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.

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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