Space Industry Adds Threats to Astronomy, Light Pollution Remains a Big Problem

Satellites provide so many benefits it’s hard to imagine life without them, from predicting our weather to guiding our travel to providing our electronic connections. But as the number of satellites in orbit expands rapidly, and the accumulation of defunct satellites and debris proliferates, they are having an increasing impact on our ability to view the night sky.

Bright satellites crossing the sky are already ubiquitous, mostly from SpaceX’s Starlink constellation but also from the company’s numerous global competitors, and the number of satellites in low-Earth orbit could climb to tens or hundreds of thousands in the coming decades. Meredith Rawls (University of Washington), presenting at the recent winter meeting of the American Astronomical Society (AAS), says the impacts will span the entire electromagnetic spectrum — pervasive in ways that we don’t expect — for years to come.

One unexpected and little-known phenomenon is ionospheric holes, which have been occurring more frequently thanks to both rocket launches and reentries.  Such holes in the ionized layer of the upper atmosphere can be an impressive sight, creating patches of sky with a red glow (dominated by emission at 630 nanometers).

While visually impressive, that glow could potentially have an impact on observations from virtually all major astronomical facilities in the continental U.S. While few astronomers have seen the effect yet, Stephen Hummel (McDonald Observatory) says that in some ways the holes could be more of a hindrance than satellite launches and deployments.

During his presentation at the AAS meeting, he says, “I did a poll of the room, maybe 60 or 70 people, and only three had actually seen an ionospheric hole or known what they look like.”

Yet, he tells Sky & Telescope, “from an astronomy perspective, these events are potentially more disruptive than satellites, as they appear quickly with little warning, are optically bright (producing their own light rather than reflecting sunlight), persist for minutes to hours, occupy a relatively large area on the sky, and a lack of accurate launch information makes them hard to predict.”

These ionospheric holes have been seen far away from the launch sites involved, he says. At McDonald Observatory in Texas, staff have observed these huge red blooms in the sky from SpaceX Falcon 9 launches at Vandenburg Air Force Base in California, about 1,000 miles to the west, as well as from the reentry of a rocket that had been launched from Kennedy Space Center, equally far to the east. With current launch patterns, the glowing patches appear regularly over the southwestern U.S., where many world-class observatories are located.

Metals in Our Atmosphere

Ionospheric holes are just one of the impacts rocket launches can have; plumes of gases and combustion products during launches as well as venting of propellant later in the mission, can also affect astronomical observations.

For example, satellites in low-Earth orbit eventually burn up during reentry into the atmosphere, leaving behind significant residues of metals and other compounds. The expected launch rate of the satellites part of SpaceX’s Starlink constellation alone could inject at least 8,000 tons of metals into our atmosphere per year – more than the natural accumulation from meteoroids. Already today, he says, 10% of stratospheric aerosols come from satellite and rocket reentries.

Because of this, the AAS in September 2024 adopted a resolution stating in part,  “The American Astronomical Society (AAS) is gravely concerned about the impacts of emissions and residual effects from extensive space launches and space object reentries upon Earth’s sky and upper atmosphere.” (Find the full resolution here.)

More recently, the AAS issued another resolution regarding “obtrusive space advertising,” another component of space economics that remains underregulated. (The full resolution is available here.) Although no company has so far made such a proposal, a Russian student project recently tested the concept. While the United States has a legal prohibition on such advertising, there is no such provision yet internationally.

Dark Skies on the Ground

Overall, diffuse brightening of the sky remains a more difficult problem to address than launches, satellites, or even space billboards. Our night sky has been growing rapidly brighter, at a rate as high as 10% per year, even affecting major observatories.

“I think it is fair to say that right now the bigger concern for ground-based astronomy is certainly light pollution, by far,” said John Barentine (Dark Sky Consulting), also speaking at the AAS meeting. “Artificial light at night emissions have been rising around the world at a fairly alarming rate.”

But he pointed out that there have been some successes in implementing regulations that may help: The European Union’s recent Nature Restoration Law, for example, “requires EU member states to reduce their light pollution by 2030,” he said at an AAS press conference. “We’ll see if that is successful. But I think in the end, it has to be the totality of all these efforts that’s eventually leading towards the same goal, I hope, which is the protection of the night sky overall.”