Space-based SRM

As anyone who has had the good fortune to witness a solar eclipse will testify, objects in space can shield the Earth from sunlight and the warmth that comes with it. Space-based SRM would seek to achieve a similar effect on a grander scale and in a​ lasting​ way, but with much less drama.  

The fundamental practical problem with this idea is that the amount of material needed is very large, and getting things into space is hard. 

Getting millions of tonnes of material into space will be very challenging 

Imagine putting enough things between the Earth and the Sun to reduce incoming sunlight by 2%, which would provide a cooling roughly equivalent to the warming offered by a doubling of the carbon dioxide level.¹ To do this, the objects put into orbit would need to have a total sun-facing area of millions of square kilometres.¹ 

Material capable of blocking sunlight need not be very heavy. One early paper² on space-based SRM suggested using thin aluminium foil, which weighs about 10 grams per square metre; more recent papers^1,3 suggest more sophisticated, lighter materials. A system using such materials would still have a total mass of tens of millions of tonnes. Since the beginning of the space age, humankind has launched only a few tens of thousands of tonnes into orbit. The largest spacecraft currently in orbit is the International Space Station, which weighs 400 tonnes. 

However, for the past few years, the rate at which material is launched into space has been growing by around 43% per year. If this trend continues, launch rates will reach 1 million tonnes per year around 2050. The current business case for increasing launch capacity – the creation of constellations of communication satellites – would not support such continued growth, but there may be other requirements that do so. If this proves to be the case, space-based projects on the scale needed for SRM might be plausible undertakings in the second half of this century. 

Leaving aside, for the moment, the matter of putting a space-based SRM system into orbit at all, what sort of orbit is best suited for such things? 

Orbital choices 

Most things launched into space end up in orbit around the Earth. The disadvantage of such orbits when it comes to blocking the sun is that an object circling the Earth spends less than half its time between the Earth and the Sun.⁴ This means less than half the constellation would be doing any cooling at a given time, so the constellation’s total sun-blocking area needs to be at least twice the area needed to block the sun.  

The lower an orbit around the Earth is, the longer the fraction of the orbit spent between the Sun and some point on the Earth. That argues for low orbits. So does the fact that it is cheaper to put objects launched from Earth into lower orbits than into higher ones.⁴  

However, there are many problems with the idea of putting satellites with a collective area of millions of square kilometres into low Earth orbit. The Earth’s atmosphere extends, albeit tenuously, well into the shell of space used by such satellites, and this exerts a drag which means the satellites either need regular reboosting or fall back to Earth.  

Avoiding collisions with the thousands of satellites already in such orbits, and the debris left over from previous space missions, could require near constant manoeuvring.⁴ The effect on both astronomy and aesthetics of satellites with a total area larger than Russia’s moving quickly across the sky all the time would be profound. 

That is why many researchers who have considered their use for space-based SRM have thought about launching sun blockers from the Moon instead.⁴  

It takes much less energy to reach an L1 halo orbit from the Moon than from the Earth. What is more, it is in principle possible to launch spacecraft from the Moon without the use of rockets or fuel by means of electromagnetic catapults similar to those which launch aircraft from the most modern aircraft carriers. It might be easier to set up tens of thousands of tonnes of mostly robotic and remotely operated industrial equipment devoted to making and launching sun blockers on the Moon than to launch tens of millions of tonnes directly from the Earth.⁴  

The long-term prospects for space-based SRM 

There is no possibility of the world developing a million-tonne-per-year launch capacity or a large lunar industrial base within the next couple of decades.⁴ Nor are such things, both of which might require investments in the $1 trillion to $10 trillion range,⁴ going to be developed on the basis of their usefulness for SRM. Were one or more groups of nations with the technical capacity to mount a global SRM effort to decide to do so in the next couple of decades, it seems certain that they would employ a technology which could be developed more quickly and at far less cost. 

That said, there are potential advantages to space-based SRM. It has no direct chemical or physical effect on the atmosphere, the oceans, or the biosphere other than those which follow from reducing incoming sunlight (though the environmental effect of the launches necessary for any specific scheme would need consideration).¹  

Unusually for an SRM technology, it is also covered by an existing governance scheme, the outer space treaty of 1967, which establishes specific national responsibilities for all space-based activities and requires that they be conducted for the benefit of all humankind. 

It is also worth bearing in mind that some scenarios imagine SRM being deployed for centuries.⁶ If over that time humankind were to develop a space-based industrial infrastructure that was capable of space-based SRM – something which is widely seen as possible, but which is by no means certain⁴ – it is conceivable that it might be used as a “second generation” system on the basis of its lower environmental side effects and, conceivably, its stable governance.