Around 15,000 satellites circle the Earth today. Before the commercial industry took off in 2019, there were only a few thousand. Counting dead spacecraft and catalogued debris, the European Southern Observatory puts the total near 32,000.
New research suggests that the plans on the table could badly damage ground-based astronomy. Proposals from companies and state-backed operators would, if fully carried out, add more than 1.7 million satellites. The study, by ESO astronomer Olivier Hainaut, warns that such numbers would impair the study of distant galaxies, potentially hazardous asteroids and planets beyond the solar system.
What the simulations show
Hainaut modelled how satellites of different sizes, brightness and orbits would show up over Paranal Observatory in Chile’s Atacama Desert, home of ESO’s Very Large Telescope.
One satellite crossing the field of view leaves a streak that can hide or distort the faint object under study. A single trail ruins little. Hundreds or thousands of them erase a lot of data and make the rest harder to process.
Reflected light does a second kind of damage. It scatters through the atmosphere and merges into a diffuse glow that brightens the whole night sky, cutting the contrast astronomers need to pick out very faint targets.
For a proposed constellation of one million fairly faint satellites, the simulations showed dozens of trails in each Very Large Telescope image taken early in the night. For some instruments the loss of usable field of view could reach 28 per cent, even if no satellite were visible to the naked eye.
Wide-field survey telescopes are more exposed, because each exposure covers far more sky. A trail bright enough to saturate a sensitive detector can also create electronic artefacts and secondary trails, contaminating more of the image than the satellite’s path alone.
Mirrors in orbit
The worst case involves spacecraft built to reflect sunlight toward the ground. The US company Reflect Orbital has proposed as many as 50,000 mirror satellites by 2035. The study estimated that fleet could make the whole night sky three to four times brighter when fully illuminated. In the modelling, a mirror satellite aiming its beam at an observer could look several times brighter than the full Moon. With the beam pointed elsewhere, it could still match Venus and disturb observations from some of the darkest sites on Earth.
A possible ceiling
The research found that astronomy could live with tolerable losses if the total satellite population stayed under about 100,000 and every spacecraft were fainter than visual magnitude seven. The authors do not present that as a hard safety line. It is a practical ceiling below which losses would be comparable to downtime from equipment failures and other technical problems.
Moving telescopes to orbit does not escape the problem. A Nature study on space-based observatories estimated that if about 560,000 proposed satellites were launched, nearly 40 per cent of Hubble Space Telescope images and more than 92 per cent of exposures from some lower-orbit observatories could contain at least one trail.
Satellites pass over every observatory, whatever local rules say about ground-level light. Researchers studying 10 nights of commissioning observations at the four-metre International Liquid Mirror Telescope at Devasthal in India found 83 orbital-object streaks. The Indian Astronomical Observatory at Hanle relies on the dark skies of Ladakh.
Astronomers accept that satellite networks widen internet access, support communications and help with disaster response. They want operators to make spacecraft dimmer, share accurate orbital data and design constellations with observatories. SpaceX has tried coatings, sunshades and changes in orientation to dim Starlink satellites, but researchers say engineering fixes cannot offset fleets in the hundreds of thousands.
