When was the last time you looked up into the night sky and saw the Milky Way? If you happen to live in one of the truly remote areas of the world, your answer might be “last night.” If you live in one of the generally “rural” areas of your country, you might remember how you used to see the Milky Way regularly, but the rise of LEDs, particularly the blue/white ones, has gradually erased the Milky Way from your nights. For the large majority of humans on our small world, the answer is “never.”
Light pollution has gradually stolen the night from us. Once powerful observatories such as Griffith Observatory have been blinded by our nocturnal illuminations, and modern telescopes must be built in the most remote areas of the world where light pollution is still manageable. Although we don’t notice it in the same way, the same is true for radio telescopes. Our days are filled with radio light, from mobile phones and Wi-Fi to the tire pressure sensors of a modern call. They all shine as bright in radio as an LED flashlight in the visible. But in recent years, both optical and radio telescopes have seen growing light pollution from another source: constellation satellites.
Companies such as Starlink have launched so many satellites that even in the most remote areas of the world, you can regularly see them near the horizon, particularly during dusk and dawn. For optical telescopes, the trails they create can be mitigated to a degree by making them less reflective. For radio telescopes, however, they pose a more serious and complex challenge.
Since Starlink and other constellations are communication satellites, they actively beam radio signals to Earth. Imagine if satellites had powerful floodlights shining over your house all the time, and you get the problem for radio observatories. One way to limit radio light pollution is to create forbidden zones where satellites don’t operate. For example, Starlink satellites go quiet over key regions of the National Radio Quiet Zone, as well as other large observatories across the world. But while this reduces the amount of light pollution, it doesn’t eliminate it.
As a recent study points out, radio signals from Starlink satellites aren’t narrowly focused. Even when they go quiet over an observatory, they are active in areas near the observatories, and stray radio light can contaminate observations. As satellite constellations become more common, this stray radio light will gradually wash out the radio sky, in much the same way that LED lights from neighboring towns diminish your view of the Milky Way.
The situation has gotten serious enough that the IAU Centre for the Protection of the Dark, Quiet Sky from Satellite Constellation Interference (CPS) has called for specific steps to be taken to save the night sky. They urge the international community to implement regulations so that ground-based astronomy can remain viable in the future.
It’s clear that satellite constellations such as Starlink are a benefit to many people in the world. For some regions, it is the only way to have a connection to the internet. In the same way, inexpensive night lighting has allowed us to have safer, more comfortable lives. But it is worth being mindful of what we can lose. Our view of the heavens has deep roots in human culture, and it is worth preserving. Balancing our history with our future is something we can all strive to do better.
Reference: Dark, I. A. U., et al. “Call to Protect the Dark and Quiet Sky from Harmful Interference by Satellite Constellations.” arXiv preprint arXiv:2412.08244 (2024).