A Starlink track is running across the sky.
A dense constellation of satellites will be required for Starlink to work. The system has over 1,500 satellites in the air and has been approved to operate 12,000 of them. Astronomers are worried about that. There's no way to completely eliminate the tracks the satellites leave across ground-based observations, despite the steps taken by the company.
How bad is the problem? The team of sholders used archival images from a survey telescope to look for Starlink tracks over the past two years. The number of images affected rose by a factor of 35, and the researchers estimate that by the time the Starlink constellation is complete, pretty much every image from their hardware will have at least one track in it.
Looking a lot.
The analysis is done at the Palomar Observatory and the hardware used for it is called the Zwicky Transient Facility. The ZTF is designed to pick up rare events. It uses software to look for objects that were absent in early images but were present in later ones. It's good for picking out dim objects in our own Solar System because of the ZTF's high sensitivity.
The ZTF relies on a wide field of view and a large camera to quickly and accurately survey the entire sky. This broad field of view increases the chance that an exposure will have a Starlink satellite in view.
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The team behind the new analysis compared the area of the sky captured in every archival ZTF image to the area of the sky captured in Starlink hardware. Software was used to detect a bright track across the image when it was identified as potentially capturing a satellite. The two-year period from November to September was covered in the analysis.
This was during the time that the Starlink constellation was being built by the company. When there were only about 100 Starlinks in the sky, it was common to have a stretch of 10 days where no one was detected. The periods were over by the time 500 were in space. In a 10-day period, the ZTF would typically image over 200 Starlink satellites.
Each blue bar represents the number of Starlink tracks. The number of Starlink satellites is tracked by the red line.
The confluence of two factors affected twilight observations. Images that capture the horizon will show more of the space occupied by Starlink. More of these satellites are likely to be fully lit because of the Sun's position. Sixty four percent of the tracks were taken low in the sky at twilight. The constellation of satellites filled in as the tracks saw explosive growth. In the late 2020s, about 6 percent of the twilight images were affected. The figure grew to 18 percent by late 2021.
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There are consequences.
The visors were put on the later generations of Starlink satellites. The research team was able to compare the visibility of the different generations and found that visors worked and satellites with visors dropped in brightness by a factor of roughly 4.5. The visibility was still higher than the target set at the workshop that was meant to address this issue.
These tracks are small and software already handles them, so they don't have much of an effect on observations. There is only a 0.04 percent chance that a rare event will be missed because it coincides with a track, according to the researchers. It's more likely to impact searches for objects within the Solar System because of the problem that is most acute in twilight observations. This would include comets and asteroids.
The problem is likely to get worse. The number of Starlink satellites is already over 10,000, and the authors estimate that every image at twilight will have a Starlink track. At which point all twilight images are likely to have four tracks, the company has indicated it would like to increase the number of satellites to over 40,000.
There is more than one company planning on this type of satellite service. As many as 100,000 of these satellites could be seen by low Earth orbit if all the companies follow through on their plans.
The picture is mixed. The growing number of satellite tracks doesn't affect the ZTF's main mission of picking out rare events caused by distant, energetic phenomena. Increasing the number of satellites won't have a big impact on observations because the percentage of events is small. A secondary science mission is already seeing a lot of light pollution, and things are only going to get worse.
The Astrophysical Journal Letters was published in 2022. The About DOIs are listed on the website.
The Caltech optical observatory has a listing image.
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