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« Ответ #990 : 15 Октябрь 2020, 23:09:24 »


Starlink already threatens optical astronomy. Now, radio astronomers are worried

The 197 radio astronomy dishes of the Square Kilometre Array (SKA) in South Africa will sit within a radio-quiet zone the size of Pennsylvania where even a cellphone is forbidden, to preserve the array’s views of the heavens. Yet that precaution won’t save the telescope, due to be completed in the late 2020s, from what may soon be overhead: tens of thousands of communications satellites beaming down radio signals straight from the heavens. “The sky will be full of these things,” says SKA Director General Phil Diamond.

The rocket company SpaceX has already launched hundreds of Starlink satellites, the first “megaconstellation” intended to provide internet service to remote areas. The satellites have aroused the ire of optical astronomers because of the bright streaks they leave across telescopes’ fields of view. Now, radio astronomers are worried, too. This week, SKA released an analysis of the impact that Starlink and other constellations would have on the array. It finds they would interfere with one of the radio channels SKA plans to use, hampering searches for organic molecules in space as well as water molecules used as a key marker in cosmology.

SpaceX is promising to address the concern. But radio astronomers are also seeking regulations. The United Nations Office for Outer Space Affairs (UNOOSA), which discussed the SKA analysis at a workshop this week, is considering ways to keep satellites from polluting the night sky with light and radio signals, not just for astronomy, but also for wildlife and the public. Astronomers also hope the International Telecommunication Union (ITU), a U.N. organization, will step in. “The radio spectrum is a resource that is being consumed by private companies that typically have no regard for science,” says radio astronomer Michael Garrett, director of the Jodrell Bank Centre for Astrophysics in the United Kingdom. “It’s only government intervention that can stop this state of affairs in my view.”

So far, SpaceX has launched more than 700 Starlinks out of an initial goal of 1440, and it has won approval for 12,000. Other operators, such as OneWeb and Amazon’s Project Kuiper, have similar ambitions. Studies suggest wide-field optical surveys will be worst affected, with satellite tracks marring most images. The team building the Vera C. Rubin Observatory, a survey telescope in Chile due to see first light next year, has been working with SpaceX to reduce the impact. The company has changed the orientation of satellites as they move up to their final orbit, painted them a less reflective color, and fitted “visors” to reduce reflections. Since August, all launched Starlink satellites have visors, SpaceX’s Patricia Cooper, vice president for satellite government affairs, told the UNOOSA workshop this week. “We’re trying to look for a path where we can coexist,” she said.

The analysis from SKA, which when complete will be the world’s largest radio observatory, highlights the new concern. The band that Starlink uses to beam down internet signals takes up a sizable chunk of frequencies from 10.7 to 12.7 gigahertz, within a range known as band 5b that is one of seven bands SKA’s South African dishes will target. The SKA analysis calculated the impact of 6400 satellites, taking into account both direct signals and leakage called “side lobes.”

The team calculated that satellite transmissions will lead to a 70% loss in sensitivity in the downlink band. If the number of satellites in megaconstellations reaches 100,000, as predicted by many, the entire band 5b would be unusable. SKA would lose its sensitivity to molecules such as the simplest amino acid, glycine, a component of proteins. “If it was detected in a planetary system that was forming, that would be a very interesting piece of information,” Diamond says. “This is a new area that SKA is opening up.” The band could also contain the fingerprints of water molecules in distant galaxies, a tracer that cosmologists use to study how dark energy is accelerating the expansion of the universe.

Since 1959, ITU has protected a number of narrow frequency bands for astronomy. But in recent decades, digital receivers have allowed telescopes to “operate over the whole spectrum,” Diamond says. “We’ve learned to coexist with transmitters,” typically by excluding them from a radio quiet zone or siting telescopes in remote areas. But they have no control over transmitters flying overhead.

Radio astronomers want the satellite operators to turn off their transmitters, move to other bands, or point them away, when they are flying over a radio observatory. Tony Beasley, director of the U.S. National Radio Astronomy Observatory, says they have been discussing these options with SpaceX. “In the next year or two, we will be doing tests where we’re going to be trying to coordinate in real time, technically, with them.” Beasley says this is a reflection of SpaceX’s corporate culture: “They want to do cool stuff; they don’t want to do any harm.”

Other astronomers don’t want to count on corporate goodwill. At the UNOOSA workshop, they pushed for two recommendations: that all future satellites in low-Earth orbit be designed to avoid beaming at radio telescopes and radio quiet zones, and that they control the leakage from their side lobes. Those recommendations, along with others discussed this week for protecting optical observatories, will be debated at a series of U.N. subcommittees next year before going to UNOOSA and, ultimately, the U.N. General Assembly for approval.

Beasley is philosophical about the situation. “SpaceX is legally transmitting inside one of their bands and there are going to be impacts for anyone trying to do radio astronomy,” he says. “These spectrum allocations represent the goals and intent of society. We make [them] to enable commerce and to enable defense and all kinds of activities. We have to come to a solution that satisfies all these to some extent.”
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