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Astropolitics
ISSN: 1477-7622 (Print) 1557-2943 (Online) Journal homepage: http://www.tandfonline.com/loi/fast20
Limits on the Space Launch Market Related to
Stratospheric Ozone Depletion
Martin Ross , Darin Toohey , Manfred Peinemann & Patrick Ross
To cite this article: Martin Ross , Darin Toohey , Manfred Peinemann & Patrick Ross (2009)
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion, Astropolitics,
7:1, 50-82
To link to this article: http://dx.doi.org/10.1080/14777620902768867
Published online: 11 Mar 2009.
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Limits on the Space Launch Market Related
to Stratospheric Ozone Depletion
MARTIN ROSS
The Aerospace Corporation, Los Angeles
DARIN TOOHEY
University of Colorado
MANFRED PEINEMANN
The Aerospace Corporation, Los Angeles
PATRICK ROSS
Embry-Riddle Aeronautical University
Solid rocket motors (SRMs) and liquid rocket engines (LREs) deplete
the global ozone layer in various capacities. We estimate global
ozone depletion from rockets as a function of payload launch rate
and relative mix of SRM and LRE rocket emissions. Currently, glo-
bal rocket launches deplete the ozone layer 0.03%, an insignifi-
cant fraction of the depletion caused by other ozone depletion
substances (ODSs). As the space industry grows and ODSs fade
from the stratosphere, ozone depletion from rockets could become
significant. This raises the possibility of regulation of space launch
systems in the name of ozone protection. Large uncertainties in our
understanding of ozone loss caused by rocket engines leave
open the possibility that launch systems might be limited to as little
as several tens of kilotons per year, comparable to the launch
requirements of proposed space systems such as spaceplanes, space
solar power, and space reflectors to mitigate climate change. The
potential for limitations on launch systems due to idiosyncratic
regulation to protect the ozone layer present a risk to space
industrial development. The risk is particularly acute with regard
to the economic rationale to develop low-cost, high flight rate
launch systems.
Address correspondence to Martin Ross, The Aerospace Corporation, M1-132, PO Box
92957, Los Angeles, CA 90009. E-mail: Martin.N.Ross@aero.org
Astropolitics, 7:50–82, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1477-7622 print
DOI: 10.1080/14777620902768867
50
Astropolitics 2009.7:50-82.
Combustion emissions from rocket launches change the composition
of the atmosphere. The changes can be divided into transient changes
near the launch site that affect air quality in the lowermost troposphere
and long-term global changes in the composition of the stratosphere. In
this paper, we are concerned with the long-term impact of rocket emis-
sions on the global ozone layer. Ozone depletion has been a critical con-
cern of nations across the globe for many decades, and large-scale
industrial processes that alter stratospheric composition are assessed with
respect to the amount of ozone depletion they would cause. When an
assessment suggests unacceptably large ozone loss for a particular process,
regulatory actions to limit or modify that process might be enacted to pro-
tect the ozone layer.
1
In this paper, we consider rocket combustion emis-
sions in the context of ozone layer protection over the next several
decades. Our calculations are not a formal assessment, but are a prelimin-
ary evaluation to identify the main areas of concern for the space industry.
These concerns include risks associated with overly conservative regula-
tion and a suggestion for new research in order to reduce the likelihood
of such regulation.
Cicerone and Stedman
2
first considered rocket emissions as a source of
ozone depletion. Subsequent studies have shown consistently that at current
launch rates, ozone depletion from rocket exhaust is insignificant compared
to other sources of ozone loss.
3
If launch rates and ozone depletion from
other sources remain at current levels, this assessment will not change.
The potential exists that the demand for launch services could increase
significantly in the future.
4
Large (factors of ten or more) increases in launch
demand could come about for a variety of reasons, including national deci-
sions regarding security, enhanced space exploration, market forces asso-
ciated with significant reductions in launch costs, or the emergence of new
markets such as space tourism, manufacturing, or solar power. Analysts gen-
erally assume that if the cost of access to orbit is reduced sufficiently, then
large, new markets will emerge for space industry and the launch market.
This development would be considered revolutionary, and it is not clear
when or if, this might occur. Nevertheless, if space transport follows the ‘‘nor-
mal’’ development path of transportation technology enters a period of con-
tinual expansion, it would be necessary to reconsider the environmental
consequences of large rockets, launched often. In this paper, we consider
the implication of such significant increase in demand for orbital launches
on the global ozone layer.
We do not consider greenhouse gas emissions from rockets. Climate
change is to some extent a separable problem from ozone depletion. While
rocket engines emit gases identified as contributing to climate change, the
amount emitted globally is trivial compared to other sources and is likely
to remain so. Annual CO
2
emissions from rockets, for example, are about
several kilotons (kt) compared to emissions of several hundred kt from
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 51
Astropolitics 2009.7:50-82.
aircraft which, in turn, is only a few percent from all CO
2
sources.
5
Space
launch emissions, even for the large growth scenarios discussed here, will
not likely be significant in future greenhouse gas regulatory schemes. As a
cautionary tale, we point out that even though aircraft are responsible for
a few percent of all CO
2
emissions, the airline industry must contend with
considerable attention and likely regulation or carbon taxation.
6
The
message to the space industry should be clear: policy and media attention
on high visibility propulsion emissions are often framed in ways that overem-
phasize the relative contribution.
7
If rockets are a minuscule contributor to the problem of climate change,
they do have a significant potential to become a significant contributor to the
problem of stratospheric ozone depletion. This follows from three unique
characteristics of rocket emissions:
1. Rocket combustion products are the only human-produced source of
ozone-destroying compounds injected directly into the middle and upper
stratosphere. The stratosphere is relatively isolated from the troposphere
so that emissions from individual launches accumulate in the strato-
sphere.
8
Ozone loss caused by rockets should be considered as the cumu-
lative effect of several years of all launches, from all space organizations
across the planet.
2. Stratospheric ozone levels are controlled by catalytic chemical reactions
driven by only trace amounts of reactive gases and particles.
9
Strato-
spheric concentrations of these reactive compounds are typically about
one-thousandth that of ozone. Deposition of relatively small absolute
amounts of these reactive compounds can significantly modify ozone
levels.
3. Rocket engines are known to emit many of the reactive gases and
particles that drive ozone destroying catalytic reactions.
10
This is true for
all propellant types. Even water vapor emissions, widely considered inert,
contribute to ozone depletion. Rocket engines cause more or less ozone
loss according to propellant type, but every type of rocket engine causes
some loss; no rocket engine is perfectly ‘‘green’’ in this sense.
Since 1987, the ozone layer has been protected by international agreements
11
that limit the production and use of substances that have been determined to
cause ozone depletion. The Montreal Protocol on Substances That Deplete
the Ozone Layer (and subsequent amendments), regulates the worldwide
production and use of ozone depleting substances (ODSs), including the
well-known chlorofluorocarbons (CFCs) and other halogen gases. The Mon-
treal Protocol is widely considered a significant success and the global phase
out of ODSs mandated by the Protocol is expected to allow the ozone layer
to recover to pre-ODS levels by about 2040. In support of the Montreal Pro-
tocol, the stratospheric science community issues a quadrennial summary,
52 M. Ross et al.
Astropolitics 2009.7:50-82.
the Scientific Assessment of Ozone Depletion,
12
describing the state of
knowledge of stratospheric composition, the factors causing ozone deple-
tion, and projections of the future ozone layer. The Quadrennial Ozone
Assessments have occasionally addressed ozone depletion caused by rocket
emissions and have determined that the current loss is ‘‘small’’ in comparison
to other sources of ozone loss so that rocket emissions are not a part of the
regulatory framework that protects the ozone layer. Later we discuss the
threshold level of ozone loss that might be considered ‘‘not small,’’ as well
as the level that might be considered ‘‘too large.’’
The recovery of the ozone layer,
13
while a favorable development, is
motivation to be concerned about ozone depletion caused by rocket emis-
sions over the next several decades. The eventual elimination of the major
sources of ozone loss (that is, ODSs) raises the question: Will sources of
ozone loss currently considered small, such as rocket emissions, eventually
be scrutinized more closely by the stratospheric protection community?
14
This would particularly apply to rocket emissions if demand for launch ser-
vices increases in coming decades, just as other sources of ozone loss
decrease due to the success of the Montreal Protocol. In addition, revisions
in our understanding of emissions, stratospheric processes, or the introduc-
tion of new propellants on a large scale (hybrid rockets, for example) may
cause changes in the estimated ozone loss for a given launch rate.
In this paper, we examine the problem of rockets and ozone depletion
from several new points of view. For the first time, we consider the pro-
blem in a long-term context that includes significant, sustainable,
15
growth
in the space industry and evolving regulatory actions associated with the
recovery of the ozone layer from past pollution. We apply the first plausi-
ble estimates of ozone loss caused by liquid propellant rockets, which will
certainly play the major role in a significant expansion of the launch indus-
try. We develop a parameterization of the steady-state global ozone loss
caused by solid and liquid propellant rocket emissions and relate the ozone
loss to amount of payload delivered to Low Earth Orbit (LEO). The model
is limited by uncertainties in the actual composition of rocket emissions,
and the stratospheric processes that bring about the ozone loss; and it
can only be used to draw conclusions of an order of magnitude. Neverthe-
less, the model is useful to examine long-term trends and investigate rocket
emission ozone loss within the context of scenarios of large increases in
launch rates, the recovery of the ozone layer, and conceptual analysis of
the econometric effect of caps on rocket emissions that might be enacted
to protect the ozone layer. We draw a number of conclusions for the global
launch industry that have implications for policymakers for both strategic
space transport planning and future protection of the ozone layer. Finally,
we also identify the most important areas where new research could reduce
the model uncertainties and so increase the ability to reliably plan for
future space systems development.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 53
Astropolitics 2009.7:50-82.
