Proposals to deploy reflective mirrors and up to 1 million additional satellites in low Earth orbit could have far-reaching consequences for human health and ecosystems, leading sleep and circadian rhythm researchers have warned. The presidents of four international scientific societies, representing about 2,500 researchers from more than 30 countries, have raised concerns in letters to the US Federal Communications Commission (FCC).
The FCC is considering plans by the start-up Reflect Orbital to illuminate parts of the Earth at night using reflective satellites, as well as applications from SpaceX that could dramatically expand satellite numbers in low Earth orbit. Reflect Orbital hopes to use satellites equipped with large reflective mirrors to redirect sunlight on to areas roughly 5km to 6km wide 'on demand', with brightness adjustable 'from full moon to full noon'. SpaceX has proposed launching up to 1 million satellites to create a giant solar-powered computing network in orbit for artificial-intelligence workloads.
In their letters, the scientists said altering the light-dark cycle could disrupt biological clocks that regulate sleep and hormone secretion in humans and animals, migration in nocturnal species, seasonal cycles in plants and the rhythms of marine phytoplankton that underpin ocean food webs. Prof Charalambos Kyriacou, a geneticist at the University of Leicester and president of the European Biological Rhythms Society, said: 'We’re saying, please think before you go through with this, because this could have global implications for things like food security. Plants need the night. You can’t just get rid of it.'
Ruskin Hartley, chief executive of DarkSky International, a non-profit focused on protecting natural night skies, said: 'While ideas like mirrors on satellites beaming “sunlight on demand” to Earth or mega-constellations of up to 1m satellites for AI datacentres may sound like science fiction, these proposals are very real.' He added that scientific studies have already shown the existing number of satellites has increased diffuse night sky brightness by roughly 10%.
Dr Miroslav Kocifaj of the Slovak Academy of Sciences said his modelling suggests these objects already add between 3 and 8 microcandela per square metre to night sky brightness. By 2035, he predicts this could rise to between 5 and 19 microcandela, approaching the threshold astronomers have set for preserving naturally dark skies. Prof Tami Martino of the University of Guelph said the key question is whether biological systems can detect the change, noting that 'circadian systems are sensitive to light levels far below what humans typically perceive as bright'.
