Since the Soviet Sputnik-1 in 1957, the first artificial satellite in Earth orbit, more than 30,000 rockets have been launched into space, including orbital and suborbital flights. These spacecraft burn fuels that leave an environmental and climate footprint. And while the number of launches may seem insignificant when compared to air and road traffic, scientists warn that there are particular reasons why spaceflight is a growing concern for the climate and the environment, especially with space tourism set to take off in the coming decades.
In the days of the US-USSR space race, everything was sacrificed for one goal: winning. Money was flowing in torrents with no need for a return on investment, and everything was disposable. The concept of sustainability had not yet become popular, and environmental impacts were not taken into account. One of the consequences of this mentality has been the accumulation of space junk: the US Space Surveillance Network has catalogued more than 28,000 objects larger than 5 centimetres in Earth orbit, of which only about 4,000 are operational satellites; the rest are junk. Many of these objects are old satellites and rocket stages containing fuel.
Today the perspective has changed radically. The need for the 21st century space race to meet economic sustainability criteria has led to the entry of private operators with plans for reusable space vehicles and ambitions to earn a profit, including through space tourism. But we are also in the midst of a climate emergency, and environmental impact is now a key consideration in all human endeavours. Against this backdrop, scientists are questioning the potential environmental and climate effects of space missions.
Total stratospheric ozone destruction
Broadly speaking, rockets and spacecraft mainly use four types of fuel: kerosene (RP-1, or Rocket Propellant-1); hydrazine (a nitrogen compound, N2H4) and its derivatives; liquid hydrogen; and various solid fuels. NASA’s new Space Launch System rocket uses solid fuel along with liquid hydrogen and oxygen, while SpaceX and Russian Soyuz rockets use RP-1. The highly toxic hydrazine, which engineers must handle in protective suits, is common in satellites and spacecraft such as SpaceX’s Crew Dragon capsule. When burned, fuels emit water vapour and nitrogen oxides (NOx); some produce so-called black carbon or soot (carbon particles), CO2, alumina (aluminium oxide, Al2O3) and chlorine gas. Re-entry of space debris into the atmosphere produces NOx due to the heat generated.
In the late 20th century, scientists became interested in the atmospheric effects of these emissions. Many of the pollutants, such as chlorine or alumina, destroy stratospheric ozone, the famous layer that protects us from the Sun’s radiation. Pioneering studies revealed that rockets propelled by solid fuels and kerosene caused total ozone destruction in the stratospheric plume left in their wake, but that there was also an effect, albeit small, from the dispersion of these pollutants.
More recently, other studies have looked at the impact of greenhouse gas (GHG) emissions—such as CO2 and water vapour— as well as black carbon and other pollutants that contribute to global warming, such as NOx. Although the amount of fuels burned by rockets is less than 1% of that used by commercial aircraft, and therefore their emissions are much lower, the problem is where they are released, atmospheric chemist Christopher Maloney, employed by the University of Colorado CIRES (Cooperative Institute for Research in Environmental Sciences), and contracted with the National Atmospheric and Oceanic Administration, tells OpenMind: “Rockets differ in that they directly release black carbon into the stratosphere. Due to the relatively slow atmospheric transport of the stratosphere, black carbon can persist in this region of the atmosphere for a much longer period of time (approximately 3-5 years).” Maloney adds that, in the case of rockets, the proportion of black carbon emitted as a proportion of their total emissions is much higher, so “fewer rockets need to be launched to match the amount of black carbon emitted by aircraft.”
New cleaner fuels
The result is that the climate impact of these emissions is comparatively much greater than terrestrial emissions: according to University College London (UCL) expert Eloise Marais, black carbon in the upper and middle atmosphere “has a warming effect 500 times greater than at levels closer to Earth.” And since, due to the large amount of fuel consumed by rockets and spacecraft, about 100 times more GHGs are emitted per passenger than on a long-haul commercial flight, scientists are alarmed by the expected growth of space tourism—in orbital and suborbital flights—by companies such as SpaceX, Blue Origin and Virgin Galactic.
In a 2022 study, Marais and her collaborators estimate that in just three years, warming due to black carbon from space tourism could account for 6% of the global total caused by this pollutant, even though such emissions would only represent 0.02%. Another 2022 study led by Maloney estimated that a 10-fold increase in emissions from space tourism could increase stratospheric temperatures by 1.5 degrees Celsius and affect global atmospheric circulation patterns. In terms of ozone depletion, Marais and her colleagues conclude that the recovery of the ozone layer achieved through the Montreal Protocol, which in 1987 imposed the phase-out of chlorofluorocarbons (CFCs), would be reduced by 16% in a decade.
Given these predictions, according to study co-author Robert Ryan of UCL, “the conversation about regulating the environmental impact of the space launch industry needs to start now so we can minimise harm to the stratospheric ozone layer and climate.” Today, researchers are looking for new, cleaner fuels. According to Maloney, “the industry is currently moving to a cleaner, methane-based rocket fuel.” “Using cleaner fuels than kerosene will help minimise the climate impact,” he adds. It is not the first time that the space sector has been a pioneer in green solutions: in 1958, the US Vanguard 1 satellite was the first practical application of solar panels, then a brand-new technology.