Scientists predict that, by the time children born in 2023 become adults, they will be able to see fewer than half the stars in the sky that we see today.
The reason? Each year, the night sky around the world is becoming about 10 per cent brighter, from the effects of satellites, growing urbanisation and more and more lights on the ground, according to a study published in Science Magazine this year.
The growing brightness of the skies, particularly the kind created by roaming satellites, poses a problem for astronomers, whose telescopes need minimal light and sound interference to collect images and data. The brightness is also a threat to the natural cycles of humans, animals and plants, and, of equal importance, to the simplest of treats: the ability to look up and see a velvet sky blanketed with sparkling stars.
The situation has become concerning enough that scientists, government officials and activists have banded together in a battle to keep at least some portion of the Earth’s skies dark and quiet.
“If this problem isn’t controlled, the night sky is going to really change,” says Dr Andrew Williams, external relations officer at the European Southern Observatory, which has its headquarters in Garching, Germany. Dr Williams is also the co-lead of the policy hub of the International Astronomical Union’s Centre for the Protection of Dark and Quiet Skies From Satellite Interference, or the IAU CPS, of which the European Southern Observatory is a contributing member.
Dr Williams adds: “This is a bit sad if you think about the impact the night sky has had on our society, from our timekeeping systems to modern agriculture practices, and religious practices still maintained by many indigenous societies.”
The sources of this growing light pollution are both ground and sky-based. Scientists have been studying ground-based light pollution for years and have developed various methods for mitigating its effects on the environment, including putting timers on lights so they are either switched off or dimmed while people sleep; placing shields around light sources to prevent diffusion; and directing the lights to shine where they are needed, often towards the ground.
Dr Richard Green, an astronomer and the assistant director for government relations at the Steward Observatory at the University of Arizona in the United States, says his observatory works with local governments to find ways to reduce their light pollution. For example, the city of Flagstaff in Arizona has established a curfew for commercial signs, and uses streetlights that emit light only in specific portions of the light spectrum. Cities including Pittsburgh and Tucson have followed, instituting their own dark skies ordinances.
But although there are established ways to mitigate light pollution from ground sources, the skies are a different story. Above us, where there were once a few thousand large satellites stationed many thousands of kilometres away from the ground, there are now swarms of small, constantly moving, low-Earth orbit satellites.
These newcomers account for the major increase in the total number of satellites floating above the Earth, from about 3,400 in 2021, to 10,590 today. Thirty-five per cent of all satellites sent to space, ever, were launched in the last three years, according to the Vienna-based United Nations Office for Outer Space Affairs (UNOOSA).
The sudden increase in satellites can be attributed to new technology that has made it easier to build “nanosats” – satellites that weigh only 25 to 50kg (compared with around six tonnes in the past) and cost a tiny fraction of the amount of an old-school satellite.
The shift seems to have begun in earnest in 2017, when a startup called Planet worked with the Indian Space Research Organisation to launch 88 satellites into orbit simultaneously, setting a new record. Since its founding, Planet has launched more than 500 satellites that capture images of the Earth’s landmass daily, manufacturing data that the company shares with local governments, farmers, fire departments and more. (Planet’s spokesperson says “contributing to light pollution is not an issue” for the company because its satellites do not take pictures at night.)
In 2019, SpaceX launched 60 Starlink satellites – a move that grabbed the attention of astronomers around the world. Worried about what was to come, various researchers and experts from astronomy, space and industry gathered forces to create the IAU CPS. The Parliament approached SpaceX, which recently put its 5,000th Starlink satellite into orbit, for comment but the firm declined.
Since 2019, entrepreneurial interest in these low-orbit satellites has only grown. As many as 100,000 more satellites could be launched in the next decade, according to UNOOSA. “Currently there’s no overarching system or regulation taking care of the number of satellites going into orbit,” Dr Williams says.
