Rockets: Air Pollution Culprits Or Justified Exploration?

Rocket launches are an integral part of the 21st century, but they are also a source of air pollution. While the amount of pollution emitted by rocket launches is relatively small compared to other industries, the impact of rocket emissions on the climate and ozone layer is a growing concern, especially with the projected increase in space travel and the space tourism industry. The pollution from rockets is released at high altitudes, up to 80 km, and can persist for several years, affecting global circulation patterns and ozone depletion. The type of fuel used by rockets also influences the level of pollution, with some companies experimenting with new types of fuel that may burn cleaner. As the space industry continues to grow, addressing the environmental impact of rocket launches and finding ways to reduce pollution will become increasingly important.

Rocket fuel

RP-1, or Rocket Propellant-1, is a highly refined form of kerosene that has been used in iconic rockets such as the Saturn, Delta, Atlas, Soyuz, SpaceX's Falcon 9, and Virgin Orbit's horizontally launched rocket. It is popular due to its stability at room temperature, relatively low cost, and lack of dangerous explosiveness. However, RP-1-fueled rockets produce soot and carbon dioxide, a greenhouse gas contributing to climate change.

Methane-based fuel, used by SpaceX's Starship, is another type of rocket fuel that has gained attention for leaving less soot residue and facilitating engine reusability. While it produces less soot than kerosene-based fuels, it still releases methane, a potent greenhouse gas.

UDMH, or unsymmetrical dimethylhydrazine, was commonly used in rockets launched from the Baikonur Cosmodrome, including the world's first artificial satellite, Sputnik 1, and the first human spaceflight, Vostok 1. UDMH is highly carcinogenic and has been dubbed "devil's venom" due to its ecological impact, turning a large area of the Kazakh Steppe into an ecological disaster zone.

The impact of rocket fuel emissions on the environment is a growing concern, and companies are experimenting with new types of fuel to reduce their environmental footprint. Virgin Orbit, for example, is exploring the use of bio-propane, a renewable biofuel, while Orbex is developing a micro-launcher that runs on biopropane, which is expected to significantly reduce emissions and soot production.

While the space launch industry's contribution to atmospheric emissions is relatively small compared to commercial aviation, the increasing launch rates and the potential for space tourism highlight the need to address the climate impact of rocket fuel emissions.

Climate impact

Rocket launches are a 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 (8 to 16 km) above the Earth’s surface. Rocket emissions pollute at extremely high altitudes, up to 80 km, and the pollutants can last for a long time in the upper layers of the atmosphere.

Rocket launches generate between 50 and 75 tonnes of CO2 per passenger. A single passenger aboard a rocket is responsible for 100 times more pollution than an airline passenger. The number of rocket flights is currently very small compared to aircraft flights: according to NASA, there were only 114 attempted orbital launches in 2020, compared to over 100,000 airplane flights per day. However, the market for space tourism is projected to grow, with current estimates putting the market at $2.58 billion in 2031, with a 17.15% growth rate each year over the next decade.

The pollutants emitted by rockets include alumina, chlorine, nitrogen oxides, hydroxyl, water vapour, and black carbon or soot. These pollutants deplete the ozone layer, which protects the Earth from the sun's harmful ultraviolet radiation, including skin cancer and weakened immune systems in humans, as well as disruptions to agriculture and ecosystems. The ozone reductions occur poleward of 30 degrees North, or roughly the latitude of Houston, in nearly all months of the year. The maximum reduction of 4% occurred at the North Pole in June. All other locations north of 30° N experienced reduced ozone throughout the year.

Some companies are experimenting with new types of fuel that may burn cleaner. For example, SpaceX's Starship burns liquid natural gas, which is almost pure methane, and produces less soot residue. However, it is not yet clear whether this type of rocket fuel will reduce environmental impacts or cause different ones.

Space junk

There are proposals to clean up space debris, such as the European Space Agency's ClearSpace-1 mission, which is set to launch in 2026. It aims to remove a 94 kg satellite from orbit by grabbing it with robotic arms and dragging it down to Earth's atmosphere where it will burn up. Another proposal involves using a ground-based laser to slow down debris so that it falls into the Earth's atmosphere.

