Astronomers alarmed by satellite that outshines all but seven stars

Observations collected from telescopes around the world confirm that the communications satellite BlueWalker 3 outshines all but seven stars, posing huge problems for astronomy

By Alex Wilkins

2 October 2023

Trail of BlueWalker 3 crossing the night sky, taken from a backyard in Tucson, Arizona

A. Block/IAU CPS

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.

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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.