STRATOSPHERIC OZONE AND REGULATORY PROTECTION
Ozone Chemistry
A detailed account of stratospheric chemistry is beyond the scope of this
paper however, a few critical concepts need to be explained in order to
justify our parameterization of rocket ozone loss. The stratospheric ozone
(O
3
) layer generally resides between 20–30 km altitudes, absorbing harmful
solar ultraviolet radiation before it reaches the Earth’s surface. Chemical and
dynamical processes that are well understood determine the vertical and
horizontal distributions of stratospheric ozone. The ozone layer results from
a long-term balance between the vertical profile of ozone production, the
vertical profile of ozone destruction, and the global circulation of strato-
spheric air.
The ozone destruction side of the balance is dominated by reactive trace
gases known as radicals. The highly reactive radicals—oxides of nitrogen,
hydrogen, bromine, and chlorine referred to as NOx, HOx, BrOx, and
ClOx—control global ozone levels by tilting the long-term balance between
ozone production and destruction in favor of the latter. Moreover, because
the radical reactions are catalytic, only trace amounts, a few parts per billion,
are able to control much greater amounts of stratospheric ozone. A single
radical molecule emitted into the stratosphere, for example, can destroy up
to 10
5
ozone molecules before being deactivated and transported out of
the stratosphere. Radicals react with ozone on very short time scales, minutes
to hours, so that direct injection into the stratosphere over a limited area (a
rocket plume, for example) will cause a prompt, localized, ozone ‘‘hole.’’
16
Particles also play an important role in ozone destruction. Chemical
reactions particle surfaces activate radicals from their reservoirs, shifting
the balance toward lower ozone levels. The strong potential for particles
to reduce ozone is demonstrated in the springtime south polar stratosphere,
where photochemical reactions on ice particles efficiently liberate ClO
x
from
reservoirs
17
and so play a role in the formation of the infamous ‘‘Ozone
Hole.’’ Such reactions are known occur on the surface of alumina and, pos-
sibly, soot particles.
18
Particles with diameter less than about 1 micron (mm)
remain suspended in the stratosphere for several years
19
and become mixed
globally by the stratospheric circulation. This means that repeated injections
of submicron particles into the stratosphere, as from global (weekly) rocket
launches for example, result in an accumulation of particles. The accumu-
lated particle surfaces increase the rates that radicals ‘‘leak’’ from their reser-
voirs and so reduce ozone levels globally.
NOx, HOx, BrOx, and ClOx radicals are produced from source gases
and reservoir gases. The sources and reservoirs can be thought of as a sort
of chemical storage for the radicals, which leak photochemically from
storage into the stratosphere, increasing the rate of ozone destruction. The
54 M. Ross et al.
Astropolitics 2009.7:50-82.
concentrations of the sources and particularly reservoirs are determined by
a steady state between fluxes across the tropopause, production from
radical-radical reactions, loss from photolysis and radical-reservoir reactions,
and direct injection from rocket engine emissions. H
2
O, emitted by all rocket
engines, is one of the most critical source gases.
20
H
2
O is the source gas for
HOx radicals but also contributes to the formation of the ice particles that
cause the polar ozone hole. Small changes in middle atmosphere water vapor
and temperature can cause large changes in stratospheric cloudiness. Ozone
loss from water vapor emissions is highly nonlinear and difficult to predict.
As a foundation for further discussion, we proceed with the understand-
ing that all types of rocket engines, solid rocket motors (SRMs) and liquid
rocket engines (LREs), emit compounds that are known to reduce ozone
to various degrees, depending on their various compositions. Rockets
engines inject all of the types of compounds mentioned above associated
with ozone loss—radicals, their sources and reservoirs, and reactive
particles—throughout all levels of the stratosphere. They are the only ozone
destroying, human-produced, compounds that are emitted into the strato-
sphere this way.
OZONE MEASUREMENT AND PROTECTION
Long-term global trends in global stratospheric ozone, the interest of this paper,
are often considered using the quantity AAGTO, the averaged global total
ozone. The AAGTO averages out spatial and temporal variability of strato-
spheric ozone into a single quantity to represent the total quantity of ozone
in the stratosphere. The AAGTO, informally known as ‘‘global total ozone,’’
is often the metric reported by models to compare predictions of the response
of the stratosphere to different emission scenarios. Stratospheric models gener-
ally agree that the present day AAGTO (global total ozone) is about 4% smaller
than the AAGTO in the era before significant ODS use, usually taken as 1980.
For this paper, we identify the term ‘‘global ozone loss’’ with a decrease
in predicted AAGTO (or equivalently, the decrease in global total ozone)
associated with a constant annual rate of rocket emissions. We further equate
the term ‘‘global ozone loss’’ with the commonly used term ‘‘ozone deple-
tion’’ in that it refers to the overall decrease in the total quantity of ozone
in the stratosphere. We use DO
3
to represent the percentage global ozone
loss, or ozone depletion, caused by human-produce compounds. As a matter
of convenience, DO
3
is numerically positive but refers to a decrease in the
total number of ozone molecules in the stratosphere. Because of the global
phase-out of ODSs mandated by Montreal Protocol regulations, the ozone
layer is expected to recover to pre-ODS levels by about 2040. Figure 1 shows
how DO
3
is expected to approach zero as the ozone layer recovers.
21
The
Montreal Protocol is widely seen as a great success; DO
3
would have reached
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 55
Astropolitics 2009.7:50-82.
40% by 2010, with widespread and serious negative effects from increased
solar ultraviolet radiation, without the regulation.
The formalism for determining which gases must be phased-out is
highly developed: widely accepted computer models of stratospheric com-
position and circulation are used to predict DO
3
per unit mass of a gas
released at the earth’s surface, and this result is used to determine the sub-
stance’s Ozone Depletion Potential (ODP), its ability to cause steady-state
global ozone loss compared to a well-understood reference gas (CFC-11).
If there is widespread agreement among the various computer simulations
of ozone response to a chemical perturbation that the ODP of a particular
compound exceeds a critical value (0.2 or higher), then it is scheduled for
reduction and elimination by the stratospheric protection community. The
models used to calculate ODPs—and so identify compounds for phase-
out—are updated routinely by the scientific and regulatory communities.
Innovations have included using a time dependant ODP or evaluating the
increase in effective equivalent chlorine (EECl) burden caused by a sub-
stance rather than simply calculating the steady state ODP.
22
For most gases
released at the earth’s surface, the regulatory process is fairly well defined
and has been operating successfully for 25 years.
The Montreal Protocol formalism is not entirely free of ambiguity and
controversy. Models do not always agree on the predicted ODP and so it
is not always clear if the ODP of a particular substance exceeds the 0.2 trigger
for phase-out. In addition, because Protocol signatories can request critical
use exemptions (CUEs) and delay phase-out schedules, some compounds
with an ODP unambiguously greater than 0.2 can nevertheless see continued
use, delaying the final phase-out of an ODS. The case of methyl bromide
(CH
3
Br) is relevant and instructive.
23
An agricultural fumigant in widespread
FIGURE 1 Anticipated recovery of global ozone assuming that the schedule of ODS phase-
out required by the Montreal Protocol is carried out as currently planned. The shaded region
represents the uncertainty over the details of the ozone layer recovery.
56 M. Ross et al.
Astropolitics 2009.7:50-82.
use, methyl bromide is the major source gas for the BrOx radical and has an
ODP between 0.4–0.8. Accordingly, the gas was scheduled for global phase-
out. Various considerations have caused some Protocol signatories to request
continued use of methyl bromide, under the CUE process. One of the argu-
ments for continued use is that despite its large ODP, the global ozone loss
DO
3
caused by methyl bromide is ‘‘small,’’ compared to other causes of
ozone depletion. This line of argument, if accepted, might be seen set a pre-
cedent for an alternate regulatory scheme for ozone protection wherein, for
some industrial compounds, the ODP value is less important than DO
3
. The
methyl bromide case suggests that 0.2% might reasonably be adopted as the
upper limit of acceptable DO
3
, regardless of a compound’s ODP, if the com-
pound in question has unique and significant economic value.
ASSESSMENT OF ROCKET PROPULSION EMISSIONS
The Montreal Protocol assessment process is even less certain for
compounds that are known to affect the ozone layer but do not fit into the
ODP formalism, which is limited to gases released at the Earth’s surface.
24
For compounds injected directly into the stratosphere, such as aircraft and
rocket emissions, assessments are done with models that predict DO
3
for a
range of ‘‘emissions scenarios.’’ Scenarios are model inputs that follow from
specification of the composition, altitude and latitude distribution, and abso-
lute quantity (e.g., fleet flight rate) of the emission. The predicted fleet DO
3
is
evaluated relative to the predicted ozone loss from other industrial sources.
This process is not formally defined and the ambiguity invariably leads to
subjective and vague assessments such as ‘‘small’’ or ‘‘large,’’ ‘‘insignificant’’
or ‘‘significant,’’ and ‘‘major’’ and ‘‘minor.’’ As we have noted for methyl bro-
mide, formal regulatory definitions of these terms have not been determined.
The level of global ozone loss DO
3
that would be considered significant
enough to require phase-out or use limitation, that is to say an international
benchmark that would invoke regulatory action, has not been specified.
