On Monday (Oct. 2), scientists with the International Astronomical Union announced that one of the brightest objects visible in the night sky is not a star or a planet, but rather the BlueWalker 3 prototype satellite.
“It is unacceptably bright for many sky observers around the world,” Meredith Rawls, co-author of a paper on the finding and member of the IAU Center for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (IAU CPS), told Space.com.
Part of an ambitious 5G communications system developed by the company AST SpaceMobile, BlueWalker 3 appears so utterly luminous from our vantage point on Earth because, in addition to a Launch Vehicle Adapter (LVA), it possesses a massive structure known as a phased-array antenna. In fact, BlueWalker 3 is deemed the largest commercial antenna system ever deployed to low-Earth orbit; it was launched there in September of 2022.
That array takes up about 64 square meters (689 square feet) of space — and because those panels are reflective, it’s almost like BlueWalker 3 is a giant mirror continuously bouncing sunlight toward our eyes.
And that mirror could soon have company. AST says it envisions about 90 BlueWalkers roaming the skies in the near future to make what experts call a “satellite constellation,” though an AST spokesperson told Space.com that, by contrast, other such constellations are expected to require thousands of satellites to achieve their coverage goals.
“We are building the first and only space-based cellular broadband network designed to seamlessly connect everyday smartphones,” the AST spokesperson told Space.com. “By connecting people, we aim to alleviate poverty, spur economic development, foster a diverse digital society and save lives.”
Naturally, however, this ultrabright satellite prototype has opened the door to concerns about whether our night sky is on the road to being littered with far too many artificial stars — especially because it’s not an isolated situation. SpaceX’s Starlink satellites, for instance, have already spurred a similar discussion. Not only might such a future be an eerie one for stargazers to live in, but it also poses a threat to the work of astronomers.
That’s because excessive, human made brightness in the sky directly impacts astronomical observations, both radio and optical.
“This satellite was launched without much consideration of its impact on astronomy,” Siegfried Eggl, co-author of the study and member of the IAU CPS, told Space.com. “The IAU CPS seeks to change that and work on solutions together with the space industry.”
In particular, astronomers are concerned about how BlueWalker 3 will affect studies conducted during twilight hours.
“These observations look towards the inner solar system and so need the sun just below the horizon,” Jeremy Tregloan-Reed, co-author of the study and member of IAU CPS, told Space.com. “One such type of observation looks for near-Earth Objects and provides an early warning to potential asteroids on a collision course with Earth. Hence, these satellites could hinder any attempt of an early warning and so, prevent us from protecting ourselves from an extinction level impact.”
The brightest of them all
Though all satellites reflect sunlight and have the potential to be disruptive, including the thousands of Starlink internet connection satellites currently shining in Earth’s lower orbit (with plenty more to come) as well as the OneWeb endeavor, which has over 600 in the region, “BlueWalker 3 is the brightest of them all,” Rawls said, “by a significant amount.”
“The good news is,” she continued, “astronomers have established dialogs with each of these companies, significant progress has been made toward darkening mitigations and there are new coordination agreements with the National Science Foundation.”
But much more work is needed throughout the industry, astronomers argue. “In the case of SpaceX and the Starlink constellation, everyone was taken by surprise by how bright they were,” Tregloan-Reed said. “Three years later and after many media reports on the the brightness of such LEO constellations and their impact to the night sky, we are seeing companies like Amazon Kuiper who are developing mitigation designs and working with astronomers prior to launch, while other companies like AST SpaceMobile appear to consider the brightness issue as an afterthought.”
AST’s spokesperson did note that the company is aware of BlueWalker 3’s brightness concerns and is working to address them by employing a list of strategies including collaborating with NASA and other astronomy groups, positioning gateway antennas at a considerable distance from radio-quiet zones vital to astronomy and using roll-tilting flight maneuvers that reduce apparent magnitude.
Per the spokesperson, AST also intends to equip its next-generation satellites with anti-reflective materials and share detailed location data to help astronomers plan their observations. However, Rawls suggests more clear-cut regulations should be put into place because coordination with the NSF, for instance, only applies to U.S.-based launches.
There is also effectively no limit, she said, on how many satellites any one company may launch or operate.
Outshining the stars
To reach their conclusions on BlueWalker 3’s impact on astronomy and the visibility of the night sky, the researchers ran an international campaign to collect sky observation data from both amateur and professional stargazers across the world. This included statistics from the United States, Morocco, Chile, Aotearoa New Zealand, Mexico and the Netherlands. According to the study, visual observations made before BlueWalker 3 launched indeed indicated the satellite would be “particularly bright,” but it wasn’t until post-deployment that scientists could really see the impact.
