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In 2021, there were 146 space launches, out of which 51 were from the U.S., 55 from China, and 25 from Russia. The launches are expected to rise, which can deteriorate the ozone layer, thus, creating a detrimental effect on the Earth’s climate.
So far, the environmental impacts of rocket launches have been largely overlooked. Many experts have claimed that the amount of pollution generated by space missions is negligible compared to other sources of pollution. For example, within one year, the aviation industry alone burns about 100 times more fuel than all rockets launched around the world, according to experts. However, the ratio might soon change as the number of rocket launches, which has more than tripled in the past decade, keeps growing in the years to come.
“Last year’s number of missions was 144 worldwide,” says William Pomerantz, vice president of special projects at Virgin Orbit, which launches small satellites horizontally from under the wing of a Boeing 747. Their rocket uses about 1/20th of the fuel of typical ground-launched, heavy-lift rockets, and recent launches include satellites now playing a key role in the collection of climate data. “Given this volume, the space launch industry remains a relatively small driver of atmospheric emissions compared with say, commercial aviation with more than 20 million flights worldwide, and other industries.”
Eloise Marais, an associate professor in physical geography at University College London, and co-author of one of the recent research papers, thinks this comparison is “erroneous”. “When we compare the amount emitted from rocket launches to aircraft, it doesn’t sound like a lot,” she says. “But this comparison was always erroneous because aircraft released their pollutants within the troposphere and the lower stratosphere, whereas rockets are releasing their pollutants all the way from the surface of the Earth to the mesophere, and when pollution is released into those upper layers it lasts for a longer time than earthbound sources.”
Projected growth in rocket launches for space tourism, moon landings, and perhaps travel to Mars has many dreaming of a new era of space exploration. But a NOAA study suggests that a significant boost in spaceflight activity may damage the protective ozone layer on the one planet where we live, thus, creating a detrimental effect on the Earth’s climate.
In June 2022, researchers from the Massachusetts Institute of Technology (MIT), the University of Cambridge, and the University College London (UCL) observed that the soot emitted from rocket launches had impacted the atmosphere by generating more heat when compared to other emitting sources.
Kerosene-burning rocket engines widely used by the global launch industry emit exhaust containing black carbon, or soot, directly into the stratosphere, where a layer of ozone protects all living things on the Earth from the harmful impacts of ultraviolet radiation, which include skin cancer and weakened immune systems in humans, as well as disruptions to agriculture and ecosystems.
The emitted pollutants are responsible for the depletion of the ozone layer in the stratosphere. Rockets are the only direct source of human-produced aerosol pollution above the troposphere, the lowest region of the atmosphere, which extends to a height of about 5 to 10 miles above the Earth’s surface.
The spacecraft sent into orbit follows a multi-stage configuration and is plunged to pass through the Earth’s atmosphere. With every stage, the auxiliary parts are separated from the launch vehicle and are discarded or reused in further explorations.
The study, led by researchers from the U.S. National Oceanic and Atmospheric Administration (NOAA), focused on the impact of fossil-fuel burning rockets, such as SpaceX’s Falcon 9. The research team used a climate model to simulate the impact of approximately 10,000 metric tons of soot pollution injected into the stratosphere over the northern hemisphere every year for 50 years. Currently, an estimated 1,000 tons of rocket soot exhaust are emitted annually. The researchers caution that the exact amounts of soot emitted by the different hydrocarbon fueled engines used around the globe are poorly understood.
The study analyzed the consequences of a tenfold increase in rocket launches, which is in line with current predictions. The researchers were interested in the soot content in the exhaust of rockets burning fossil fuels. Currently, rockets inject about 1,000 tons of soot per year into the otherwise pristine upper layers of Earth’s atmosphere. This pollutant accumulates at high altitudes over the years and absorbs heat, which can lead to the warming of those atmospheric layers.