Assessments of aerospace combustion emissions are complicated further
by uncertainty over who would have regulatory authority, the Montreal
Protocol or the International Civil Aviation Organization (ICAO), which
recommends environmental standards for aircraft emissions.
Subjective assessments have advantages and disadvantages for the
regulatory process, especially for poorly understood emissions without
feasible alternatives. The indeterminacy of subjective assessments provides
flexibility, an important advantage for successful regulation. A disadvantage
of subjective assessments is that the associated uncertainties, under the rubric
of ozone protection, can lead to distortions of design or investment
decisions. This could possibly even include exploitation of the uncertainty
by proponents of one system in competition with another.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 57
Astropolitics 2009.7:50-82.
The case of supersonic transport (SST) aircraft emissions
25
is illustrative
as a cautionary tale regarding lack of knowledge, exploitation of the uncer-
tainties, and the resulting idiosyncratic behavior of the policy apparatus.
Early research suggested that SSTs could cause a large global ozone loss
and this claim was used by various factions to argue against full-scale SST
development. Decades after the SST was cancelled, knowledge of turbine
emissions and stratospheric processes improved to show how SST ozone loss
concerns could be successfully mitigated with technology (low emission
engines) and operations (cruise altitude). However, the lay perception has
remained that the SST was cancelled because of ‘‘environmental’’ issues.
(As an aside, global emissions from subsonic aircraft are predicted to cause
a small increase in global total ozone and so are not of special concern from
an ozone protection standpoint.)
The point here is that the SST debate demonstrated how the reality of a
public policy issue often equals the public perception. If the uncertainty in
some environmental effect is only potentially large or the associated policy
ambiguous, then the effect becomes a policy problem. Space launch systems
represent a situation similar to that of SSTs with respect to ozone layer
protection.
26
Ozone loss from rocket emissions could be large in some sce-
narios and rockets are not included in the present assessment and regulatory
regime. Formal metrics such as ODP and EECl are not meaningful for rockets.
These metrics do not account for direct emission into the stratosphere,
particles, or gases other than chlorine or bromine compounds. Thus, as for
SSTs in the past, computer models are used to predict the change in global
total ozone for a particular rocket fleet scenario and are informally evaluated
in a qualitative and subjective sense.
Compared to SST emissions however, the level of research applied to
predict ozone loss from rocket emissions has been minuscule and out of
balance with reality. Consider that the global space launch industry is well
established and likely to grow while the SST industry is nil and unlikely to
grow at all. The 2002 WMO Ozone Assessment briefly mentions (1) that
SRM emissions at current launch rates likely have a ‘‘small’’ impact on ozone
and (2) that LRE emissions may have some additional impact that is largely
unknown. The wording of the 2002 Assessment implies that 0.1% can be
taken as the threshold for ‘‘small’’ ozone loss, even less than the 0.2% implied
by some arguments in the ongoing methyl bromide discussion.
In this work, we are interested in quantitatively evaluating different sce-
narios of launch activity so for the purpose of discussion we need to define
the threshold that differentiates between ‘‘small’’ or ‘‘large’’ ozone loss, even
if there is currently no such designation formally. We consider 1% an unam-
biguous upper limit that defines unacceptably large DO
3
. The upper limit
could be smaller however. Following the methyl bromide arguments,
the upper limit on DO
3
might be as low as 0.2%. We also note, for some
environmental concerns the ICAO standard is ‘‘no worse than Concorde’’
58 M. Ross et al.
Astropolitics 2009.7:50-82.
the only supersonic transport to see operational status. Concorde’s DO
3
at the
peak of fleet operations was certainly small,
27
less than 0.01%. As we show
below, the present day impact of many of the world’s rocket systems likely
exceeds the ‘‘Concorde’’ standard with regard to ozone loss. In order to
compare different future launch market scenarios, we assume two values
of DO
3
,1% and 0.2%, that represent a possible upper bound that defines
the limit of ‘‘acceptable’’ ozone loss. If the predicted global ozone loss
DO
3
does not exceed 0.2%, we assume regulatory concerns would be nil.
In contrast, if predicted DO
3
exceeds 1%, we assume that regulatory factors
would almost certainly limit launch operations. For launch systems, this reg-
ulation could take a number of forms such as caps on launch rates or limits
on propellant type combinations. We emphasize that these trigger limits of
0.2% and 1% are theoretical and completely particular to this work; our
purpose here is to foster discussion regarding the matter and show how a
regulatory limit on global ozone loss from launch systems could affect space
launch development.
STRATOSPHERIC IMPACT OF ROCKET EXHAUST
Overview and Methodology
A full description of the complex processes that mix, transport, and chemi-
cally process rocket emissions into the global stratosphere is beyond the
scope of this work However, it is of interest to briefly review the available
information on rocket emissions and how the ozone layer is affected. With
this background, we present approximate descriptions of the global ozone
loss DO
3
for rocket emissions based on available data and models. The avail-
able information is sparse and approximate; so our analysis must be consid-
ered in the context of large uncertainties. This is particularly so for liquid
propellant engines. The alternative to our work is to make no progress at
all. Accordingly, in addition to our conclusions, we highlight the many areas
where further research is required.
To first order, rocket engine exhaust consists of chemically inert com-
pounds (N
2
and CO
2
), radicals (NO, OH, Cl), radical sources and reservoirs
(HCl, H
2
O), intermediate underoxidized compounds (H
2
, CO) and alumina
or soot. The relative combinations of these compounds in the exhaust
depend on propellant type; four main propellant types are in wide use,
one solid, and three liquid. We must distinguish between rocket exhaust
(hot gases and particles at the nozzle exit) and rocket emissions (the cold
plume wake that mixes into the stratosphere). In the lower stratosphere, fuel
rich rocket exhaust is modified in the hot plume by intense secondary com-
bustion reactions driven by atmospheric oxygen mixing into the plume. This
‘‘afterburning’’ governs the conversion of H
2
to H
2
O, CO, and soot to CO
2
,
and net production of ozone destroying radicals.
28
Afterburning is vigorous
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 59
Astropolitics 2009.7:50-82.
in the lower stratosphere, lessens with altitude, and stops in the upper strato-
sphere and so rocket emissions are highly variable with altitude. Afterburn-
ing is not well understood—especially with respect to the minor
components that most affect ozone.
Table 1 shows the first order emission compositions for the four main
propellant types. Parentheses show the common names for the different
propellant types. Table 1 acknowledges afterburning by reporting H
2
and CO in the exhaust as converted to H
2
O and CO
2
, respectively, and
net production of radicals. We emphasize that plume models have never
been validated with respect to the net emission of radicals, soot, or the
details of the alumina particles sizes. One recent measurement suggests
that the models in fact underestimate the production of NO in the Space
Shuttle SRMs or LREs.
The emissions presented in Table 1 cause prompt and deep ozone loss
(approaching 100%) in the immediate plume wake, caused by the radical
emissions, over areas of hundreds of square miles lasting several days after
launch. These stratospheric ‘‘ozone mini-holes’’ have been well observed
in situ by high altitude aircraft plume sampling campaigns. It is not known
if the cumulative effect of the small ‘‘ozone holes’’ is significant compared
to the global steady-state chemical effects of the emissions.
Beyond the prompt plume wake ozone destruction, second order pro-
cessing of rocket combustion products occurs during the weeks and months
after launch. The plumes are transported and mixed into the global strato-
sphere and lose their identity as distinct air masses. This intermediate mesos-
cale phase would be characterized by complex plume-atmosphere
interactions among radicals, reservoirs, and sinks. Significant influences from
alumina or soot particles are expected, possibly involving the creation of new
H
2
O related particles. The details of this processing will be highly variable
according to altitude and even time of day of launch and certainly has a large
influence on the steady-state global ozone loss. A few chance observations of
aged plumes confirm the importance of the mesoscale processing. No studies
have been done on this aspect of rocket emissions.
TABLE 1 Approximate Emission for the Four Main Propellant Types Given as Mass Fraction
for each Component.
Inert Inert OH source Radicals Radical reservoirs Particles
N
2
CO
2
þ CO H
2
O þ H
2
ClOx, HOx, NOx HCl Alumina soot
NH
4
ClO
4
=Al (solid)
0.08 0.27 0.48 0.1 0.15 0.33
LOX=H
2
O (cryogenic) ——
1.24 0.02
——
LOX=RP-1 (kerosene) —
0.88 0.30 0.02
—
0.05
UDMH=N
2
O
4
(hypergolic)
0.29 0.63 0.25 0.02
— Trace
Note: The Total Mass Fraction Exceeds Unity because of the assumption that air mixed into the plume oxi-
dizes CO and H
2
.
60 M. Ross et al.
Astropolitics 2009.7:50-82.
One should now appreciate that rocket emissions are complex, variable,
and not well understood. Global models of the stratospheric response to
rocket emissions have not taken into account at all the influence of afterburn-
ing and mesoscale processing. This is because a rigorous analysis of these
effects has never been done. The information in Table 1 should not be taken
to predict the emission of a given rocket engine type. Rather, the main points
of Table 1 are that (1) SRMs emit relatively large quantities of gases and
particles that deplete ozone; (2) LREs emit smaller amounts of gases and
particles that potentially deplete ozone; and (3) all rocket engines emit
water vapor that will deplete ozone.
For our purposes, we make the problem tractable, at least to first order,
by making several simplifications:
1. We ignore the prompt plume wake ozone ‘‘mini-holes’’ and the mesoscale
mixing and processing. We only consider the situation presented in pub-
lished models of rocket emissions, the long-term, cumulative, steady-state
ozone loss DO
3
from all rocket launches.