Mila Francisco Ferrada, a diplomat who represents Chile at the United Nations in Vienna, has been fighting to put dark and quiet skies on the agenda of the UN Committee on the Peaceful Uses of Outer Space, where she is an alternative representative. But the committee works on consensus and, so far, Iran and Russia have blocked efforts to move forward.
“The question is more than, are we doing enough? It’s also, are we doing it fast enough?” Francisco Ferrada says. At this point, she adds, dark and quiet skies will not get on the committee’s agenda until at least 2025 because of subcommittee and committee timelines. For now, she and her team are not sitting back. “We can work with the same actors – the state, the private sector, astronomers – in an informal way now, so that once we have this on the agenda, we can immediately present recommendations and solutions,” she says.
Companies such as SpaceX, OneWeb, Planet, Amazon’s Project Kuiper and Virgin Orbit have rushed to invest resources and money into the nanosat market, in the hope of creating faster, smoother internet connections that will enable everything from video games and Zoom meetings to streaming.
“Jeff Bezos wants anyone anywhere in the world to be able to order something from Amazon at any time,” Dr Williams says. “For that, you need a global internet network that’s accessible anywhere. You need satellites that are low and very numerous.”
Low-orbit satellites also have massive potential for state actors. SpaceX’s Stаrlink satellites, for example, have been crucial to Ukraine’s war effort, helping the military co-ordinate drone strikes and gather intelligence. “In the old days, you could strike a ground station and disable communications infrastructure,” Dr Williams says. “Now you can carry around a suitcase, a little thing that opens up, and then you have internet.”
Older satellites the size of buses hang out about 36,000km above Earth and are “parked in specific slots, where they put up massive antennae and beam down to one single place on earth”, explains Dr Williams. New, smaller satellites the size of sofas operate at low orbit, or about 300 to 1,200km above Earth, where signals from the ground can reach them more quickly. They whizz around the sky, passing internet signal from one to the next as if in a relay race. Doing this without interrupting internet service requires many satellites.
Having a low orbit means the signal from Earth has a shorter distance to travel to the satellite — but it also means the satellites are more visible to humans.
Until recently, astronomers only had to worry about a total of around 3,000 to 4,000 satellites, most of them far away and stationary. Based on the rate that companies are shooting satellites into space and their plans for the future, Dr Williams says, soon enough, anyone anywhere on Earth will have about 5,000 satellites floating above them.
As Dr Stefan Wallner, an astrophysicist at the Institute of Construction and Architecture at the Slovak Academy of Sciences in Bratislava, explains, many of these satellites reflect the sun’s rays, increasing atmospheric brightness. “During sunset or sunrise, the satellite is at an angle where it reflects the sunlight towards the ground, towards us,” he says. “It has the same effect as light pollution.”
As the new satellites do their rounds, there is a high chance they will create streaks in images taken by telescopes such as the Hubble Space Telescope or the Vera Rubin C Observatory, a massive telescope currently under construction in Chile.
Dr Green says: “Without special treatment, every satellite in low-Earth orbit that’s illuminated by the sun in a way we can see can make a streak in your telescope’s focus.”
Sometimes the streaks can be removed, but other times they compromise the image so much that it must be discarded.
Low-orbit satellite constellations are also contributing to overall light pollution, which makes it difficult to establish new observatories. Wallner says: “If you’re searching for a location for a new observatory, the International Astronomical Union has a strict rule that if the night sky is brighter than 10 per cent compared with its natural background, you should not plant telescopes there any more. Now this is happening globally.”
Low-orbit satellites use a radio frequency band that is adjacent to one reserved for radio astronomy, a branch of astronomy that involves the study of the unique radio waves that are emitted by stars, planets, galaxies and molecules of gas. Spillovers can cause problems with data collection. “They’re broadcasting signals all the time, everywhere,” Wallner says. “They emit noise that’s uncontrolled.”
