(Photo: Researchers say that a ‘planetary sunshade’ placed between Earth and the Sun could cool down the planet by 1 °C. Credit: Mongpro/Getty)
In August, entrepreneur Elon Musk suggested in a social-media post that Earth could be cooled by launching satellites into space to shade the planet from the Sun. These “sentient satellites”, he wrote, “can solve global warming for ~1B years”.
It’s unclear whether Musk, the chief executive of aerospace firm SpaceX in Starbase, Texas, meant for this to be taken seriously. But some researchers are earnestly investigating space-based methods of reflecting sunlight away from Earth to cool the planet as temperatures rise to increasingly dangerous levels.
A non-profit organization called the Planetary Sunshade Institute in Los Angeles, California, published a report this month that researchers say is the most detailed analysis yet of such geoengineering methods. The report — which summarizes studies from a consortium of engineers and climate scientists, including some from leading space agencies such as NASA — estimates that it could become viable to begin building a ‘planetary sunshade’ in as few as 20–40 years. The timeline would depend on the rate at which heavy-lift rockets are developed that can haul millions of tonnes of material into deep space to create the sunshade. (SpaceX, which is not involved in the report, did not respond to a query from Nature about whether it is conducting research on this topic.)
A lot of people think this idea is “just fantasy”, says Chantal Cappelletti, a space-systems engineer at the University of Nottingham, UK, who is part of the consortium. But the falling cost of space launches have made the prospect seem more feasible, she says.
Not so science fiction
Blocking sunlight to cool the planet through solar geoengineering has long been controversial. Proposals have included releasing sunlight-reflecting particles into Earth’s atmosphere, which could affect the planet in poorly understood ways, such as disrupting precipitation and plant growth. Space-based approaches could come with unique risks, including the pollution unleashed from many thousands of rocket launches. Critics also say that focusing attention on geoengineering distracts from the urgent need to cut greenhouse-gas emissions to address climate change.
But solar geoengineering has received more attention from researchers as Earth looks set to hurtle past 1.5 °C of warming above the planet’s pre-industrial baseline temperature. The “accelerating climate crisis” is pushing the world towards solar geoengineering, says Morgan Goodwin, a climate activist and executive director of the Planetary Sunshade Institute.
The report is the group’s biggest project so far, funded by a £400,000 (US$530,000) grant from the UK Advanced Research and Invention Agency (ARIA), a body sponsored by the government to fund high-risk, high-reward research. That grant is part of a larger £56.8-million ARIA research programme on solar geoengineering.
Daniele Visioni, a climate scientist at Cornell University in Ithaca, New York, says that he had previously dismissed space-based approaches as science fiction. But after attending an engineering workshop on such approaches earlier this year, he was persuaded that they deserve more attention. However, “I still wouldn’t put this as something feasible before the end of the century”, he says.
Between Earth and the Sun
The report considers the engineering requirements and climatic effects of six ‘architectures’ that could be used to build a massive sunshade in space that would be big enough to block about 1% of the solar radiation reaching Earth, which climate models suggest would cool temperatures by 1 °C.
Two main types of architecture were: launching a constellation of satellites, or heliogyros, that would form a sunshade (see ‘Shading the planet’), and launching material to create a huge cloud of dust that would dim the sunlight reaching the planet. A point just over two million kilometres away from Earth is considered the optimum distance to place each option to shade the planet using minimal material.
The heliogyros, each 40 km in diameter, would use the centrifugal force of their own rotation to support their paper-thin reflective blades. They could be powered by the pressure of photons from the Sun hitting the blades. Unlike aerosol particles injected into Earth’s atmosphere, the satellites could be controlled remotely to reflect different amounts of sunlight by changing the positions of the blades. “You can decide when you want to shade, when you don’t want to shade,” Cappelletti says.
The report estimates 1 °C of cooling would require about 5,000 such satellites floating in formation to create a sunshade, built from nearly 70 million tonnes of material. That’s around 1,500 times more mass than the total mass of everything that has ever been launched into space previously, according to one data source. Building the constellation would involve launching satellite parts from Earth and completing their construction in space using materials mined from the Moon. The low-gravity conditions on the lunar surface would make heavy-lift launches more efficient and less costly.
Another option considered in the report is launching a reflective cloud of dust or engineered particles to an orbit point between the Sun and Earth. The cloud could be prevented from drifting by a fleet of ‘shepherd’ spacecraft using a repulsive electric field. This approach might have the benefit of launching hundreds of times less mass than the heliogyro strategy. However, the report concluded that the idea — which is being explored by researchers at NASA’s Jet Propulsion Laboratory in Pasadena, California — is more speculative than are satellite-based sunshades.
There are too many unknowns to make any reliable cost estimate for these sunshades, the report says. But Cappelletti says that a sunshade might cost tens of trillions of dollars. “It‘s not something that we can’t do. It’s something that requires a lot of launch,” she says.
Ren Ecosystem