2. We assume that ozone loss caused by SRMs is separable from ozone loss
caused by LREs. As Table 1 shows, SRMs emit much greater amounts of
reactive chemicals and particles. Thus, we can be certain (despite the
relative lack of LRE models) that SRM emissions will affect ozone much
more significantly than emissions from any of the three LRE types.
3. We consider LREs together as a class so that the ozone loss they cause can
be described with a single parameter, representing the ‘‘characteristic’’
LRE impact. Each LRE type certainly reduces ozone uniquely and
unequally, but all LREs cause some ozone loss that is an order of
magnitude smaller than SRM ozone loss. This approximation is rough,
but it is consistent with the sparse LRE emissions data, plume data, and
model predictions.
4. We describe the ozone loss for SRMs and the LREs based on generaliza-
tions deduced from published models that describe perturbations to the
global stratosphere from rockets and supporting plume measurements.
While significant progress was made on the problem of rocket emissions
in the 1990s, little additional work has been done since the 2002 WMO
Ozone Assessment. The literature is sparse. Published calculations of
DO
3
for rocket emissions are very limited—none account for variability
associated with afterburning, mixed propellant types, chemical processing
between the early plume wake and steady-state phases, and LRE emis-
sions, in general.
5. We assume that the steady-state ozone loss DO
3
from rockets can be
approximated by the linear addition of the long-term steady-state effects
of all launches on a global basis. This simplification ignores several sec-
ond order effects such as the effect of launching from different latitudes,
seasons, times of day, and the changing background stratosphere.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 61
Astropolitics 2009.7:50-82.
6. We only consider rocket propulsion systems currently in wide use. We do
not consider proposed propellant combinations such as LOX=methane,
hybrid systems, or hypersonic propulsion systems. These are a small part
of the present day global emissions. They are likely to be more significant
in coming decades however. Airbreathing hypersonic systems in particu-
lar are of great interest for ozone loss since they would emit much of their
combustion products directly into the peak of the ozone layer at altitudes
near 20 miles and are often discussed as the first stage of proposed high
flight rate, reusable launch systems.
29
A hypersonic-based launch system
would deposit a larger fraction of the total combustion products into
the stratosphere than rocket systems. Much research and development
is currently being invested into hypersonic systems, without much regard
to their impact on the ozone layer.
We emphasize the uncertainty in specification of rocket emissions and their
effects on the stratosphere. This is especially true for the cryogenic and ker-
osene systems that are usually assumed harmless to the ozone layer.
30
Signif-
icant data collected in LRE plumes strongly suggest that LRE emissions
contain significant amounts of reactive NO, OH, and particles; furthermore,
they cause the formation of ice particles in the middle and upper
atmosphere.
31
These data indicate that the assumption that ‘‘LREs are
green’’
32
as far as ozone is concerned are not correct.
PARAMETERIZATION OF GLOBAL OZONE LOSS DO
3
With the problem simplified, we write an expression for the global ozone
loss DO
3
from rockets as the sum of the effects of SRM and LRE combustion
emissions as
DO
3
¼ d
S
M
S
þ d
L
M
L
ð1Þ
where DO
3
is the steady-state percent decrease in global total ozone for an
atmosphere with rocket emissions compared to one without. M
S
and M
L
are the annual stratospheric emission (kiloton) from SRMs and LREs, respec-
tively. d
S
and d
L
are ozone loss coefficients that relate DO
3
and emission mass
for SRMs and LREs, respectively. For convenience, we assume that the units
of M and d are kilotons (kt) and percent per kiloton (% per kt) of rocket
combustion emitted into the stratosphere. M
S
and M
L
are assumed annual
values. Both terms on the right-hand side implicitly include the effects of
both gas and particle emissions of the two propellant types. The parameters
d
S
and d
L
are admittedly not as sophisticated as the standard metrics for
ozone depleting substances such as ODP or EECl but they can reasonably
applied understand trends and comparisons to the major sources of ozone
loss such as halogens. Indeed, one of the goals of this work is to argue the
62 M. Ross et al.
Astropolitics 2009.7:50-82.
need for the creation of a more robust and considered formalism to assess
rocket emissions.
We estimate the values for d
S
and d
L
by summarizing previous research
on the emissions from rocket engines and the response of the stratosphere to
those emissions. For some cases, the relevant research has not been done,
and so we make use of general arguments to estimate the correct values
for d
S
and d
L
. The emissions M
S
and M
L
are either specified explicitly or esti-
mated as a function of LEO payload. The ozone loss coefficients d
S
and d
L
are
acknowledged only an approximate description of the relationship between
rocket emissions and global total ozone loss. Our definition of d
S
and d
L
averages out a number of second order variables including latitudinal and
seasonal details of launches and nonlinear effects in stratospheric chemistry
and dynamics. d
S
and d
L
are too coarse to be considered as an ODP analogue
for rockets. As we point out below, much work lies ahead in order to derive a
comprehensive and sensible way to compare the global total ozone loss from
different rockets.
Some work has been done to model the effects of SRM emissions and so
we have some confidence in our assumed value for d
S
of 1.5 10
2
% per kt.
This value follows from a number of detailed steady-state stratospheric mod-
els that include SRM chlorine gas and alumina particle emissions.
33
The
ozone loss from chlorine emissions from SRMs is well understood, however,
the ozone loss from alumina particles is only poorly understood. Chemical
reactions on alumina particles promote production of ClOx from its reser-
voirs, similar to reactions on certain ice particles that contribute to the south
polar ozone hole. While the chemistry is known, the steady-state alumina
loading is not. Only alumina particles smaller than 1 mm remain in the stra-
tosphere for years and contribute to the steady-state ozone loss. The fraction
of SRM alumina particles that meet this criteria been variously reported as
between 1% and 30%.
34
Here we assume the alumina sub-micron mass frac-
tion equals 5%, in the middle of the range of possible values. The models
show that for a mass fraction of 5%, the alumina and chlorine contribute
about equally to SRM ozone loss. Our assumed value for d
S
of 1.5 10
2
%
per kt can be considered moderately conservative.
Little work has been done to model LRE emissions or their effects on
ozone. We have only moderate confidence in our assumed value for d
L
of
3 10
4
% per kt. This value follows from a detailed model of hypergolic
emissions where the global ozone loss, almost completely due to NO emis-
sions, approximately equaled 2% of the ozone loss from SRM emissions, sui-
tably scaled by propellant mass.
35
We assume that all three LRE types can be
characterized under the assumption that the ozone loss coefficient d
L
equals
2% of d
S
. This assumption is speculative, though plausible. OH and NO emis-
sions from kerosene and cryogenic engines are unknown but are plausibly
present at the percent level with after burning and are certainly present at
the tens of percent level without afterburning. Soot from kerosene engines
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 63
Astropolitics 2009.7:50-82.
might also contribute to ozone loss. We will say that the actual ozone loss
factor for LREs is not likely to exceed our assumed value greatly, but neither
can we rule this out because of the nearly complete lack of data and models.
In the event that additional data and model results show that after burning
kerosene and cryogenic engines do not emit significant amounts of NO,
OH, or chemically active particles, then the ozone loss would be determined
by H
2
O vapor alone. In that case, our calculations would overestimate ozone
loss by about a factor of ten.
It is convenient to restate equation (1) in terms of the total annual mass
of rocket exhaust emitted into the stratosphere M
T
and the ratio f where:
M
T
¼ M
S
þ M
L
f ¼
M
S
M
T
ð2Þ
where f represents the fraction of total stratospheric emission from SRMs. We
then write the global ozone loss as
DO
3
¼ M
T
ð f d
S
þð1 f Þd
L
Þð3Þ
GLOBAL OZONE LOSS FOR VARIOUS LAUNCH
FLEET SCENARIOS
Ozone Loss and Rate of Orbital Payload Delivery
We are interested in the global ozone loss caused by the total emissions from
all launches from the earth’s surface and so ‘‘fleet’’ refers to the total of all
rocket types worldwide. The assumed values for d
S
and d
L
are used in equa-
tion (1) to calculate the global steady-state ozone as a function of the exhaust
mass emitted annually above the tropopause. Figure 2 shows global ozone
loss DO
3
for an SRM fleet and an LRE fleet; DO
3
from mixed fleets would
lie between the two extremes. The ozone losses in Figure 2 require about
three years of roughly constant emissions to develop as exhaust gases and
particles to reach a steady state with stratospheric processes, a requirement
that the global launch industry meets. Presently, SRMs contribute about
one third of annual global rocket emissions and so they dominate current
ozone loss. In recent years, the average of annual SRM emissions is about
3 kilotons per year so that Figure 2 shows that current launch related ozone
loss is about 0.03%, well below the 0.2% loss threshold we assigned earlier to
designate small (or equivalently, minor) DO
3
. The figure makes clear that
even the 1% loss threshold we assigned for unacceptably large ozone loss
can be reached for sufficiently large emissions of either SRMs or LREs.
In order to understand how rocket ozone loss relates to the annual, glo-
bal weight of material placed into orbit, we must account for the relationship
64 M. Ross et al.
Astropolitics 2009.7:50-82.
between launch emissions and payload weight. While each launch vehicle
will have slightly different relationships between payload and emissions,
detailed studies of several specific launch vehicles show that an approximate
relationship between stratospheric emission M
S
or M
L
and first and second
stage propellant masses M
1
and M
2
is given by
M
S ; L
¼
1
3
M
1
þ M
2
: ð4Þ
Figure 3 shows estimated stratospheric emission as a function of LEO pay-
load mass for a variety of launch vehicles (active and inactive). The relative mix
of SRM and LRE propellant burned in the stratosphere for each of these vehi-
cles is highly variable, from purely SRM to purely LRE. Since the engines used
by the various launch vehicles have different efficiencies (e.g., specific
impulse) and each launch vehicle has been designed for different missions
(LEO, GEO transfer, high inclination, reusability) and has different launch site
latitude, the definition of payload in this context is somewhat arbitrary. In addi-
tion, a given launch vehicle can take on different stage configurations (e.g.,
strap-on stages) so that the relationship shown is only approximate.