Ozone depletion

The ozone layer, situated in the stratosphere, acts as a protective barrier for life on Earth, shielding it from the sun's harmful ultraviolet (UV) rays. In the 1980s and 1990s, this layer was severely damaged due to chlorofluorocarbons (CFCs), chemicals found in aerosols and refrigeration. As a result of coordinated global action and legislation, the ozone layer is now recovering. However, the increasing number of rocket launches has raised concerns about potential ozone depletion.

Rocket launches emit gases and particulates that can accumulate in the stratosphere and alter the atmosphere. While some rocket propellants, such as liquid oxygen and liquid hydrogen, produce mainly water vapour and have minimal environmental impact, others, like kerosene, release black soot and alumina. These emissions can remain in the stratosphere for years, affecting the ozone layer in two ways. Firstly, black soot acts like a "black umbrella," absorbing sunlight and warming the stratosphere. Secondly, alumina particles reflect sunlight back into space, triggering chemical reactions that destroy the ozone.

The magnitude of ozone depletion caused by rocket emissions is not yet fully understood due to limited research in this area. According to Martin Ross, an atmospheric scientist at The Aerospace Corporation, the problem has been relatively unnoticed as rocket launches were considered rare. However, with the increasing frequency of launches and the emergence of new space race participants, there are growing concerns about the potential impact on the ozone layer.

To address these concerns, researchers and industry experts are calling for more comprehensive and focused scientific studies on rocket engine emissions and their effects on the atmosphere. By improving our understanding of atmospheric circulation and the behaviour of particles injected into the stratosphere, we can better assess the potential risks associated with rocket launches. Additionally, increased scrutiny under the Montreal Protocol, an international agreement to phase out ozone-depleting materials, may be warranted as the global launch rate continues to rise.

In conclusion, while rocket launches have become more frequent, their impact on the ozone layer is not yet fully understood. To ensure the sustainability of the global launch industry, coordinated global action and further research are necessary to protect the Earth's atmosphere and the vital ozone layer that shields us from harmful UV radiation.

Alternative fuels

The rocket industry has a significant impact on our climate, and companies are experimenting with new types of fuel that burn cleaner. For instance, SpaceX's Starship burns liquid natural gas, which is almost pure methane. This methane-based fuel leaves less soot residue and makes it easier to reuse engines. However, it is unclear whether this rocket fuel will reduce environmental impacts or cause different ones. Blue Origin's BE-4 rocket also runs on liquified natural gas, and while it produces a lot less soot than kerosene-fuelled rockets, it still releases methane, a potent greenhouse gas.

Hybrid rockets that use specific oxidizer-fuel combinations are considered a green alternative to current propulsion systems. They do not release very toxic or polluting exhausts, only less harmful substances such as carbon monoxide/dioxide and soot. However, as the space industry becomes more prevalent, the use of current green propellants may become insufficient if the industry follows stricter environmental rules. Therefore, alternative sustainable solid fuels for hybrid rockets that are not derived from fossil fuels and are ideally carbon-neutral are being investigated.

The chemicals used in space propulsion have often been toxic, carcinogenic, and corrosive. For example, the fuel UDMH (unsymmetrical dimethylhydrazine), used in many rockets that blasted off from the Baikonur Cosmodrome, was dubbed "devil's venom" by scientists due to its highly carcinogenic nature. It is blamed for turning a large area of the Kazakh Steppe into an ecological disaster zone.

The pollution caused by rocket emissions can seem insignificant compared to other global challenges and the benefits the space industry brings. The percentage of fossil fuels burned by the space industry is only about 1% of that burned by conventional aviation. However, as the space tourism industry grows, the impact of rocket emissions will become more significant. Approximately two-thirds of propellant exhaust is released into the stratosphere and mesosphere, where it can persist for at least 2-3 years and contribute to ozone depletion through chemical reactions.

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