Space mirrors

This idea was first considered in the 1980s as a way to cool Venus, and was adapted in 1989 by James Early who suggested creating a large shade (2000km diameter) to block the suns radiation.  The cost of something like this is huge (between $1-10trillion) so although it is being considered as a way to reverse climate change, it remains hugely controversial.

How it works
Essentially, objects are sent into space to block the suns rays and lower Earth's insolation, making this a SRM technique.  Proposed designs include a huge, single piece shade or a shade composed of trillions of smaller objects.  As well as just deflecting radiation, these objects could potentially be used as solar power satellites.
Whether the shade is 1 huge object or made of trillions of smaller ones, both would be situated in space at the Sun-Earth Lagrangian point (L1):

L1 lies about 1.5million km from Earth, and is the only point where an object here will have an orbital period exactly the same as Earth's.  Another way to think of it is that any object closer to the sun than Earth will eventually 'overtake' and move away, because the distance round the sun is shorter.  At L1, Earth's gravity intensifies and weakens the pull of the sun.  Therefore, if the space mirrors are placed just right; they will maintain position between Sun and Earth and be able to prevent some of the Sun's  radiation reaching Earth.

A cloud of small spacecraft
Firstly I'll look at creating the shade out of trillions of small objects.  Roger Angel is a key researcher in this field and has obtained several grants to further the research into developing this technique.  One paper assesses the feasibility of such a huge project..
Starting with the conclusion, Angel (2006) declares 'the same massive level of technological innovation and financial investment needed for the sunshade could, if also applied to renewable energy, surely yield better and permanent solutions." Which makes me wonder why he bothers with any of this and doesn't spend time researching renewables instead, but I digress.
The concept is a 'cloud' of free-flying spacecraft at a length of ~100,000km along the L1 point.  Instead of being mirrors, transparent material would be used to deflect the path of radiation (to minimise radiation pressure which may throw them off orbit).  This should reduce the sun's heat input by 1.8%, enough to reverse the effects of global warming.

• Total sunshade mass = ~20 million tonnes.
• Transportation cost = $50 per kg.
• The instruments are built on the ground and sent up to avoid the need of an unfolding mechanism.
• Each instrument weighs roughly 1g and would be launched in stacks of 800,000
• Each instrument would last 50 years

So to transport all 20 million tonnes of sunshades, 20 million launches would be required, at the cost of ~$600 billion, and may take more than a century.  It costs $30 billion to build a launcher, and $150 billion in energy costs to use it.
The environmental impact is terrible if electricity is required, coal would be the worst case scenario, needing 30kg of coal for every 1kg of spacecraft launched.  So the technique worsens the emissions problem, and although it will offset the emissions it makes from launch it doesn't remove carbon from the environment!
And all this can be yours for a few trillion dollars.  I think this is all very clever but I must agree with Angel, maybe investing trillions in a permanent solution would be more sensible.





Alternatively, Early (1989) proposes a 2000km diameter shield, 10μ thick made by moon materials placed in the L1 to block 2% radiation from the Sun.  Similarly, this project is also extortionately priced, in the range of $1-10 trillion.  As far as I can tell it is similar to Angel's proposition in terms of proportion of radiation blocked and cost, the main difference being more of the work in terms of constructing the shield would have to occur in space, which in itself is less feasible.

So overall then, this is the most expensive geoengineering technique I've come across so far, at between $1-10 trillion.  The vagueness in the figure itself highlights the fact more research is needed for this idea if it is ever to be put into practice.  The simplicity of it; sending an object into space to block radiation is amazing but the infrastructure and technology required to achieve it is impractical.  I feel there might be other ways more suitable to reversing climate change, but still don't think this should be ruled out as it has huge potential to reverse over 150 years of damage.


Angel, R. (2006) 'Feasibility of cooling the Earth with a cloud of small spacecraft near the inner Lagrange point (L1)', Proceedings of the National Academy of Sciences in the United States of America, 103, 46, 17184-17189.


Early, J. T. (1989) 'Space-based solar shield to offset greenhouse effect', Journal of the British Interplanetary Society, 42, 567-569.