Application of the ideal rocket equation for a 5 kt payload, two stage
vehicle with specific impulse of 300 sec and structural mass fraction of 0.1,
along with equation (4), and consistent with the overall level of approxima-
tion for this discussion allows us to write
M ¼ 12 M
LEO;
ð5Þ
where M is the total stratospheric emission, and M
LEO
is the LEO payload.
Figure 3 shows that this relationship is consistent with estimated emission
FIGURE 2 Global ozone loss DO
3
(%) as a function of annual rocket emission into the strato-
sphere (kt per year) for different values of f, the fraction of total stratospheric emissions from SRMs.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 65
Astropolitics 2009.7:50-82.
across a variety of launch vehicles and so represents the relationship
between LEO payload and stratospheric emission in a robust, if coarse, man-
ner. Note that for the Space Shuttle, we assume the useful payload rather
than the total mass of the Orbiter with payload placed into orbit. This means
that the Space Shuttle emits several times more stratospheric emission per
payload mass than the general trend for other launchers. Greater strato-
spheric emission for reusable launch vehicles (RLVs) compared to expend-
able vehicles would be characteristic of RLV systems in general and so in
this sense RLVs are more harmful to ozone than expendables.
We can combine equations (3) and (5) to write an expression for global
total ozone loss DO
3
as a function of mass placed into low earth orbit (LEO)
M
LEO
as
DO
3
¼ 12 M
LEO
ðf d
S
þð1 f Þd
L
Þ: ð6Þ
We apply equation (6) to investigate how much global ozone loss the
current global launch market causes, how this fits into other sources of ozone
loss, and how this might change in the future as the space industry grows.
Figure 4 shows the range of global ozone loss for mixed SRM and LRE sys-
tems as a function of payload delivered to LEO. In recent years, the global
launch industry has annually lifted nearly 1 kt into LEO and about one quar-
ter of the stratospheric emission has been from SRMs. (Note that GEO pay-
loads typically require about six times their final GEO weight to be placed
into LEO parking orbits.) Like Figure 2, Figure 4 shows that the current ozone
loss from the global launch fleet is about 0.03% with almost all of this is due
to the SRM component of the exhaust. As discussed above the stratospheric
protection community has deemed this level of ozone loss ‘‘not significant.’’
6
FIGURE 3 Stratospheric propulsion emission as a function of LEO payload for fourteen
different launch vehicles. The line indicates stratospheric emission estimated according to
equation (5).
66 M. Ross et al.
Astropolitics 2009.7:50-82.
Interestingly, Figure 4 shows the situation that would have developed
had the Space Shuttle met its original goal of weekly launches. In this case,
DO
3
from Space Shuttle launches would have approached the 0.2% limit for
small ozone loss. Designed and built prior to the era of international protec-
tion of stratospheric ozone and when the impact of SRM emissions on ozone
was not understood, global ozone loss was not a factor in Space Shuttle
design or operations planning. If weekly launches of the Space Shuttle were
proposed now, it is not clear how the stratospheric protection community
would respond. We point this out to emphasize that, as new discoveries
improve our understanding of rocket emissions and stratospheric processes,
ozone depletion considerations might influence launch systems design and
operations during the typical multi-decade life of launch systems.
Global fleet launch rates sufficiently large to exceed the 0.2% threshold
for any propellant type mix are not likely without revolutionary changes in
the space industry. We cannot know when or how such changes might come
about though serious planning and discussions are ongoing.
36
Various pro-
posals for very large space infrastructure projects, space tourism, or RLVs
with very high launch rates have been seriously proposed and significant
resources have been invested. All of these proposals would result in large
launch vehicles with very high launch rates (i.e., daily) and so it is of interest
to evaluate them based on their impact on the ozone layer. One often dis-
cussed concept is the use of Solar Power Satellite (SPS) systems for electricity
production. A recent study
37
indicates that the required annual launch
demand for an economically viable SPS system would be about 25 kt per
year, over many years, perhaps indefinitely. Figure 4 shows that this SPS
FIGURE 4 Steady-state ozone loss as a function of LEO payload rate for different values of f,
the fraction of stratospheric emission from SRMs. The square shows the situation that would
have emerged for the original Space Shuttle design goal. The circle illustrates the approximate
situation for the global launch business in recent years.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 67
Astropolitics 2009.7:50-82.
launch system would be constrained to use liquid propellant systems in order
to ensure that DO
3
remains below the 0.2% threshold; an SPS launch system
could not make significant use of SRMs in the stratosphere. Since we are able
to define LRE ozone loss in only approximate term, we cannot rule out that
even an all liquid propellant fleet at SPS launch rates might result in signifi-
cant global total ozone loss, depending on the actual effect of a specific LRE
type. The ambiguity of SPS launch illustrates the need for an improved
understanding of rocket emissions of all types. With the current knowledge
gaps, we cannot predict with any confidence which (indeed, any) large
launch or space systems are plausible with respect to ozone depletion.
THE CARRYING CAPACITY OF THE STRATOSPHERE
Figure 4 raises the notion that the stratosphere is characterized by a ‘‘carrying
capacity’’ that represents an upper limit to the amount of material that can be
placed into earth orbit using conventional rocket propulsion without causing
excessive ozone loss. We have argued that the upper limit of acceptable DO
3
could reasonably be considered 0.2–1%, though we reiterate that there is no
formally defined value or even a regulatory formalism to determine ‘‘too
much ozone loss’’ for a particular global industrial process or system. For a
value of DO
3
considered excessive as small as 0.2% our results show that
upper limits on material that can be placed into earth orbit by conventional
chemical propulsion systems are about 1 and 28 kt per year for SRM and LRE
launch systems, respectively. Our ability to calculate the actual carrying capa-
city of the stratosphere is controlled by several poorly known parameters, the
global ozone loss parameters d for each propellant type and the maximum
value of global ozone loss DO
3
allotted to launch systems by regulatory
actions.
In the event that an upper limit on DO
3
is ever assigned to the global
launch industry, the limit would necessarily be globally distributed, perhaps
on a national basis. Any given nation then would have to operate under a
limit that would be even smaller than the global limit. The carrying capacity
of a given nation then would be determined by the mix of propellant types
used by its launch systems. It is unlikely that the carrying capacity will be
approached in the near or even intermediate future but it could be
approached after several decades of significant launch market growth, espe-
cially for the scenarios of revolutionary changes in the space industry. The
details of the true carrying capacity of the stratosphere for a given propellant
(or combinations of propellants) will change as new models and data
become available, however Figure 4 shows us that it is not possible, within
the context of protecting the ozone layer from excessive loss, for the launch
market to grow arbitrarily large. Given the anticipated recovery of the ozone
layer, the potential for large scale space infrastructure projects, and the
68 M. Ross et al.
Astropolitics 2009.7:50-82.
potential growth in non-traditional space markets (tourism, for example),
policy makers should begin to consider how a vigorously growing launch
market might affect the ozone layer. Further, they need to define a formalism
for evaluating rocket emissions without prejudice with respect to any one
nation’s launch systems.
OZONE LOSS FOR VARIOUS LAUNCH MARKET SCENARIOS
In order to predict the relative contribution of rocket emissions to global total
ozone loss from all sources, especially in view of the declining influence of
ODSs (Figure 1), we must assume a scenario of future demand for launch
capacity. Predicting launch demand, even in the near term, has proven diffi-
cult. Widespread expectations of a surge in commercial launch demand in
the 1990s where overoptimistic and the demand did not materialize. Studies
of long-term (e.g., decades) launch demand have been based on various
approaches including historical comparisons to other transportation systems
such as railroads or aircraft and economic analysis of emerging market
demand based on price and demand relationships One simple approach is
to assume that the relative mix of SRM and LRE propulsion systems will
not change significantly over time and apply a constant growth rate to the
current value of global ozone loss of 0.03%.
Figure 5 shows the sum of global ozone loss from ODSs (declining leg)
and space launch (increasing legs) for illustrative rates of launch demand
increases: zero, doubling each decade, and tripling every decade. If such
trends appear and persist over the next several decades, around 2035 rocket
emissions would represent an ozone depletion mechanism comparable to
the declining ODSs. Figure 5 makes the point that, while currently insignifi-
cant compared to ODSs, the very success of the Montreal Protocol increase
the relative ozone depletion from rocket emissions.
A more sophisticated approach to predicting launch market growth
links launch price ($ per pound launched into LEO) to the quantity of mate-
rial launched into LEO (kt launched per year) and so allows us to predict
how DO
3
from rockets might change as a function of launch price. A detailed
econometric analysis of the launch market is beyond the scope of this paper;
however, we can illustrate some basic notions of how limitation on the
launch market associated with ozone depletion might affect the space econ-
omy. The organizing principal is that as launch price declines, new demand
will appear and, over a sufficiently long time, launch capacity and demand
are in equilibrium with an appropriate law of supply and demand. However,
as we have seen launch rates cannot rise arbitrarily high without causing too
much ozone loss. Thus, ozone depletion from rocket emissions could be
considered a market externality that has the potential to lead to inefficient
allocation of investments or failure of the launch market.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 69
Astropolitics 2009.7:50-82.