But it’s not just scientists who are affected by the satellites and growing brightness of the night skies. Light pollution poses problems for animals and plants that rely on light to regulate sleep, nourishment and reproduction. “Seventy per cent of animals are nocturnal,” Wallner says. “It’s only logical that, if we are stealing natural light, they have to change their behaviours and ways of living.”
With increased light pollution, there are fewer places where nocturnal animals and insects can live, he says. Plants, too, are affected. “If it’s not fully dark any more, plants will also photosynthesise at night,” he says, referring to the process by which plants convert light, carbon dioxide and water into sugar and oxygen. “For example, we’ve seen that trees lit by parking lights stop losing leaves in winter and stay green throughout the year, as if it’s daytime all day and all year. It’s not a natural rhythm. These plants can die very quickly.”
Light pollution can also negatively impact humans, for example by reducing the amount of melatonin, the sleep hormone, our bodies produce. When it is irregular, Wallner says, people often suffer from sleep disorders and other health consequences.
Light pollution is also harmful for something less quantifiable: the experience of looking up and seeing stars. A study published in Science Magazine that geo-mapped light pollution around the world found that 60 per cent of European residents live in places where they cannot see the Milky Way.
“Humanity has enveloped our planet in a luminous fog that prevents most of Earth’s population from having the opportunity to observe our galaxy,” the paper’s authors wrote.
Part of the issue is that the light from large urban centres travels easily and can sometimes reach even remote places. “With very good atmospheric conditions, light can travel more than 100km [perhaps more than] 200km,” Dr Wallner says. “A national park close to a city will be affected, for example.”
Luckily, at least one company – SpaceX – has been responsive to these concerns. To make their satellites less bright, engineers have developed several innovative ideas, including coating the satellites in low-reflectivity black paint and placing mirror-like film on their surfaces that reflects sunlight into space rather than down to Earth.
But, Dr Williams says, “SpaceX is a unique company. They own their launcher; they build all the satellite components mostly themselves. If they need to make changes, they can do it quite rapidly. Other companies use third-party launch providers and buy components off the shelf, so it’s much harder to fix things on the fly.”
There’s no overarching regulation taking care of the number of satellites going into orbit
For IAU CPS, the goal, Dr Williams says, is to find a way to codify effective practices for reducing the light and noise pollution caused by the satellites and to create guidance that could be turned into EU regulation. To that end, the IAU CPS has been working with the European Space Agency to add dark and quiet skies as a topic to the agency’s clean space project. The ESA regularly works with the EU, and is a “big spender”, Dr Williams says. “If they have stringent requirements, that’ll have a big effect,” he adds.
The IAU CPS has also successfully advocated for inclusion of a line calling for “measures to prevent light pollution” in the EU Parliament’s authorisation for a new satellite constellation programme that is expected to reach full operational capacity by 2027 (the programme is a public-private partnership, with €2.4bn of the funding coming from the EU budget). The companies competing in the programme’s tender are required to develop strategies to minimise disruptive visual brightness.
“Though we’re only talking about 200 satellites, that’s still something substantial,” Dr Williams says. "The European space industry will start designing satellites to fit these requirements. I think that will create technical know-how that can influence future satellite projects.”
Dr Green says he hopes that companies will eventually be required to show they are aware of the need for dark and quiet skies, and that they are willing to work to maintain them somewhat. “If you’re going to put 500 to 30,000 satellites up there, sign this agreement with a science agency that you’ll try your best to minimise the impact,” he suggests.
But convincing most industry actors to make changes in the name of dark and quiet skies is tough. When it comes to new technology, Dr Williams says, “whoever is first gets to set the rules. The companies know this and they know that if they can be the ones to get all their assets up there to learn how to operate, they can influence the government regulations that catch up eventually.”
That is why timing is crucial. Francisco Ferrada is calling for more urgency. “We’re already late,” she says. “We have to protect our space now; otherwise, it will be ruined before we know it.”