The Case for a Planetary Sunshade
It’s Less Sci-Fi Than I Expected
Earlier this month I was at the University of Nottingham for the first-ever Planetary Sunshade Workshop, a gathering of about 70 researchers, engineers, entrepreneurs, and policy people convened with the International Academy of Astronautics and the Planetary Sunshade Institute. I was keen on coming because I wanted to understand where the sunshade field actually is, and whether there’s a plausible path to it becoming real in a timeframe that matters.
A planetary sunshade, if you haven’t encountered the concept, is exactly what it sounds like: reflective structures positioned in space between Earth and the Sun to reduce the amount of solar energy reaching us. The idea has been around since at least 1989, when James Early at Lawrence Livermore National Laboratory wrote a short paper laying out the basic physics of placing a reflector at the L1 Lagrange point, a gravitationally stable spot about 1.5 million kilometers from Earth, roughly 1% of the distance to the Sun. Early’s paper was concise and elegant, but it stayed almost entirely theoretical for 35 years.
That makes this workshop a seminal moment. Ross Centers, founder of the Planetary Sunshade Institute, opened by framing it as the transition from “a collection of papers and interesting ideas” into a field. A field has contributing thinkers, conferences, shared vocabulary, competing design proposals, and institutional credibility. The IAA co-sponsorship matters here because it anchors the sunshade conversation in aerospace engineering rather than atmospheric science, which is a different credibility base and a different audience entry point than the one SAI research has used.
Within the SRM world, the sunshade has long been the idea people acknowledge politely and then move past. Stratospheric aerosol injection is cheaper, faster, more studied, and deployable on a timeline that matches the urgency of the Earth’s energy imbalance. The sunshade, by comparison, requires building things in space at a scale humanity has never attempted. When I’ve talked with people in the SAI community about sunshades, the general take has been something like: sure, interesting, but we have more pressing problems. And the sunshade community itself largely agrees. Ross has said explicitly that he won’t support sunshade development without SAI proceeding in parallel, because if there’s a gap in coverage while the sunshade scales up, you risk exactly the kind of abrupt warming you’re trying to prevent.
So I went to Nottingham with genuine curiosity but also with the zero-sum question in mind: if funding is limited, why should any resources go here instead of toward SAI?
The answer I came away with surprised me.
What you’re actually building
Among all the presentations I saw, there were two main design families that dominated the technical discussions.
The first is solar sails: very thin reflective foils, massively large, positioned to block or redirect incoming sunlight. Solar sails have real flight heritage. NASA has tested several, from the small NanoSail-D experiment in 2010 through the Advanced Composite Solar Sail System launched in 2024, which demonstrated deployment and controlled orbital maneuvering using photon pressure. The engineering challenge with solar sails at sunshade scale is control. Photons exert pressure on the sail surface, and at the dimensions you’d need, maintaining precise positioning is a hard, unsolved problem.
The second design is the heliogyro: imagine a wheel with long blades instead of spokes, where each blade rotates on its own axis while the entire structure spins. The spin keeps the blades taut through centrifugal force, which may be better for the control problem. This is the design the Planetary Sunshade Institute favors for further research.
The diameter of a single heliogyro unit would be roughly 40 kilometers long. To shade about ~1% of incoming sunlight, you’d need over 5,000 of these units, with a total system mass of about 67 megatons. That is just an insane amount of mass to get into space. Which means boosting all of it from Earth probably isn’t the right pathway. According to an estimate presented at the workshop, 99.96% of the mass would need to come from the moon, with only about 23 kilotons launched from Earth (roughly 230 Starship flights for the sunshade components alone, with more needed for the lunar mining and factory infrastructure). If all that is true, then it follows that you can’t have a sunshade without industrializing the moon first.
The engineering is early. People at this workshop were presenting design concepts for the first time, and nobody is building hardware. The research projects represented here were largely preliminary, and there have been no major collaborations between space agencies on this. But there is now an organized technical community doing real engineering work, not just writing speculative papers, and the IAA affiliation gives that work institutional grounding.
SAI now, sunshade later
The sequencing logic I kept hearing at the workshop runs like this: SAI is the better overall near-term intervention. It’s deployable within a decade, relatively inexpensive (I’ve written about this before, but it’s estimated to cost roughly $10 billion per year for significant cooling), and the physics are (relatively) well understood. The sunshade is the long-term replacement, coming online in the second half of this century as SAI is gradually phased out.
Ross framed the two as “harmonious and complementary approaches” and described the relationship using a cycling metaphor: the sunshade rides in SAI’s peloton until conditions allow it to break away as the lead vehicle. His target is what he called “parity-of-readiness,” meaning that by the time SAI deployment begins, the sunshade community should already have demonstrators flying and a buildable plan on the shelf.
