The Issue of Space Debris
39
Fernand Alby
Contents
39.1 Introduction .............................................................................. 680
39.2 The Situation in Orbit ................................................................... 680
39.2.1 Where Space Debris Comes From ............................................. 680
39.2.2 Debris Inventory ................................................................ 681
39.2.3 Lifetime ......................................................................... 682
39.3 Risks Associated with on the Ground and in Orbit ..................................... 684
39.3.1 Risks on the Ground ........................................................... 684
39.3.2 Risks in Orbit ................................................................... 684
39.4 Actions to Be Implemented ............................................................. 685
39.4.1 Knowing the Situation ......................................................... 685
39.4.2 Protection ....................................................................... 687
39.4.3 Stopping Production of Debris ................................................. 690
39.4.4 Removing Old Debris .......................................................... 695
39.5 Conclusion ............................................................................... 697
References ....................................................................................... 697
Abstract
Since 1957, human space activities have placed a great deal of objects in orbit
around Earth. Debris represents a growing risk for operational satellites, in case
of collision, and on the ground when it reenters the atmosphere. This situation
calls for action, namely, in the following four areas: obtaining accurate knowledge
of the situation, protecting satellites and populations, reducing as far as
possible the creation of new debris, and cleaning up in space by removing the
largest objects. The prevention measures mainly consist in post-mission management
for satellites and launchers. Measures have been developed, and have
met with broad consensus. However, to ensure more systematic application,
F. Alby
CNES, Toulouse, France
e-mail: fernand.alby@cnes.fr
K.-U. Schrogl et al. (eds.), Handbook of Space Security,
DOI 10.1007/978-1-4614-2029-3_10,
# Springer Science+Business Media New York 2015
679
legal mechanisms are also being established, States being liable in the event of
an incident. Protection actions are also needed, but offer only partial solutions:
these actions involve setting up services for preventing the risk of collision and
predicting atmospheric reentries. However, due to collisions between debris
objects, these actions alone will not be enough to stabilize the debris population:
cleanup actions will eventually be necessary.
39.1 Introduction
Since 1957, human space activities in orbit have produced a large amount of waste,
known as space debris, which has become a real problem – to the extent that there is
now an urgent need for regulation, while also developing the means to one day
begin cleanup.
The purpose of this chapter is to describe the situation in space and the risks
associated with this debris, and then to review the various actions to be
implemented in the short and medium term.
39.2 The Situation in Orbit
39.2.1 Where Space Debris Comes From
More than 5,000 spacecraft have been launched since Sputnik, resulting in a large
amount of waste being put into orbit. The different types of space debris include
(Rathgeber et al. 2010):
• Operational satellites and abandoned satellites which remain in orbit around Earth.
• Upper stages of the launchers used to place these satellites in orbit.
• Operational debris, objects intentionally released during space missions: covers
used to protect instruments during the launch phase, systems used to attach
solar panels or antennas before deploying them in orbit, separation devices and
straps, etc.
• Fragmentation debris: debris produced when objects in orbit collide with space
debris or meteorites, and from the accidental or voluntary explosions of
spacecraft.
• Propellant residue: solid propellant motors used to carry out transfers in orbit,
especially between a transfer orbit and geostationary orbit, which release small
alumina particles during thrust periods. This problem is especially critical at the
end of the thrust period when combustion becomes unstable, at which point slag
measuring several centimeters can be ejected into space.
• Debris from the ageing of materials in space. The space environment is very
harsh, with major temperature differences between shade areas and areas
exposed to the Sun, atomic oxygen and ultraviolet rays, etc. The ageing process
causes large amounts of debris to be produced (separation of photoelectric cells,
weathering of thermal protection covers, and peeling of paint, etc.).
680 F. Alby
There are other, more anecdotal sources of debris which have also had
a signiﬁcant impact on the population of objects in orbit:
• In 1961 and 1963, as part of the Midas 4 and Midas 6 experiments, the US Air
Force planned to release several million copper needles (West Ford Needles)
into orbit at an altitude of around 3,000 km. The goal was to create a ring of
dipoles around Earth to act as a passive reﬂector for military communications.
Only the second experiment was partly successful. The needles then formed
clusters, 65 of which could still be seen from the ground in 1998.
• In the 1980s, the Soviet Union used Radar Ocean Reconnaissance Satellites
(RORSAT) equipped with nuclear reactors. At the end of their mission, the cores
of these reactors were re-orbited at altitudes between 900 and 1,000 km to allow
their radioactivity to decrease before they fell back into the atmosphere. Leaks in
the cooling circuit were found on 16 of these satellites, which resulted in drops of
liquid sodium potassium (measuring between 1 mm and several centimeters)
being released into orbit.
39.2.2 Debris Inventory
Knowledge of the debris population is obtained using radar or optical observation
means, on the ground or in orbit, and by studying the effect of debris on surfaces
that have spent time in space.
The debris is generally broken down into the following categories:
• “Large” objects, measuring more than 10 cm in low orbit and 1 m in geostationary
orbit: these objects are listed individually, or catalogued, by space
surveillance systems (see paragraph 3.1). Around 15,000 are routinely cataloged
by the US surveillance network. However, their total number is estimated at
20,000, not all of them being catalogued.
