WASHINGTON — Developers of large satellite constellations say it may be impossible to meet brightness goals established by astronomers to minimize the impact of those satellites on their constellations.
In a paper submitted to the arXiv preprint server at the end of June, astronomers affiliated with the International Astronomical Union’s Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference, or IAU CPS, found that most satellite constellations in development are brighter than the recommended limits set by the group.
That limit is magnitude 7 for satellites at altitudes of 550 kilometers or less, with the limit growing fainter at higher altitudes. That limit was set by astronomers, working with satellite developers, with a particular emphasis on minimizing effects on the new Vera C. Rubin Observatory in Chile with its large field of view and sensitive camera.
The study found that only OneWeb met that limit with an average apparent magnitude of 7.85, right on the adjusted limit for its satellites at altitudes of 1,200 kilometers. Other satellite systems were often significantly brighter than that.
The worst offenders are the BlueBird satellites from AST SpaceMobile, with an average apparent magnitude of 3.3. Those satellites, at times, can be brighter than magnitude 2, meaning they can be seen by the naked eye even in relatively light-polluted skies. However, only a handful of such satellites are in orbit today.
None of the various series of Starlink satellites from SpaceX, by far the largest constellation in operation, meet the magnitude 7 limit. The “V2 Mini” satellites being launched currently have average apparent magnitudes ranging from 5.16 to 6.24.
The two Chinese megaconstellations now being launched, Guowang and Qianfan, also significantly exceed brightness limits despite being at much higher altitudes than BlueBird and Starlink satellites. The Qianfan satellites have an average apparent brightness of 5.76 and Guowang satellites 5.07. Both are at altitudes of around 1,000 kilometers.
The paper does not include satellites from Amazon’s Project Kuiper constellation, which started launching its operational satellites in April. The first of those satellites are only now reaching operational orbits around 600 kilometers altitude.
Mitigating interference to both optical and radio astronomy is a core “level zero requirement” for Project Kuiper, said Josef Koller, principal space safety advisor at Amazon Kuiper, during a session of the AIAA ASCEND conference last month.
He said that Amazon has tried different approaches to reducing the brightness of its satellites, including some technologies like dielectric mirror stickers previously demonstrated by SpaceX on its Starlink satellites. “They did not work for us because we are tuned to a different frequency band,” he said.
The company did find an alternative solution from another vendor it could apply to its flat panel antennas that did not attenuate radio signals but did reflect sunlight away from the ground. He said Amazon is also building its own observatories to allow it to monitor the brightness of its satellites and adjust their design if needed.
Koller argued that it is difficult to minimize the brightness of satellites. “Satellites are incredibly complex form factors on the outside. They’re not just a box,” he said. “You just cannot put a sticker on everything.”
He suggested that may make it very difficult, if not impossible, to meet the magnitude limit recommended by astronomers. “It’s really, really difficult to achieve,” he said, citing the complexities of satellite design and the need for satellites to still carry out their primary mission while incorporating those design changes.
“It is a designed outcome that we at Kuiper will work towards, but it is something that is a very tall order and very difficult to achieve,” he said.
Others on the panel said that the magnitude limit was still a worthwhile goal. Yaswant Devarakonda, a senior advisor in the State Department’s Office of Space Affairs, said that magnitude limit comes up in international meetings, like the UN’s Committee on the Peaceful Uses of Outer Space, as well as in one-on-one meetings with nations.
“It’s a goal. It’s not necessarily set in stone,” he said. “It’s still something we should strive towards, and having an open dialogue with countries is really useful.”
“We’ve seen a lot of positive movement in this direction,” said Ilsa Mroz, regulatory affairs manager at Planet. She called for more discussions on what international standards should be and compliance to them. “If we can get everyone on the same page about even just what those definitions are, that would take us a million miles into the future about how we think about this and how we can work towards compliance.”