The night sky is clearing

The night sky is very lightly polluted by satellites and other objects launched into space by people.

Researchers write this in the magazine Monthly Notices from the Royal Astronomical Society: Letters. Satellites, space debris and other objects orbiting the earth literally lit up the night sky. In some places, the night sky is therefore more than 10% brighter than it would have been in the absence of these objects, the researchers estimate. And in many cases, satellites alone cause an area to be classified as “light polluted” by official standards.

Telescopes against our eyes
This is not the first time that researchers have studied the impact that satellites and other objects orbiting the Earth have on our night sky. For example, great attention has already been paid to the constellations of satellites under construction, which will eventually have to number thousands or even tens of thousands of satellites. These satellites already appear regularly on telescope images – often in the form of bright dots or dashes – and thus interfere with the observations of astronomers.

For the new study, however, the researchers did not examine how satellites have (permanently) changed the view of telescopes. Instead, they looked at how every object in orbit – from satellites to space junk – changed the night sky for us as observers. Our eyes are not sharp enough – like telescopes – to observe and be disturbed by individual satellites. But you can see the combined effect that all of these objects have on the brightness of the night sky.

light
“When the angle between the sun, a space object and an observer on Earth is correct, the space object intercepts sunlight and reflects it on the night side of Earth,” says researcher John Barentine. “We can see large objects that intercept large amounts of sunlight (like the International Space Station, editor’s note) like a point of light in the sky. But there are also many, many smaller objects (orbiting the earth, editor’s note); some are only a few centimeters or millimeters in length. Although an observer cannot perceive them as separate light sources, they do contribute to the light that your eye receives. The result of all these light sources – large and small – together, is a brighter night sky (see sidebar).

The night sky is never completely dark. “In addition to the light from the cosmos, the earth’s atmosphere itself also glows very faintly, as chemical reactions take place above the planet.” The night sky is therefore always a little “lit”. So the light that satellites and space debris reflect is on top of that.

Surprising
For this study, scientists looked at how much brighter the night sky has become due to satellites and space debris. “Since it had never been studied, we weren’t sure what to expect,” Barentine explains. “We were surprised that the effect was so big; much bigger than we thought. We have carefully reviewed our model and calculations to make sure we haven’t made any mistakes. But after these checks, the result was still that space objects ensure that the night sky becomes at least 10% brighter than it already is due to natural light sources in and beyond Earth’s atmosphere. .

Before the mega-constellations
This estimate is based on a model (see box), which in turn is based on the most recent data available to researchers on the number of objects in orbit and the size of these objects. However, this data dates back to the time before construction began on huge constellations of satellites, such as Starlink (from SpaceX). Meanwhile, SpaceX alone has launched some 1,500 satellites. But their effect on the night sky has not yet been taken into account. “In fact, we are revealing what the situation was like before the era of huge constellations of satellites began. This is why we also argue that you should see this 10% increase in night sky brightness as a lower limit; the night sky will only brighten up for years to come. “

Check
The researchers used a model to get a feel for the impact of satellites and space debris on our night sky. And based on that data, the model predicts that the night sky had already become about ten percent lighter before the mega-constellations were built. “We haven’t yet been able to verify the model’s predictions through observations,” says Barentine. “It is practically impossible from the surface of the Earth, because of the sources of natural light in the atmosphere.” To verify the results, the observations must in fact be made from space. “One option would be to do it from the International Space Station. We are currently investigating this possibility. “

If you look at the starry sky with the naked eye, you won’t immediately notice that the night sky has become around 10% brighter. Things will change when the night sky gets even brighter. Then the Milky Way’s bright star clouds, which you can still see well at a great distance from light-polluted cities, may be hidden from view. “In a truly” virgin “place (ie little polluted by light, editor’s note), you can see up to 6,000 stars spread across the night sky”, explains Barentin. “If the night sky is 10% brighter, that number will be slightly smaller. But if the night sky got 50% brighter, it would be 1000 less. It is important. “

Upper limit cannot be predicted
It is difficult to estimate the brightness of the night sky. “A major uncertainty is the effect of space debris,” notes Barentine. Because more and more objects are placed in orbit around the Earth, you can expect more and more collisions between objects in the future. “But we don’t yet have a good idea of ​​the frequency of collisions if the number of satellites orbiting the Earth increases.”

Hopefully he will not succeed. But nothing is preventing companies and organizations from putting objects into orbit around the earth at this time; there are no international or national laws prohibiting or limiting this. “Basically it all comes down to the question: does space belong to all of us or does space really belong to those who use it? No international agreement has yet been concluded on this subject. And so, companies like SpaceX, Amazon and OneWeb continue to boldly build their mega-constellations. “As long as countries and companies are allowed to put objects – without too many restrictions – into orbit around Earth, it is It’s hard to imagine that the situation we describe in our research will improve. We have long worked to limit the influence of artificial light sources on Earth on the night sky. And these efforts are now undermined by light sources beyond. above our planet which are much more difficult to combat. I don’t mean to say that it is “too late” to change the situation, but with each launch of a new satellite it becomes more and more difficult. “