Once BlueWalker 3’s antenna array was totally unfolded in space, its brightness reached a peak magnitude of about 0.4. In this centuries-old magnitude system, smaller numbers are brighter than larger numbers — a magnitude 0 object is 100 times brighter than a magnitude 5 object. Super bright objects can even go into the negatives (a full moon has a magnitude of around -12.6, while the sun shines around magnitude -27).
For context, most satellite constellations we can see in space have magnitudes between 4 and 6, the study states, likening BlueWalker 3’s magnitude of 0.4 to the stars Procyon and Achernar. These stellar bodies appear to us on Earth as the brightest stars in the constellations Canis Minor and Eridanus, respectively.
Giant antenna array aside, even the satellite’s Launch Vehicle Adapter itself appeared to reach a brightness of about magnitude 5.5, which the team says is four times brighter than the current International Astronomical Union recommendation of magnitude 7. In part, this recommendation is based on a brightness level that’s expected to be particularly challenging for observations with the Vera C. Rubin Observatory’s LSSTCamera when it begins its decade-long survey of the southern sky next year, among other things.
“Fainter than this limit would allow either the use of software such as AI to remove the satellite from an image whilst recovering the astronomical data underneath, or at least to mask the affected pixels,” Tregloan-Reed said. “Above this limit means that the satellite will most likely saturate the pixels, making it impossible to recover the data underneath the satellite.”
BlueWalker 3 is over 400 times brighter than that target.
“At the moment, essentially all constellation satellites are brighter than the 7th magnitude recommendation set by the IAU,” Eggl said. “So, in some sense, they are all troubling. Their aggregate effect on sky brightness is yet another issue.”
Starlink satellites, for instance, are individually only about 0.7% as bright as BlueWalker 3 (yes, they’re still visible to the unaided eye in the night sky) yet SpaceX wishes to create a constellation of about 40,000 of them, Tregloan-Reed said: “The overall impact, particularly to the sky glow, is worrisome.”
Now what do we do?
Ultimately, the goal of the team’s work is to bring attention to this issue in an effort to encourage both scientists and satellite engineers to think about how to protect our dark skies.
Satellite manufacturers should find ways to mitigate the brightness these modules emit in low-Earth orbit, the team says, but telescope facilities ought also to consider ways of working around the brightness issue. “On the one hand, astronomers are pretty experienced at dealing with ‘noise’ in our data,” Rawls said, thinking about unwanted signals from moonlight, camera electronics, clouds and terrestrial light pollution, for instance.
So, BlueWalker 3 is just one more source of noise to contend with.
Rawls also mentioned that scientists are already developing various tools to combat the issue such as what are known as image analysis pipelines, streak identification programs and telescope scheduling algorithms.
“On the other hand,” she added, “building all of these tools requires real resources and would not be necessary if the sky were not filling with bright satellites at an astonishing rate.”
Tregloan-Reed also suggested a few solutions for satellite engineers to consider, one of which is quite interesting. The concept basically requests mirroring the satellite market with the energy market, where consumers can select what energy supplier they wish to use based on the suppliers’ green credentials.
For instance, satellite operators can advertise that their satellite constellation has the lowest impact to the night sky, which would hopefully help consumers make more informed choices about which of these constellations’ services, if any, they want to use. Another idea would be to target geostationary orbit rather than low-Earth orbit for satellite constellations.
“If BlueWalker 3 was placed in GEO at an altitude of 35,000 kilometers instead of its current 500 kilometers, it would instantly become 0.02% of its current brightness, an order of magnitude fainter than the 7th magnitude limit set by the IAU,” he said.
To start aiding astronomers in avoiding noise in their data, the team also made sure to look into how brightness levels of BlueWalker 3’s array changed depending on the satellite’s location. Potentially, this could help observatories plan for how to conduct astronomical research in a future in which the night sky is overwhelmed with an abundance of artificial light.
“Most surfaces are naturally reflective to some degree, and the type of reflection and how bright it appears at various locations on Earth is tricky to accurately predict ahead of time,” Rawls said. “It is not necessarily something you would think of when you sit down to design a satellite.”
“Preserving dark skies has to be a priority for darkening mitigations to be developed, tested, and deployed.”
The paper was published on Oct. 2 in the journal Nature.