• Objects measuring between 1 and 10 cm: based on statistical observations, the
total is estimated at several hundred thousand.
• Objects measuring between 1 mm and 1 cm, which are counted in tens of
millions.
Figure 39.1 shows the breakdown of catalogued objects by category (source:
public catalogue of the US Space Surveillance Network). It should be noted that
active satellites make up only 6 % of the population of catalogued objects in space.
Figure 39.2 (Source: NASA Orbital Quarterly News, volume 16, issue 1 January
2012) shows the progression over time of the number of objects catalogued by the
space surveillance network in the United States. The total number of objects is the
sum of the four components shown in the graph: fragmentation debris, spacecraft,
mission-related debris, and rocket bodies. Two events in particular added large
amounts of waste: the voluntary destruction of the Fengyun 1C satellite in January
2007, and the collision between the active Iridium 33 satellite and the abandoned
Cosmos 2251 satellite in February 2009.
Figure 39.3 (Source: NASA Orbital Quarterly News, volume 16, issue 2, April
2012) shows the progression in the total mass of artiﬁcial objects in orbit around
39 The Issue of Space Debris 681
Earth, and in each of the four categories mentioned above. The progression is
almost linear, climbing steadily since the 1980s. The two events mentioned above
impacted the number of objects, but not the total mass.
39.2.3 Lifetime
The lifetime of objects in orbit depends on their altitude, due to the effect of
atmospheric drag.
17000
Monthly Number of Objects in Earth Orbit by Object Type
Total Objects
Fragmentation Debris
Spacecraft
Mission-related Debris
Rocket Bodies
16000
15000
14000
13000
12000
11000
10000
9000
8000
NumberofObjects
7000
6000
5000
4000
3000
2000
1000
Year
0
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
2010
2012
Fig. 39.2 Progression in the number of catalogued objects as a function of time
operational
satellites; 6%
abandoned
satellites; 22%
upper
stages; 17%
operational
debris; 13%
fragments; 42%
Fig. 39.1 Breakdown of
catalogued objects
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Atmospheric density decreases more or less exponentially in function of the
altitude. Traces of atmosphere are still present in low orbits, and these molecules
slow the passage of orbiting objects. Reducing their speed causes them to lose
altitude, in which case they meet even greater resistance, since the lower the
altitude, the higher the atmospheric density will be. Ultimately, as the cycle
continues, the object will be captured by the atmosphere and fall into the dense
layers. This phenomenon is signiﬁcant in low orbit (at altitudes below 1,000 km)
and nonexistent in geostationary orbit. In the case of the International Space Station
(ISS) located at an altitude between 350 and 400 km, the lifetime would be between
6 months and a year without maneuvers. Altitude-boosting maneuvers are made
regularly to offset this disturbance. At a higher altitude, around 800 km, the lifetime
of a satellite is around 200 years and beyond; lifetime increases quickly the higher
the altitude.
However large differences in orbital lifetime estimation may be observed due to
the inﬂuence of solar activity. The Sun’s ultra-violet radiation causes excitation of
the different molecules in the atmosphere resulting in the temperature being
increased and in overall dilatation of the atmosphere. Important variations of the
solar activity are observed during the 11-year cycle of the Sun. For instance, at
400 km altitude, according to the NASA MSISE-90 model of the upper atmosphere,
the density during high-activity periods may reach 100 times the density value
during low-solar-activity periods. This explains why a large number of satellite
decays are observed during periods around the maximum. In addition, shorter term
variations exist and lead to large dispersions when predicting reentries.
7.0
Monthly Mass of Objects in Earth Orbit by Object Type
(without STS)
6.0
5.0
4.0
3.0
MassinOrbit(millionsofkg)
2.0
1.0
0.0
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1992
1990
1994
1996
1998
2000
2002
2004
2006
2008
2010
2012
Year
Total Objects
Fragmentation Debris
Spacecraft
Mission-related Debris
Rocket Bodies
Fig. 39.3 Progression in the mass of objects in orbit
39 The Issue of Space Debris 683
39.3 Risks Associated with on the Ground and in Orbit
39.3.1 Risks on the Ground
When the spacecraft reenters the atmosphere, encountering the dense layers of the
atmosphere at a very high speed, it is subjected to strong aerothermodynamic
forces. Appendages, such as antennas and solar panels, are torn off at an altitude
of around 90 km, and at around 75 km the spacecraft disintegrates. The debris is
exposed to a very high heat ﬂux: most of the materials are vaporized, but some
components can survive these conditions, e.g., materials such as titanium, steel,
ceramics, or components which are shielded by others (masking effect). It is
generally estimated that 20–40 % of the mass in orbit reaches the ground in the
form of debris. This debris is distributed along the track mainly as a function of its
surface-mass (S/m) ratio. The impact zone on the ground is typically
1,000–1,500 km long (in the direction of the track) and 50–80 km wide. The
random fall-back of these pieces is a risk on the ground if they fall onto inhabited
areas. Although debris is regularly found on the ground, thus far, the random
reentry of debris has never caused any injury or damage.
When the object that is going to fall presents a major risk due to its mass or the
materials it is made of, the usual procedure is to conduct a controlled reentry. This
involves one or several deceleration maneuvers in order to guide the object’s fall to
a chosen impact zone on the ground, as was done for the MIR station in March 2001
and the European ATV, for example.
