Eggshell Particles Aim to Cool Earth by Reflecting Sunlight
A chef sprinkling a pinch of anti-caking powder into a bag of flour and a physicist launching particles into the stratosphere seem to share nothing in common. Yet both rely on the same unassuming material: amorphous silica. The chef prevents clumps; the physicist hopes to prevent a boiling planet. This is the wager at the heart of Stardust Solutions, a company proposing to dim the sun itself.
The Powder That Mimics a Volcano
Nature has already run this experiment. In 1991, Mount Pinatubo erupted in the Philippines, blasting so much sulfur dioxide into the atmosphere that global temperatures dropped by 0.9 degrees Fahrenheit (0.5 degrees Celsius) for more than a year. [2] Stardust Solutions wants to replicate that cooling effect artificially, but with a twist: instead of sulfur dioxide, they propose spraying engineered particles designed to be safer and more controllable. The company, founded in 2023 by two former Israeli government nuclear physicists, has raised $75 million from investors to pursue this vision, according to company statements. Their approach involves high-altitude aircraft or balloons releasing tiny particles into the stratosphere, where these specks would reflect the sun's energy back into space before it can warm the Earth's surface. It is a form of solar geoengineering, a field that has moved from academic theory to commercial reality in just a few years. Stardust has developed two specific particles for this task. The first uses amorphous silica, a compound commonly found in food additives, toothpaste, and cosmetics. The second combines a core of calcium carbonate-the main ingredient in eggshells-with a shell of amorphous silica. Both substances occur naturally and can be manufactured synthetically, which the company argues makes them safer than the sulfur dioxide that volcanic eruptions naturally produce. The choice of materials matters because sulfur dioxide carries baggage. While it cools the planet effectively, it also contributes to acid rain and depletes the ozone layer. Stardust's CEO Yanai Yedvab said the company wanted to develop something "that will be better, will be safer, that could be controlled." [1] The company published its particle designs in scientific papers in 2024, though these papers still await peer review. The safety argument draws on unusual precedents. In 2016, the US Department of Homeland Security released amorphous silica into the New York City subway system as part of a test simulating the impacts of a potential biological attack. The choice of material for that test, Yedvab noted, highlights how safe silica is considered to be by government agencies responsible for public health.
The Price Tag of Planetary Shade
The numbers involved in this enterprise are staggering. Every million tons of particles would produce 0.9 degrees Fahrenheit of cooling, according to Yedvab's estimates. That cooling would require annual replenishment, meaning the deployment would need to continue year after year to maintain the effect. Each million-ton deployment would cost roughly $10 billion-a figure Yedvab acknowledges is substantial, though he argues it remains a small fraction of the damage caused by climate-related disasters. [Pic1] The company has so far limited its testing to laboratory conditions. Outdoor tests are the next step, but the history of such experiments suggests the path forward will be difficult. Harvard scientists abandoned plans for test flights in Arctic Sweden in 2021 after intense local opposition. A rogue experiment by US startup Make Sunsets in Mexico in 2022 prompted the Mexican government to ban solar geoengineering experiments entirely. Stardust hopes to begin outdoor tests within the next few years, but Yedvab insists these would only run "under supervision and direct guidance of governments." Yedvab compares the testing process to developing life-saving drugs. The approach would begin with very small-scale tests with clearly defined success criteria before graduating to larger experiments. The company says it is already engaging with policymakers to build a regulatory framework around testing. Decisions on whether the technology is ever deployed belong "solely in governments," Yedvab said. But experts raise concerns beyond the regulatory hurdles. David Keith, a professor of geophysical sciences at the University of Chicago known for his solar geoengineering research, said Stardust is "greatly overclaiming to say that they're confident the risks are lower than with sulfur." [4] One specific worry involves what researchers call the "hitchhiker hypothesis"-the idea that particles in the air can pick up metals or other substances that could cause harm when people breathe them in. Kelly Wanser, executive director of SilverLining, a nonprofit climate research organization, said Stardust's particle design takes these risks into account, but whether the particles will work at scale and how they might behave in the atmosphere remains unclear. [5] The gap between laboratory conditions and the chaotic reality of the stratosphere is immense, and no amount of controlled testing can fully bridge that uncertainty.
The Peril of a Sun That Cannot Be Turned Off
The most alarming scenario associated with solar geoengineering has a name: termination shock. This describes what would happen if the world began deploying these particles and then suddenly stopped. The cooling effect would vanish within about a year, and the planet would face "a catastrophic, sudden warming playing out over just a few years," said Raymond Pierrehumbert, professor of planetary physics at the University of Oxford. [6] Stardust argues its technology can avoid this trap. The particles fall out of the atmosphere after roughly a year, which means "the level of cooling remains a continuing, adjustable choice in the hands of governments and termination shock can be avoided," Yedvab said, though independent researchers note this adjustability has never been tested at scale. The system could be tuned up or down gradually, allowing for a measured approach to reducing deployment rather than an abrupt halt. Pierrehumbert remains skeptical of the entire enterprise, particularly the involvement of private profit motives. "When the whole world is at stake, satisfying the needs of venture capitalists for profits should not enter at all into decisions about what, if anything, should be deployed," he said. [6] The concern is not merely philosophical: proprietary technology developed by a for-profit company could create conflicts between financial incentives and the transparency needed for legitimate research. Keith echoed this concern in a blogpost last year, writing that "proprietary technology and profit motives are directly at odds with the transparency needed to legitimize research." [4] Sarah Doherty, an associate professor of atmospheric sciences at the University of Washington who leads a solar geoengineering research team, said "the current science does not support making financial bets on, or driving society toward, specific approaches." [7] Yedvab counters that most groundbreaking technologies benefiting humanity have been developed by for-profit companies, often building on academic research. But the stakes here are different from previous technological revolutions. Scientists warn that solar geoengineering could alter rainfall patterns and shift monsoons, with major repercussions for food production across the globe. A 2023 study led by researchers at Harvard University concluded that deploying the technology would bring "profound systemic uncertainty" and the "potential to destabilize the global climate system further [Pic2] The intersection of atmospheric physics, public health, and economics creates questions that no single discipline can answer alone. The particles might cool the planet, but at what cost to weather patterns that billions of people depend on for their food? The technology might be adjustable, but who decides the setting when nations disagree? The company might have the best intentions, but what happens when venture capital demands returns faster than governments can build consensus? These questions extend beyond technical feasibility into governance, equity, and the shared fate of a planet with only one atmosphere.