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As reflective satellites fill the skies, students are making sure astronomers can adapt

As reflective satellites fill the sky, UArizona students allow astronomers to adapt

Grace Halferty, a senior graduating this summer with a bachelor’s degree in aerospace and mechanical engineering and the lead author of the paper, showed the instrument researchers built to measure the brightness and position of SpaceX Starlink satellites. Credit: Kyle Mittan/University of Arizona

As satellites crawl through the sky, they reflect light from the sun back to Earth, especially during the first few hours after sunset and the first few hours before sunrise. As more companies launch satellite networks into low Earth orbit, a clear view of the night sky becomes rarer. Astronomers in particular are trying to find ways to adapt.

With that in mind, a team of students and educators from the University of Arizona completed a comprehensive study to track and characterize the brightness of satellites, using a ground-based sensor they developed to measure brightness, speed and to measure the paths of satellites through the sky. Their work could be useful to astronomers, who — if notified of incoming bright satellites — can close the shutters of their telescope-mounted cameras to prevent light trails from tainting their long-exposure astronomical images.

The research team was led by planetary science professor Vishnu Reddy, who also, along with systems and industrial engineering professor Roberto Furfaro, leads the university’s Space Domain Awareness lab, which tracks and monitors all kinds of objects in orbit around the Earth and the Moon. characterizes.

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 from the Royal Astronomical Society. The study describes how the team created a satellite tracking device to measure the brightness and position of SpaceX Starlink satellites and compared those observations with government satellite tracking data from the Space Track Catalog database.

“Until now, most of the photometric or brightness observations available were done with the naked eye,” Halferty said. “This is one of the first comprehensive photometric studies to be peer-reviewed. It’s challenging to track the satellites with traditional astronomical telescopes because they’re so bright and fast-moving, so we built what is basically a small sensor with a camera lens itself because nothing off the shelf was available.”

The team took 353 measurements from 61 satellites over two years and found that the positions of Starlink satellites, as recorded in the government’s Space Track Catalog, only differed on average by 0.3 arcseconds from the UArizona calculations. An arcsecond in the sky is about the size of a dime held 2.5 miles away. The small difference is likely due to natural delays in government data, Reddy said. Because that data is based on estimated orbits calculated days earlier, rather than real-time observations, positioning errors can accumulate.

“This suggests there is hope that astronomers can use this data to close the shutter of their telescopes in time amid the growing chaos in the sky above,” Reddy said.

A great traffic jam

Starlink is a large network of satellites, also known as a megaconstellation, operated by SpaceX with the aim of providing worldwide internet coverage. SpaceX began launching Starlink satellites in 2019. More than 2,700 Starlink satellites have been launched today – a fraction of the target total of 42,000 satellites.

Other examples of satellite constellations are 31 GPS satellites and 75 iridium communication satellites. Other entities have plans to launch more satellites into low and medium orbit in the coming years. For example, Amazon plans to launch 3,000 satellites and the Chinese government plans to launch 13,000. These satellites will orbit no higher than 22,000 miles above the Earth.

The problem with satellites is that they require energy from solar panels, which can reflect sunlight off ground-based telescopes, and in turn affect astronomical observations from telescopes around the world. About 30% of all telescope images will be affected by at least one satellite track once the Starlink constellation is complete, said study 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 studies,” he said.

These satellites reflect even more right after launch, while still being relatively low and densely clustered before spreading through their orbit over time. They are often as bright as Saturn or Jupiter, two of the brightest objects in the night sky. As they maneuver into higher orbits, they become slightly weaker.

A moving target

SpaceX has employed a number of different methods to darken its Starlink satellites. For example, VisorSat satellites rely on a shadow to block out additional sunlight, making them 1.6 times dimmer. DarkSat satellites, on the other hand, rely on an anti-reflective coating that makes them 4.8 times weaker. However, DarkSats got too hot, so SpaceX moved away from that particular method. As of August 2021, all Starlink satellites are VisorSats.

“While these adjustments are steps in the right direction, they also don’t dim the satellites enough for astronomical studies,” said research team member Adam Battle, a graduate student studying planetary science.

In July, SpaceX announced new strategies. One involves mirrors that reflect sunlight off the Earth and another involves the use of dark building materials. Reddy’s team plans to investigate how effective these methods are at reducing the reflection of sunlight toward Earth.

While it’s helpful for astronomers to know exactly where satellites are, closing the cameras adds overhead to telescope operations. Measurements become less efficient when astronomers have to close the shutter or discard contaminated images. For example, a study that would take five years to complete could take 10 to 20% more time if the efficiency of the study decreases. The costs will continue to rise as more satellites are launched, Reddy said.

The team plans to build on the success by studying the brightness of the latest generation of Starlink satellites in four different colored filters – the same filters used in astronomical surveys of the sky to collect different information from stars, planets and more. track down. To accomplish this, the team teamed up with Tucson-based small company Starizona to build a sensor that can take pictures of satellites in four colors at once.

“Working with local small businesses is a win for us because it gives our students the ability to quickly prototype and bring a new system online,” said Reddy.

SpaceX launches Starlink satellites from California

More information:
Grace Halferty et al, Photometric characterization and orbital accuracy of Starlink satellites: implications for ground-based astronomical studies, Monthly Notices from the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac2080

Provided by the University of Arizona

Quote: As reflective satellites fill the sky, students allow astronomers to adjust (2022, Aug. 3), retrieved Aug. 3, 2022 from https://phys.org/news/2022-08-satellites-students-astronomers.html

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