39.3.2 Risks in Orbit
Objects in orbit travel at a very high speed (8 km/s for an object in low circular
orbit). At speeds such as this, the kinetic energy of even a small piece of debris
is very high: at 10 km/s, a 1 mm aluminum sphere has the same kinetic energy
as a riﬂe bullet. So it is understandable that satellite operators worry about the
risks of collisions between their precious satellites and the debris they regularly
encounter. Collisions are not a ﬁgment of our imagination: their effects can be
seen on spacecraft and any surfaces that have spent time in space and returned
to Earth. For example, a large number of impacts, luckily small ones, have been
found on space shuttles, on the solar panels of the Hubble telescope, and on the
LDEF (Long Duration Exposure Facility) spacecraft. They have also been
spotted on the International Space Station during extravehicular activities.
Unfortunately, collisions between large objects (catalogued objects) also occur,
and have major consequences in terms of producing new debris. The most recent
example is the 10 February 2009 collision between the Iridium 33 satellite and
an old Russian satellite (Cosmos 2251) abandoned in space. The consequence
was of course the destruction of both objects, and the creation of a great
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deal of debris: around 1,400 new objects measuring over 10 cm have been
catalogued, and the smaller pieces of debris are far more numerous.
When a collision occurs, it is generally considered that:
• Debris larger than 1 mm can cause perforations: the effect on a satellite depends
on the location of the impact and can result in equipment failure.
• Debris larger than 2 cm can result in loss of the satellite (lethal collisions) due to
the force of the impact, its dissipation in the structure and projection of particles
inside the satellite at very high speed.
• Debris larger than 10 cm not only results in loss of the satellite, but also produces
a great deal of debris (catastrophic collisions).
39.4 Actions to Be Implemented
Now that we have a clear picture of the situation and the associated risks, we will
discuss the implications and identify the actions to be implemented. There are four
types of solutions:
• First, having the best possible knowledge of the debris population, how it is
distributed and its characteristics. This involves space surveillance activities for
the largest objects and modeling activities for smaller objects (statistical models
indicating the particle ﬂux).
• Once the situation has been ascertained, the next step is protection: protection
against small debris thanks to shielding and adapted architecture, protection
against large objects by avoiding collisions in orbit or during launches, and
protection on the ground by monitoring atmospheric reentries.
• In addition to these actions, we must also stop creating new debris which will
exacerbate the problem in the medium term. Thus, we must apply these prevention
measures to satellites and launcher stages and most importantly, manage
their disposal when they are no longer in use.
• Finally, in the longer term, we will surely have to do clean up in space, i.e.,
retrieve and remove the largest objects abandoned in orbit before the prevention
measures went into effect.
In the following paragraphs, these four types of actions are described in greater
detail.
39.4.1 Knowing the Situation
The goal of space surveillance is to inventory the objects above a certain size which
are in orbit around Earth. This inventory (catalogue) provides information on the
origin of the objects (name, launching country) and their trajectory (orbit parameters)
so that the object can be looked up later. To obtain this information, different
types of sensors have to be used: ﬁrst, detection tools with a wide ﬁeld of vision
39 The Issue of Space Debris 685
allowing objects above a certain size to be seen as they pass by, and their orbit to be
roughly calculated so they can be found again later, and second, tracking tools with
a narrow ﬁeld of vision which can follow a given object in order to take
trajectography measurements and better deﬁne its trajectory. These detection and
tracking tools basically consist in radars for objects in low orbit and telescopes for
objects in higher orbits. They may be located on the ground or in orbit.
The main source of information is the Space Surveillance Network (SSN) set up
by the United States, which provides the most complete information. Russia has
a similar system for which very little information is available. There is also the
ISON telescope network, which provides a detailed catalogue of the objects in
geostationary orbit. Finally, France has a limited-capacity network: the Graves
system.
The core of the SSN is the JSpOC (Joint Space Operations Center) located at the
Vandenberg Air Force Base. The JSpOC is responsible for programming the
sensors and collecting and then analyzing the data in order to compile and manage
the catalogue. The SSN can follow objects measuring around 10 cm in low orbit
(and potentially objects measuring around 5 cm at low altitude and in inclined
orbits) and objects measuring around 1 cm in geostationary orbit.
To create and manage the catalogue, the SSN uses two different orbitography
models: (1) general perturbations (GP), an analytical model based on a simpliﬁed
representation of forces, and (2) special perturbations (SP), a model based on
numerical integration with a more accurate representation of forces. Only the less
precise information produced using the GP model is available on the Space Track
website. This information is set out in TLE (Two-Line Element) form: the object’s
SSN and COSPAR number, and the mean orbit parameters on two lines (see details
and format on the Space Track website). TLEs provide only a rough idea of the
orbit: the degree of uncertainty can be up to several kilometers on creation of the
TLE, and gets worse over time. This information is not accurate enough to reliably
predict risks of collision.
The population of smaller debris, below the size threshold, is no longer deﬁned
deterministically, but statistically: this information is obtained from ﬂux models
such as ORDEM (NASA) or Master (ESA).
For a given date and orbit, these models provide the ﬂux on the different surfaces
of a spacecraft, according to the size or mass of the debris in question. The ﬂux is
the number of impacts per surface unit (m2
) and per time unit (year). The models
also indicate the direction and speed of the impacts.