The study found that a tenfold increase in the amount of soot injected into the stratosphere every year would after 50 years lead to an annual temperature increase in that layer of 1 to 4 degrees Fahrenheit (0.5 to 2 degrees Celsius). The stratosphere is the layer of the atmosphere just above the lowest troposphere. The study found that the projected warming would slow down subtropical jet streams, bands of strong wind circling the planet at the lower edge of the stratosphere that influence the African and Indian summer monsoons.
Warmer temperatures in the stratosphere would also degrade the protective ozone layer, which blocks harmful ultraviolet radiation from the sun from reaching the planet’s surface. The researchers found that the tenfold increase in concentrations of soot in the stratosphere would especially affect ozone concentrations in the Northern Hemisphere, particularly locations at least from 30 degrees north of the equator.
The research team also simulated two larger emission scenarios of 30,000 and 100,000 tons of soot pollution per year to better understand the impacts of an extremely large increase in future space travel using hydrocarbon-fueled engines, and more clearly investigate the feedbacks that determine the atmosphere’s response. Results showed that the stratosphere is sensitive to relatively modest black carbon injections. The larger emission simulations showed a similar, yet more severe disruptions of atmospheric circulation and climate loss than the 10,000 metric ton case.
“Our work emphasizes the importance of ozone depletion caused by soot particles emitted by liquid-fueled rockets,” Ross said. “These simulations change the long-held belief that spaceflight’s only threat to the ozone layer was from solid-fueled rockets. We’ve shown that particles are where the action is for spaceflight’s impacts.”
While the new research describes the influence that soot in rocket exhaust has on the climate and composition of the stratosphere, the scientists said it represents an initial step in understanding the spectrum of impacts on the stratosphere from increased space flight.
Combustion emissions from the different rocket types will need to be evaluated, they said. Soot and other particles generated by satellites burning up when they fall out of orbit is also a growing, poorly understood source of emissions in the middle-to-upper atmosphere. These and other topics will need further research to produce a complete picture of space industry emissions and their impacts on Earth’s climate and ozone.
Even though rockets running on fossil fuels are still the most common today, new technologies are already in use or being developed that seem to have a lower environmental impact. Now there is a race on to develop alternatives to existing fuels like RP-1 and UDMH, and liquid methane appears to be in the lead. Several new rocket engines, including SpaceX’s Raptor and the European Space Agency’s Prometheus engine, have been designed to use this gas as a fuel because it has a higher performance than other fuels, meaning the rocket can be smaller and produce less soot when it’s launched. Its lower cost means the price of a rocket launch can be reduced, too.
For example, the combination of liquid oxygen and liquid hydrogen, which is used in Blue Origin’s New Shepard suborbital rocket, emits only water vapor. Also, the combination of liquid oxygen and methane, if burned efficiently, generates very little pollution, according to experts.
Several rocket start-ups are at a relatively early stage of experimenting with sustainable alternatives to RP-1 made from waste plastic or biomass. Such start-ups typically focus on the easier tasks of reducing their carbon footprint and protecting the environment around the space port, as well as the harder job of cutting emissions in the stratosphere.
Orbex is a UK-based low-cost launch company with a rocket factory in Forres, near Inverness in Scotland. Orbex plans to launch its small rocket called Prime up to 12 times a year from Space Hub Sutherland in the far north of the country. The fuel its rocket runs on is bio-propane, a renewable biofuel created as a waste product from the production of biodiesel. Orbex’s rocket could end up with around 90% fewer emissions than an RP-1-fuelled launch. It should also produce less soot than rockets burning its kerosene cousin.
The other option to reduce the industry’s atmospheric impact is to explore new ways of launching satellites, horizontally like Virgin Orbit, or even in a sling shot, as Nasa is exploring. Like a child’s toy, this will work by attaching a rocket payload to the end of a huge arm that will be accelerated by electric motors to very high speeds, flinging the rocket out into space.
References and Resources also include:
https://www.bbc.com/future/article/20220713-how-to-make-rocket-launches-less-polluting
