Bluewalker 3 satellite is one of the brightest objects in the sky, observations show

The craft is the latest to raise concerns about the loss of the night sky to constellations of satellites.
By | Published: October 2, 2023 | Last updated on October 3, 2023

The night sky is changing with the stunning growth in commercial satellites — and one of the satellites that concerns astronomers the most is Bluewalker 3.

The prototype craft, launched by the Texas-based startup AST SpaceMobile last September, was the subject of a global observing campaign that published results today in Nature Astronomy. The observations confirm what astronomers realized as soon as the satellite unfurled its massive antenna last November: Bluewalker 3 is one of the brightest objects in the night sky, rivaling the most brilliant stars.

“While there is only one BlueWalker 3 so far, it is important to characterize its brightness and understand its impact, since many groups are planning to launch many bright satellites to low-Earth orbit in the near future,” says Meredith Rawls, a study co-author and astronomer at the University of Washington, where she works with the Vera C. Rubin Observatory. Those groups are growing in number: In addition to existing constellations from SpaceX and OneWeb, Amazon is planning for over 3,200 satellites as part of its Project Kuiper; its first prototype satellites are set to launch on Friday.

Cell towers in space

AST SpaceMobile’s goal is to build a satellite constellation that provides global 5G internet service directly to smartphones — unlike SpaceX’s Starlink service, which requires a terminal to receive the satellite signals. Last month, the company used Bluewalker 3 to demonstrate what it says is “the first-ever 5G cellular connectivity from space directly to everyday smartphones,” connecting a voice call from Maui, Hawaii, to Madrid, Spain. The company says it has also demoed 4G video and data transmission with Bluewalker 3.

The craft is essentially a cell tower in space, and its antenna is much larger than traditional communications satellites. At 692 square feet (64.3 square meters), the antenna is nearly the size of a badminton singles court (747 square feet [69.4 square meters]).

All this surface area reflects sunlight — a lot of it.

Bluewalker 3’s large brightness is due to its massive phase-array antenna, made of individual antennas that work together to form independent transmission beams to mobile phones. Credit: AST SpaceMobile

According to the newly published observations, the brightness of the satellite peaked at magnitude 0.4, which puts it within the top 10 brightest stars — comparable to Procyon, Achernar, and Betelgeuse. That means it’s about 900 times brighter than objects that shine at magnitude 7, which is the satellite brightness limit that the International Astronomical Union (IAU) recommends to head off the most severe effects on astronomy. It’s also just outside the limits of the human eye.

Astronomers already had strong concerns about the prospect of satellite megaconstellations like Starlink, which could consist of tens of thousands of satellites when fully deployed. Though AST plans a more modest constellation of about 90 satellites, the sheer brightness of the Bluewalker 3 prototype caused astronomers to sound the alarm.

“BlueWalker 3 is a big shift in the constellation satellite issue and should give us all reason to pause,” said Piero Benvenuti, the director of the IAU Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference (CPS), in a statement last December.

The new observations flesh out the visual characteristics of the craft, including how its brightness changes over time. “I think this paper is particularly valuable because it brings together observations from around the globe with instruments ranging from large professional telescopes to expert visual observers,” said Rawls, who co-leads the IAU’s SatHub initiative, part of CPS, which coordinated the observing campaign.

Mitigation efforts

Astronomers hope to forge a working relationship with the growing number of satellite firms to manage their effects. A goal of SatHub is to create a database of satellites and their predicted motions. This would allow astronomers to plan their observations around satellites in orbit — dodging the streaks they trace in the sky or temporarily halting data-collecting when one passes through its field of view.

Researchers have already worked with SpaceX to identify strategies to avoid the worst effects on astronomy, particularly large survey telescopes like Rubin, which will image the entire northern sky every three nights. The IAU recommendation of keeping the brightness of on-station satellites below magnitude 7 or dimmer allows Rubin astronomers to mask out image streaks while preserving the accuracy of the data; brighter than that, and knock-on effects in the sensors can seriously degrade the data quality.