As these small particles cannot be observed from the ground, knowledge of them
comes only from information provided by debris detectors (very rare), or (primarily)
from examining surfaces which have spent time in space after their return to
Earth: LDEF (Long Duration Exposure Facility), Eureca, Space Shuttle, Hubble
solar panels, etc. These observations are only possible in very low orbits, which
allow the models to be correctly “readjusted” for the corresponding altitudes. For
other altitudes, the models are “extrapolated” without there being any means of
verifying their accuracy: given the absence of measurements, the degree of uncertainty
is surely very high.
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39.4.2 Protection
39.4.2.1 In Orbit
With the number of objects in space steadily increasing, predicting the risk of
collision in orbit has become one of the primary tasks of the control centers which
monitor and manage satellites. The annual risk of losing a satellite in a collision
is no longer negligible, as shown in Table 39.1 for two satellites in low orbit
(Klinkrad 2006):
It must be kept in mind that a collision would result not only in the destruction of
both objects but also create large quantities of debris. For example, the collision
between the Iridium 33 and Cosmos 2251 satellites created two clouds of debris: the
2012 SSN catalogue listed 492 pieces of debris from the Iridium 33 satellite and
1,361 pieces of debris from Cosmos 2251.
To manage this risk, operators use the available space surveillance data.
These data allow them to foresee dangerously close passes several days in advance,
to calculate the risk, and to conduct an avoidance maneuver, slightly altering
the satellite’s trajectory in order to ensure a safety distance from the hazardous
object.
The surveillance process is fairly time consuming due to the inaccuracy of the
available data: it generally involves a ﬁrst-level of automatic surveillance which
detects potential risks, which must then be more closely analyzed by orbitography
experts. If the risk appears serious, trajectography measurements are requested
from available radar means (usually military means): these measurements provide
better knowledge of the hazardous object’s trajectory, assisting the operators in
taking the decision as to whether or not an avoidance maneuver must be
implemented. The entire prediction process spans several days (typically 3). It
should be pointed out that the avoidance maneuver changes the monitored satellite’s
trajectory, which generally means that its mission must be interrupted. In the
case of an observation satellite, this can be a major constraint. A maneuver will then
be needed to return the satellite to the nominal orbit before resuming its mission. All
of this requires signiﬁcant means: experts, controllers, radars, calculation means
and TM/TC stations, etc., and uses propellants, reducing the satellite’s lifetime by
as much. To reduce the impact of these maneuvers, a planned maneuver (such as
a position maintenance maneuver) can sometimes be anticipated, i.e., a maneuver
that would have been necessary in any event can be implemented ahead of schedule,
which limits the use of propellants.
To allow the risk of collision to be predicted more reliably, since 2011, the JspOC
has dispatched collision alerts in the form of Conjunction Summary Messages (CSM)
to operators: these messages are drafted using precise information and contain the
Table 39.1 Annual
probability of collision with
objects >10 cm and with
catalogued objects
Objects > 10 cm Catalogued objects
ENVISAT 0.015 0.0073
ERS2 0.0039 0.0021
39 The Issue of Space Debris 687
characteristics of the close pass and associated dispersions (covariance). With this
information, the operators are able to calculate the probability of collision.
This process is illustrated by the following ﬁgures from risk-of-collision surveillance
for 18 satellites carried out at CNES (French Space Agency) in 2010: the
automatic process identiﬁed 353 risks with a probability of collision higher than
10À4
. In addition, 92 alerts were received from JSpOC. After analyzing these cases,
21 requests for radar measurements or support to JSpOC were issued (probability
of collision higher than 10À3
), and in the end, 13 avoidance maneuvers were
carried out.
Risk-of-collision surveillance is not limited to close passes with “large”
catalogued objects whose trajectories are known. Because they are so numerous,
smaller objects represent a greater risk for satellites. Moreover, they are not
catalogued, which means they cannot be avoided. Shielding has been developed
to protect them, but due to the increased mass this implies, this solution is
reserved for several special spacecraft, such as the International Space Station.
Satellites are usually not shielded, but their walls provide a certain degree of
protection.
39.4.2.2 During the Launch Phase
During the launch phase and the ﬁrst orbits, the launcher’s last stage and the
satellites placed in orbit cross orbits used by other operators: this is especially
true of a geostationary transfer orbit with a perigee in a low orbit and an apogee at
an altitude of around 36,000 km. These newly injected objects will not be listed on
the catalogues for some hours (typically 48 h), which means that other space users
have no way of monitoring the risk of collision between these new objects and their
satellites. This is especially important for manned spacecraft (such as the ISS)
whose control center cannot monitor the risk from these objects.
The launch operator alone has information on the planned trajectory, and can
therefore predict the risks of collision. This prediction must take into account all
objects placed in orbit (launcher stages, satellites, structural components) for
a period of approximately 48 h. In the event of risk, postponing the launch time
by several seconds ensures a safety distance between objects. After this 48-h period,
it is considered that the new objects are catalogued, and that each operator can carry
out their own surveillance.
