Greenhouse gas emissions: a menace for low Earth orbit?

Accord­ing to new ana­lyses car­ried out by aerospace engin­eers at MIT, increas­ing green­house gas emis­sions will make it pro­gress­ively more dif­fi­cult to sus­tain satel­lite oper­a­tions in low Earth orbit. This is because car­bon diox­ide (CO₂) and oth­er green­house gases have the effect of ‘shrink­ing’ the ther­mo­sphere, a lay­er of the upper atmo­sphere where most satel­lites cur­rently orbit. This con­trac­tion decreases the dens­ity of the ther­mo­sphere, which in turn reduces atmo­spher­ic drag, a force that pulls used satel­lites and oth­er space-borne objects down to alti­tudes where they can burn up. Wil­li­am Park­er, a gradu­ate stu­dent research­er at AeroAstro, dis­cusses the find­ings of this research.

Wil­li­am Park­er. This reduc­tion in drag means that space junk will stay in the ther­mo­sphere for longer peri­ods of time, so lit­ter­ing this import­ant region and increas­ing the risk of inter-satel­lite col­li­sions. Our ana­lys­is shows that if we con­tin­ue to emit green­house gases at the cur­rent rate, these emis­sions will reduce the num­ber of satel­lites we can safely oper­ate in the years ahead.

The Earth’s upper atmo­sphere plays a crit­ic­al role in clear­ing space debris: for most objects, atmo­spher­ic drag is the only nat­ur­al remov­al mech­an­ism. How­ever, as green­house gas emis­sions increase, the upper atmo­sphere is cool­ing and con­tract­ing, retreat­ing from the region where we depend on it to clean up debris.

We know that green­house gases cause warm­ing near the Earth’s sur­face by trap­ping heat that would oth­er­wise escape into the upper atmo­sphere. How­ever, these gases make it easi­er for the upper atmo­sphere to radi­ate energy into space, lead­ing to long-term cool­ing and con­trac­tion at high­er altitudes.

To mod­el the effects of this shift­ing energy bal­ance, we used sim­u­la­tions of the entire atmo­sphere to exam­ine how changes in sur­face CO₂ con­cen­tra­tions affect the tem­per­at­ure and dens­ity struc­ture in the region of the atmo­sphere where satel­lites orbit (that is, up to 2,000 km from the Earth’s sur­face). This region is known as low Earth orbit. These satel­lites are import­ant for provid­ing essen­tial ser­vices for the Inter­net, com­mu­nic­a­tions, nav­ig­a­tion and weath­er fore­cast­ing, to name but a few.

We sim­u­lated sev­er­al green­house gas emis­sion scen­ari­os for the 21^st Cen­tury to study how they impact atmo­spher­ic dens­ity in the ther­mo­sphere and the asso­ci­ated drag. For each alti­tude range or ‘shell’ of interest, we mod­elled the orbit­al dynam­ics and the risk of col­li­sion between satel­lites based on the num­ber of objects with­in the shell. We then used this data to determ­ine the ‘car­ry­ing capa­city’ of each shell, that is, the max­im­um num­ber of satel­lites it can sustain.

We ana­lysed sev­er­al scen­ari­os: one in which green­house gas con­cen­tra­tions remain at the year-2000 level; and oth­ers in which emis­sions increase accord­ing to the Inter­gov­ern­ment­al Pan­el on Cli­mate Change (IPCC) Shared Socioeco­nom­ic Path­ways (SSPs) to the end of the cen­tury. We found that the lat­ter scen­ario could sig­ni­fic­antly reduce the car­ry­ing capa­city. Indeed, the sim­u­la­tions pre­dict that by 2100, the capa­city of regions at alti­tudes of between 200 and 1,000 km could be reduced by 50 to 66%. This situ­ation could even occur before the end of the cen­tury, espe­cially as some regions of the atmo­sphere are already becom­ing crowded with satel­lites. ‘Mega­con­stel­la­tions’ such as SpaceX’s Starlink, which com­prises fleets of thou­sands of small Inter­net satel­lites, are an example of this type of satellite.

Cli­mate change was already dis­rupt­ing the status quo, and now we are see­ing a massive increase in the num­ber of satel­lites launched in recent years. Indeed, more satel­lites have been launched in the last five years than in the pre­vi­ous 60 years com­bined. There are now more than 10 000 satel­lites in low Earth orbit.

We already knew that the ther­mo­sphere nat­ur­ally con­tracts and expands every 11 years as a res­ult of the Sun­’s nat­ur­al cycle of activ­ity. When the Sun is in a phase of low activ­ity, the Earth receives less sol­ar radi­ation and its upper atmo­sphere tem­por­ar­ily cools and con­tracts. It then expands again when sol­ar activ­ity increases. Research­ers wanted to find out how the ther­mo­sphere behaves in response to green­house gases in addi­tion to this nat­ur­al sol­ar cycle. Early stud­ies revealed that the ther­mo­sphere should shrink, thereby redu­cing atmo­spher­ic dens­ity at high alti­tudes. We have been able to meas­ure changes in drag on satel­lites in recent years, and these meas­ure­ments have shown that the ther­mo­sphere is con­tract­ing in a way that can­not be explained by sol­ar activ­ity alone.

Low Earth orbit is becom­ing increas­ingly con­ges­ted and con­tested, so accur­ate atmo­spher­ic mod­els are essen­tial for pre­dict­ing the long-term evol­u­tion of this debris. These mod­els must there­fore take into account changes in the upper atmosphere.

Our work demon­strates that vari­ab­il­ity in the spa­tial envir­on­ment­al plays a key role in determ­in­ing the tip­ping points for sus­tain­able activ­ity in low Earth orbit. It also high­lights the advant­ages of exploit­ing lower alti­tudes, where atmo­spher­ic drag allows for rap­id remov­al of any prob­lems in the event of fail­ure, thereby redu­cing long-term satel­lite pol­lu­tion. The con­clu­sions of our study under­score the urgent need for inter­na­tion­al coordin­a­tion in man­aging space traffic, without which we risk cross­ing dan­ger­ous thresholds that could lead to a tragedy of the commons.

Interview by Isabelle Dumé

Ref­er­ence:

• Green­house gases reduce the satel­lite car­ry­ing capa­city of low Earth orbit, Nature Sus­tain­ab­il­ity 8 363–372