‘Say hello to my little friend.” – Scarface
Let’s do some math on metallic reflective solar radiation modification.
Solar radiation modification (SRM) is defined on Wikipedia.
Many schemes for modifying Earth’s atmospheric and surface temperatures have been proposed under the umbrella of SRM, some of which involve mirrors or shades, some of which involve aerosols.
MEER [1] proposes tiling cities and countrysides with reflective mirrors.
Someone else wrote a rather daft paper proposing that solar-powered reflective stratospheric drones could precisely apply shade to specific locations like urban heat islands.[2]
We’ll call the above general strategies “metallic reflective” SRM.
A different avenue of SRM, namely stratospheric aerosol injection (SAI), exploits the aerosol precipitation leverage by injecting substances high in the atmosphere where they can trigger the formation of water droplets as clouds — far more mass of cloud than launched aerosol. Make Sunsets is using balloons and SO2 for this purpose. [3]
Others propose using giant hoses, cannons, aircraft, and/or rockets to export those aerosols high enough to stay aloft for some time.[4]
This blog entry will ignore SAI as there’s currently no standardized metric comparing SAI (kilograms of SO2 injected) versus MEER (mirrors installed at sea level). Also, nobody is doing SAI at scale (yet).
Back to mirrors.
Reflect Orbital is a startup that wants to sell reflected sunlight.
Their satellites will orbit the Earth’s solar terminator in perpetuity and reflect sunlight to Earth that would’ve otherwise sailed right past it. There has been some debate about the economic viability of Reflect’s product, but let’s give them the benefit of the doubt.
If they can reflect sunlight towards Earth, their same technology in a different orbit can also reflect sunlight away from Earth.
Thickness matters
Solar shields proposed in the past, e.g. orbiting at L1, are close to a few micrometers or nanometers thick, just like Reflect Orbital’s proposed satellite membranes. [5] The thinner the better, as long as they’re strong enough.
The problem with L1 sunshields is that most of the sunlight they reflect wouldn’t have hit the Earth in the first place – in other words, most of the shadow is wasted on empty space. This dramatically inflates the launch mass.
Is the idea dead? Plenty of great YouTube videos on the topic suggest so.
November 2025: Enter Elon Musk, stage left.
Space mirrors are back from the dead.
SpaceX’s CEO claims that large fleets of his company’s Starlink satellites could be used to provide shade to Earth.[6] SpaceX does have the advantage of a massive in-house orbital launch system doing hundreds of flights per year with reusability. Does this change the economics at all?
Let’s first estimate how cost-effective Starlink satellites are as shade.
One Starlink V2 satellite: cross section ~60 square meters.[7] Weight: 800 kilograms.
Let’s pretend it’s a flat plate of aluminum. 800 kg yields 0.29 m^3; dividing by 60 m^2 yields 4.8 millimeters effective thickness. That’s 5,000 times thicker than the 1 micron shield membranes proposed for use at L1.
Area of a sphere
Let’s move on to the surface level surface area math that should be surprising only to readers under the age of 10. How much area are we talking about?
Suppose, using a wild guess as to the solar constant sensitivity of our atmosphere, that SpaceX gets the world to agree they want to dim the sun by 1% globally. How much surface needs to be covered? How many satellites are needed?
Earth’s radius is 6371 km. The surface area of Earth is 500 million square kilometers. 1% of that is 5 million square kilometers (167 billion Starlink V2 satellites, for those that prefer normal units of area).
Covering 1% of Earth’s surface with a 1 micron thick layer of aluminum would amount to 5*10^6 square kilometers * 10^-9 kilometers, or about 5*10^-3 cubic kilometers of aluminum. More comprehensibly: 5 million cubic meters; about 13 million tons worth. Assuming a spot price of $2700/ton, that’s 35 billion dollars of metal.
Let’s not even think about the plastic film, silicon chips, antennas, radios, solar panels, or anything. For now, let’s pretend the satellites are just aluminum.
What would it cost to launch this?
Starship (in the present) promises 100 metric tons of payload to orbit in reusable mode for about $10 million – in the long term, assuming full reuse. $10MM/100t translates to $100k/ton to LEO.
100 metric tons of aluminum costs $270k – a rounding error next to the launch cost. Don’t ask me what it costs to convert 100 tons of aluminum into a 1-micron-thick structure.[8]
13 million tons to orbit at $100k/ton is 1.3 trillion dollars and 130,000 launches. That’s 100 launches a day for 3.5 years. SpaceX might need a few more launch sites.
CO2 impact
Let’s say the Starship/Super Heavy stack burns 1,000 tons of methane fuel per launch. 1,000 tons of methane oxidizes to around 2,750 tons of CO2, so 130,000 launches will produce a modest 0.35 gigatons of CO2. Humanity releases around 40 gigatons of CO2 per year, making anything SpaceX does a rounding error. Spread over 3.5 years, this launch effort would only grow the world’s CO2 emissions by 0.25%.
If 1% solar attenuation is all we need to reverse the course of climate change, increasing our emissions by 0.25% for a few years doesn’t actually seem like a bad trade.
Economics of aluminum
It costs 37 times more to launch the aluminum to orbit ($100k/ton) than to buy it ($2700/ton). Can we even afford to buy 13 million tons of aluminum?
(VisualCapitalist, USGS)
Annual worldwide production was just under 70 million tons in 2024. Spread over 3.5 years, SpaceX would eat about 5% of the global Al market.
We could expect soda can prices to go up a bit.
Evaluating the original claim in the present
Elon Musk’s original claim was that a fleet of Starlink satellites could shade the Earth.
We already estimated that a fleet of 1 micron thick reflective satellites would cost about a trillion dollars and take three years to launch.
The Starlink V2’s effective thickness is 5,000x that, so the price would need to also increase by 5,000x. 5 quadrillion dollars is a lot. I don’t have that kind of money. So, Starlink V2 can’t do it.
StarMembrane V42 might have a chance, but unless we can mine it entirely from asteroids, it’ll still cost a quadrillion dollars and likely dump a measurable amount of aluminum oxide dust [9] into the atmosphere.
Should we do it?
References
[1] Mirrors for Earth Energy Rebalancing
[2] It’s not published.
[3] Their work has no measurable effect but has triggered much gnashing of teeth.
[4] https://podcasts.apple.com/us/podcast/srm-hoses-hyde/id1529459393?i=1000730122226
[5] https://x.com/elonmusk/status/1985201007413080569
[6] https://en.wikipedia.org/wiki/Space_sunshade
[7] https://lilibots.blogspot.com/2020/04/starlink-satellite-dimension-estimates.html; assuming double the 30m2 area of the Starlink V1 satellite.
[8] A 100-ton aluminum sunshield with a thickness of 1 micron would be 35 square kilometers across. Send me an email when you figure out how to make that unfold reliably!
[9] https://pmc.ncbi.nlm.nih.gov/articles/PMC10139039/
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