Rocket launches pollute the atmosphere

Rockets for space flights have a greater impact on the climate than is often assumed: they produce harmful nitrogen oxides, contribute to the depletion of the ozone layer and accelerate global warming. The volume of such space flights is still small, but scientists expect an enormous increase through the emerging space tourism, which companies such as Space-X, Blue Origin and Virgin Galactic are planning.

A team led by Robert Ryan from University College London has calculated that three years of space tourism could be enough to produce twice as many climate-damaging emissions as all scientific space missions.

The consequences of rocket flights for the Earth's atmosphere are difficult to assess. Because direct measurements in those zones of the atmosphere through which a rocket flies are not possible. The scientists use computer models, which incorporate a great deal of knowledge about the properties of individual layers of the atmosphere and the chemical reactions that take place there.

Ryan and colleagues used the Geos-Chem model, which also includes higher layers of air such as the stratosphere and mesosphere. For their study, which was presented in the specialist journal "Earth's Future", they primarily examined soot particles and the effects on the ozone layer in the upper stratosphere.

"Rocket launches are routinely compared to greenhouse gas and air pollutant emissions from the aircraft industry, which our work shows to be wrong," explains co-author Eloise Marais from University College London. The simulations showed that soot particles from the burned fuel in the stratosphere are about 500 times more effective at heating the Earth than near the ground. Although rockets only contribute 0.02 percent of global soot emissions, they already account for six percent of soot-related global warming.

The researchers are also critical of the impact of rocket launches, the falling back of burnt-out rocket stages and the return of spacecraft on the ozone layer. This protects the earth from aggressive ultraviolet solar radiation and is subject to special protection under the Montreal Protocol of 1987.

According to the simulations, the rocket launches in 2019 reduced the ozone layer by 0.01 percent. After ten years of space tourism, growing at a modest rate of 5.6 percent per year, ozone loss over the northern polar region was calculated to be 0.15 percent. This would be about a tenth of the amount the ozone layer over the North Pole has recovered after implementation of the Montreal Protocol.

"The only part of the atmosphere that shows strong ozone recovery under the Montreal Protocol is the upper stratosphere," Ryan points out. "And it is precisely those who are hit hardest by the effects of rocket emissions." The scientist advocates urgently discussing the effects of space tourism on climate change now.

Knud Jahnke from the Max Planck Institute (MPI) for Astronomy in Heidelberg doesn't think much of space tourism anyway: "Such space flights are harmful to the climate and a pure waste of resources, just because a few rich people have discovered it as a status symbol."

Ioannis Kokkinakis and Dimitris Drikakis from the University of Nicosia in Cyprus have described the exhaust plume of rockets in more detail in the journal "Physics of Fluids". The effects of the hot combustion gases are very different at the simulated altitudes of 10, 30, 50 and 67 kilometers. Because the chemical composition of the air changes - and above all, the density decreases enormously. At an altitude of 70 kilometers, a modern rocket emits as much carbon dioxide (CO2) in a one-kilometer flight segment as there is in 26 cubic kilometers of air at this altitude.

In particular, the researchers calculated the formation of nitrogen oxides (nitrogen monoxide and nitrogen dioxide) from the rocket exhaust gases. In the higher atmosphere, fewer nitrogen oxides tend to form because the exhaust gases expand and cool quickly in the thin air. For this reason, the temperature of around 920 degrees Celsius, at which point oxygen from the exhaust gas and the air combine with the nitrogen in the air to form nitrogen oxides, is only briefly exceeded.

At an altitude of up to ten kilometers, the higher air pressure keeps the rocket's exhaust plume together and therefore hot, so that nitrogen oxides can form. The number of nitrogen oxides in the rocket trail is so high that, according to the World Health Organization (WHO), it would be harmful to human health.

"We hope that commercial airlines such as SpaceX, Virgin Galactic and Blue Origin, and their associated engine manufacturers, will take these effects into account in future designs," Drikakis said.

Even if the greenhouse gas footprint of scientific space flights is manageable, more and more astronomers are thinking about how their science can be made more climate-friendly. In a study presented in the journal "Nature Astronomy", researchers from the University of Toulouse led by Jürgen Knödlseder examined the carbon footprint left by the astronomical research infrastructure.

They estimate the amount of greenhouse gases released into the atmosphere by astronomers' activities at about 20.3 million tonnes of CO2 equivalent per year. This expression means that the global warming potential of other gases such as methane is converted to the potential of CO2. Broken down to the individual astronomers, this is 36.6 tons of CO2 equivalent per year. If you consider that nature on Earth can only offset two tons of CO2 per person per year, astronomy is currently 18 times that.

Even if research on space missions cannot avoid rocket launches, scientists could do more to protect the climate, for example at observatories. MPI researcher Jahnke gives an example from the European Southern Observatory (Eso) in the Atacama Desert in Chile: For a long time it was not possible to connect the facilities on Mount Paranal directly to the Chilean power grid. Therefore, the high-tech telescopes and the entire infrastructure had to be supplied with diesel units. But that has since changed.

Two years ago, Jahnke and his colleagues determined the CO2 footprint of his own institute. For 2018, he came up with a value of 18.1 tons of CO2 equivalent per astronomer. 47 percent of the greenhouse gas emissions came from the more than 1000 official flights that he and his colleagues completed this year.

"We are a small scientific community and we have to work together internationally," says Jahnke, explaining the large number of flights. However, the corona pandemic has shown that many personal meetings between cooperation partners can be replaced by video conferences in projects. Jahnke advocates doing this more in the future.

Other major items in the CO2 balance of the MPI for Astronomy are the consumption of electricity, especially for calculations on supercomputers, and heating the buildings. "It's good that we now have these numbers, because only what can be measured can also be managed," says Jahnke.

For him it is important that basic research also strives for more climate friendliness. If many areas of society laboriously cut greenhouse gas emissions, astronomy will also feel increasing social pressure anyway.