Satellites reflect light from the sun back to Earth during the first few hours after sunset and the first few hours before sunrise. A clear view of the night sky is becoming harder to find as more companies launch satellites. Astronomers are looking for ways to adapt.
With that in mind, a team of University of Arizona students and faculty has completed a comprehensive study to track and characterize the brightness of satellites, using a ground-based sensor they developed tomeasure satellites' brightness, speed and paths through the sky. If they were notified of bright satellites, they could close the shutters of their telescopes to prevent light trails from tainting their images.
The research team was led by Vishnu Reddy, a professor of planetary sciences, and Roberto Furfaro, a professor of systems and industrial engineering.
The lead author of the study is a senior who is graduating this summer with a degree in mechanical engineering. The study shows how the team created a device to measure the position and brightness of the satellites and compared it to government data.
Most photometric or brightness observations were done by naked eye. One of the first comprehensive photometric studies to go through peer review is what this study is about. Because the satellites are so bright and fast moving, it is difficult to track them with traditional telescopes, so we built a small sensor with a camera lens ourselves.
The position of Starlink satellites as recorded in the government's Space Track Catalog only differed by an average of 0.2 arcs seconds from the UArizona calculations, according to the team's two-year measurement of 61 satellites. The size of a dime is 2.5 miles from the sky. Natural lag times in the government data may be to blame for the small difference. The positioning errors are caused by the fact that the data is based on estimates rather than actual observations.
The hope is that the data can be used to close the shutter of the telescopes in a timely manner.
A lot of traffic jams.
The goal of Starlink is to provide global internet coverage. The first Starlink satellites were launched in the year 2019. A fraction of the planned 42,000 satellites have been launched.
There are 31 gps satellites and 75 iridium satellites in the constellation. There are plans for more satellites to be launched in the next few years. The Chinese government and Amazon both plan to launch thousands of satellites. The satellites won't go higher than 22,000 miles above Earth.
The problem with satellites is that they need power from solar panels, which can reflect sunlight at ground-based telescopes, and in turn, impact astronomy. When the Starlink constellation is complete, about 30% of all telescope images will be impacted by at least one satellite trail.
The problem will only get worse with the addition of other constellations.
After launch, these satellites are even more reflective because they are still relatively low and clustered tightly. They are bright enough to be seen in the night sky as a pair of planets. They become fainter as they move into higher altitudes.
A moving object.
There are a number of different ways to dark the Starlink satellites. VisorSat satellites are 1.6 times fainter because they rely on a shade to block the sun's light. DarkSat satellites use an anti-reflective coating that makes them fainter. DarkSats got too hot, so the method was changed. All Starlink satellites are now called VisorSats.
Adam Battle, a graduate student studying planetary science, said that the modifications are steps in the right direction, but don't dim the satellites enough for astronomy surveys.
New strategies were announced by the company. Mirrors that reflect sunlight away from Earth is one of the methods used. The team plans to study the effectiveness of these methods.
Overhead costs for telescope operations are increased when the cameras are shut down. Astronomers get less efficient when they have to close the shutter. If the efficiency of the survey goes down, it will take 10% to 20% more time to complete. As more satellites are launched, costs will go up.
The team plans to build upon its success by studying the brightness of the latest generation of Starlink satellites in four different colored filters. The team built a sensor that can take pictures of satellites in four different colors at the same time.
Working with local small businesses gives our students an opportunity to quickly prototype and bring a new system online.
More information: Grace Halferty et al, Photometric characterization and trajectory accuracy of Starlink satellites: implications for ground-based astronomical surveys, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac2080 Journal information: Monthly Notices of the Royal Astronomical Society
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