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Raumfahrt - The true impact of SpaceX’s Starlink constellation on astronomy is coming into focus

26.03.2020

New data gives a glimpse into a future with 12,000 more satellites

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The Ckoirama Observatory in Chile, while a trail of Starlink satellites pass overhead.
Photo by Rodrigo Maluenda

Ever since SpaceX launched its first batch of internet-beaming satellites last year, astronomers have watched with dread as the company continued to blast more spacecraft into orbit. Could this ballooning constellation of bright satellites fill the night sky with artificial light and muck up observations of the Universe for years to come? Now, new data is partially validating what many astronomers have feared since that first launch.

Up until now, people have been somewhat in the dark about the true impact of SpaceX’s internet-from-space project called Starlink, which envisions nearly 12,000 of these satellites orbiting Earth. SpaceX’s satellites are super bright compared to others, and astronomers have been worried that with so many luminous satellites in the sky, the odds of one passing in front of a telescope and obscuring an image will increase.

It turns out, some astronomers have reason to be concerned. Certain types of astronomy may be more negatively affected than others, one peer-reviewed study shows, particularly those kinds that scour large swaths of the sky over long periods of time looking for faint, faraway objects. That means scientists looking for distant objects beyond Neptune — including the hunt for the mysterious Planet Nine — might have trouble when Starlink is complete. Additionally, Starlink may be much more visible during twilight hours, or the first few hours of the night, which could be a major problem in the hunt for massive asteroids headed toward Earth. “It depends on what science you’re doing, and that’s really what it comes down to,” Jonathan McDowell, an astrophysicist at Harvard and spaceflight expert who wrote the study accepted by Astrophysical Journal Letters, tells The Verge.

Meanwhile, scientists are also learning if SpaceX’s effort to mitigate the brightness of its satellites is actually going to work. The company coated one of its satellites in an attempt to make it appear less visible in the sky. Now, the first observations of that satellite are being published, and the coating is working — but it might not be enough to make everyone happy. “It doesn’t solve the issue,” Jeremy Tregloan-Reed, a researcher at the University of Antofagasta and lead author on the study, which is undergoing peer review at Astronomy and Astrophysics Letters, tells The Verge. “But it shows that SpaceX has taken on board astronomers’ concerns, and it does appear to be trying to solve the situation.”

HOW STARLINK WILL AFFECT THE ASTRONOMERS

For astronomers, light is everything. Observing celestial objects in different wavelengths of light is the best method we have for exploring the Universe. That’s why adding artificial light to the sky freaks out so many scientists. Some astronomers take long-exposure images of the sky, gathering as much light as possible from distant objects — and when a bright satellite reflecting light from the Sun passes overhead, it can leave a long white streak that ruins the picture.

Of course, the sky is a big canvas, and one tiny satellite isn’t going to be a major headache. A host of factors dictate exactly how and when satellites will be a problem. A satellite’s size, shape, height, and path around Earth all affect exactly how much light it reflects from the Sun and where people will see it the most. Meanwhile, the time of year and the time of night determine how much sunlight is shining on a satellite at any given moment.

To figure out Starlink’s exact impression on the night, McDowell made a comprehensive simulation based on what we know about where all of the Starlink satellites are going. Ahead of launching its constellation, SpaceX had to file multiple requests with the Federal Communications Commission, detailing where the company planned to send all of its spacecraft. Using that information, McDowell came up with a snapshot of which areas will see the most satellites overhead and what times of night will be the worst for observations.

In the more northern and southern latitudes, Starlink satellites will dominate the horizon during the first and last few hours of the night. In the summertime, it’ll be much worse, with hundreds of satellites visible for those in rural areas away from city light pollution. “Where I live in [Boston], I can see the planes hovering over Logan [Airport] on the horizon,” says McDowell. “That’s what it will look like, but it’ll be satellites and it’ll be a lot of them.” SpaceX declined to comment for this story.

While people living in cities and towns won’t really notice, this spells bad news for those hunting really distant faint objects using long exposures. “The longer that you have the shutter open for, the more that you’re likely to have an observation impeded by one of these streaks that are quite bright,” Michele Bannister, a planetary astronomer at the University of Canterbury in New Zealand who helped McDowell with his research, tells The Verge. That means those hunting Planet Nine and objects at the edge of the Solar System have some cause for alarm.