Given the hypothetical choice (with all costs and political constraints momentarily ignored) between injecting sulfate particles into the stratosphere and putting a reflector in space, I’d likely lean toward the reflector. SAI is clearly physically and politically plausible, but it introduces atmospheric pollutants, carries real risks of disrupting regional precipitation, and would likely make the sky noticeably whiter (which I think is both minor and maybe the most sad aspect of SAI). Ross compared SAI to chemotherapy and the sunshade to precision immunotherapy; I think that overplays how precise a sunshade would actually be, but the directional intuition feels correct. Reflection from space avoids stratospheric chemistry and aerosol side effects, can be adjusted or even shut off, and doesn’t add new particulates into the atmosphere. It introduces different risks rooted in large-scale space operations, dual-use and security concerns, and the politics of who controls a planet-scale piece of infrastructure in orbit. But on the overall physics and just straight vibes of the whole thing, the sunshade feels far more elegant to me.
Why the economics changed
One of the biggest obstacles to taking the sunshade seriously has always been cost. We’re talking about an estimated trillion+ dollars to build. In a world where climate philanthropy struggles to raise even tens of millions for SAI or climate stabilization research, the idea that anyone would fund building thousands of 40-kilometer structures in space for climate reasons alone has been, to put it generously, aspirational.
What shifted my thinking at this workshop was what I’d call the convergence argument: the economic forces pushing toward space industrialization have nothing to do with climate, and as it happens, they’re building exactly the infrastructure a sunshade would require.
If you look at the growth curves for compute and energy demand, I just don’t see any reason they’re going to flatten. And it’s not only compute. We’re electrifying everything as we decarbonize1, which means energy demand goes up even as emissions go down. This is the story of human civilization: energy demand constantly increases. If you play those curves out over time, generating energy off-planet starts to become cost-competitive, and eventually necessary. And though it may seem ludicrous to mention today, there is an actual thermodynamic ceiling on how much energy you can produce on Earth’s surface before waste heat alone becomes a climate problem. Current global energy use is around 18 to 20 terawatts. At roughly 25 times that level, waste heat adds about 1 watt per square meter of radiative forcing, which is significant. At 75 times current use, you’re at 3 watts per square meter, enough on its own to substantially warm the planet even in a fully decarbonized world. That ceiling is still far off, but the market forces pushing us toward off-planet energy generation will kick in well before we reach it.
The early stages of this transformation off-planet are already underway. Overview Energy is developing space-based solar power, with satellites in geosynchronous orbit collecting continuous sunlight and beaming it back to Earth. They’ve completed an airborne demonstration and are targeting a first orbital demo in 2028. Starcloud is building orbital data centers with large solar arrays and radiative cooling for GPU-heavy AI workloads, betting that abundant solar energy and the vacuum of space as a heat sink make orbital compute cost-competitive with terrestrial facilities. These are early companies working on early technology, but they represent a capability we’re going to have to develop regardless: generating and consuming energy off of Earth. And building that capability is also building the industrial base you’d need for a sunshade.
And then there’s SpaceX, which is raising what will likely be the largest IPO in history next month. At a recent keynote, Elon Musk discussed long-term plans for lunar industrialization, including a concept for a “terafab” and a mass driver(!) on the moon2. SpaceX isn’t the only company in this domain, but their ambitions set the scale for what the industry is heading toward. Just this week, NASA held a conference laying out their own phased moon base timeline: first crewed mission by 2029, semi-annual missions through 2032, and sustained human presence beyond that, with cargo capacity scaling from 4,000 kilograms to 150,000 kilograms per delivery.
The moon has aluminum and silicon in abundance. If you’re already harvesting lunar resources for commercial purposes, building orbital infrastructure for energy and compute, and launching at the frequency that SpaceX’s cost curves suggest, the sunshade stops being a standalone climate megaproject. It becomes a climate application built on commercial infrastructure that’s coming regardless of whether anyone cares about Earth’s energy balance.
To be clear, I don’t want to overstate this. The trend lines point in this direction, and you can see how the pieces fit together, but any of a dozen things could slow or redirect the trajectory of space industrialization. What I can say is that when I arrived at Nottingham, I assumed we’d probably build a sunshade eventually, but I couldn’t tell whether that meant the 2040s or the 22nd century. I didn’t yet see where it fit within our current trajectory. I left the workshop thinking the overlap between space industrialization and climate need gives it a much more concrete path than the pure-climate-funding frame ever could.
