What’s the Impact of the Space Industry on Climate Change?

ULA’s Delta IV Heavy rocket uses a combination of liquid hydrogen and liquid oxygen for fuel. Image courtesy of ULA.

In honor of Earth Day, we asked Marty Ross, Aerospace rocket emissions expert studying the effects of aerospace propulsion on the Earth’s atmosphere, about the climate impacts of rocket launches, and how the increased pace of launch might change those outcomes.

There is a lot of information about the impact of airplane emissions on global warming. Do rocket emissions have a similar impact?

The concept of “carbon footprint” is often used as an easily understood measure to compare the climate impact of one kind of transportation system to another. However, because the emissions and technologies of each transportation system, i.e., automobiles, ships, aviation, and space, are unique in time and location, so are their climate impacts. Each transport system is associated with different emissions consisting of gases and particles emitted into the atmosphere at different altitudes, each with different lifetimes. The definition of “carbon footprint” or “carbon equivalent” depends on fully understanding the specific science associated with each system’s emissions and atmospheric response. There is no universally agreed-upon definition of these concepts for spaceflight emissions.

Sustainability has not been much of a concern for space systems development. Just like their jet engine cousins, rocket engines emit a variety of gases and particles into the atmosphere that can have regional and even global consequences. Even so, the environmental impacts of launch vehicles are typically disregarded by comparing jet and rocket fuel consumption in an overly simplified way.

The CO₂ emission from rockets is insignificant in the global picture. Aviation represents about 3 percent of the annual global CO₂ emission. Rockets burn less than 0.01 percent of the fuel that aircraft burn every year and emit less CO₂ than jets do per kilogram of fuel, so rockets emit less than 0.01 percent of the CO₂ than aviation.

CO₂ is only part of the story, however. Rockets emit large quantities of soot and alumina particles directly into the very sensitive stratosphere where aircraft rarely fly. For rockets, these particles can have a much greater climate impact than CO₂. Rockets also emit water vapor into the upper atmosphere which causes climate forcing. The climate forcing from a rocket’s particle emissions is likely smaller than aviation’s impact but remains an unanswered question. Future research will allow scientists to calculate the total amount of climate impact caused by rockets from CO₂, H₂O, and particles. Then it may be possible to carefully compare aviation climate change with rocket climate change.

The stratosphere, containing the ozone layer, is susceptible to pollution from soot and alumina from rocket launches, though more study is needed to estimate the climate impact. Atmosphere diagram courtesy NASA.

Will the burgeoning Space Tourism industry have a big carbon footprint?

Climate impacts depend on the propellant type and the many different types of rockets used to carry private space travelers. The intense heat of spacecraft reentry can also disturb the atmosphere. There is insufficient scientific investigation of space industry emissions to define a “carbon footprint” for launching passengers into space and returning them to Earth. Scientific research that clearly determines the net climate impact needs to be performed for rocket launches and spacecraft reentries to define and calculate a carbon footprint for private space travelers.

The several types of suborbital rockets that are commonly associated with space tourism are similar to the rockets used to place spacecraft into orbit, except that they are very much smaller. Each suborbital launch uses only a few percent of the fuel of a typical orbital launch. The global impacts of many hundreds of suborbital “Space Tourist” launch vehicles per year of any type would be small compared to the effects of present-day orbital launches.

What kind of emissions is specific to space launch?

Rocket engine exhaust, like jet exhaust, is mostly carbon dioxide (CO₂) and water vapor (H₂O), and the global impacts of these emissions are well understood. CO₂ emitted at any altitude is a “long-lived” greenhouse gas (GHG), adding to the atmospheric GHG burden. H₂O is a “short-lived” GHG. These components of rocket exhaust are less than 0.01 percent of aviation’s CO₂ and H₂O outputs. Though the climate consequences of the CO₂ and H₂O parts of rocket emissions are not significant, other components of spaceflight emissions might be.

Solid-rocket motors (SRMs) emit chlorine gas, the most serious threat to stratospheric ozone. Chlorine damage to the ozone was the reason “spray cans” of the last century were banned. Direct plume sampling 20 years ago by NASA aircraft showed SRM plumes creating dramatic ozone mini holes persisting for several days after a launch. The mini holes do disappear as plumes mix into the global stratosphere and the total amount of chlorine emitted by all SRMs annually is very small compared with the amount released by the long-banned spray cans, so chlorine from SRMs is not a serious threat to the ozone layer currently.

What is the climate concern about spaceflight’s particle emissions?

For spaceflight, particles uniquely play the dominant role in their impact on Earth’s climate and the ozone layer. Three types of particles characterize spaceflight emissions: (1) black carbon soot particles mainly from hydrocarbon fueled rocket engines, (2) alumina (Al₂O₃) particles from SRM rockets, and (3) dust particles of largely unknown composition produced during reentry “burnup”.