Additionally, asteroid hunters are going to be extra affected by this constellation, says McDowell. “They’re really hosed, because they need to look at twilight,” he says. Scientists looking for asteroids orbiting near Earth often look for these objects near the Sun; they observe just after sunset when they can see the part of the sky near the Sun that’s too bright to see during the day. “That’s where the problem with illuminated Starlink satellites is the worst,” he says. “Even from regular 30-degree latitude observatories, they’re going to have serious problems.”

As for what that means for these astronomy fields, one obvious concern is that a potentially hazardous asteroid could go unnoticed until it’s too late to act appropriately. It’s also possible observers will have to take expensive countermeasures to get the kinds of images they want. “It may mean you have to observe twice as long, if you have to throw away half your data,” says McDowell. “So that’s expensive. Or you may need to make changes to your telescope design, to stop reflections from a satellite.”

The silver lining here, at least, is that McDowell’s study found that Starlink may not really have a big effect on a lot of other astronomers’ work, especially those who only look at small slices of the night sky for certain periods of time. But his work does fly in the face of what SpaceX CEO Elon Musk has said about Starlink and its astronomy repercussions. “I am confident that we will not cause any impact whatsoever in astronomical discoveries. Zero,” Musk said during a space conference at the beginning of March. “That’s my prediction. And we’ll take corrective action if it’s above zero.”

Despite Musk’s brazen proclamation, the truth is SpaceX has already taken some corrective action, but new research shows it may not be enough to silence all of the company’s critics.

A COAT OF NO COLORS

On its third Starlink launch in January, SpaceX included a satellite that had been painted with an experimental coating, meant to darken the spacecraft’s reflectivity. Nicknamed DarkSat, the spacecraft has been of particular interest to amateur satellite trackers. Various observatories have taken images of DarkSat as it’s passed overhead to gauge just how much fainter it appears compared to its cohort.

The answer, it seems, is that DarkSat is indeed darker but only slightly. Once it reached its final orbit, the satellite appeared 55 percent fainter compared to another bright Starlink satellite, according to Tregloan-Reed’s study. That’s based on the initial observations he made using a telescope at the Ckoirama Observatory in Chile. “The DarkSat coating does push the satellite beyond being able to be seen with the naked eye,” says Tregloan-Reed.

That’s a big reduction, but 55 percent may not be enough for some observatories. The Vera Rubin Observatory in Chile is still under construction, but it has the massive task of surveying the entire night sky. “It’s going to be able to give us the history of the Solar system in absolutely intricate and amazing detail,” says Bannister of the survey. “And I think that’s definitely something that is under threat.” People at the observatory have estimated that the Starlink satellites would need to be even fainter than DarkSat in order to truly stay out of the way and not saturate the images gathered.

The good news is that SpaceX has hinted that more extreme countermeasures may be on their way. During its latest launch, a SpaceX employee noted that while the coated satellite showed “a notable reduction” in brightness, a future Starlink satellite may be equipped with a sunshade to further reduce reflectivity. “We have a couple other ideas that we think could reduce the reflectivity even further, the most promising being a sunshade that would operate in the same way as a patio umbrella, or a sun visor — but for the satellite,” Jessica Anderson, a lead manufacturing engineer at SpaceX, said during the live stream.

Tregloan-Reed says he’s hopeful about some kind of shade. “If that was to work then in theory it would block out the sunlight completely,” he says.

Still, that doesn’t solve every single astronomy problem because even a darkened satellite can still be a nuisance. Astronomers searching for planets beyond our Solar System, for instance, often take very sensitive measurements of distant stars, looking for dips in their brightness that might indicate a foreign planet passing by. If a satellite, even a dark one, were to pass in front of a star someone was observing, it could throw off the search for these alien worlds.

No matter what, it seems that a giant constellation is going to have some kind of negative impact on someone — it can’t be helped. And looking at the big picture, SpaceX isn’t alone in its attempt to create a mega-constellation of satellites. The company just gets the most attention because it’s proposing the largest number of spacecraft, and its vehicles are big, bright, and lower in the sky compared to other proposed constellations. Others like OneWeb and Amazon want to also fill the sky with internet-beaming vehicles.

Such a large influx of artificial bright spots is really the heart of the issue. “I understand the importance of Starlink; I can see the benefits of worldwide internet,” says Tregloan-Reed. “It’s just the sheer numbers that are worrying me.”

Quelle: The Verge

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