Particularly during the first few hours after sunset and the first few hours before sunrise, satellites bounce sunlight back to Earth as they move across the sky. A pristine night sky is getting harder to find as more businesses send networks of satellites into low-Earth orbit. Particularly astronomers are looking for strategies to adapt.
To track and quantify the brightness of satellites, a group of the University of Arizona students and faculty conducted thorough research. They used a ground-based sensor they designed to detect the brightness, speed, and trajectories of the satellites as they moved through the sky.
If astronomers were made beforehand of the arrival of bright satellites, they might be able to close the shutters on the cameras mounted on their telescopes to avoid light trails contaminating their long-exposure astronomical photographs.
The research team was led by professor of planetary sciences Vishnu Reddy, who also co-leads with professor of systems and industrial engineering Roberto Furfaro the university’s Space Domain Awareness lab, which tracks and characterizes all kinds of objects orbiting Earth and the moon.
Grace Halferty, a senior graduating this summer with a bachelor’s degree in aerospace and mechanical engineering, is the lead author of the study, which is published in Monthly Notices of the Royal Astronomical Society.
In the article, it is explained how the scientists built a satellite tracking tool to gauge the brightness and location of SpaceX Starlink satellites and then compared those observations to information about government satellite tracking from the Space Track Catalog database.
“Until now, most photometric or brightness observations that were available were done by the naked eye,” Halferty said. “This is one of the first comprehensive photometric studies out there to go through peer review. The satellites are challenging to track with traditional astronomical telescopes because they are so bright and fast-moving, so we built what’s basically a small sensor with a camera lens ourselves because there was nothing off the shelf available.”
The scientists took 353 measurements of 61 satellites during a two-year period and discovered that the positions of the Starlink satellites as listed in the government’s Space Track Catalog only varied from UArizona calculations by an average of 0.3 arc seconds.
An arc second on the sky is about the size of a dime held 2.5 miles away. The tiny difference is probably due to natural lag times in the government data, Reddy said. Positioning inaccuracies can accumulate because the data is based on predicted orbits that were calculated days before to the observations.
Until now, most photometric or brightness observations that were available were done by the naked eye. This is one of the first comprehensive photometric studies out there to go through peer review. The satellites are challenging to track with traditional astronomical telescopes because they are so bright and fast-moving, so we built what’s basically a small sensor with a camera lens ourselves because there was nothing off the shelf available.
Grace Halferty
“This suggests that there is hope that astronomers can use these data to close the shutter of their telescopes in time amid the growing chaos in the skies above,” Reddy said.
A stellar traffic jam
Starlink is a large network of satellites, also called a mega constellation, operated by SpaceX with the goal of providing global internet coverage. SpaceX started launching Starlink satellites in 2019. Today, more than 2,700 Starlink satellites have launched a fraction of the intended total of 42,000 satellites.
31 GPS satellites and 75 communication-focused Iridium satellites are two further examples of satellite constellations. In the upcoming years, further satellites will be launched by other organizations into low and medium-Earth orbit. For instance, the Chinese government and Amazon both intend to launch 13,000 satellites. These satellites will orbit the planet at a maximum altitude of 22,000 miles.
The issue with satellites is that they need to be powered by solar panels, which can interfere with astronomical observations made by telescopes all around the world by reflecting sunlight at ground-based telescopes.
About 30% of all telescope images will be impacted by at least one satellite trail once the Starlink constellation is complete, said research team member Tanner Campbell, a graduate research assistant in the Department of Aerospace and Mechanical Engineering.
“As other constellations are added, the problem will only get worse for ground-based astronomical surveys,” he said.
These satellites are even more reflective immediately following launch, when they are still quite low and closely grouped before they gradually disperse throughout their orbit. They frequently shine as brightly as Saturn or Jupiter, two of the stars with the highest brightness at night. As they maneuver into higher orbits, they become slightly fainter.
A moving target
SpaceX has deployed a few different methods to darken its Starlink satellites. For example, VisorSat satellites rely on shade to block additional sunlight, making them 1.6 times fainter. DarkSat satellites, on the other hand, rely on an anti-reflective coating that makes them 4.8 times fainter. However, DarkSats got too hot, so SpaceX moved away from that specific method. Since August 2021, all Starlink satellites are VisorSats.
“While these modifications are steps in the right direction, they also don’t dim the satellites enough for astronomical surveys,” said research team member Adam Battle, a graduate student studying planetary science.
In July, SpaceX announced new strategies. Both involve employing darker building materials and mirrors to bounce sunlight away from Earth. The effectiveness of these techniques at lowering sunlight reflection back to Earth will be investigated by Reddy’s team.
Even though being able to pinpoint satellite locations is useful to astronomers, the act of turning off the cameras increases the overhead costs for telescope operations. When astronomers have to shut the shutter or discard tainted photographs, surveys are less effective.
For example, a survey that would take five years to complete could take 10% to 20% more time if survey efficiency is down. Costs will continue to increase as more satellites are launched, Reddy said.
By examining the brightness of the most recent Starlink satellites through four distinct colored filters, the same ones used in scientific surveys of the sky to extract various information from stars, planets, and more, the team hopes to build on its previous achievements.
To do this, the group has collaborated with Tucson-based startup Starizona to develop a sensor that can concurrently capture images of satellites in four different colors.
“Working with local small businesses is a win for us as it provides our students an opportunity to rapidly prototype and bring a new system online,” Reddy said.
