Q&A: a light pollution expert
WITH A LIGHT POLLUTION EXPERT Light scattering from satellites and orbital debris could be greatly increasing background light pollution, affecting astronomical observations
Why do we want to prevent light pollution?
What people think about first is protecting the night sky, but we have an increasing understanding of the significance of light pollution as part of the overall threat to our environment. We know that it has an effect on wildlife ecology, has a potential influence on human health, public safety and energy security – it’s not simply a matter of astronomy.
How do satellites increase light pollution? Until recently we weren’t thinking about this issue. It came to the fore about two years ago with the first launch of the SpaceX Starlink constellation satellites; to see 60 objects at a time moving across the sky was startling. For astronomers the bright streak of a satellite running through their images cost data, but it was limited to the part of the image where the satellite actually appeared.
Our new paper takes a different view of that subject and instead asks, what is the collective contribution from all of these things in the sky? We discovered this was raising the background level of brightness in the sky by about 10 per cent. That was a startling result, and it’s a lower limit because it was based on the assumption that there were only a few thousand objects in orbit around Earth. We know that over time that number will increase.
How much will that alter the naked-eye view?
It is unlikely that an average observer at a very dark location will notice this effect. There are sources of light, particularly in Earth’s atmosphere, that are natural in origin and are more significant contributors of light. Satellites might be contributing something like 10 per cent above the background brightness, but the natural processes in the atmosphere that are generating light could be 50 per cent or higher. I’m concerned about five or 10 years from now, when there might be a 100,000 satellites in Earth orbit. Our model predicts that the contribution above the background brightness becomes higher. Does that start to impact what stargazers see? It’s very likely, and it’s going to make it harder to see faint stars.
Is this a problem for professional observatories? Yes, because they can see fainter things than most amateur photographers. If you want to achieve a certain minimum brightness of objects in your image and you have a telescope of a given size, one way to do that is by taking a longer exposure to get more photons from the target object. If you are observing something that’s very faint, it may take hours of exposure to get to that amount of signal. However, if the level of background brightness in the sky increases, you will need an even longer exposure and take more time to get the same result. In other words, you may not be able to observe as many objects in a given amount of observing time.
Will space-based observatories be affected?
It is a concern for space-based observatories. At least one Hubble Space Telescope image has a Starlink trail through it. Most of the satellites that are intended to be launched will be at higher orbits. Above 600km or so, a satellite will be in sunlight for almost all its orbit in the summer, so it will be illuminated all night long.
What is being done to mitigate the issue?
There is an ongoing dialogue between the space industry and the astronomical community. Operators like SpaceX have been making changes to try and make their objects less reflective, but there’s not a lot that can be done to curb the effect that we’re talking about in our paper. We are looking at how much the satellites raise the background brightness level of the sky. The reflectivity of the satellites is part of that and reducing that reflectivity will lower the impact of the diffuse sky brightness, but it doesn’t eliminate it. The only way to avoid this problem would be to have fewer objects orbiting our planet.