SpaceX’s Starlink Internet service will require a close constellation of satellites to provide uniform low latency connectivity. The system already has over 1,500 satellites in orbit and has received approval to operate 12,000 of them. And that worries astronomers. Although SpaceX has taken steps to reduce the impact on its hardware, there is no way to eliminate the traces the satellites leave across ground-based observations.
How bad is the problem? A team of astronomers has been using archival footage from a survey telescope to search for Starlink tracks over the past two years. During that time, the number of affected images increased by a factor of 35, and researchers estimate that once the planned Starlink constellation is complete, virtually all images from their hardware will have at least one trace in them.
The hardware used for the analysis is called the Zwicky Transient Facility (ZTF) at the Palomar Observatory. ZTF is designed to capture rare events, such as supernovae. It does this by scanning the entire sky repeatedly, with software that monitors the resulting images to look for objects that were absent in early images but that appeared later. The high sensitivity of the ZTF makes it good for selecting weak objects, such as asteroids, in our own solar system.
To facilitate the task of quickly scanning the entire sky, the ZTF relies on a very wide field of view and an equally large camera. Unfortunately, this wide field of view also increases the likelihood that an exposure will have a Starlink satellite in sight.
To determine how often the presence of these satellites was detected by the ZTF cameras, the team behind the new analysis took data on the orbits of all Starlink hardware and compared them with the area of the sky captured in each ZTF archive image. . Once an image was identified as potentially capturing a satellite, software was used to detect the presence of a bright trace across the image. All in all, the analysis covered an approximately two-year period from November 2019 to September 2021.
This was at the time when SpaceX was quickly building its Starlink constellation, and it certainly shows. At the start of the study period, when there were only about 100 Starlinks in orbit, it was relatively common to have a stretch of 10-day observations where no one was detected. At the time when 500 were in orbit, these periods were in the past. And once there were over 1,500 Starlink satellites in orbit, the ZTF would generally image over 200 in a 10-day period.
Twilight observations were particularly affected due to the confluence of two factors. Images that capture the horizon will include angles that show much more of the space occupied by Starlink orbits. And because of the position of the Sun, several of these satellites are likely to be fully illuminated. As a result, about 64 percent of the tracks depicted were taken low in the sky in the twilight. These tracks also experienced explosive growth as the constellation of satellites was completed. By the end of 2020, only about 6 percent of twilight images were affected. By the end of 2021, that number had grown to 18 percent.
In response to complaints from the astronomical community, SpaceX placed visors on later generations of Starlink satellites. The research team was able to compare the visibility of these different generations and found that the visors worked – satellites with visors decreased in brightness by a factor of about 4.6 (the exact number depended on the wavelength). However, the visibility was still higher than the goal set at a workshop to solve this problem.
Because these tracks are small and software already identifies and handles them, they do not have much of an effect on observations. The researchers estimate that at present, there is only a 0.04 percent chance that a rare event will be missed because it coincides with a track. However, because the problem is most acute at dusk observations, it is more likely to affect searches for objects in the solar system. This will include comets and asteroids – including asteroids that formed around other stars.
But again, the problem is likely to get worse. SpaceX already has approval to increase the number of Starlink satellites to well over 10,000; the authors estimate that at 10,000, each image in the twilight will likely contain a Starlink trace. SpaceX has indicated that it would eventually like to increase the number to over 40,000 satellites, at which point all twilight images are likely to have four tracks.
And SpaceX is not the only company planning this kind of satellite services. If all the companies involved follow their plans, low orbits around the Earth could see as many as 100,000 of these satellites.
Overall, the picture is mixed. ZTF’s main mission – to select rare events caused by distant, energetic phenomena – is largely unaffected by the growing number of satellite tracks. And because the percentage of events is currently small, tripling the number of satellites will not have a dramatic impact on observations. But a secondary science mission is already seeing a lot of light pollution, and things are only getting worse.
The Astrophysical Journal Letters, 2022. DOI: 10.3847 / 2041-8213 / ac470a (About DOIs).
List image of Caltech Optical Observatories / IPAC