SRMs cause so much ozone loss that if all launches were on SRMs, the
market could not likely grow at all beyond the current situation, and so SRMs
are not likely to play a major role when proposing large increases in payload
delivered to orbit. Whether large growth comes about with expendable or
reusable systems they will almost certainly have to be LRE systems, at least
from an ozone loss point of view. Accordingly, we do not consider SRMs
any further in this paper and concentrate our attention on LRE emissions
for the remainder of this paper.
One measure of market performance is total revenue. In this case, we
mean the combined revenue (in dollar terms) for launch providers world-
wide, though it could mean an individual country or whatever organization
is allotted the particular level of ozone loss (e.g., 0.2% or 1%). Assuming for
the moment that launch price P and launch demand Q are independent, we
write an expression for the total launch market revenue R, given by
R ¼ QP; ð7Þ
where P is launch cost ($ per pound to LEO); and Q is identified with M
T
, the
total mass of payloads launched into LEO. The carrying capacity concept
implies that there is an upper limit on Q, which in turn implies an upper limit
to total revenue R for a given price P
R
max
PQ
max:
ð8Þ
This limit on total launch market revenue R
max
is shown in Figure 6 for
an LRE-based launch fleet with Q
max
determined for 0.2% and 1% maximum
allowable DO
3
. Figure 6 shows that for a launch price of 100 $ per lb, the
FIGURE 5 Global ozone loss from 2005–2050 for different launch growth scenarios. The solid
line show declining ozone loss from past ODS emissions. The dash-dot, dot, and dashed lines
show increasing or stabilizing ozone loss from launch emissions for the cases of no growth,
doubling per decade, and tripling per decade, respectively.
70 M. Ross et al.
Astropolitics 2009.7:50-82.
maximum possible total revenue for a launch industry based on LRE systems
equals about 15 and 70 G$, respectively. Also noted in Figure 6 is the approx-
imate present day market condition with launch price of about 10,000 $ per
lb and gross revenue of about 14 G$.
In real-world markets operating near supply and demand equilibrium, Q
can be related to P with an assumed specification of market elasticity E
describing how demand changes with changing price. Various studies of
the relationship between price P and quantity Q for the launch market have
been done and one common result is that, at least near the current price of
about 10,000 $ per lb, the launch demand is not very elastic with respect to
changes in price.
38
The studies often conclude than the launch market is
inelastic until some threshold price is reached, below which the demand
for payload quantity will (perhaps) strongly increase as price decreases.
The threshold price and the rate of demand increase below the threshold
are only speculation but the notion of a demand elasticity threshold seems
widely accepted. Using these concepts, we relate launch quantity demand
and price as
Q ¼ Q
0
P
E
; ð9Þ
where E is the elasticity of the launch market, and the constant Q
0
is deter-
mined assuming that Q does not increase until price falls below 2,000 $
per lb. E has the usual definition of fractional change in quantity per frac-
tional change in price; E ¼ 1 corresponds to an inelastic market. For discus-
sion purposes we assume three levels of market elasticity, low, medium,
FIGURE 6 Revenue R (G$) as a function of launch price P ($ per lb) for an LRE based system
(i.e., f ¼ 0.) The solid and dashed lines show the maximum possible revenue for maximum
permissible global ozone loss DO
3
of 1% and 0.2%, respectively. The current global launch
market economic condition of 10,000 $ per lb and 20 G$ is indicated.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 71
Astropolitics 2009.7:50-82.
and high elasticity E ¼ 1.5, 2, and 2.5 respectively. Figure 7 shows the result-
ing equilibrium relationship between Q and P. This analysis does not address
how the market gets to equilibrium, only that it does by the usual market
forces and the assumption of equilibrium provides a reference for discussion.
The values of E we assume are speculative but follow the results of other stu-
dies. One RLV cost study, for example, assumes elasticity between our high
and medium cases. Another study assumes a demand curve that resembles
our low case.
We are interested in how the market limitations implied by Figure 6
relate to total revenue. Figure 8 shows total launch market revenue as a func-
tion of launch price for LREs, with the limits associated with 1% and 0.2%
global ozone loss limitation indicated. Ignoring the global ozone loss limita-
tions for the moment, Figure 8 shows that revenue decreases until the 2,000 $
per lb elasticity threshold is reached. This conundrum of a ‘‘trough’’ in launch
market revenue has been previously cited in launch market studies. It means
that the launch market will have to ‘‘jump over’’ a trough in revenue, near the
price of 2,000 $ per lb, through technology investments that would be unpro-
fitable for a long period of time. Investors financing such a jump would need
high confidence that a predictable launch market, with known limitations
including regulatory ones, will be found on the low price side of the trough.
Figure 8 shows that for LRE systems, total revenue can respond favor-
ably for some combinations of elasticity and global ozone loss limits. How-
ever, the stratospheric carrying capacity limits how much revenue recovery
is possible. For example, for a global ozone loss limit of 1% and medium
elasticity, revenue can increase by a factor about 3 for a price of about 100
$ per lb. On the other hand, if the global ozone loss limit is only 0.2%,
revenue can barely recover to the present day value of about 10 G$.
FIGURE 7 Equilibrium demand for launch quantity Q as a function of launch price P for three
scenarios of launch market elasticity. Values of E equal to 1.5, 2, and 2.5 correspond to low,
medium, and high elasticity, respectively. An inelastic market (E ¼ 1) is assumed between
2,000 and 10,000 $ per lb.
72 M. Ross et al.
Astropolitics 2009.7:50-82.
Our results have important implications for the development of the
space and launch industry. Analysis of future launch markets (reusable
launch systems particularly) implicitly assume that the only limitations on
launch rates are technological or economic. Studies of launch economics
typically assume that launch rates are free to rise to meet any quantity
demand if launch costs can be reduced sufficiently to spur demand. Indeed,
these analyses usually show that launch demand must increase by orders of
magnitude in order to justify the development of a reusable system. Our ana-
lysis shows however that considerations of stratospheric ozone depletion
represent a very significant factor that should be considered in econometric
analysis of large-scale space systems.
While a detailed econometric analysis is beyond the scope of this work,
reflecting on the development of a low-cost launch system against an evol-
ving ozone regulatory environment illustrates our point. Our putative launch
system is consistent with the scale of future launch systems often presented
for econometric analysis. We suppose that a corporation decides to commit
the financial resources to develop a low-cost launch system that could profit-
ably operate in a launch market with medium quantity demand elasticity,
E ¼ 2. The objective of this new launch system is to ‘‘jump’’ across the trough
in revenue and profitably operate at a price of 100 $ per lb, demand of 400 kt
per year (10,000 flights per year), generating revenue of 80 G$ per year.
Development would take about ten years and a positive return on investment
would take even longer. The decision to proceed with decades long devel-
opment and investment critically depends on (among other things) an
assumption that normal market forces will freely respond to a low-cost
launch service and modulate demand over several decade life-cycle of a
launch system.
FIGURE 8 Total revenue R(G$) as a function of launch price P($ per lb) at demand-price
equilibrium. R(P) is shown for elasticity E values of 1.5, 2.0, and 2.5. The heavy solid, dashed,
and dotted lines show R(P) for DO
3
<0.2%,1%<DO
3
<0.2%, and DO
3
>1%, respectively.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 73
Astropolitics 2009.7:50-82.
Consider, however, the implication of unanticipated ozone protection
regulation for our conjectural launch system. Suppose that as the system
becomes operational, new international regulations are adopted to
strengthen protection of the ozone layer. Suppose that new regulations
restrict the global total ozone loss DO
3
resulting from any particular industrial
activity, including space launch, to not exceed 1%. Figure 8 shows that under
this regulatory regime, the new launch system would be limited to launching
only about 278 kt into orbit per year, much less than the design assumption
of 400 kt per year. The new launch system could not reach the design eco-
nomic state. To realize the design revenue of 80 G$, the launch price would
have to rise to about 145 $ per lb, violating the original design assumption. If
our conjectural launch system is limited by the lower maximum DO
3
equal to
0.2% (perhaps because a global launch limit of 1% is divided among the
world’s various national launch agencies) then the economic situation
becomes very distorted. In this case, the new launch system is limited to
launching 30 kt per year so that price must increase to about 1,300 $ per lb
to realize the design revenue of 80 G$. This price is so high so as to comple-
tely obviate the need to invest resources to develop the 100 $ per lb system in
the first place.
This launch market thought experiment clearly demonstrates that for
plausible values of the two main uncertainties of concern, the amount of
ozone loss caused by LRE exhaust (the value of d
L
) and the magnitude
of maximum allowable global ozone loss (a cap on launches, for example)
could have a significant impact on the economic viability of possible large-
scale, low-cost launch systems. We emphasize that the value we assume for
d
L
is only one plausible value from a range of possible values. It is also
plausible, though perhaps less likely, that the actual value of d
L
is much
smaller than the value we have assumed; we simply do not have sufficient
data. For the best case with d
L
equal to the lower limit from H
2
O emissions
alone and an ozone loss limit of 1%, the launch limit is about 2500 kt per
year. At this level, it is difficult to imagine that ozone loss regulation would
play any role in space development. However, for the worst case with d
L
equal to the upper limit and an ozone loss limit of 0.2%, only about
30 kt could be placed into orbit per year, a level certain to have a strong
influence on the economic and technological basis for large scale space
systems.
