This image from the ISS shows strong tropospheric storm clouds pushing up against and being blocked at the tropopause, illustrating the decoupled and isolated nature of the stratosphere from the troposphere below. This relative isolation allows rocket emissions to accumulate in the stratosphere. Credit: NASA

This op-ed originally appeared in the June 4, 2018 issue of SpaceNews magazine.

In recent years, governments, intergovernmental organizations, and businesses have begun to focus on the challenge posed by orbital debris. As often seems to be the case, we appear to be a decade or two too slow in coming to consensus on the risks. If we had foreseen a half-century ago the challenges that orbital debris presents today, what would we have done differently? Combustion emissions from launch vehicles present the space industry with a comparable concern that we can begin to address now, before it grows and becomes a potential impediment to space access.

Most human-generated pollution is concentrated on or near the surface of the Earth, whether on land, sea, or in the troposphere, the lowest layer of the atmosphere. However, rockets emit a variety of gases and particles directly into all levels of the stratosphere, the only industrial activity to do so. The stratosphere extends roughly from 10 to 50 kilometers above the Earth’s surface and contains the Earth’s ozone layer. The global civil aviation fleet generally cruises in the troposphere, only occasionally polluting the stratosphere directly.

Among the most consequential emissions are soot and alumina, which are long-lived and accumulate in the stratosphere. These accumulations promote chemical reactions and absorption and scattering of sunlight that modify the composition and flow of radiation in the stratosphere. Ultimately, these processes reduce stratospheric ozone, warm the stratosphere, and cool the Earth’s surface. Little is known about these particle accumulations and their contributions to stratospheric ozone depletion and thermal perturbations because of a lack of consistent and focused research.

Since 1987, emissions of ozone-depleting pollutants are highly regulated by international agreement through the Montreal Protocol on Substances That Deplete the Ozone Layer. Even with recent advances in reusability and the introduction of large launch vehicles and new launch sites around the globe, rocket launches occur irregularly so that concerns about the damage done to the ozone layer by rocket emissions have not elicited regulation. But with projections that the global launch rate will at least double in the coming decade, increased scrutiny under the Montreal Protocol is likely. Increased concerns about the environmental impact of rocket launches, provoked by perceptions of a rapidly growing launch industry, could result in international calls for launch limitations or the phase-out of propellants that the launch industry has come to depend on.

The timing and intensity of a regulatory backlash as launch rates increase is impossible to predict accurately, especially because the science of rocket emissions is still not well understood. Rather than allow a legal and regulatory process to unfold in the absence of high-quality, peer-reviewed data, governments and the launch industry should conduct the scientific research needed to fill the knowledge gaps. This will allow the launch community to engage in future far reaching discussions regarding the impacts of rocket emissions with the support of empirical data and computer models that carry the imprimatur of the rocket engineering and atmospheric science communities.

The launch industry has enjoyed freedom of action with respect to rocket engine emissions since the start of the space age. Studies of future launch architectures, market demand, and lifecycle costs rarely consider regulation of emissions as a potential future risk factor. Even when emissions are considered, the impacts are examined on a system-by-system basis; the cumulative impact of the global launch fleet is not acknowledged. The net impacts of the global launch industry, across all propellant types, are the parameters of interest to international regulators and, therefore, the global impacts create the regulatory risk.

Space Shuttle Endeavour bursts through the clouds on its way to the International Space Station in May 2011. Credit: NASA
Space Shuttle Endeavour bursts through the clouds on its way to the International Space Station in May 2011. Credit: NASA

In addition to acknowledging the risks and potential unintended consequences of launch emissions for ozone and the flow of radiation in the atmosphere, the space industry must recognize the extent that other emerging actors may interact with the stratosphere. For example, so-called “geoengineering” or “climate intervention” schemes propose to inject particles into the stratosphere to intercept sunlight and mitigate the warming effects of carbon dioxide and other greenhouse gases.

