Swarms of satellites are harming astronomy. Here’s how researchers are fighting back

SpaceX and other companies plan to launch tens of thousands of satellites, which could mar astronomical observations and pollute the atmosphere

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 launched¹.

Sources: Jonathan’s Space Pages (**https://go.nature.com/4IV22FD**; **https://go.nature.com/4225MKX**)

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 observations².

Starlink satellites leave streaks in a 2019 image taken by a 4-metre telescope at the Cerro Tololo Inter-American Observatory in Chile.Credit: 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 trail³.

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)⁴. 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.