Our work shows that the commonly held assumption that LRE emissions
have no environmental impact is not accurate. LREs, particularly kerosene
and cryogenic systems, are often described as ‘‘environmentally friendly’’
or ‘‘green’’ propellants
52
and this is generally true compared to SRMs. SRM
emissions are a fairly well known cause of global total ozone loss. We reiter-
ate the point that emissions from all rocket propellant types cause some glo-
bal total ozone loss, even if only from water vapor emissions. Further, even
LRE ozone loss will be considered significant by the regulatory apparatus
74 M. Ross et al.
Astropolitics 2009.7:50-82.
charged with protecting the ozone layer at high enough flight rates. Analysis
of proposed launch systems or emerging space markets such as tourism or
space power generation commonly assume that the only limit to the potential
number of launches or the amount of material that can be launched into orbit
is economic. This is not necessarily true. We have shown that protection of
the ozone layer presents a potentially serious limit to growth of the space
transportation market and that this limit might be low enough so as to influ-
ence the economic value of investments in new launch systems. The problem
is that we do not have sufficient information on the actual effects of rocket
emissions on ozone to eliminate this possibility. Challenges to the growth
and sustainability of space exploration
51
need to consider the risk of overly
conservative limits related to ozone depletion that might arise out of a lack of
accurate information on rocket emissions.
How do rockets currently compare to existing aerospace propulsion
emissions? We are sure that rockets cause ozone loss. We are relatively sure
that global aircraft emissions currently cause a small increase in global
ozone,
39
making a comparison between them with regard to ozone deple-
tion is not strictly meaningful. Rockets cause a few percent of all ozone
depletion, about the same relative impact that aircraft have on radiative
forcing of the atmosphere.
40
And as we noted, there is great policy pressure
for aircraft to reduce their greenhouse gas emissions, possibly at great cost
to the industry.
41
How do rockets currently compare to other ODS sources? As we have
noted, ozone loss from rockets is currently small compared to all ODSs on
a global basis. However, can also compare rockets, a specific industrial pro-
cess, to specific applications of ODSs that require exemptions from Montreal
Protocol phase-outs. One example is Metered Dose Inhalers (MDIs), hand-
held medical devices that historically used CFCs as a propellant. United States
companies have been granted Essential Use Exemptions for CFC use in MDIs
in recent years. In 2008, the exemption, obtained after much diplomatic
negotiation, was for 0.03 kt
42
of CFCs. Now the annual emission of United
States SRMs is about 3 kt into the stratosphere and while there is no common
metric for comparison, there is little doubt that the integrated impact from
SRMs was larger than the integrated impact from the CFCs covered under
the MDI exemption.
In these two examples, we see that policies to protect the global atmo-
sphere might be seen as having contradictory or inconsistent positions. A
small contributor to climate change (air transport), might be regulated, while
a small to ozone depletion (rockets) is not regulated. Montreal Protocol
exemptions are required for one activity (CFC-based MDIs) while another
activity with larger ozone loss (SRMs) is not subject to any regulatory atten-
tion. Indeed, new SRM-based launch vehicles are being developed by the
European Space Agency and NASA so that SRM ozone depletion might even
expand even as the CFC exemptions continue.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 75
Astropolitics 2009.7:50-82.
POLICY IMPLICATIONS
The purpose of space policy is to provide long-term direction for the space
industrial base in order to support national interests and promote growth.
Space policy should characterize opportunities and risks, with regard to
the growth of the space transportation industry, as well as recommend steps
to increase opportunity and reduce risk. We have demonstrated that the
international imperative to protect the ozone layer presents a long-term risk
to the space launch industry in coming decades. Should the space industry
enter a phase of rapid growth, ozone loss caused by high launch rates could
become large enough to attract the attention of the regulatory apparatus pro-
tecting stratospheric ozone. The risk of limitation on launch systems due to
ozone depletion is certainly many decades away. Nevertheless, the risk is
not zero, applies to all rocket engine types, and the timescale is no longer
than typical launch systems design and life cycle timescales.
This risk of launch limitations comes about for two reasons: uncertainty of
impact and uncertainty of regulation. The range of uncertainty regarding
ozone loss from liquid propellant rocket engines is too large to support clear
assessment of future launch systems. Plausible assumptions within the ranges
of uncertainties can lead to the conclusion that excessive ozone loss might limit
payload delivery rates to a level only a factor 30 greater than current rates.
Meanwhile new rocket propulsion systems that could affect the ozone layer
such as hybrid propellants and hypersonic propulsion are being developed
and promoted without regard to ozone impacts. The level of ozone loss from
rocket emissions, globally or nationally, that would be considered unaccepta-
bly large is also very uncertain. The international organizations that regulate
ozone depleting substances and the environmental effects of aerospace sys-
tems have not addressed the issue in a quantitative way that planners can apply
to analysis of space launch growth. Together, these uncertainties make it so
that we cannot know what sort of space transportation infrastructure, launch
rates and propulsion systems, will be acceptable several decades from now.
Even though the Montreal Protocol does not define the ODP for aero-
space combustion emissions, some estimates of an equivalent SRM ODP have
been proposed and depending on any of several assumptions in the model,
the value can exceed 0.2 SRM emissions, and are therefore, in some sense,
similar to methyl bromide in that the calculated ODP likely exceeds 0.2—
the value that triggers regulatory action. Continued use of methyl bromide
might indicate the success of arguments that as long as a compound’s DO
3
is ‘‘small’’ compared to other sources, then continued use may be appropriate,
irrespective of the calculated ODP. Such arguments might be applied SRM
emissions in the future. The inherent weakness of this kind of argument is that
if the comparison DO
3
shrinks or is eliminated then ‘‘small’’ may become
‘‘large.’’
76 M. Ross et al.
Astropolitics 2009.7:50-82.
We wish to make clear that this work is not calling for limitation of any
kind on the launch market. Rather we wish to point out that our current level
of technical and regulatory understanding allows that such limitations are
likely to emerge at some point in this century. Nor is this work considered
overly alarmist regarding ozone depletion from rocket exhaust. Rather we
wish to point out some of the long-term sensitivities and vulnerabilities of
the space industry to ozone depletion considerations. Policy and investment
decisions typically rely on the assumption that the future is likely to be similar
to the recent past and our shows one way that this assumption could fail.
Current United States policy regarding ozone loss from rockets is to
accept that the Montreal Protocol does not require any action because rocket
emissions are not on the list of proscribed substances. New space systems
must only consider ozone loss as part of an Environmental Impact Analysis
(EIS) required by the National Environmental Policy Act (NEPA). Under
NEPA, which has no enforcement mechanism, each space system is consid-
ered independent of each other and there is no formal cumulative analysis.
For example, one space system may require several launches over a number
of years. The ozone loss from these few launches is, of course, insignificant
and so the EIS for that system contains a finding of no significant strato-
spheric impact. Under NEPA, each system considers only its own contribu-
tion to ozone loss as insignificant and there is no accounting for the
cumulative loss of all systems or programs, either nationally or globally. In
addition, the EIS process does not require a peer reviewed consensus in
the scientific community regarding predicted ozone loss as the Montreal Pro-
tocol does; NEPA only requires a period of public comment. Finally, a final
EIS of record is not required until a system is essentially ready for deploy-
ment (long after development investments have been made) and so does
not require consideration of the effects of proposed or in development
systems whose ozone impact is in the (perhaps far) future.
The policies of other space faring countries regarding ozone loss from
rocket emission are not as clear, except insofar as the Montreal Protocol does
not require any action. In the early and mid 1990s, the impact of SRMs was
much more uncertain than now and the issue was occasionally raised at the
Meeting of the Parties to the Montreal Protocol, but it has not been subject to
recent attention. The recent lack of attention is due mainly to the fact that
SRM emissions are much better understood than in the 1990s; the estimated
upper limit on d
S
has been reduced by a factor of about ten since then. How-
ever, if the issue of rocket emissions becomes significant, the Parties to the
Montreal Protocol might attempt to cap ozone loss from rocket emissions.
It is not at all clear how such a cap might be configured or specified. The
Montreal Protocol allocates ODS production and consumption on a ‘‘coun-
try’’ basis but this approach would be entirely inadequate for rocket emis-
sions. Space launch is a globally integrated activity whereby a rocket
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 77
Astropolitics 2009.7:50-82.
produced in one country, launches from a site in a second country, using pro-
pellant produced in a third country, and carries the payload of a fourth coun-
try. A ‘‘by country’’ approach would be difficult to implement for rockets.
A related concern, illustrating the possibly contentious nature of the
problem, is that the relative mixes of SRM and LRE emissions vary widely
from country to country. The Russian Proton (hypergolic) and European
Ariane V (SRM and cryogenic), for example, launched about the same orbital
payload in 2007. However, the Ariane V likely causes about 25 times more
(steady state) ozone loss than the Proton. Disparities such as this, coupled
with the lack of validated models and undefined assessment metrics, raises
the potential for economic competition to become conflated with regulatory
actions.
The implications of our work for policy makers are two-fold:
1. Space policy should help establish the way for launch market develop-
ment unfettered by uncertain regulatory concerns. The confused experi-
ence of the supersonic transport, where science and policy were not
synchronized with engineering systems development, should be avoided
for emerging space transportation technologies. Policy should establish
the goal of obtaining a sufficiently robust scientific understanding of
rocket emissions and their impact on stratospheric ozone to support
analysis and development of large space systems. This includes in parti-
cular reusable launch vehicles, new propellants, and hypersonic sys-
tems. In order to achieve this goal, space policy should encourage
research efforts to identify and close knowledge gaps regarding rocket
emissions. Our work shows that some guidance is required from the
technical side of the equation on the composition and impact of rocket
emissions.
