In the next few months, from its perch atop a mountain in Chile, the Vera C. Rubin Observatory will begin surveying the cosmos with the largest camera ever built. Every three nights, it will produce a map of the entire southern sky filled with stars, galaxies, asteroids and supernovae — and swarms of bright satellites ruining some of the view.
Astronomers didn’t worry much about satellites photobombing Rubin’s images when they started drawing up plans for the observatory more than two decades ago. But as the space around Earth becomes increasingly congested, researchers are having to find fresh ways to cope — or else lose precious data from Rubin and hundreds of other observatories.
The number of working satellites has soared in the past five years to around 11,000, mostly because of constellations of orbiters that provide Internet connectivity around the globe (see ‘Satellite surge’). Just one company, SpaceX in Hawthorne, California, has more than 7,000 operational Starlink satellites, all launched since 2019; OneWeb, a space communications company in London, has more than 630 satellites in its constellation. On paper, tens to hundreds of thousands more are planned from a variety of companies and nations, although probably not all of these will be launched.
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Satellites play a crucial part in connecting people, including bringing Internet to remote communities and emergency responders. But the rising number can be a problem for scientists because the satellites interfere with ground-based astronomical observations, by creating bright streaks on images and electromagnetic interference with radio telescopes. The satellite boom also poses other threats, including adding pollution to the atmosphere.
When the first Starlinks launched, some astronomers warned of existential threats to their discipline. Now, researchers in astronomy and other fields are working with satellite companies to help quantify and mitigate the impacts on science — and society. “There is growing interest in collaborating and finding solutions together,” says Giuliana Rotola, a space-policy researcher at the Sant’Anna School of Advanced Studies in Pisa, Italy.
Timing things right
The first step to reduce satellite interference is knowing when and where a satellite will pass above an observatory. “The aim is to minimize the surprise,” says Mike Peel, an astronomer at Imperial College London.
Before the launch of Starlinks, astronomers had no centralized reference for tracking satellites. Now, the International Astronomical Union (IAU) has a virtual Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS), which serves as an information hub and to which researchers, including Peel and Rotola, volunteer their time.
One of the centre’s tools, called SatChecker, draws on a public database of satellite orbits, fed by information from observers and companies that track objects in space. Astronomers can use SatChecker to confirm what satellite is passing overhead during their observations. The tool isn’t perfect; atmospheric drag and intentional manoeuvring can affect a satellite’s position, and the public database doesn’t always reflect the latest information. For instance, the BlueWalker 3 satellite from telecommunications firm AST SpaceMobile in Midland, Texas, launched in 2022 and was sometimes brighter than most stars; yet uncertainty of its position was so great at times that astronomers had difficulty predicting whether it would be in their field of view for their night-time observations.
Starlink satellites leave streaks in a 2019 image taken by a 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile.
CTIO/NOIRLab/NSF/AURA/DECam DELVE Survey
Tools such as SatChecker help telescope operators to avoid problems by allowing them to target a different part of the sky when a satellite passes overhead or by simply pausing observations as it flies by. It would aid astronomers if SatChecker had even more accurate information about satellite positions, but there are constraints on improving the system. SatChecker data come from the US Space Force, which draws on a global network of sensors that tracks objects in orbit and issues updates on satellite locations as often as several times a day. The frequency of these updates is limited by factors such as how often a sensor can observe an object and whether the sensor can distinguish what it’s looking at.
Currently, satellite streaks are a relatively minor issue for telescope operators. But the problem will grow as satellite numbers continue to increase drastically, meaning more observation time will be lost, and this issue will be magnified for Rubin.
Fixing the streaks
Rubin, which cost US$810 million to build, is a unique case because it scans large swathes of the sky frequently — meaning it can detect rapidly changing phenomena such as incoming asteroids or cosmic explosions. Astronomers don’t want to be fooled by passing satellites, as happened in 2017 when researchers spotted what they thought was a γ-ray burst — high-energy flashes of light — from a distant galaxy but turned out to be sunlight reflecting off a piece of space junk.
Rubin’s powerful camera, coupled with its 8.4-metre telescope, will take about 1,000 nightly exposures of the sky, each about 45 times the area of the full Moon. That’s more wide-field pictures of the sky than any optical observatory has ever taken. Simulations suggest that if satellite numbers in low Earth orbit rise to around 40,000 over the 10 years of Rubin’s survey — a not-impossible forecast — then at least 10% of its images, and the majority of those taken during twilight, will contain a satellite trail.
SpaceX took early steps to try to mitigate the problem. Working with Rubin astronomers, the company tested changes to the design and positions of Starlinks to try to keep their brightness beneath a target threshold. Amazon, the retail and technology giant based in Seattle, Washington, is also testing mitigations on prototype satellites for its planned Kuiper constellation. Such changes reduce, but don’t eliminate, the problem.
To limit satellite interference, Rubin astronomers are creating observation schedules to help researchers avoid certain parts of the sky (for example, near the horizon) and at certain times (such as around twilight). For when they can’t avoid the satellites, Rubin researchers have incorporated steps into their data-processing pipeline to detect and remove satellite streaks. All these changes mean less time doing science and more time processing data, but they need to be done, astronomers say. “We are really looking forward to getting data from Rubin and seeing how it turns out,” Peel says.
