One of the Brightest Stars in the Sky is Actually a Satellite
By Evan Gough
Back in the 70s, kids used to look up at the summer sky and try to be the first one to shout, “Satellite!” That seems like a relic from the past now, alongside Polaroid cameras and astronauts on the Moon. These days, it’s rare to spend any amount of time looking at the sky without seeing a satellite, or several of them.
A new satellite is emphasizing that fact. It’s a prototype communications satellite with a roughly 700-square-foot antenna, and it’s brighter than most stars.
BlueWalker 3 is a prototype satellite built and launched by AST SpaceMobile. They launched it in September 2022, and at first, it had very little impact. Then, in November, the satellite unfurled its antennae. The antenna array is as large as a billboard on the side of the highway, and astronomers immediately became concerned.
“It’s like exactly what astronomers don’t want,” said astronomer Meredith Rawls of the University of Washington. Rawls helps run the International Astronomical Union’s Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. “It’ll show up as a super bright streak in images and potentially saturate camera detectors at observatories,” she said at the time.
Now, a team of astronomers has quantified BlueWalker 3’s effect on the night sky in a new research article. It’s titled “The High Optical Brightness of the BlueWalker 3 Satellite,” and it’s published in Nature. The lead author is Sangeetha Nandakumar, a Ph.D. student at the Universidad de Atacama in Chile. Rawls is a co-author of the study. Their work is based on an observing campaign involving amateur and professional astronomers around the world, including the Steward Observatory and the Leiden Observatory.
When BlueWalker 3 unfolded itself, it became one of the brightest objects in the night sky. The brightness of objects in the sky is measured in magnitude. Magnitude isn’t measured in units; it’s logarithmic, and a smaller number is brighter. BlueWalker 3 is so bright it set a record and broke the International Astronomical Union’s guidelines.
“From our observations, we learned that AST Space Mobile’s BlueWalker 3—a constellation prototype satellite featuring a roughly 700 square-foot phased-array antenna—reached a peak brightness of magnitude 0.4, making it one of the brightest objects in the night sky,” said Siegfried Eggl, who coordinated the new research. “Although this is record-breaking, the satellite itself is not our only concern. The untracked Launch Vehicle Adapter had an apparent visual magnitude of 5.5, which is also brighter than the International Astronomical Union recommendation of magnitude 7.”
There’s growing concern among astronomers about the proliferation of satellites, especially constellations of satellites like Elon Musk’s Starlink system. BlueWalker 3 is the prototype for a constellation of satellites like it. And Starlink and BlueWalker aren’t the only constellations in orbit. It’s getting crowded up there, and more satellites are on the way.
“We cannot accurately predict where all the satellites will be years in advance. Just accepting recurring data loss in multi-billion-dollar observatories is not an option either.”
Siegfried Eggl, IAU’s Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference.
This short video shows BlueWalker 3 at 19:52:45, 19:52:56, 19:53:18, 19:53:29. Starlink-4781 is visible at 19:52:54 and 19:53:26, leading BlueWalker 3. Starlink-4016 is parallel and slightly behind BlueWalker 3 at 19:53:34.
Aside from the Sun and the Moon, Venus is the brightest object in the sky, with a magnitude of around -4 at its brightest. Naked-eye stars are between -1 and +6 magnitude, so the dimmest stars are around magnitude 6. At magnitude 0.4, BlueWalker 3 is far brighter than most naked-eye stars.
Why is it so much brighter?
It’s because of the antenna array. BlueWalker 3’s array is 693 sq. ft. For comparison, a single Starlink satellite is roughly the size of a table.
The satellite’s large size translates into visible light pollution.
BlueWalker 3 is just one sky object. Alone, it can’t cause much havoc. But it’s not alone.
“One might think if there are bright stars, a few more bright satellites won’t make a difference. But several companies plan to launch constellations,” Eggl said. “For example, Starlink already has permission to launch thousands of satellites, but they’ll probably get their full request of tens of thousands granted eventually.”
These communication satellites occupy the Low-Earth Orbit (LEO), which is where most of our satellites orbit. LEO isn’t strictly defined by altitude because the Earth isn’t truly spherical. The most-used definition for LEO is an orbit with a period of 128 minutes or less, meaning an object in LEO orbits the Earth a minimum of 11.25 times per day.