The main difﬁculty in predicting risk of collision during a launch lies in
considering the dispersions affecting the orbit parameters of various objects at
injection: propagating these dispersions over 48 h results in signiﬁcant amounts
of error around each body, and could close the launch slot completely if all
catalogued objects were considered in the analysis. This is why predicting the
risk of collision during the launch is generally limited to manned spacecraft and
certain satellites of particular interest.
39.4.2.3 On the Ground
As indicated in paragraph 2–1, when a spacecraft disintegrates on reentry, the
resulting fragments represent a risk on the ground. Controlled reentries allow the
688 F. Alby
fall-back area to be deﬁned, thereby avoiding risk for populations. However, most
reentries are uncontrolled, with no control over the fall-back area. For example, in
2011, of the 499 listed reentries, 25 were controlled and 474 were uncontrolled. The
latter category included 63 satellites and launcher stages, i.e., slightly more than
one uncontrolled reentry of a large object per week.
The debris fall-back area spans several hundred kilometers along the orbit path
and measures a few tens of kilometers in width (typical values: length
1,000–1,500 km, width 50–80 km).
In the case of a natural (random) reentry, it is impossible to predict the exact
location of the fall-back area, due to the lack of accurate information onseveral factors:
• The atmospheric density r and its variability below an altitude of 200 km
• The object’s attitude (i.e. its orientation with respect to the velocity vector)
which deﬁnes the drag surface S: the object can be rotating, or stabilized, or have
a variable orientation, inducing a possible lift effect
• The aerodynamic coefﬁcient CD
• The mass m
This makes it impossible to accurately estimate the main perturbation (atmospheric
drag), which is proportional to rCDS/m.
In terms of how precisely fall-back time can be predicted, the generally accepted
uncertainty margin is 10 % of the remaining time to fall-back. For example, 10 days
before fall-back, the uncertainty margin is Æ1 day (i.e., anywhere within the limits
of the inclination), and 10 h before, the uncertainty margin is Æ1 h.
This 10 % margin has been conﬁrmed by the atmospheric reentry exercises
organized each year by the Inter-Agency Space Debris Coordination Committee
(IADC), in which the orbits returned by each of the agencies are pooled, and the
fall-back predictions are compared.
Thus, to summarize, if the uncertainty in the fall-back time is taken to be 10 % of
the remaining time to fall-back, this gives the following dispersions on the position
of the exact impact area:
• 48 h prior Æ4.8 h or approximately Æ3 orbits
• 24 h prior Æ2.4 h or approximately Æ1.6 orbits
• 12 h prior Æ1.2 h or approximately Æ32,000 km
This uncertainty value of 10 % could be reduced to around 5 % if better
knowledge of the trajectory were available thanks to more measurements more
evenly distributed along the entire orbit. In the best-case scenario, several hours
before reentry, the uncertainty is around 30 or 40 min, corresponding to uncertainty
on the impact point of slightly less than one revolution. It should be noted, however,
that the debris cannot fall outside this 50- to 80-km-wide strip located beneath the
orbit path.
Within several hours to several days after reentry, the Space Track website
indicates the position of the observed passage point at 80 km, with an accuracy
value of Æ1 min (Æ500 km). Thus, after the fact, we can delineate an area of
approximately 2,000 Â 100 km which is liable to have been affected by the
fragments. This additional information may be useful should it be necessary to
determine liability.
39 The Issue of Space Debris 689
39.4.3 Stopping Production of Debris
The problem of space debris is mainly an issue in low orbit and geostationary orbit:
• Region A: Low Earth Orbit (LEO) is where the highest concentration of space
objects is found (density curve), especially at altitudes of 700–900 km. This is
the area in which two major events occurred, resulting in the creation of large
quantities of debris: (1) voluntary destruction of the Fengyun 1C satellite in
January 2007 and (2) the collision between the Iridium 33 satellite and Cosmos
2251 in February 2009.
• Region B: geostationary orbit is a very speciﬁc area (circular and equatorial
orbit, with a period equal to the period of the Earth’s rotation). In order to share
this unique resource between operators, the longitudes and frequencies need to
be managed. Moreover, due to the distance from Earth, the effect of atmospheric
drag is null, and debris created in this area will stay there and drift, crossing paths
with positions occupied by operational satellites.
39.4.3.1 Principle of Prevention Measures
The main space agencies represented within the IADC have identiﬁed two regions
in space to be protected (see Fig. 39.4):
• The Low Earth Orbit (LEO)-protected region is the volume that extends from the
Earth’s surface up to a spherical shell of 2,000 km altitude above the equator.
Fig. 39.4 Protected regions in space (Region A – Low Earth Orbit, Region B – Geostationary
Orbit)
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• The geosynchronous (GEO)-protected region is a segment of the spherical shell
deﬁned by the following: the altitude extent is bounded by the geostationary
altitude Æ200 km (35,786 Æ 200 km above the equator) and the latitude extent is
bounded by Æ15 deg (centered on the equator).
The prevention measures implemented by satellite or launcher operators can be
broken down into three categories:
• No longer intentionally releasing objects in space (mission-related objects): e.g.,
covers which protect optical systems during launch, various other covers,
springs and straps, etc., which used to be released after injection into orbit.
This category also includes the alumina particles ejected by solid propellant
motors, used for transfers into geostationary orbit, for example. In particular,
these motors can release slag measuring several centimeters at the end of
combustion: ejected at a low speed, this debris stays close to the orbit being
used. Pyrotechnical cutting systems are also a potential source of debris when
they are activated: using “clean” systems, which trap the debris produced, should
reduce the creation of this type of debris.