Currently, rocket soot and alumina are the largest sources of particle emissions; reentry particles are expected to play a large role in the future. These particles accumulate in the stratosphere and form distinct, though invisible to the naked eye, layers according to their size and composition, absorbing and scattering sunlight before it reaches Earth’s surface causing climate forcing. Because the sunlight intercept occurs high up in the stratosphere, the climate impacts are complex and difficult to understand.

Are some rocket fuels better or worse than others?

There are four main types of rocket propellant types in use today, each being a mixture of fuel and oxidizer, and while the data vary from year to year, in 2022 more than half of all rocket fuel burned was hydrocarbon-based. In addition to hydrocarbon kerosene, other types of rocket fuel include solid (SRMs), hypergolic, and hydrogen. A fifth type, methane fueled, will soon be used as well.

Each propellant has a unique emissions fingerprint, and each causes a different kind of impact on the atmosphere. Any comparison of the global impact of one rocket type to another is complicated by a lack of complete emissions data, insufficient models of the atmosphere, different efficiencies for putting payloads into orbit, and a lack of standard metrics on which to base comparisons. The attention from scientists and policymakers needed to clearly define which propellant is more “green” has not yet been done.

From the current research, the available scientific data suggests that hydrogen-fueled rocket engines, which emit mostly water vapor, present the smallest effect to the global atmosphere. Research continues both to understand all rocket emissions and to define appropriate comparison metrics.

Soyuz rocket plume seen from the International Space Station shows complex diffusion patterns of soot, ice particles, and exhaust gases in the upper atmosphere. Images courtesy NASA / Christina Koch @Astro_Christina

What about reentry of rocket stages and other space debris? Does this affect the atmosphere, too?

Orbital debris returning to Earth does not disappear on reentry. Some spacecraft parts will survive and reach the surface of the Earth. Most of the reentering mass vaporizes into a hot gas that condenses into a spray of small particles — bright reentry plumes high in the atmosphere mean particle production.

Reentry is as much of an emission as a launch. Unlike the chemically simple particles from launch, particles from reentering space junk are a zoo of complex chemical types. Particles from vaporizing propellant tanks, computers, solar panels, and other exotic materials will form around an 85-kilometer altitude, then drift downward, accumulating in the stratosphere along with the launch’s soot and alumina.

The growing number of low earth orbit (LEO) satellites constellations, each with thousands of spacecrafts, all use reentry “burnup” as the way to dispose of old satellites that no longer function. After these constellations are fully deployed, hundreds of tons of nonfunctioning satellites will be brought in for disposal every year. Most of this mass will become particles in the atmosphere affecting climate and ozone, similar to launch’s black carbon and alumina particles. Very little is known about this reentry dust production and how it could affect climate and ozone.

Space debris from defunct Starlink satellites reentering over Puerto Rico, February 7, 2022. Credit: Eddie Irizarry/Sociedad de Astronomía del Caribe

With the frequency of launches and number of satellites in orbit increasing every year, is this an impending problem? What can be done to mitigate these issues?

Scientists do not yet know with certainty about spaceflight’s growing impacts. If the space industry were to freeze as it is today — not grow or innovate any further — scientists could say, within the present scientific uncertainties, that rocket and reentry emissions will not present a significant future impact on climate or ozone. But spaceflight is growing and innovating at a very rapid pace and the scientific information we have today is not at all sufficient to have confidence in predicting spaceflight’s future impacts.

Space industry emissions themselves are too poorly known to answer fundamental questions about the environmental impact and sustainability. An appropriate scientific program would include models and measurements of launch and reentry plumes, detailed atmosphere models of emissions—from newly created plumes to final steady-state global mixing—and laboratory measurements of the microphysics of all the different particle types generated from launch to reentry.

Sustainability needs to be a fundamental aspect of space system development. In the future, every industry will be expected to be transparent and responsible with respect to climate impacts to ensure that policymakers have the right information to avoid a climate emergency. It will be easier to guarantee freedom of action for space systems if the environmental impacts of every stage in spaceflight’s life cycle are carefully evaluated ahead of time.

Aerospace is serving as a thought leader and center of cooperation among space industry stakeholders and the scientific community to carry out the new research needed to obtain measurements of key parameters describing launch and reentry emissions.

Have a question about launch emissions, satellite reentry, or (literally) anything else in space? Aerospace probably has an expert on the topic! Tweet at us @AerospaceCorp with your questions and comments.

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The Aerospace Corporation

The Aerospace Corporation

We operate the only federally funded research and development center (FFRDC) committed exclusively to the space enterprise.