For other observatories, the IAU CPS is working on tools to help astronomers identify and correct satellite streaks in their data. One is a new database of crowdsourced observations of satellite brightnesses called SCORE, which is currently being beta tested and is planned for wider release in the coming months. This will help scientists to work backwards — they might see something puzzling in their past observations and be able to work it out, Peel says.
The database “is definitely a very valuable tool” because it’s one of few that have data freely available, says Marco Langbroek, a space-tracking specialist at Delft University of Technology in the Netherlands. As a beta tester, Langbroek has added a number of entries to SCORE, including measurements of a NASA solar sail that changes in brightness as it tumbles through space. Going forwards, he says, SCORE will be most useful if a lot of astronomers contribute high-quality observations to the database, thereby building up a resource over time.
Tuning things out
Astronomers who work in the radio portion of the electromagnetic spectrum face extra challenges when it comes to satellites.
Big radio telescopes are typically located in remote regions, to be as far as possible from mobile-phone masts and other technological infrastructure that leak radio emissions. But satellites can’t be avoided. “If signals are coming from the sky, they’re always there,” says Federico Di Vruno, an astronomer at the Square Kilometre Array Observatory in Jodrell Bank, UK, and co-director of the IAU CPS.
When satellites transmit signals, the electromagnetic interference can overwhelm faint radio signals coming from the cosmos. One solution is to re-direct or temporarily turn off satellite transmissions. The US National Radio Astronomy Observatory and SpaceX have been working on ways to accomplish this, and the company now momentarily redirects or disables transmissions when Starlinks pass above sensitive telescopes including the Green Bank Telescope in West Virginia. The method requires voluntary buy-in by all partners, plus a lot of data sharing and intensive programming by the companies and by the astronomers, but it does reduce interference. It has been successful enough that small group of radio astronomers visited China last month to discuss the strategy with satellite operators and scientists there.
An image made from multiple exposures shows streaks from Starlink satellites, the International Space Station and other satellites over a site in Wales.
But as soon as one solution is found, fresh challenges appear. One is the rise of ‘direct-to-cell’ satellites, which function like mobile-phone towers in space and can transmit to areas on the ground that otherwise don’t have coverage. Optical astronomers worry about these because they are physically large and therefore bright, and they are a big problem for radio astronomers because direct-to-cell transmissions are extremely powerful. If one of those hits a radio observatory, “the telescope might be blind for a little bit”, Di Vruno says. So astronomers and satellite operators are discussing how they can share information about these as well, to avoid each other when a satellite passes over an observatory.
Another emerging challenge is ‘unintended’ emissions — which happen when satellites ‘leak’ radiation in wavelengths far outside the bands typically used for transmissions and other tasks. Early tests for the Square Kilometre Array radio telescopes, which are under construction in Australia and South Africa, discovered such leakage coming from Starlinks and other satellites.
Many of these unintended emissions are at the low frequencies that are used in some studies including those of the early Universe. So far, astronomers haven’t come up with a good solution, other than scheduling telescopes to not record data when a satellite passes through the part of the sky being observed. In the future, it is possible that authorities such as the International Telecommunication Union might be able to issue regulations on this, as it already does for other shared uses of the electromagnetic spectrum.
Cleaning up the atmosphere
Astronomers aren’t the only researchers concerned about the impacts of satellite constellations. In the past few years, a growing number of atmospheric scientists have been warning that these fleets will pollute Earth’s upper atmosphere during launches and then when their orbits decline and they burn up. Researchers are just starting to get to grips with the scope of this pollution, says Connor Barker, an atmospheric chemist at University College London (UCL).
The point of satellite constellations is to have lots of satellites in orbit, but refreshing them when new technology comes along means that the pace of launches and re-entries will accelerate. In February alone, an average of four Starlink satellites a day re-entered the atmosphere and burned up.
Each re-entry adds chemicals to the upper atmosphere. In a 2023 study, researchers reported that measurements made during high-altitude aeroplane flights detected more than 20 chemical elements in Earth’s upper atmosphere that probably came from satellite re-entries, including aluminium, copper and lead. Other work has found that satellite constellations contributed around 40% of many types of carbon emission from the space industry in 2022, including black carbon particles and carbon dioxide that could contribute to warming the atmosphere. It’s not yet clear how much this warms the planet or contributes to other environmental problems. Some early analyses suggest that satellite launches could contribute a small but measurable amount of ozone destruction.
There are no regulations on satellite atmospheric pollution. Barker and his colleagues at UCL say a good first step towards a solution is to get better estimates of the scope of the problem. They have been building an emissions inventory for rocket launches and satellite re-entries, carefully tallying up the contaminants involved and estimating the altitudes at which they enter the atmosphere. “Even though this is currently a relatively small industry that’s having a relatively small impact on the atmosphere, we should still be aware of it,” says Eloise Marais, an atmospheric chemist at UCL.
Researchers are trying to raise the profile of these and other concerns linked to satellite fleets. Some of these issues were discussed in February in Vienna, at a meeting of the United Nations Committee on the Peaceful Uses of Outer Space. It was the first time that the committee formally discussed the impacts of satellite constellations on astronomy.
No major actions were taken, as expected for these early discussions. But “now all of the member states know of dark and quiet skies”, Di Vruno says. That in itself, he says, is a success.
This article is reproduced with permission and was first published on March 18, 2025.