• Reducing the risks of explosion in orbit: this means avoiding the accidental or
voluntary explosion of spacecraft during their in-orbit lifetime. This period
covers not only the operational mission phase, but also their post-mission life
in orbit after the withdrawal from service phase. More than 220 fragmentations
in space have been listed thus far, representing the primary source of debris.
Many of these fragmentations had to do with the propulsion system or batteries.
After mission termination, the object is “passivated” and then abandoned in
space. To passivate means to make the object inert in order to eliminate the risk
of subsequent explosion due to an internal cause (such as accidental mixing of
propellants or battery overcharge) or external cause (such as a debris or meteorite
impact against a pressurized tank). The passivation process consists in
emptying all propellants remaining on board, lowering the pressure in all tanks
(e.g., pressurization gas), and discharging and isolating the batteries to prevent
accidental recharging.
• Managing the end-of-life orbit: the goal is not to leave objects in space for “too
long,” due to the risks of collision and debris production. For satellites in low
orbit, the best solution is to conduct a controlled reentry, which immediately
frees up the orbit and minimizes the risks on the ground, by having the debris fall
into an ocean. In practice, this operation requires a large amount of propellants,
which increases rapidly with altitude. Table 39.2 indicates the amplitude of the
maneuver and the amount of propellant required to go from a circular orbit at
a given altitude to a reentry orbit with perigee 0 in the case of a 2-t satellite and
a speciﬁc pulse of 290 s:
Table 39.2 De-orbiting maneuver and quantity of propellant required according to altitude
Altitude (km) 300 400 500 600 700 800 900 1,000
DV (m/s) 89 117 145 172 198 223 248 272
Propellant mass 62 81 99 117 134 151 167 182
39 The Issue of Space Debris 691
When controlled reentry is not possible, the accepted practice is, insofar as
possible, to limit the time objects spend in orbit. The current maximum
recommended period is 25 years. At the end of the operational mission, operators
must maintain the ability to maneuver, in order to move the object into a loweraltitude
orbit so that the wear from atmospheric drag will cause it to fall out of orbit
within 25 years. Another solution consists in transferring objects above the
protected region, i.e., to an altitude of over 2,000 km. Reentry into the atmosphere
is no longer an option for satellites in geostationary orbit, due to the quantity of
propellants this would require. Thus, this solution consists in freeing up the useful
orbit by transferring objects to a “graveyard” orbit 200 km above the protected
region. Once they have been transferred, the objects must then be passivated.
39.4.3.2 International Cooperation
End-of-life operations are complex and represent a considerable workload for
operators. Some of the main difﬁculties involved are:
• The difﬁculty of accurately estimating the quantity of propellants remaining in
the tanks on board: given the uncertainty associated with the different estimation
methods, greater margins are taken to ensure that the end-of-life operations can
be carried out, which reduces the mission duration accordingly.
• The need to maintain control of the satellite, especially during passivation: risk
of degrading the orbit attained or losing the attitude when the tanks are emptied.
• The difﬁculty of deciding to stop the mission of a satellite which is still operating
correctly: the operator may tend to prolong the mission a bit, at the risk of not
being able to carry out the end-of-life operations.
• The fact that these operations need to be taken into account right from the
satellite or launcher design phase (necessary systems), which leaves open the
question of spacecraft already in orbit.
Implementing these measures represents additional costs for operators: reducing
operational life, cost of operations, additional systems to be included in spacecraft
designs, deoptimizing launcher trajectories, etc.
Operators are of course willing to implement the prevention measures . . . on the
condition that their competitors be subject to the same requirements. Thus, the
challenge is to reach a general consensus so that all actors are applying the same
rules. There have been discussions on this subject at various levels:
• The United Nations provides the natural framework for discussions between
States. The Scientiﬁc and Technical Subcommittee (STSC) of the Committee on
Peaceful Uses of Outer Space (COPUOS) has addressed the issue of space
debris. Work in this area was completed in June 2007, with the publication of
the UN-COPUOS Space Debris Mitigation Guidelines (reference A/AC.105/
C.1/L.284) which sets out seven high-level guidelines to apply in space. This
document was then ratiﬁed by the United Nations General Assembly on 10 June
2008 (A/RES/62/217). In 2010, the STSC recalled the importance of ensuring
the safe and sustainable future use of outer space and decided to establish
a dedicated working group. The working group will prepare a report on the
long-term sustainability of outer space activities containing a consolidated set of
692 F. Alby
current practices and operating procedures, technical standards and policies
associated with the long-term sustainability of outer space activities. On the
basis of all the information collected, the working group will produce guidelines,
which could be applied on a voluntary basis by States, either individually or
collectively; international organizations; national nongovernmental organizations,
and private sector entities to reduce the risks to space activities for all
participants and to ensure that all countries have equitable access to outer space
(United Nations 2011). The report should be submitted to the COPUOS in 2014.
