Carbon footprint and space activities: an ambiguous common ground

Des­pite their sig­ni­fic­ant car­bon foot­print, the space industry remains import­ant for sci­ence and soci­ety. How­ever, it is still neces­sary to pri­or­it­ise its uses, because while space tour­ism has no soci­et­al value, obser­va­tion mis­sions are cru­cial to our under­stand­ing of the Earth. Loïs Miraux, an inde­pend­ent research­er and spe­cial­ist in the envir­on­ment­al impacts asso­ci­ated with the space industry, and Jür­gen Knödlseder, CNRS research dir­ect­or at the Insti­tute for Research in Astro­phys­ics and Plan­et­o­logy, share their expertise.

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Jür­gen Knödlseder. In light of the devel­op­ment of com­mer­cial space flights, Car­ba­jales-Dale and Murphy cal­cu­lated the car­bon foot­print of manned space flights¹. Over the entire life cycle, they estim­ate that the cli­mate cost amounts to 1,500 kg of CO₂ equi­val­ent per hour. This is the hourly equi­val­ent of 60 to 100 dies­el buses run­ning at the same time. For me, space tour­ism is not a priority.

Loïs Miraux. Space tour­ism dam­ages pub­lic per­cep­tion. It sug­gests that this is a mod­el to fol­low and may dis­cour­age people from tak­ing action to reduce car­bon emis­sions in their daily lives.

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LM. Space travel is the only human activ­ity that impacts the upper atmo­sphere, par­tic­u­larly the stra­to­sphere [Editor’s note: loc­ated between 12 and 50 km above sea level]. Some life cycle assess­ments (or LCAs) have been car­ried out, but very few have been made public.

Last June, as part of its roadmap for decar­bon­ising the industry, the CNES revealed the very first glob­al study on the car­bon foot­print of the space sec­tor at nation­al level. The industry’s annu­al emis­sions amount to 1.8 mil­lion tonnes of CO2 equi­val­ent (Fig­ure 1), or 0.3% of France’s nation­al emis­sions. These fig­ures are high­er than pre­vi­ous estim­ates made by sci­ent­ists. Only one study from 2018 assesses the sector’s foot­print on a glob­al scale: emis­sions over one year are estim­ated at 6 mil­lion tonnes, or 0.01% of glob­al anthro­po­gen­ic emis­sions².

Respons­ib­il­ity for the dif­fer­ent phases – rock­et con­struc­tion, launch, fuel pro­duc­tion, etc. – var­ies depend­ing on the type of vehicle. The CNES estim­ates that the man­u­fac­ture and trans­port of rock­ets and satel­lites account for the largest share of emis­sions. Oth­er authors³ estim­ate the radi­at­ive for­cing [Editor’s note: cor­res­pond­ing to the warm­ing effect on the atmo­sphere] of rock­et launches at 16 mW/m². By way of com­par­is­on, the radi­at­ive for­cing of avi­ation cur­rently stands at 100 mW/m².

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LM. Some applic­a­tions of the space sec­tor are essen­tial, par­tic­u­larly sci­entif­ic mis­sions to study the Earth sys­tem and nat­ur­al dis­aster man­age­ment. The ques­tion of pri­or­ity uses must be addressed.

JK. Many applic­a­tions of the space sec­tor are vital to mod­ern soci­ety. I am con­vinced that Earth obser­va­tion mis­sions bring great­er bene­fits to the plan­et than they cause harm. But this cost-bene­fit cal­cu­la­tion is dif­fi­cult and requires debate and polit­ic­al arbit­ra­tion. Sig­ni­fic­ant effort is also needed with regard to the data pro­duced, as much of it is not used and, by pool­ing it, space mis­sions could be optimised.

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LM. When it comes to tech­no­lo­gic­al devel­op­ments, reusable launch­ers nat­ur­ally spring to mind. I con­duc­ted a life cycle ana­lys­is for CNES, which revealed that the cli­mate impact was zero, but pos­it­ive (20 to 30% sav­ings) in terms of resources. Reusable launch­ers are heav­ier due to the addi­tion­al com­pon­ents and fuel required for land­ing, which reduces their cli­mate benefits.

We lack com­pre­hens­ive know­ledge regard­ing the effects of rock­ets on the upper atmo­sphere. Rock­ets release large quant­it­ies of particles (soot and alu­mina) into the upper atmo­sphere. These particles remain there much longer than when emit­ted into the lower atmo­sphere: the warm­ing effect of soot is 500 times great­er in the upper atmo­sphere than in the lower atmo­sphere. Almost all of the cli­mate impact asso­ci­ated with rock­et launches is linked to the emis­sion of these particles. Reusable launch­ers will not help in this regard.

JK. One of the little-known effects that con­cerns us relates to the ozone lay­er. The impact of space activ­it­ies could become com­par­able to that of human activ­it­ies before the Montreal Pro­tocol was imple­men­ted, which enabled its recovery.

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LM. Tech­no­logy will not be able to com­pletely cir­cum­vent this. Even with the best fuels, rock­ets will always emit water vapour and nitro­gen oxides, as well as met­al particles when leav­ing or re-enter­ing the atmo­sphere⁴, affect­ing the cli­mate and ozone layer.

Added to this is the evol­u­tion of the sec­tor. His­tor­ic­ally, satel­lite mis­sions covered sci­entif­ic, mil­it­ary and tele­com­mu­nic­a­tions needs in a more bal­anced way. But the num­ber of satel­lites in orbit has soared, par­tic­u­larly with Starlink: it has ris­en from around 2,000 in the 2010s to just under 13,000 today. Between 2019 and 2024, the amount of fuel con­sumed by rock­ets has more than tripled. If the cur­rent pace con­tin­ues, the cli­mate impact of space travel could reach that of avi­ation today.

UNCERTAIN

JK. The prob­lem with space is mainly linked to the expo­nen­tial growth in the num­ber of launches. Tech­no­lo­gic­al devel­op­ments such as reusable launch­ers are driv­ing down costs and pro­du­cing a rebound effect: the num­ber of launches is increas­ing. I think inter­na­tion­al reg­u­la­tion is neces­sary; it is not reas­on­able to allow private act­ors to con­gest space.

Sci­ent­ists, even though they account for only a small part of the sector’s car­bon foot­print, must also ask them­selves ques­tions. We have assessed the car­bon foot­print of astro­phys­ics: it amounts to around one mil­lion tonnes of CO₂ equi­val­ent per year, or 36 tonnes of CO₂ equi­val­ent per year per astro­nomer. It is a very high-impact field of research. Space mis­sions dom­in­ate the foot­print, par­tic­u­larly the send­ing of probes to explore our plan­et­ary sys­tem. I ques­tion the value of this type of research; soci­ety should be able to decide how much of the remain­ing car­bon budget should be alloc­ated to it.

In the end, we have a huge amount of data that has nev­er been exploited. For the past few years, I have been work­ing exclus­ively on the archives. We have made some excit­ing dis­cov­er­ies, par­tic­u­larly regard­ing the hypo­thes­is of the ori­gin of positrons at the centre of the Milky Way.

Interview by Anaïs Maréchal