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Astronomy is under threat like never before. A significant new study, which was conducted by astronomers from the European Southern Observatory (ESO), has warned that if the number of satellites in Earth’s orbit exceeds 100,000 (i.e. those with below naked eye visibility) then the human race may lose its ability observe the night sky with modern telescopes.
The issue of satellite streaks appearing in astronomical photos taken from telescopes on the ground has been steadily increasing over the past few years as the number of satellites continues to grow. The biggest culprit has been those in Low Earth Orbit (LEO), primarily mega constellations from SpaceX’s Starlink broadband network, as well as Amazon LEO, OneWeb and more.
The International Astronomical Union (IAU) currently recommends that LEO satellites should have a maximum brightness of magnitude +7 at altitudes of up to 550km. On this scale, the brightest objects actually have the smallest numbers (e.g. brilliant Venus can reach up to -4.6, while the North Star is much dimmer at +2). If satellites are too bright then that can make it much harder to picture the night sky and do other things, such as to spot dangerous asteroids or detect key celestial events.
One other way around this is of course to launch space-based telescopes, but they’re exceedingly complex, hugely expensive and cannot realistically replace the huge coverage that exists across ground-based telescopes. The new ESO report has now brought this debate back into a sharper focus and warned of the “devastating consequences for astronomy” if current plans were to reach even a fraction of their targets. Current proposals exist to launch over 1.7 million satellites into orbit (1 million of these form part of SpaceX’s orbital data centres plan)!
According to the study, no more than 100,000 faint satellites, below naked eye visibility, should orbit Earth, to “safeguard our ability to observe the night sky with modern telescopes“. The study is the first to compute the extent to which large and bright satellite constellations — which have also raised concerns about their impacts on health and the environment — would affect astronomical observations by making the night sky brighter.
“Until now we have managed, but it’s getting worse,” stresses Olivier Hainaut, who has been involved in developing recommendations to mitigate the impact of satellite constellations on astronomy. While companies like SpaceX have taken measures to make their satellites less bright, such as by modifying them and working with SOME large observatories to move satellites out of telescope viewing arcs, the current satellite proposals will ultimately go “beyond the limit” of what astronomy can withstand.
ESO Statement
The new study shows that, for a large fraction of each night, hundreds of satellites would be visible and, at certain times, up to several thousand, similar to the number of stars seen with the naked eye in good conditions. Other planned satellite constellations such as E-Space’s Cinnamon and China’s CTC-1 and 2 would add hundreds of thousands more satellites into orbit, compounding the problem.
Reflect Orbital, a US start-up, aims to launch a constellation of very large mirror-like satellites to provide sunlight at night, with reflected beams that span at least five kilometres on Earth’s surface. They intend to start with a prototype satellite in orbit this year and plan to increase their satellite population to 50 000 by 2035. These satellites would be the brightest ever in orbit, with damaging consequences for dark skies on Earth.
Hainaut’s calculations show that the full constellation would fill the night sky with hundreds of very brightly visible satellites. Seen from within a reflected beam, the satellite delivering sunlight would appear four times brighter than the full Moon. Even if no satellite points its beam directly at an observer, each would be as bright as the planet Venus, the ‘morning star’. From a light-polluted city, like Munich, Germany, these hundreds of satellites would be the only ‘stars’ visible in the night sky.
These proposals, combined with others considered in the study, would dramatically brighten the night sky, hindering humankind’s ability to observe faint cosmic targets, including far-away galaxies, some Earth-like planets around other stars, and even asteroids potentially dangerous to Earth.
In order to compute the impact of all this and other effects of satellite constellations on astronomical observations, Hainaut simulated the positions, motion and brightness of all present and planned satellite constellations. For example, for Starlink, he found that dozens of trails would appear in each image taken two hours into the night with ESO’s Very Large Telescope (VLT) at Paranal Observatory in Chile, representing field-of-view losses of up to 28%. This assumes that the satellites would be faint enough not to be seen with the naked eye in good conditions. If they are just a little brighter, some instruments would be even more affected.
Long story short. The study found that these negative impacts can only be avoided by limiting the total, of both existing and future satellites, to 100,000 satellites faint enough not to be seen with the naked eye from a dark site.
“This is not a hard number, like 99,999 is good and 100,001 is bad: clearly I’d prefer 50,000,” clarified Hainaut. “But 100,000 causes losses at about the level of other technical losses, such as equipment failure.” However, he adds, the satellites must be fainter than visual magnitude 7; should some of them be too bright — above the minimum threshold for naked-eye visibility — the total number would need to be much lower.
The big problem is that, thus far, major world governments and regulators haven’t really acknowledged the seriousness of the problem and still seem to approve the launch of mega constellations without consideration for the wider ramifications.
The ESO, in collaboration with the UK’s Royal Astronomical Society (RAS) and the International Astronomical Union (IAU), are now using the study as the basis for their submissions to counter future launch proposals. But it remains unclear whether they will have the kind of impact necessary to walk the situation back toward some common sense.