• More technical discussions are undertaken by space agencies within the
framework of the IADC (Inter-Agency Space Debris Coordination Committee),
made up of the following 12 agencies: ASI (Agenzia Spaziale Italiana), CNES
(Centre National d’Etudes Spatiales), CNSA (China National Space Administration),
CSA (Canadian Space Agency), DLR (German Aerospace Center),
ESA (European Space Agency), ISRO (Indian Space Research Organisation),
JAXA (Japan Aerospace Exploration Agency), NASA (National Aeronautics
and Space Administration), NSAU (National Space Agency of Ukraine),
ROSCOSMOS (Russian Federal Space Agency) and UKSpace (UK Space
Agency). In 2003, IADC published the IADC Space Debris Mitigation Guidelines,
a document which describes the prevention measures in detail. This
document deﬁnes the protected regions in space, e.g., and explains that 25
years is the recommended maximum period for objects to stay in low orbit
after the end of their operational life.
• Finally, for the practical application of these recommendations by manufacturers
and operators, norms and standards will have to be developed for use in drawing
up contracts. ISO completed an important task in drafting Standard 24113
(Space Systems-Space Debris Mitigation) published in 2010, which contains
all of the rules relating to space debris. This document is based on a series of
implementing standards describing how to apply them and proposing veriﬁcation
solutions and methods. A third level of documents, the technical notes,
completes the body of work with substantiating information.
Other initiatives are also ongoing such as the International Code of Conduct for
Outer Space Activities proposed by the European Union. The project was launched
in 2008 as a means to achieve enhanced safety and security in outer space through
the development and implementation of transparency and conﬁdence-building
measures. The proposed Code would be applicable to all outer space activities
conducted by States or nongovernmental entities, and would lay down the basic
rules to be observed by space faring nations in both civil and defense space
activities. Discussions open to the participation of all UN Member States, will
begin in 2012 with a view to adopt the Code in 2013.
39.4.3.3 Application Mechanisms
The IADC recommendations were ﬁrst adopted in the regulatory documents of
space agencies, e.g., NASA, JAXA, and CNES standards, and the European Code
of Conduct for Space Debris Mitigation (ASI, BNSC, CNES, DLR, and ESA).
These documents applied to these agencies’ projects, but not to the activities of
39 The Issue of Space Debris 693
private manufacturers and operators. Thus, the private sector was free of any
obligation. However, the measures were generally applied by “responsible”
operators.
This regulatory gap was mentioned in resolution 62/217 of the General Assembly
of the United Nations dated 1 February 2008 (document A/RES/62/217), which
approves the COPUOS Space Debris Mitigation Guidelines and states the need for
a legal framework for States which are liable for the activity of their nationals:
“. . .and invites Member States to implement these guidelines through relevant
national mechanisms.”
This need had already been stated in 1967 in the Space Treaty (Treaty on
Principles Governing the Activities of States in the Exploration and Use of Outer
Space, Including the Moon and Other Celestial Bodies). Article VI stipulates that
“the activities of non governmental entities in outer space, including the Moon and
other celestial bodies, shall require authorization and continuing supervision by the
appropriate State Party to the Treaty.”
Thus, the States are liable in the event of damage caused on the ground or in
space as a result of their nationals’ activity. In response to this situation, States are
gradually creating legal instruments allowing them to monitor the space activities
for which they may be held liable. The United States, e.g., has set up a licensing
system managed by three organizations: the FAA (Federal Aviation Administration)
for launch operations, the NOAA (National Oceanic and Atmospheric Administration)
for Earth observation satellites, and the FCC (Federal Communications
Commission) for radiocommunications satellites. The United Kingdom has also set
up a similar licensing system. In France, the Parliament passed the Space Operations
Act (Loi sur les Ope´rations Spatiales) in June 2008, which came into effect on
10 December 2010. Other countries are taking similar initiatives and setting up
equivalent systems.
The goal of these legal instruments is to set up a national authorization and
monitoring system for space activities carried out under the State’s jurisdiction, or
for which the State is internationally liable under the United Nations
Treaty. These instruments apply to satellite and launch operators, and their
purpose is to ensure personal safety and public health and to protect property
and the environment on Earth, in the atmosphere and in orbit. In this regard,
requirements relating to the safety (risks on the ground) and prevention of space
debris play a key role.
Although quite different in form, these various texts are basically equivalent in
their content, and particularly with regard to debris prevention, they comply with
the IADC Mitigation Guidelines and ISO standard 24113.
Nevertheless, the situation is far from perfect: the satellites and launchers in
operation today were designed before these regulatory texts were published. Thus,
it is not always possible to apply some of these rules, such as passivating used
helium tanks to pressurize propellant tanks. For this reason, the texts generally
provide for a transitional period during which operators must show that they have
made their best efforts considering the existing design. Down the road, another
problem may arise if all countries, without exception, do not implement equivalent
694 F. Alby
systems. To evade the requirements, operators could decide to base themselves in
countries without regulations or with much less stringent regulations. This would
create a situation of unfair competition, much like that existing in maritime law
with ﬂags of convenience.
39.4.4 Removing Old Debris
Prevention measures will be effective in signiﬁcantly reducing or eliminating the
creation of new debris, but this will not solve the problem of “old” debris already in
orbit. Various studies are being carried out to estimate long-term debris population
growth. The models developed by various agencies are based on variety of assumptions
on the number of future launches, mission types, satellite size and lifetime,
how thoroughly prevention measures are applied, the number of accidental explosions,
etc. All of these models show the situation continuing to worsen.
