SFor decades, astronomers have been lamenting the increasing brightness of the night sky due to artificial lighting in our cities – without much success. Meanwhile, there is another threat to celestial exploration: the rapidly growing number of man-made objects—satellites and debris—in near-Earth space. In the current edition of journal “Nature Astronomy” astronomers are therefore once again sounding the alarm with a whole series of articles: it is high time to save the night sky for research and also as a cultural heritage of mankind.
As recently as January, Christopher Kyba from the German Research Center for Geosciences in Potsdam and his colleagues reported that the so-called light pollution from artificial light sources is increasing significantly more than previously assumed: The observations collected by amateur researchers from all over the world show an annual increase in sky brightness of 9.6 percent. Miroslav Kocifaj from the Slovak Academy of Sciences and his colleagues are now presenting another worrying number. In regions for which there is insufficient data, the brightness of the night sky has so far been underestimated by up to two and a half times.
The constantly growing number of artificial objects orbiting our earth is also leading to a brightening of the night sky. About 36,500 objects – from intact satellites to space debris – with sizes over ten centimeters are in orbit. In the range of one to ten centimeters, it could even be a million, estimate John Barentine from the University of Utah in the USA and his colleagues. All of these bodies reflect sunlight – larger ones then appear as luminous lines on sky images, the smaller ones diffusely brighten the sky background.
And that creates a dilemma for astronomers. “The general increase in diffuse background light requires longer exposure times to reach certain detection limits,” explain Barentine and his colleagues, “but this in turn increases the likelihood that streaks from larger objects will interfere with the images.” And it is already foreseeable that this will happen the situation further deteriorated. For example, SpaceX is planning to launch a total of 42,000 satellites for the Starlink communications network.
In the future, short flashes caused by reflective surfaces on satellites could also become a particular problem, as many astronomical projects are looking for short-term phenomena in the sky. A satellite flash can then trigger a false alarm. Only recently did astronomers believe they had encountered an exotic astrophysical phenomenon. But it was just a satellite. The researchers fear that such false alarms could soon become rampant.
However, there is no solution to these problems in sight. Since the introduction of electric light, astronomers have withdrawn farther and farther from the cities with their observatories – today the large observatories are far away from civilization on high mountain peaks. But today there are “almost no remote places on earth that simultaneously have all the characteristics required for the construction of an observatory – no light pollution, a large number of clear nights and a quiet atmosphere.” Fabio Falchi from the University of Santiago in Spain and his colleagues.
An international working group of astronomical organizations and institutions recently asked the United Nations Committee on the Peaceful Uses of Outer Space to set up an expert group to protect the dark sky. But whether artificial lighting and the number of satellites will be capped in the foreseeable future, as Falchi and his colleagues would like, is literally written in the increasingly poorly visible stars.
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