A prototype satellite for a proposed space-based mobile phone network is brighter than all but seven stars in the night sky. The satellite and others like it could force ground-based telescopes to either track and dodge satellite trails or prolong their observations to gather enough unspoilt data.

In 2022, Texas-based company AST SpaceMobile put its BlueWalker 3 satellite into orbit to test the feasibility of a mobile phone network orchestrated from space. Early observations of BlueWalker 3’s 64-square-metre reflective antenna suggested it was brighter than most stars in the sky.

That has now been confirmed by a year-long monitoring campaign using telescopes in the US, Chile, the Netherlands and New Zealand. Siegfried Eggl at the International Astronomical Union in France and his colleagues have found that BlueWalker 3 is even brighter than previous measurements and has a similar brightness to the brightest stars in the constellations Canis Minor and Eridanus.

This presents a significant threat to ground-based astronomy, says Eggl, especially for wide-angle surveys of the night sky, such as one that will be carried out at the Vera C. Rubin Observatory in Chile. “If there’s a bunch of these really bright objects, I think the danger is that there’s going to be potential large-scale data loss, if not damage to detectors, if that’s not mitigated,” he says.

Some of this might be avoided if telescopes adopt tracking and avoidance strategies, but these methods can still fail because satellites are hard to track. This might mean that telescopes would need to periodically shut down while a satellite moves over their line of sight, potentially delaying observations or skewing datasets.

Eggl and his team also found that BlueWalker 3’s brightness fluctuated over time, depending on the attitude, or angle, at which it faced the sun. Companies like AST SpaceMobile have control over this angle, so they play a significant role in making sure that the satellite reflects minimal light, says Eggl.

In a statement, an AST SpaceMobile spokesperson said it is “collaborating with NASA and certain astronomy working groups to develop advanced industry solutions, including potential operational interventions.” It is also avoiding broadcasts in areas sensitive to radio astronomy, the spokesperson said, as well as controlling the satellite’s attitude to minimise brightness and developing anti-reflective coatings for its next generation of satellites, which it says will number around 90.

Many more observations are needed to work out the full extent of the impact on astronomy, especially for longer wavelengths of light, such as the radio band. Satellites like BlueWalker 3 are equipped with powerful radio transmitters, so they have the potential to do more damage to radio-based astronomy, says Eggl. “For optical light, these things are as bright as the stars in the sky, but for radio astronomy, they can be as bright as the sun,” he says.

Comprehensive observation campaigns like this one are essential, says Aparna Venkatesan at the University of San Francisco, California, because “an individual satellite’s brightness is a complex, time-varying function of its design, area, viewing angle and altitude”.

“With ever more satellite launches planned by a rapidly rising number of state and private space actors, we must now consider not only the light pollution and electromagnetic interference from a diverse range of individual satellites, but the aggregate effects of all the satellite constellations slated for low Earth orbit,” she says.