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SpaceX launch highlights threat to astronomy from ‘megaconstellations’

Debris objects in low-Earth orbit (LEO).

Space is already congested — but companies including SpaceX and Amazon have plans to launch thousands of satellites into orbit in years to come (artist’s impression).Credit: ESA

Spaceflight company SpaceX is set to launch 60 communications satellites into orbit today as the basis for a web of spacecraft designed to provide global Internet access. But many astronomers worry that such ‘megaconstellations’ — which are also planned by other companies that could launch tens of thousands of satellites in the coming years — might interfere with crucial observations of the Universe. They fear that megaconstellations could disrupt radio frequencies used for astronomical observation, create bright streaks in the night sky and increase congestion in orbit, raising the risk of collisions.

SpaceX aims to launch its second set of these satellites — called Starlinks — from Cape Canaveral, Florida, just before 10 a.m. local time; the first 60 went up in May. But these launches are just the beginning: by the end of 2020, there could be hundreds of Starlinks in orbit, and SpaceX envisions thousands in the years to come. Other companies such as Amazon, headquartered in Seattle, Washington, and London-based OneWeb are planning launches that altogether could more than double the number of existing satellites. They are meant to bring fast, reliable Internet to underserved communities worldwide, with other potential applications, including improved satellite Internet service for military planes.

Although it’s not clear how many of the planned megaconstellations will actually be built, several researchers have begun to analyse how the satellite networks could affect astronomy. The situation doesn’t seem as bad as initially feared, but might still dramatically shift how some astronomers do their jobs.

Sky streaks

Satellites are not a new challenge for astronomy; the US military tracks and releases information on nearly 20,000 objects that are in orbit. Many are small objects that aren’t visible to the naked eye and don’t interfere with astronomical observations. “What’s really concerning is how bright all these new satellite constellations will be,” says Patrick Seitzer, an astronomer at the University of Michigan in Ann Arbor.

Within the next year or so, SpaceX plans to launch an initial set of 1,584 Starlink satellites into 550-kilometre-high orbits. At a site like Cerro Tololo, Chile, which hosts several major telescopes, six to nine of these satellites would be visible for about an hour before dark and after dawn each night, Seitzer has calculated.

Most telescopes can deal with that, says Olivier Hainaut, an astronomer at the European Southern Observatory (ESO) in Garching, Germany. Even if more companies launch megaconstellations, many astronomers might still be okay, he says. Hainaut has calculated that if 27,000 new satellites are launched, then ESO’s telescopes in Chile would lose about 0.8% of their long-exposure observing time near dusk and dawn. “Normally, we don’t do long exposures during twilight,” he says. “We are pretty sure it won’t be a problem for us.”

But an upcoming, cutting-edge telescope could be in bigger trouble. The US Large Synoptic Survey Telescope (LSST) will use an enormous camera to study dark matter and dark energy, asteroids and other astronomical phenomena. It will survey the entire visible sky at least once every three nights, starting in 2022. Because the telescope has such a wide field of view, satellites trailing across the sky could affect it substantially, says Tony Tyson, an astronomer at the University of California, Davis, and the LSST’s chief scientist.

He and his colleagues have been studying how up to 50,000 new satellites — an estimate from companies’ filings with the US government — could affect LSST observations. Full results are expected in a few weeks, but early findings suggest that the telescope could lose significant amounts of observing time to satellite trails near dusk and dawn.

Paint it black

There are other impacts beyond losing observing time. Bright satellite streaks can saturate the camera’s sensors, creating false signals. The streaking problem would be worse in summer, when satellites are visible for longer — introducing a seasonal bias that would harm LSST studies that require building up statistical significance over time, including studies of dark matter and dark energy.

There are ways to manage these issues, says Paul Dabbar, the under secretary for science at the US Department of Energy, which funds the LSST camera. Companies operating the satellites could provide astronomers with detailed information on where they are in the sky at any given time so that observers could schedule around the expected satellite trails. The Earth-facing surfaces of the satellites could also be painted a dull black, which would make them appear fainter.

SpaceX says that it is “taking steps to make the base of Starlink satellites black to help mitigate impacts on the astronomy community”, but did not say whether the upcoming launch will involve darkened Starlinks. The company also told Nature that it is sharing information on the position of its satellites in the US military catalogue, and talking with astronomy groups around the world to assess the effects and evaluate mitigation strategies.

Radio interference

Radio astronomers face a second set of challenges. They observe the Universe in wavelengths of light that are also used for satellite communications. The use of such frequencies is regulated, but the huge number of planned satellites complicates the situation, says Tony Beasley, director of the US National Radio Astronomy Observatory in Charlottesville, Virginia. As satellites communicate with ground stations, their signals could interfere with radio-astronomy observations, rendering the astronomy data useless.

The observatory is talking with SpaceX and OneWeb about the frequencies that those megaconstellations will use for their broadcasts. Companies might decide to shift the frequencies at which they broadcast away from those used for radio astronomy. Another idea is for satellites to temporarily shut off their communications as they pass over radio-astronomy facilities.

A further issue with megaconstellations is that the sheer number of satellites will complicate efforts to manage growing congestion in space. Even if only some of them are eventually launched, it will worsen the space-junk problem. Amazon has estimated that if 1 in 20 of its planned satellites fails, there is a 6% chance of collision with another orbiting object, which would generate more space debris.

The first batch of Starlinks have already caused some congestion. In September, the European Space Agency (ESA) had to manoeuvre its Aeolus wind-mapping satellite out of the way of a Starlink satellite. The Starlinks are supposed to automatically move away from potential collisions, but a communications glitch between ESA and SpaceX meant each didn’t know what the other was doing. The incident highlighted the fact that satellite operators don’t have a universal strategy if two active satellites are on a potential collision course, says Holger Krag, head of ESA’s space-debris office in Darmstadt, Germany.

He and his colleagues are hoping to help develop a global collision-avoidance system that automatically detects potential crashes and orders satellites to move to safer locations. “We would like to see that in two to three years,” Krag says.

But with the megaconstellations already becoming reality, the operators are running out of time.

Nature 575, 268-269 (2019)



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