HOUSTON — A prototype satellite launched by AST SpaceMobile a year ago is at times one the brightest objects in the night sky, raising new concerns about its impact on astronomy.
In an advanced copy of a paper to be published by the journal Nature released Oct. 2, astronomers documented several months of observations of BlueWalker 3, a spacecraft launched into low Earth orbit by AST SpaceMobile in September 2022. The spacecraft subsequently deployed a 64-square-meter antenna to support direct-to-device communications.
The paper analyzed observations of the spacecraft made in the months following its launch by astronomers around the world. Shortly after the spacecraft deployed that antenna in November 2022, the spacecraft’s brightness increased from magnitude 6, the limit of naked-eye observations in dark sites, to magnitude 0.4. That made the satellite one of the brightest objects in the night sky, similar in brightness to the stars Procyon and Achernar.
The spacecraft later dimmed, likely because of changes in its orientation, only to brighten again, returning to magnitude 0.4 in April 2023. The satellite’s brightness is a function of several factors, astronomers said, including its elevation above the horizon, range to the observer on the ground and solar phase angle.
“These results demonstrate a continuing trend towards larger, brighter commercial satellites, which is of particular concern given the plans to launch many more in the coming years,” said Siegfried Eggl of the University of Illinois at Urbana-Champaign, one of the authors of the study, in a statement released by the International Astronomical Union Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference, or IAU CPS. That center works to study the impact of satellite constellations on astronomy and methods to mitigate those impacts.
A spokesperson for AST SpaceMobile, in a statement to SpaceNews, did not directly address the brightness observations of BlueWalker 3 reported in the paper. However, the company said it is collaborating with “NASA and certain astronomy working groups to develop advanced industry solutions, including potential operational interventions,” to address those concerns.
AST SpaceMobile said it is working to reduce the brightness of its satellites with “roll-tilting flight maneuvers” to minimize sunlight reflected to the ground. It is also planning to add anti-reflective materials to future satellites.
The company also noted that its constellation will be far smaller, in terms of number of satellites, than others, with only about 90 satellites needed “to provide substantial global coverage.” OneWeb has more than 600 satellites in orbit while SpaceX’s Starlink constellation is approaching 5,000 satellites in orbit.
Those satellites, though, are substantially fainter than BlueWalker 3. SpaceX has worked with astronomers on ways to reduce the brightness of its Starlink satellites and signed a coordination agreement with the National Science Foundation (NSF) in January to collaborate on ways to address the brightness of its larger V2 satellites.
At a Sept. 19 meeting of the Astronomy and Astrophysics Advisory Committee, an NSF official said that the agency was finalizing similar coordination agreements with Amazon and OneWeb for their constellations, and praised the Federal Communications Commission for putting requirements in updated licenses for smaller constellations operated by Iceye and Planet that those companies also coordinate with the NSF.
The NSF, though, has not announced a coordination agreement with AST SpaceMobile. At that committee meeting, Connie Walker, co-director of the IAU CPS and an astronomer at the NSF’s National Optical-Infrared Astronomy Research Laboratory, noted that a paper on observations of BlueWalker 3 would soon be published in Nature and that its conclusions had been shared with AST SpaceMobile in advance, but did not discuss the contents of the paper.
Astronomers also remain worried about radio astronomy interference from AST SpaceMobile spacecraft, which use spectrum allocated for terrestrial communications. Those frequencies are close to those used for radio astronomy, said Federico Di Vruno, the other co-director of the IAU CPS, and radio telescopes located in “radio-quiet zones” on the ground intended to avoid terrestrial interference could still be susceptible to interference from satellite transmissions.
AST SpaceMobile said it will avoid broadcasts from its satellites into or adjacent to the U.S. National Radio-Quiet Zone in Virginia and West Virginia, as well as “other radioastronomy locations as required or needed, including those not officially recognized.” The company will also avoid radio-quiet zones for its gateway antennas.
While the potential for optical and radio astronomy interference from satellite constellations alarms astronomers, they also recognize they will have to co-exist with such systems.
“The astronomical community understands the need for greater connectivity and improvements to internet access, particularly for rural and underserved communities,” said Jeremy Tregloan-Reed, a co-author of the paper at the Universidad de Atacama in Chile, in the IAU CPS statement. “However, that progress has to be balanced against the negative impact that bright satellites can have on the night sky.”