The Vera C. Rubin Observatory on Cerro Pachón in Chile is scheduled to come online next year. Credit: Rubin Observatory/NSF/AURA/B. Quint

In a statement emailed to Astronomy and attributed to an AST spokesperson, the company emphasized its mission of bringing broadband internet to the estimated 2.6 billion people who don’t have access to it. “At AST SpaceMobile, our mission is to democratize access to knowledge and information regardless of where people live and work. … By connecting people, we aim to alleviate poverty, spur economic development, foster a diverse digital society, and save lives.” It continued: “Solving significant problems for humanity often comes with challenges. In this case, we are working to address the concerns of astronomers,” including collaboration with “NASA and certain astronomy working groups.”

AST said it is using roll and tilt maneuvers on Bluewalker 3, which reduce its apparent brightness; these maneuvers can orient the craft so it reflects light away from ground-based observatories. And AST said it plans to build its next generation of satellites with anti-reflective materials.

The firm also says it is sharing ephemerides of its craft to help astronomers avoid getting satellite streaks in their observations. This is something AST did not do immediately, including for Bluewalker 3’s launch vehicle adapter (LVA), a piece of equipment that housed the antenna before it unfolded. The LVA was jettisoned shortly after launch and appeared in the campaign’s observations. Though much dimmer than Bluewalker 3 itself, it was still four times brighter than the IAU’s guidelines and would have been visible to the naked eye in a dark site. The authors of the new study noted that four days passed before the object appeared in public satellite catalogs. “This poses additional challenges to mitigation efforts by ground-based observatories, since satellite avoidance requires a complete and highly accurate set of satellite orbits,” the team wrote.

The global nature of the team’s observing campaign allowed it to characterize just how accurate and timely this orbital information currently is for Bluewalker 3 — key information for astronomers if they are to thread their telescopes between satellite streaks in their observations. The team found that the published orbital parameters for Bluewalker 3 gave a predicted position that was off on average by 7.2 arcminutes, or nearly one-quarter the width of the Full Moon. And due to atmospheric drag and solar storms, the predicted positions get worse over time by roughly 0.4 arcminute every hour. The results highlight the importance of updating the orbital information quickly and regularly, the team writes.

An invisible threat

As concerning as the optical measurements are to astronomers, they’re just one part of the threat that Bluewalker-type satellites pose to astronomy. The other is invisible: The cellular transmissions it will send can disrupt radio astronomy and observations.

Radio astronomers already go to great lengths to avoid cell phone interference. Some areas have established radio quiet zones where most radio traffic is prohibited, like the National Radio Quiet Zone (NRQZ) that surrounds the Green Bank and Sugar Grove facilities in West Virginia. And some radio bands are protected internationally for radio astronomy. But those regulations do not extend to space. And as with optical astronomy, there is a new generation of radio arrays that may be affected — like the Square Kilometre Array, whose antennas will span distances of tens of miles in both South Africa and Australia.

The Green Bank Telescope lies deep inside the U.S. National Radio Quiet Zone. Credit: Jay Young for Green Bank Observatory

AST said it is “avoiding broadcasts within or adjacent to the [NRQZ] and other radio astronomy locations as required or needed, including those not officially recognized,” and also that it plans to put ground-based gateway antennas for their network “at a considerable distance from the NRQZ and other radio-quiet zones vital to astronomy.”

It’s an issue that will require a lot more research and strategizing to work around, the team behind the new paper writes. Bluewalker 3’s gateway downlink transmissions operating on frequencies directly adjacent to the radio-astronomy protected band of 42.0–42.5 GHz, the team notes.

And even satellites that don’t broadcast near astronomically important frequencies leak electromagnetic radiation from their basic circuitry that can interfere with observations; a study published this August found that Starlink satellites emit unintended radiation that encroaches on a radio-astronomy protected band.