Regulation of such geoengineering activity is already under discussion. Space launch operators, as contributors of stratospheric emissions, could get swept up into these discussions, which involve the same types of particulate matter associated with rocket emissions. Any resulting regulations or guidelines must include adequate consideration of launch activities, which will require a better understanding of rocket emissions than we have today.

To improve that understanding, industry should encourage and support scientific research on rocket engine emissions and how they affect the atmosphere. There has been little research to date. The few research papers that have appeared in recent decades mostly point out the knowledge gaps rather than add to the knowledge base. The research has been unfocused, disorganized, and not suited to the needs of the launch industry. As it stands today, the scientific community can predict ozone depletion attributable to rocket emissions to no better than an order of magnitude. In an environment of growing launch rates, new propellants, larger, reusable launch vehicles, and the emergence of other stratospheric polluters, this is not sufficient. Lack of accurate information inevitably invites distorted competitive claims and unwarranted and overly restrictive regulation.

A vigorous research program would be guided by the goal to collect high confidence information and data that describe rocket emissions as inputs into global atmosphere models and would include the following components:

  • Stratospheric plume measurements using in situ and remote sensing instrumentation.
  • Laboratory measurements of particulate emission microphysics.
  • Test stand measurements of engine exit plane exhaust composition.
  • Modern rocket engine combustion, plume chemistry, and global atmosphere models.

All of the instrumentation, models, and expertise to carry out this research already exists within the engineering and scientific communities. The in situ and test stand measurements would validate combustion and plume models. Validated models permit the development of emission profiles for particular rocket engine types. These profiles, with various growth assumptions, would be used to construct global emission projections. Finally, the global emissions scenarios would provide data to construct input profiles for modern three-dimensional whole atmospheric chemistry and climate models in order to estimate ozone loss, climate forcing, and a variety of secondary effects such as changes in the global circulation and cloud formation.

A policy to promote objective and vigorous research, across the full range of propellant types, will provide the space industry with the information required to take ownership of the problem and exert strong influence on the future debate. By accepting the reality of the risk to freedom of action presented by rocket emissions, and promoting a full and complete scientific understanding of the global impacts, the industry can best inoculate itself from attempts to regulate or limit launch development and operations and disassociate itself from other polluters.

There is historical precedent for such an approach. In order to promote supersonic civil aviation development, during the 1990s NASA partnered with the aviation industry to carry out the High Speed Research (HSR) program. One of the goals of HSR was to understand how High Speed Civil Transport (HSCT) aircraft would affect stratospheric ozone. Earlier HSCT efforts in the 1970s were severely and wrongly hampered by knowledge gaps with respect to ozone depletion. HSR demonstrated the airframe, engine, and operational combinations that would minimize ozone impacts and permit (if the economics had been convincing) unregulated development and deployment. The launch industry should organize around a similar approach and partner with the scientific and regulatory communities to determine how space launch can freely develop while minimizing the risks of regulatory intervention.

As launch rates and launch vehicle sizes increase, the impact of rocket emissions approaches a “tipping point” when international regulation becomes likely, probably beginning with efforts to protect the ozone layer or limit stratospheric pollution to ward off geoengineering. If the launch industry moves quickly to support the necessary scientific research and fully understand these impacts – in concert with other private-sector and government stakeholders – it is more likely that future regulation will be well-informed and as limiting as possible.

As with other large-scale ventures, the application of specialized expertise is essential to anticipating the risks and needs of the enterprise and to managing the impacts on society. With irrefutable data, modeling, and analyses, emissions-related regulations or limitations can be anticipated and configured to ensure that space-based capabilities and systems continue to enhance and improve human life and extend the space industry’s progress made over the past six decades.

Martin Ross is a senior project engineer in civil and commercial launch programs at the Aerospace Corporation. James Vedda is a senior policy analyst at the Aerospace Corporation’s Center for Space Policy & Strategy. The opinions expressed here are the authors’ own.