Proposals to deploy reflective mirrors and up to one million additional satellites in low Earth orbit could have far-reaching consequences for human health and ecosystems, according to leading sleep and circadian rhythm researchers. The presidents of four international scientific societies, representing about 2,500 researchers from over 30 countries, have raised concerns in letters to the US Federal Communications Commission (FCC).
The regulator 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. The scientists warn that altering the natural light-dark cycle could disrupt biological clocks regulating 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.
Reflect Orbital hopes to use satellites equipped with large reflective mirrors to redirect sunlight onto areas roughly 5km to 6km wide 'on demand', with brightness adjustable 'from full moon to full noon'. The company says the system could extend solar energy production into the evening and provide lighting for construction projects, disaster response and agriculture. Meanwhile, SpaceX has proposed launching up to one million satellites to create a giant solar-powered computing network in orbit for artificial-intelligence workloads.
Ruskin Hartley, chief executive of DarkSky International, said: 'While ideas like mirrors on satellites beaming sunlight on demand to Earth or mega-constellations of up to one million satellites for AI datacentres may sound like science fiction, these proposals are very real.' He noted that scientific studies have already shown existing satellites have increased diffuse night sky brightness by roughly 10%.
Dr Miroslav Kocifaj of the Slovak Academy of Sciences said satellites affect the night sky in two main ways: individual satellites can leave streaks in telescope images, while sunlight reflected by satellites and debris brightens the sky. His modelling suggests these objects already add between 3 and 8 microcandela per square metre to night sky brightness, which could rise to between 5 and 19 microcandela by 2035, approaching the threshold 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. 'If the night sky becomes permanently brighter, the consequences could be profound,' she warned.
