As reflective satellites fill the sky, students ensure astronomers can adapt – ScienceDaily

As satellites crawl across the sky, they reflect light from the sun back to Earth, particularly in the first few hours after sunset and the first few hours before sunrise. As more companies bring satellite networks into low-Earth orbit, a clear view of the night sky is becoming increasingly rare. Astronomers in particular are trying to find ways to adapt.

With this in mind, a team of students and faculty from the University of Arizona conducted a comprehensive study to track and characterize the brightness of satellites. It used a ground-based sensor they developed to measure the satellites’ brightness, speed and paths through the sky. Their work could be helpful to astronomers, who, when notified of incoming bright satellites, could close the shutters of their telescope-mounted cameras to prevent light trails from distorting their long-exposure astronomical images.

The research team was led by Vishnu Reddy, Professor of Planetary Sciences, who, along with Roberto Furfaro, Professor of Systems and Industrial Engineering, also directs the university’s Space Domain Awareness Lab, which tracks and characterizes all types of objects that make up the Earth and the earth orbit moon.

Grace Halferty, a senior who will be completing her bachelor’s degree in aerospace and mechanical engineering this summer, is the lead author of the study, which will be published in Monthly Bulletins of the Royal Astronomical Society. The study describes how the team developed a satellite tracking device to measure the brightness and position of SpaceX Starlink satellites and compared those observations to government satellite tracking data from the Space Track Catalog database.

“Until now, most of the photometric — or brightness — observations that were available were made with the naked eye,” Halferty said. “This is one of the first comprehensive photometric studies to be peer-reviewed. The satellites are difficult to track with traditional astronomical telescopes because they’re so bright and fast, so we basically built a small sensor with a camera to self-object because there wasn’t anything off the shelf.”

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

“This suggests there is hope that astronomers can use this data to timely close the shutters on their telescopes amid the growing chaos in the sky,” Reddy said.

A star jam

Starlink is a large satellite network, also called a mega-constellation, operated by SpaceX to provide global internet coverage. SpaceX began launching Starlink satellites in 2019. More than 2,700 Starlink satellites were launched today – a fraction of the planned total of 42,000 satellites.

Other examples of satellite constellations include 31 GPS satellites and 75 Iridium satellites for communications. Other companies have plans to launch more satellites into low- and medium-Earth orbit in the next few years. For example, Amazon is planning to launch 3,000 satellites and the Chinese government is planning 13,000. These satellites will orbit no higher than 22,000 miles above the earth.

The problem with satellites is that they require energy, which is harvested from solar panels, which reflect sunlight on ground-based telescopes and in turn can 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 Tanner Campbell, a member of the research team and a research associate at the Institute of Aerospace and Mechanical Engineering.

“As other constellations are added, the problem for ground-based astronomical surveys will only get worse,” he said.

These satellites are even more reflective right after launch while still relatively low and dense before spreading out over 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 few different methods to eclipse its Starlink satellites. For example, VisorSat satellites rely on a shadow to block 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 was getting too hot, so SpaceX moved away from that particular method. As of August 2021, all Starlink satellites are VisorSats.

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

In July, SpaceX announced new strategies. One is mirrors that reflect sunlight away from Earth, and another uses darker building materials. Reddy’s team plans to study how effective these methods are in reducing the reflection of sunlight back to Earth.

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

The team plans to build on its success by examining the brightness of the latest generation of Starlink satellites in four different colored filters – the same ones used in astronomical sky surveys to tease out different information from stars, planets and more. To achieve this, the team worked with a small Tucson-based company, Starizona, to build a sensor that can capture images of satellites in four colors simultaneously.

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

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