2. Policymakers in the stratospheric protection community should begin to
consider how to assess rocket emissions formally, in comparison to each
other and in comparison to other substances that deplete the ozone layer.
It is unlikely that rocket emissions could be brought under any existing
metric definition. Even the most fundamental definition of such a metric
is not clear. The explicit coupling between economic gain (payload)
and loss of ozone complicates the problem. Since each country places dif-
ferent emphasis on SRMs and LREs, a globally acceptable metric is needed
before putative space industry growth raises rocket emissions above the
minor category as far as ozone loss in concerned. Our work shows that
some guidance is required from the ozone protection policy side of the
equation on the level of acceptable ozone loss.
Policy makers in the space travel and stratospheric protection communities
might begin to consider how the launch industry can best fit into whatever
regulatory regime emerges as the earth’s ozone layer recovers and the global
78 M. Ross et al.
Astropolitics 2009.7:50-82.
demand for access to earth orbit grows. One direction that ozone protection
could take is an emissions trading scheme in which each national space
agency is allocated a certain quantity of DO
3
and can sell unused allocations
or buy allocations as needed. Another direction would be an ‘‘ozone loss tax’’
on launch providers or payload providers. Systems such as these are already
emerging in response to climate change with air transport providers possibly
subject to a ‘‘carbon tax’’ that would have major impacts on their profitability
and aircraft fleets.
43
In a future paper we will address potential methods to
apportion or regulate launch ozone loss including accounting of ozone loss
by launch provider and payload provider, especially in comparison to cur-
rent ODS special use exemptions and possible ODS trading schemes.
Refinement in our understanding of ozone loss from rocket propulsion
is critical for understanding the political economy of large scale growth in
space transport. A systematic effort to accomplish this can be implemented
at modest cost. A program of stratospheric plume measurements, plume
wake and stratosphere model investigations, and laboratory measurements
on alumina and soot particles can be effectively coordinated within existing
NASA, NOAA, and USAF research efforts and research infrastructures. This
notion was proven in the 1990s first by the Rocket Impacts on Stratospheric
Ozone (RISO)
44
and Atmospheric Chemistry of Combustion Emissions Near
the Tropopause (ACCENT)
45
efforts and most recently by the Plume Ultrafast
Measurements Acquisition (PUMA) effort.
46
Prior to RISO, ACCENT, and
PUMA, uncertainties regarding ozone loss from SRMs were much greater
than present day uncertainties regarding LREs and these efforts returned
high-value emissions data at relatively low cost. Renewed and vigorous
rocket emissions research would have the goal of reducing the uncertainties
of LRE ozone loss to the same level as SRMs and provide a predictive
capability for evaluating the ozone loss associated with emerging propulsion
technologies.
SUMMARY
Space travel’s impact on stratospheric ozone can be relatively significant in
comparison to other industrial activities because rockets uniquely emit ozone
destroying compounds throughout the stratosphere. Both solid and liquid
fueled rockets cause ozone loss. Based on existing data, models, and general
principals of rocket combustion and stratospheric chemistry we constructed
a simple description of the relationship between rocket combustion emis-
sions and ozone depletion and then related ozone depletion to the mass
of payload placed into LEO. Because stratospheric rocket emissions are
not fully understood, our description is necessarily uncertain, especially with
respect to liquid propellant engines. Even so, we draw several conclusions
and provide guidance for future work.
Limits on the Space Launch Market Related to Stratospheric Ozone Depletion 79
Astropolitics 2009.7:50-82.
Present day global ozone loss caused by rocket emissions is dominated
by SRM emissions and is almost certainly less than 0.1%, insignificant relative
to other sources of ozone loss at the present time. The relative impact of
rocket emissions will likely increase over the next several decades as the
requirements of a growing space industry grow and the ozone layer recovers
from past use of ozone depleting substances that have been now banned by
international agreements.
Global ozone loss associated with space development scenarios that
assume large increases of payload delivered to orbit could be significant,
even using liquid propellants. Growth of a factor of one hundred could cause
several percent global ozone losses, likely large enough to trigger attention
by the international stratospheric protection community. Regulation of
launches might take the form of limitation of the number of types of launches
or mass of payloads and might apply globally or nationally. Such limits
would present significant distortions in what is usually assumed to be an
emerging free market for launch services. One implication of launch limits
associated with ozone depletion is to increase the difficulty of recovering
large investment to reduce launch cost through increased launch rates.
Because of the large uncertainty over the impacts of liquid propellant rockets
on ozone, and the lack of a clear process to assess the ozone loss caused by
rocket emissions, the potential for limitation on space transportation cannot
be eliminated. This potential presents a long-term risk that space develop-
ment could be hampered by overly aggressive ozone protection efforts that
might arise from a lack of information on rocket emissions. Policy makers in
both the space development and stratospheric protection communities
should begin to better understand ozone loss from rocket emissions, how
to quantify those losses, and how to manage the loss if the space transport
business grows significantly in the future.
ACKNOWLEDGMENTS
This work was supported at The Aerospace Corporation by the NASA
Airborne Sciences Program. The authors thank reviews and comments by
J. Vedda, P. Zittle, R. Seibold, J. Skratt, and G. Law.
NOTES
1. S. Anderson and K. Sarma, Protecting the Ozone Layer. (London, UK: United Nations Environment
Program, 2002): 274–288.
2. R. Cicerone and D. Stedman, ‘‘The Space Shuttle and Other Atmospheric Chlorine Sources,’’ NTRS
paper 75A35353, 1974.
3. World Meteorological Organization, Scientific Assessment of Ozone Depletion, (Geneva,
Switzerland, 2002).
4. A large body of literature explores the potential for large and accelerating growth in the space
industry. Examples include: ‘‘Commercial Space Transportation Study,’’ CSTS Alliance (Langley, VA: NASA,
80 M. Ross et al.
Astropolitics 2009.7:50-82.
1994): 230–256; J. R. Wertz, ‘‘Economic Model of Reusable Vs Expendable Launch Vehicle,’’ IAF Congress,
(Rio de Janeiro, Brazil, 2000); J. Penn and C. Lindley, ‘‘Spaceplane Design and Technology Considerations
Over a Broad Range Of Mission Applications,’’ IEEE paper 0-7803-4311-5 (1998).
5. J. Penner et al., Aviation and the Global Stratosphere, (Cambridge, MA: Cambridge University
Press, 1999): 176–225.
6. A number of news, analysis, and commentaries have noted that the public and regulatory atten-
tion focused on aviation emissions as a source of climate change is out of balance, on a percentage basis,
with their actual contribution.
7. European Space Agency (ESA), ‘‘Suborbital Spaceflight ESA Position Paper 14,’’ (April 2008). It is
interesting that ESA felt the need to discuss the environmental effects of so-called space tourism when the
industry does yet even exist.
8. D. Waugh and T. Hall, ‘‘Age of Stratospheric Air: Theory, observations, and models,’’ Revs. of Geo-
phys. (2002): 46–68. The washout rate of material deposited in the stratosphere is usually considered to be
in the range of 3–5 years but this parameter is not clearly understood and is likely variable from year to year.
9. Daniel J. Jacob, Atmospheric Chemistry (Princeton, NJ: Princeton University Press, 1999), Ch. 10
is a good introduction to stratospheric chemistry. J. Farman, et al., ‘‘Large losses of total ozone in
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ozone loss.
10. M. N. Ross, et al., ‘‘Observation of stratospheric ozone depletion in rocket plumes,’’ Nature 390,
(1997): 62–65.
11. Andersen and Sarma (note 1).
12. World Meteorological Organization, Scientific Assessment of Ozone Depletion, (Geneva, Switzer-
land:WMO, 1990, 1994, 1998, 2002, and 2006).
13. Strictly speaking, ozone depletion has only stopped worsening. The stratospheric chlorine load is
clearly decreasing approximately as expected. See Fahey et al., ‘‘Twenty Questions and Answers About the
Ozone Layer: 2006 Update,’’ (Geneva, Switzerland: World Meteorological Organization, 2007).
14. The Technology and Economic Assessment Panel of the Montreal Protocol meets on a regular
basis to review new data and determine which substances are to be phased-out and to approve Special
Use Exemptions requested by signatory countries.
15. J. A. Vedda, ‘‘Challenges to the Sustainability of Space Exploration,’’ Astropolitics 6 (2008): 22–49.
16. Rocket plume ozone holes have been observed in situ and on the ground, minutes to hours after
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17. S. Solomon, et al., ‘‘On the Depletion of Antarctic Ozone,’’ Nature 321 (1986).
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19. The actual steady state accumulation of particles depends on particle size, density, and assumed
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20. D. Kirk-Davidoff, et al., ‘‘The effect of climate change on Ozone Depletion Through Changes in
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37. Space-Based Solar Power as an Opportunity for Strategic Security, (Washington, DC: National
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39. Penner et al., (note 5). Also see ‘‘Aviation Climate Change Research Initiative,’’ (Federal Aviation
Administration, 2009). The increase is mainly in the northern hemisphere.
40. Ibid, 150.
41. US Airlines face $9 Billion Carbon Bill by 2020, Flight International 16 May, 18, 2008.
42. S. J. DeCanioa and C. Norman, ‘‘Economics of EUEs for metered-dose inhalers under the Montreal
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43. US Airlines face $9 Billion Carbon Bill by 2020, Flight International, 2 May 2008.
44. M. N. Ross, ‘‘Rocket Impacts on Stratospheric Ozone (RISO),’’ Symposium on the Effects of Aircraft
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82 M. Ross et al.
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