Indeed, collisions between objects will produce new debris, which will in turn
create new collisions, and so on. This chain reaction or cascading effect (also
known as the Kessler syndrome, after Don Kessler, the NASA writer who revealed
this phenomenon in 1978) will mainly occur in the region between 700 and
1,000 km of altitude, where debris is densest.
Figure 39.5 (Liou 2001) shows the progression over time in the number of
catalogued objects, for three assumptions:
20000
Reg Launches + 90% PMD
Reg Launches + 90% PMD + ADR2020/02
Reg Launches + 90% PMD + ADR2020/05
LEO Environment Projection (averages of 100 LEGEND MC runs)
18000
16000
14000
12000
10000
8000
6000
EffectiveNumberofObjects(>10cm)
4000
2000
0
1950 1970 1990 2010 2030 2050 2070
Year
2090 2110 2130 2150 2170 2190 2210
Fig. 39.5 Simulated LEO population growth as a function of time
39 The Issue of Space Debris 695
• No active debris removal
• Active debris removal starts in 2020 and two objects are removed each year
(ADR 2020/02)
• Active debris removal starts in 2020 and ﬁve objects are removed each year
(ADR 2020/05)
These three simulations are based on the assumptions that (1) the number of
launches will continue at the same rate as over the past 8 years and (2) post-mission
disposal (PMD) measures will be effectively applied with a 90 % success rate.
If the assumptions for long-term growth prove to be true (i.e., inexorable
population growth even with full application of the prevention measures), cleanup
in space will be necessary: abandoned objects will have to be removed. The largest
objects, which are potential sources of more debris in the event of collisions, will
have to be removed ﬁrst. The studies available at present show that 5–10 large
objects per year would have to be removed to stabilize the debris population.
Various solutions, some more exotic than others, have been proposed by many
different authors, but certain technical difﬁculties still need to be resolved:
• Approaching and capturing a noncooperative spacecraft, which is likely to
feature a complex rotation movement: the capturing solutions involve systems
of nets, harpoons, claws or robotic arms, etc.
• Attaching a de-orbiting system: solid propellant kit, electrodynamic cable,
inﬂatable surface, sail, etc.
No-contact solutions have also been proposed: e.g., a laser (on the ground or on
board) to reduce objects’ speed, electrostatic attraction between the chaser and the
target, “blowing” the target via an electric propulsion system installed on the
chaser, etc.
The technical feasibility of these solutions has yet to be demonstrated. Once
more in-depth studies have been carried out, one or several designs will have to
be selected, and missions demonstrating the critical technologies will have to be
carried out before the ﬁrst operational mission can be planned.
In any case, the cleanup spacecraft will have to be able to move between orbits in
order to reach different debris, which will be fairly complex.
Aside from these technical challenges, other difﬁculties of political, legal, and
economical nature will also have to be considered:
• Political difﬁculties: active debris removal operations could be used as a cover for
military activities. This conﬁrms the need for international agreement and transparency
between the various actors. Also, certain countries may feel singled out or
reproached when it comes to cleaning up the objects they have abandoned in space.
• Legal difﬁculties: there is currently no international consensus on the deﬁnition
of the term “debris”: according to the United Nations Treaties, objects in space
forever remain the property of their launching State, so prior authorization is
needed before touching an object belonging to another State.
• Economic difﬁculties: these cleanup missions will probably be fairly costly.
Who will pay for them, and in what form? States will probably also ask
themselves why they should remove their objects from space when other States
do not do the same, or worse, fail even to apply the prevention measures.
696 F. Alby
39.5 Conclusion
The space debris issue is a growing concern for all space-faring nations: the
increasing population of objects orbiting the Earth represents a collision risk to
operational satellite and also a risk on the ground in case of fragments surviving the
reentry.
Ongoing actions aim at knowing the situation (observations, modeling) better
and at protecting satellites through shielding and collision avoidance. In parallel,
important actions are necessary to reduce the production of new debris through the
implementation of mitigation measures such as the disposal of satellites and upper
stages at the end of their operational life.
However, the problem being global, the solutions shall be agreed by all.
An international consensus is therefore necessary:
• In the short term, the same rules shall be applied by all space actors.
Mitigation measures have already been deﬁned and approved at the international
level. National regulation systems shall be now implemented by each
country to ensure their immediate application by all operators.
• In the middle term the space community has to conﬁrm the future instability
of the space environment, even if the mitigation measures are fully applied,
and to conﬁrm the need to remove from orbit several objects per year.
• In the longer term active debris removal missions will require an increased
international cooperation due to complex technical, economical, legal, and
political issues.
References
Klinkrad H (2006) Space Debris: models and risk analysis. Springer, Berlin
Liou JC (2001) An active debris removal parametric study for LEO environment remediation.
Adv Space Res 47:1865–1876
Rathgeber W, Schrogl KU, Williamson RA (2010) The fair and responsible use of space, an
international perspective, vol 4, Studies in space policy. Springer (European Space Policy
Institute), New York
United Nations (2011) General Assembly, terms of reference and methods of work of the working
group on the long-term sustainability of outer space activities of the scientiﬁc and technical
subcommittee, A/AC.105/L.281/Add.4 Annex II
39 The Issue of Space Debris 697