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By Roger Highfield on

How sustainable is space tourism?

Roger Highfield, Science Director, discusses a new study that suggests the soot released by space tourism could have a bigger impact on the climate than the soot from aviation.

Space tourism and commercial experiments on satellites and space stations are increasingly driving what’s known as the low-Earth orbit economy.

However, as the industry gears up for rapid growth, evidence that our cosmic back yard needs to be developed in a more sustainable way has come from a study by researchers from UCL, the University of Cambridge and the Massachusetts Institute of Technology.

‘This is an important paper that flags a fast-evolving concern with the environmental impacts of space exploration (terrestrial and extra-terrestrial),’ comments Doug Millard, curator of space at the Science Museum. ‘Benefits accrued from an ever-essential space infrastructure (satellites) in low earth orbit do need to be balanced with the uncomfortable fact that space, like Earth, needs also to be developed in a sustainable manner.’

A two stage SpaceX Falcon 9 Block 5 rocket, like many launchers, is driven by the combustion reaction between liquid oxygen and kerosene fuel, hydrocarbons composed of carbon and hydrogen atoms, to form carbon dioxide, water vapour and black carbon particles – soot – in the rocket’s exhaust.

Nitrogen oxides (NOx), pollutants, also form due to a reaction between nitrogen – which becomes reactive at high temperature – and oxygen molecules. These oxides are also produced when the rocket’s reusable components return to Earth, and by meteorites, due to extreme temperatures produced by friction. This is an issue even for rockets that only burn hydrogen and oxygen to make water vapour.

Rocket launches release pollutants and greenhouse gases into several atmospheric layers. These persist for much longer – years – in the middle and upper atmosphere, compared with when equivalent pollutants are released at or near the Earth’s surface, the troposphere, where they linger for weeks.

Launch of the SpaceX rocket for Axiom Mission 1. Credit: SpaceX
Launch of the SpaceX rocket for Axiom Mission 1. Credit: SpaceX

Published this month in the journal Earth’s Future,  Drs Robert Ryan and Eloise Marais from UCL, Chloe Balhatchet of the University of Cambridge and Dr Sebastian Eastham of the Massachusetts Institute of Technology used a 3D model of atmospheric chemistry – the chemical processes in the Earth’s atmosphere – to explore the impact of rocket launches and re-entry, and weigh up the possible impact of space tourism.

Total carbon dioxide emissions from one launch per passenger are around 100 times those from a typical long-haul flight, though the overall impact of launches is tiny compared with global aviation. Co-author Dr Marais said: ‘Rocket launches are routinely compared to greenhouse gas and air pollutant emissions from the aircraft industry, which we demonstrate in our work is erroneous because it misses the different behaviour of soot at high altitudes.’

Currently, an estimated 500 tons of rocket soot is generated annually, less than those from the aircraft industry. However, when released into the middle and upper atmosphere, soot has a warming effect 500 times greater than when released closer to Earth because it persists longer and partly because there are typically no clouds and few to no aerosols competing with soot for the sun’s rays.

‘Soot particles from rocket launches have a much larger climate effect than aircraft and other Earth-bound sources, so there doesn’t need to be as many rocket launches as international flights to have a similar impact,’ she said.

Furthermore, when these particles reach the ozone layer in the upper reaches of the atmosphere, the stratosphere, they convert ozone to oxygen, depleting the fragile stratospheric ozone shield that protects the planet from the sun’s ultraviolet radiation, which has harmful effects linked with skin cancer and weakened immune systems in humans, as well as disruptions to agriculture and ecosystems.

For their study, the researchers collected information on the chemicals from all 103 worldwide rocket launches in 2019, as well as data on reusable rocket and space junk re-entry. They also constructed a plausible scenario of how space tourism would develop. This included routine launches by Virgin Galactic, Blue Origin and SpaceX.

These data were incorporated into their 3D atmospheric chemistry model to explore the impact on climate and the ozone layer.

The team showed that warming due to soot is 3.9 milliwatts of power per square metre over a decade, which is dominated by emissions from kerosene-fuelled rockets. However, this more than doubles (7.9 milliwatts per square metre) after just three years of additional emissions from space tourism launches, due to the use of kerosene by SpaceX and hybrid synthetic rubber fuels by Virgin Galactic.

Under a scenario of daily or weekly space tourism rocket launches, the team found that the impact on the stratospheric ozone layer threatens to undermine the recovery experienced after the successful introduction of the Montreal Protocol.

Adopted in 1987, the Protocol global ban on substances such as CFCs, chlorofluorocarbons, that deplete the ozone layer is considered one of the most successful international environmental policy interventions. Co-author Dr Ryan said: ‘The only part of the atmosphere showing strong ozone recovery post-Montreal Protocol is the upper stratosphere, and that is exactly where the impact of rocket emissions will hit hardest. We weren’t expecting to see ozone changes of this magnitude, threatening the progress of ozone recovery.’

There are still many unknowns that have to be understood in more detail, from more accurately projecting the growth of the industry to the impact of burning other carbon containing rocket fuels like liquid methane and bio-derived fuels on climate and ozone. Soot and other pollutants generated by satellites burning up when they fall out of orbit is also a growing, poorly understood source of emissions.

‘We are developing this work further by seeing if we can detect air pollution and returning space junk with Earth observing satellites,’ said Dr Marais.

The team calls for urgent research to assess the consequences of commercial space activities for the planet down below.