A new study of Starlink satellites’ impact on astronomy gives the community cause for both relief and concern.

SpaceX’s Starlink is a network of satellites designed to provide broadband internet to remote areas. The company has 1,848 working satellites currently in orbit, most of them about 500 kilometers (310 miles) above the ground, but it has applied to fly 40,000 more. And that’s just Starlink — other companies such as Amazon’s Kuiper, UK’s OneWeb, and China’s Guowang are applying to send tens of thousands more satellites to an increasingly crowded low-Earth orbit.

That growing crowd has the potential to seriously affect astronomy, not to mention space. Now, astronomers working with the Zwicky Transient Facility, a telescope and camera set up to scan the entire Northern Hemisphere sky every two days, have examined images taken between November 2019 and September 2021, looking for the telltale streak of a Starlink satellite crossing the field of view. They’ve published their results in Astrophysical Journal Letters.

Thousands of Trails

First the bad (though unsurprising) news: The number of satellite trails found in astronomical images, particularly during twilight hours, is escalating. “In 2019, 0.5% of twilight images were affected, and now almost 20% are affected,” says study lead Przemek Mróz (University of Warsaw, Poland). Eventually, as the Starlink constellation grows, nearly all twilight images will contain a Starlink trail, the team predicts.

Taking images during twilight is particularly important for studies of potentially hazardous objects, whose orbits cross Earth’s — a specialty of the Zwicky Transient Facility. Asteroids and comets near or approaching Earth will also be near the Sun in the sky, so twilight imaging is essential if we are to find and track them.

Also, while Starlink satellites are at low orbits and thus affect only twilight images, other constellations can (and will) go to higher altitudes. OneWeb, which is already operating 394 satellites (out of 6,372 planned) at 1,200 km, will affect images taken later at night, during what should be true darkness.

“The value of a careful analysis of observational data is that it provides ground truth for the modeling predictions on which the policy concerns have been based,” says Richard Green (University of Arizona), who has helped organize the astronomy community’s dialog with the satellite industry. “They paid particular attention to the kinds of observations that are most relevant to the search for potentially hazardous asteroids, which is a Congressional charge to NASA and a specific concern of the United Nations’ Committee on the Peaceful Uses of Outer Space (COPUOS).”

Science Not Affected (Yet)

There’s also good news: Despite the large and increasing number of satellite streaks, it’s not affecting ZTF science.

That’s because — unlike some other observatories — single satellite trails are faint enough that they don’t saturate the ZTF detector. That means that the automatic pipeline, which already removes airplane trails and the like, can easily mask a satellite streaking across an image. So while a few pixels are lost (about 0.04% of the detector area), the image as a whole is still useful.

“We estimate that once SpaceX deploys the entire constellation of 42,000 satellites, every ZTF image taken during twilight would be contaminated with about four satellite streaks,” says Mróz, “which means that about 0.2% of all pixels would be lost (and so there is a 0.2% chance that ZTF may miss a transient during twilight).” Even then, because ZTF re-images the entire sky every two days, they’d still likely pick up whatever they missed.

A key aspect to this has been SpaceX itself working to decrease the brightness of its satellites. Their new “VisorSat” model, which uses a shield to block reflected sunlight, is on average 4.6 times fainter than the original model. But it’s still not quite at the 7th-magnitude level that a congress of astronomers and industry professionals, known as SATCON, has recommended for satellites at 500 km up.

The community has chosen that limit because that’s the brightness that will saturate ultra-sensitive wide-field detectors, such as the one to be installed at the Vera Rubin Observatory. At the moment, most Starlink satellites would breach the 7th-magnitude limit, overwhelm the detector, and create “ghost trails.” There’s no easy way to mask satellite trails, as ZTF has done, if the trails themselves echo across an image.

“Industry’s best efforts to date haven’t reached the target that lowers the impact on astronomical data to the needed level,” Green says. “How to codify the current good intentions, or to continue to provide positive incentive for industry’s cooperation in meeting the technical challenges, remains a challenge.”

Impacts on Sky, Space, and Earth

While the new study focused on the satellites’ impact on professional astronomy, concerns remain in other arenas. Not least of these is simple stargazing; on the current course, one team of astronomers has predicted that one of every 15 lights future observers will see in the sky will be a satellite.

There’s also space-traffic control: The growing number of satellites in low-Earth orbit, particularly below 600 km, is causing increasing numbers of near-misses between satellites and even space stations. The increase appears to be linear with time (ignoring Starlink passes near other Starlinks), according to tweets from Hugh Lewis (University of Southampton, UK). China has even filed an official complaint with the United Nations’ COPUOS, noting two near misses between Starlink satellites and its Tianhe space station (which had crew aboard at the time).

Another concern is the satellites’ impact on Earth itself. Satellites that reenter our atmosphere burn up, eliminating them as space debris. But in the process, they deposit metals in the upper atmospheric layers. “Mass doesn’t just vanish,” said Aaron Boley (University of British Columbia, Canada) at the UN’s conference on Dark and Quiet Skies in October; his research appears in Scientific Reports.

Meteors burn up in the atmosphere, too, but Boley notes that with enough space activity, humans could become the dominant source of metals in the upper layers, which could have unintended consequences.

“That’s not to say we can’t access and use space,” he said, “but we have to understand the implications.”

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