in detail, so one from each method will be selected.
Figure 7.1 A concept diagram of the different debris capturing methods (Shan et al, 2015)
7.1.1 Tentacles capturing
The tentacle technique clamps onto debris by using a robotic arm for initial capture, or without a robotic
arm where it embraces the target before contact. Removing the robotic arm creates technical problems
with precision when dealing with tumbling objects, but cost, complexity and mass have made a non-
robotic arm version more attractive; experiments have proven the concept to be viable but requires
improvements in precision when capturing an object (Shan et al, 2015). When the object is captured it is
then manually deorbited by a chaser satellite and can then move onto a new target. The advantages of
such a system are that it is easy to test on the ground, is an available technology and allows a controlled
re-entry of the object. The disadvantages are that a rendezvous with an object is complicated, it may
bounce off the target or collide, and create more debris, so accurate position and velocity data is
required prior to commencing capture (Shan et al, 2015).

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Figure 1 Four competing design concepts for tentacle capture (Shan et al, 2015)
7.1.2 Electrostatic force field (tractor)
This concept is largely based on work undertaken to deflect asteroids. However in principle, the design
could be useful for removing large objects, particularly from Geostationary Earth Orbit (GEO).
Figure 7.3 Design concept for an electrostatic force field (Shaub & Sternovsky, 2014)
This concept works by establishing a tractor beam between the two objects where the tractor would
bombard the debris with electrons, giving it a negative charge of around 10 kilo-volts. The positively
charged tractor could then lock-on and tow the debris to a higher less populated orbit, or into an orbit
that will allow it to re-enter Earth’s atmosphere. This method will be able to stabilise a tumbling object
and reduce the chances of a collision due to a stand-off distance of between 15-25 metres and would
also be reusable for other removal missions, making it cost effective. However, it would not be useful for
capturing smaller objects as their smaller surface area would not provide the necessary electrostatic
force required for capture (Shaub & Sternovsky, 2014).
7.2 Debris removal methods
This method is useful for capturing smaller debris, particularly clusters of debris caused by collisions and
explosions. As shown in figure 7.4, they range from space environment-based to non-space
environment-based.

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Figure 7.4 A concept diagram of the different debris removal methods (Shan et al, 2015)
7.2.1 Drag augmentation system
This concept works on increasing the surface-to-mass ratio of the debris and thereby increasing the
atmospheric drag in LEO. It works by surrounding debris with foam, fibres or by attaching an inflating
ball.
Figure 7.5 Drag augmentation concepts from left to right: Foam, inflation and fibre-based methods (Shan et al 2015)
Although it works well for small debris, it is useful for larger objects too and is a relatively low-cost
solution. The drawbacks are that, re-entries will not be controlled, which provides an unacceptable risk
on the ground and there is also a risk that these applications could breakup, resulting in an actual
increase in debris (Shan et al, 2015).
7.2.2 Laser system
Unlike other proposals this method entails a ground-based solution which fires pulsed laser beams at the
target which is designed to reduce the velocity of the debris, meaning it will reduce its altitude leading to
an inevitable re-entry. There is risk from the further breakup of the debris and the system would be
limited to objects larger than 1 cm and objects with a mass less than 500 kg, it is also limited to a
maximum range of 500 km for small debris and 1000 km for larger objects (Phipps et al, 2012). It would
take many weeks to de-orbit just one piece of debris and the ability to accurately detect the smallest
pieces of debris would be necessary (Shan et al, 2015)

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Figure 7.6 A ground-based laser system designed to reduce debris velocity (Phipps et al, 2012)
8. Conclusion
For over 50 years the amount of objects in orbit around the Earth have been increasing, creating a man-
made hazard for humans working in orbit and to a lesser extent endangering the population on the
ground. Although the risk to the ground-based human population is slim, subsequent explosions of
rocket booster stages and collisions between satellites, have made missions more perilous to
astronauts. Debris is mitigated for by agencies, by way of regular tracking of large amounts of debris,
built-in shielding, avoidance manoeuvres when necessary and controlled re-entries of satellites
whenever possible. The creation of the IADC has helped to focus efforts on tackling the problem by
introducing a 25 year de-orbiting of decommissioned satellites and making agreements that all discarded
rocket boosters depressurise unspent fuel to prevent further fragmentations. This will go some way to
reducing the build-up of debris, but not eradicate it, as future collisions between satellites are highly
likely. This means that debris will continue to rise, further increasing the risk to both astronauts and the
population on the ground.
Much of this debris will not be able to make controlled re-entries or is likely to remain in orbit for
hundreds to thousands of years, so it is imperative that it be physically removed from orbit. To this
extent, the IADC have an important role in improving and promoting the cooperation of numerous space
agencies and coordinating efforts for present mitigation solutions in the form of policies, procedure and
the tracking of debris. Although there is no defined time frame for the physical removal of debris it is
imperative that research and development be expedited in order to begin the clearing of Earth’s orbit via
technical solutions as soon as possible. The continued presence of space debris and the current inaction
only risks making the problem worse and increases the probability of human casualties in the future.
(4975 words)

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