No more dark place on earth – Unexpectedly high light pollution also from diffuse reflections from satellites


Orbital scattered light: The diffuse reflections from satellites and space debris contribute surprisingly much to terrestrial light pollution, as a study has now revealed. According to this, this orbital scattered light brightens our nights by at least ten percent – worldwide. As a result, there is no longer an absolutely dark place on earth. The planned mega-constellations for the satellite Internet will make this problem worse, warn astronomers.

More than 80 percent of the world’s population already live under an abnormally bright night sky, because the artificial lighting and its scattered light brighten the darkness. There are almost no dark areas in Germany either. In Germany and around the world, light pollution is also increasing, as measurements show. The brightening of the nights not only hinders the astronomical observation of the sky, it also disrupts the internal clock of humans and animals.

Number of objects circling in orbit and officially recorded. © NASA Orbital Debris Program Office

Nocturnal scattered light from orbit

But as it now turns out, earthly light sources are not the only cause of nighttime light pollution. Satellites and larger space debris in Earth orbit can also brighten up the nights. The reason: “At these heights, space objects on the night side of the earth also remain illuminated by sunlight,” explain Miroslav Kocifaj from the Comenius University in Bratislava and his colleagues. “This means that these objects appear as light stripes in telescope images.”

In addition to these punctual signatures, the diffuse reflection of sunlight on the satellites also causes scattered light. With nearly 3,400 active satellites and tens of thousands of larger pieces of space debris, this orbital scattered light could be enough to brighten the darkness of the night. Whether this is the case and how much was previously unknown.

This is why Kocifaj and his team have now examined this orbital light pollution in more detail for the first time. On the basis of the known number and size of orbital objects, they calculated in their model how much sunlight is reflected by such an object on average and what proportion of it is directed towards the earth as scattered light.

Above the critical threshold

The result: The existing satellites and orbital scrap parts alone brighten up an average of around 16 to 20 microcandela per square meter during the night. “This amount of light corresponds to around ten percent of the natural brightness of the night and is thus above the critical threshold that the International Astronomical Union (IAU) classified in 1979 as an acceptable upper limit for light pollution at astronomical locations,” the researchers explain.

And the true level of orbital light pollution could be even greater. Because for their calculations, Kocifaj only considered the known satellites and scrap pieces. “But by no means all orbital objects have been identified so far, so that their number could be significantly higher,” they explain. “Our estimates are therefore only the lower limit.”

Crossed the “red line” for astronomy?

But this means that the night sky, even far from earthly light sources, is now brighter than it should be. “Unlike ground-based light pollution, this diffuse light is visible from much of the earth’s surface,” explains co-author John Barentine of the International Dark-Sky Association. There is no longer a really dark place anywhere, because satellites and space debris surround almost the entire globe.

For many astronomical observatories a “red line” could already be crossed, from which orbital light pollution makes observations difficult. “Astronomers set up their observatories far away from the cities in order to have the darkest possible sky, but this form of light pollution also reaches these places,” says Kocifaj. In addition, the team only examined the visible wave range of light – how strongly, for example, infrared observations are disturbed is still unknown.

Satellite internet will compound the problem

But it gets even worse: The planned mega-constellations of thousands of new satellites for orbital broadband internet could significantly intensify this form of diffuse light pollution, warn the astronomers. The points of light and bright stripes of the satellites from Starlink, Amazon, OneWeb and Co are already visible in the sky and disrupt the astronomical observations.

If tens of thousands of these mini satellites orbit in near-Earth orbit as planned in the near future, their reflections will brighten the night sky accordingly. “Our results suggest that then far more people than just astronomers lose sight of the night sky,” says Barentine. He hopes that their results will help satellite operators look for ways to curb this trend. (Monthly Notices of the Royal Astronomical Society, 2021; doi: 10.1093 / mnrasl / slab030)

Quelle: International Dark-Sky Association

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