The zero-sum question, revisited
I posed the zero-sum funding question to many people at the workshop. If five million dollars could go to SAI research, or marine cloud brightening, or ice sheet protection, or sunshade research, where should it go? The honest answer is almost certainly one of the nearer-term stabilization interventions, where even single-digit millions can make meaningful impact now. Most sunshade researchers would probably agree.
Two responses changed how I think about the question, though. The first was about different money. The SpaceX IPO is going to create enormous new wealth among people who are interested in space for space reasons, not climate reasons. Those newly wealthy people represent a different potential funding pool than climate philanthropy, and they might be more naturally drawn to the sunshade than to atmospheric particle injection.
The second was the technology-tree argument, and it resonated with the part of me that spent years playing strategy games as a kid. If you’ve ever played a game like Civilization, you know that certain technologies require prerequisites, and you can’t skip the early steps just because you need the later ones urgently. Solar sail control systems, heliogyro dynamics, and lunar resource extraction techniques all have their own development timelines. Getting that work started now, even modestly, is how you have capability ready when the commercial space infrastructure matures enough to make full-scale deployment practical.
The governance question
On the third day of the workshop, there were breakout sessions covering different aspects of the field, from technical requirements to financing to governance. I participated in the governance breakout, with about a dozen people for 80 minutes. I’ve sat in on, read, and watched a lot of governance discussions about SRM over the past year, and this one was a genuinely useful exercise that surfaced some of the more interesting ideas I’ve encountered. The group ran a backcasting exercise from the year 2100: imagine a sunshade is operating successfully, and describe the governance system that made it possible. Then work backward.
The model that drew the most energy in the room was that of an independent central bank: an institution given a mandate (temperature stabilization for a thriving civilization), operating with its own resource mobilization capacity, technology-agnostic across a defined toolkit, making evidence-based decisions independently of direct political control while remaining accountable to a representative framework. The Bank of England was the reference case. The room explicitly rejected a UN-agency model as lacking the autonomy and enforcement power the task requires.
The group was already grappling with the hardest problems. Weaponization came up repeatedly: a sunshade is dual-use technology, and whoever controls it controls Earth’s thermal management from orbit. Then there’s what someone in the room called the “Sunshade Industrial Complex” risk, the concern that at trillion-dollar scale, the institution maintaining the sunshade develops its own incentive to keep expanding it. The group’s response to that was interesting: the capability itself persisting isn’t the problem, because climate management is going to be a permanent need. What you have to guard against is the logic of “we need an ever-larger sunshade” becoming self-justifying.
I’ll be recording a podcast with Morgan Goodwin, the executive director of the Planetary Sunshade Institute, and Ross Centers next week, where we’ll dig deeper into both the technical and governance questions. More on that soon.
Where I landed
I went to Nottingham thinking the sunshade was genuinely interesting but pretty far out. I couldn’t see the path to it becoming real in the way I can see that path for SAI or marine cloud brightening or even ice sheet interventions. I came away thinking the trajectory of space industrialization makes this far more plausible than I’d appreciated, in ways that have nothing to do with the usual arguments for or against space-based reflection.
SAI research remains the right near-term priority. Nobody at the workshop disagreed with that. The sunshade isn’t going to help with the warming we’ll face in the 2030s and 2040s. But if you zoom out to the rest of this century, to the question of what long-term temperature management actually looks like at the scale and duration we’re going to need, the sunshade is where the economic trend lines and the climate need meet. The world is unlikely to fund a trillion-dollar climate project any time soon. But it may build the infrastructure for other reasons, and a planetary sunshade becomes the highest-value climate application you can run on it.
The next workshop is in two years and I plan to go back. My favorite thing about doing this work is meeting the people who are directly building toward these solutions, and this was one of those rooms. There were 70 people from around the world who showed up to a university in the English Midlands to work on something that most of the climate world still treats as science fiction. I think they might be early, but I don’t think they’re wrong.
If you really want a deep-dive on where electrification is going, you must read Packy McCormick’s The Electric Slide essay. It’s nearly book-length, but the historical background is incredibly useful, and it follows along the same technology tree narrative that I deployed in this article.
Robert Heinlein fans rise up! The Moon is a Harsh Mistress is one of my favorite sci-fi novels, and it popularized the concept of a mass driver on the moon.
Thanks Paul, interesting write-up. I worry about the timelines given an impending recession from the Hormuz effect and the gradual, then accelerating GDP destruction as the planet warms. The event in two years time will be interesting to see if spirits have dampened.
Interestingly, our policy at GlobalCarbonReward.org also requires central bank participation. Perhaps the winds of change blow favorably..