LEO is getting crowded. There are around 5,000 satellites in LEO now, with many more thousands to come. Satellites are a powerful technology, and we’re not about to stop launching them. So what can be done? “The expected build-out of constellations with hundreds of thousands of new, bright objects will make active satellite tracking and avoidance strategies a necessity for ground-based telescopes,” the authors write in their article.
The number of communication satellites is expected to multiply quickly in the coming years.
Starlink intends to launch tens of thousands more satellites in the near future. In the US alone, the FCC is dealing with requests to launch 400,000 more individual satellites. Amazon just launched the first two prototype satellites in its Project Kuiper constellation, and companies in Europe want their own constellations, too. Most of these will be much smaller than BlueWalker 3, but the cumulative effect of all of these satellites could transform our night sky. For the astronomy community, that transformation is anxiety-inducing.
“BlueWalker 3 is so bright that most of the big telescopes, such as the Rubin Observatory, believe it could obliterate large parts of exposures,” Eggl said. “They already have to avoid observing Mars and Venus for the same reason, but we know where the planets are, so we can dodge them. We cannot accurately predict where all the satellites will be years in advance. Just accepting recurring data loss in multi-billion-dollar observatories is not an option either.”
But is the onus all on the astronomy community to deal with all of these objects streaking across the sky?
There are ways to mitigate the disruptive light from our ballooning fleet of satellites. Starlink engaged early with the astronomical community to look for solutions. They tried painting one black, but it absorbed too much heat. They also tried sunshades made out of dark foam and changing the angle of the satellites. They also tried mirrors. The effort continues.
“If SpaceX can make the solar panels point in a different direction to avoid glints, or use these mirror tricks, they might solve a lot of the problems we have with the optical flaring of Starlink satellites,” Eggl said.
“Starlink is looking at making their satellites’ surfaces darker, which absorbs more and reflects less visible sunlight. But the absorption generates heat. The satellites then have to emit infrared light, which means observations in optical wavelengths don’t have as large of a problem, but infrared observations might. And heat is one of the biggest engineering problems that we have in space. So, painting everything black comes with repercussions,” Eggl said.
Starlink’s solutions might not work for BlueWalker 3 anyway. That satellite needs to be pointed at Earth for maximum efficiency. “With other providers, it’s not quite as easy,” Eggl said. “AST has gigantic satellites, with hundreds of square feet of electronic phased arrays, that they need to communicate with cell phones on the ground.”
The Vera Rubin Observatory is coming online soon, and it’ll be especially affected by satellites. Its job is to repeatedly image the entire night sky and look for transients. All the light from all these moving objects will challenge the Observatory’s work. Even if satellite darkening techniques improve, the trails are still a problem.
A paper published in 2020 calculated some of the outcomes. “Future Starlink darkening plans may reach g ~ 7 mag, a brightness level that enables nonlinear image artifact correction to well below background noise. However, the satellite trails will still exist at a signal-to-noise ratio ~ 100, generating systematic errors that may impact data analysis and limit some science.”
Only collaboration can solve this growing problem. And so far, the space industry seems cooperative. “We are trying to work with the space industry, where possible, Eggl said. “We want to solve this together so it’s a collaborative effort that everybody can sign onto because that’s the fastest route to get things done.”
It’s good that Starlink is working with the astronomy community. But what if companies keep launching much larger satellites like BlueWalker 3? Remember, it’s just a prototype. AST SpaceMobile says BlueWalker 3 is the predecessor to a fleet of satellites. BlueWalker 3 is “designed to operate directly with standard, unmodified mobile devices,” the company writes on their website. That’s why its array is so large. What would the sky look like with a fleet of these things?
Light pollution from satellites might not seem important to people struggling with inflation and other concerns. But problems don’t go away just because we don’t think about them. We already know that for city dwellers around the world, and that’s most people, the night sky is becoming increasingly inaccessible. That’s largely because of Earthly light pollution. But the sky is the sky, and both issues are intertwined.
We may be reaching the day when sky-gazing morphs into satellite-spotting. If Starlink, AST SpaceMobile, OneWeb and others go through with their plans, then it may get difficult to lose yourself in the natural night sky without satellites streaking across your view.
Instead of contemplating nature, we’ll be contemplating satellites.
The post One of the Brightest Stars in the Sky is Actually a Satellite appeared first on Universe Today.
October 7, 2023 at 12:57AM
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