Broadband satellites being launched by SpaceX and other companies will inevitably have a negative impact on astronomers' ability to observe the night sky, according to a new report by astronomers. There are no mitigation strategies that can completely eliminate the satellites' impact on astronomical observations—other than not launching satellites at all—but the report includes recommendations for how satellite operators can minimize disruption and how observatories can adjust to the changes.
The report released this week is titled, "Impact of Satellite Constellations on Optical Astronomy and Recommendations Toward Mitigations." The report resulted from the recent Satellite Constellations 1 (SATCON1) workshop, which was organized jointly by the National Science Foundation's NOIRLab and the American Astronomical Society (AAS). SpaceX engineers participated in the online workshop, but the report was written by members of the SATCON1 Scientific Organizing Committee and represents their consensus views. The committee members hail from NOIRLab, AAS, the Lowell and Steward observatories in Arizona, the Rubin Observatory in Chile, the University of Michigan, UC-Davis, Smith College, and the Association of Universities for Research in Astronomy (AURA).
The report said:
"Changes are required at both ends: constellation operators and observatories. SpaceX has shown that operators can reduce reflected sunlight through satellite body orientation, Sun shielding, and surface darkening. A joint effort to obtain higher-accuracy public data on predicted locations of individual satellites (or ephemerides) could enable some pointing avoidance and mid-exposure shuttering during satellite passage. Observatories will need to adopt more dynamic scheduling and observation management as the number of constellation satellites increases, though even these measures will be ineffective for many science programs."
SpaceX has so far launched over 600 satellites and OneWeb has launched 74. Both companies plan to eventually launch tens of thousands of satellites into low-Earth orbits and provide broadband to areas that lack fast wired service. Amazon is also planning to launch thousands of satellites. Because of their low-Earth orbits (LEO), the satellites will provide lower latency than traditional satellite networks.
Musk predicted no impact
SpaceX CEO Elon Musk said in March that he is "confident that we will not cause any impact whatsoever in astronomical discoveries." He said the satellites are visible immediately after launching because "they're tumbling a little bit" and essentially "blink" or "reflect in ways that is not the case when they're on orbit." Once satellites stabilize and raise their orbits, they shouldn't cause problems for astronomers, Musk claimed.
But it's been over a year since SpaceX began launching broadband satellites, and astronomers have now "accumulated enough observations of constellation satellites like those being launched by SpaceX and OneWeb and run computer simulations of their likely impact to begin to understand the magnitude and complexity of the problem," the SATCON1 report said.
"If the 100,000 or more LEOsats proposed by many companies and many governments are deployed, no combination of mitigations can fully avoid the impacts of the satellite trails on the science programs of current and planned ground-based optical-NIR [near-infrared] astronomy facilities," the report said. "Astronomers are just beginning to understand the full range of impacts on the discipline. Astrophotography, amateur astronomy, and the human experience of the stars and the Milky Way are already affected."
Report authors said they "performed simulations of representative LEOsat constellations" in order to reach their conclusions. Recommendations for minimizing harms to astronomy "are based on work by and collaboration between astronomers and SpaceX," but these are "intended for a broad audience, and especially the satellite constellation industry as a whole," the report said.
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Altitude is important
The satellites' impact on astronomy will be affected by their altitude. Satellites orbiting at altitudes below 600km (like those being launched by SpaceX) are not as harmful to observations as those orbiting above 600km (like those launched by OneWeb). Amazon's plan calls for altitudes of 590km, 610km, and 630km.
"LEOsat constellations below 600km are visible for a few hours per night around astronomical twilight from observatories at middle latitudes, but they are in Earth's shadow and invisible for several hours per night around local solar midnight, with some satellites visible during the transitions. This visibility pattern causes these constellations to most heavily impact twilight observers," the report said.
With sub-600km satellites being closer to the Earth's surface, they are "brighter than the same satellites would be at higher orbital altitudes" and "more likely to exceed the unaided-eye brightness threshold if operators fail to design with this criterion in mind," the report said. However, satellites "above 600km are an even greater concern to astronomers because they include all the impacts mentioned above, but can also be illuminated all night long. Full-night illumination causes these high-altitude constellations to impact a larger set of astronomical programs."
Satellite constellations will be brightest "near the horizon and during twilight," disproportionately impacting "searches for near-Earth objects (NEOs), distant Solar System objects and optical counterparts of fleeting gravitational wave sources," the report said, adding:
"Depending on constellation design, LEO satellites can also be visible deep into the night, broadening the impact to encompass all astronomical programs. We find that the worst-case constellation designs prove extremely impactful to the most severely affected science programs. For the less affected programs, the impact ranges from negligible to significant, requiring novel software and hardware efforts in an attempt to avoid satellites and remove trails from images."
Satellite constellations like OneWeb's, with orbits of 1,200km, "present particularly serious challenges; they will be visible all night during summer and significant fractions of the night during winter, fall, and spring, and will have negative impacts on nearly all observational programs," the report said. (OneWeb, which is going through a bankruptcy and sale, is also using medium-Earth orbits of 8,500km.)
So far, SpaceX has permission to launch nearly 12,000 satellites, OneWeb has approval for 2,000, and Amazon has approval for 3,236. SpaceX has applied to the Federal Communications Commission for another 30,000 satellites and OneWeb has applied for another 47,844.
Action plan
The report listed these methods of minimizing the impact on astronomy:
1. Launch fewer or no LEOsat constellations. This is the only option identified that can achieve zero impact.
2. Deploy satellites at orbital altitudes no higher than ~600 km.
3. Darken satellites by lowering their albedo, shading reflected sunlight, or some combination thereof.
4. Control each satellite's attitude in orbit so that it reflects less sunlight to Earth.
5. Remove or mask satellite trails and their effects in images.
6. Avoid satellite trails with the use of accurate ephemerides [that provide data about a satellite's position relative to Earth].
Satellite operators should also "make their best effort to avoid specular reflection (flares) in the direction of observatories," the report said.
The report's recommendations for observatories focus heavily on development of new software. Observatories should support "development of a software application available to the general astronomy community to identify, model, subtract, and mask satellite trails in images on the basis of user-supplied parameters," the report said. They should also support development of observation-planning software for "the general astronomy community that predicts the time and projection of satellite transits through an image, given celestial position, time of night, exposure length, and field of view, based on the public database of ephemerides."
Researchers also provided recommendations for work that observatories and satellite providers can do together, such as "an immediate coordinated effort for optical observations of LEOsat constellation members, to characterize both slowly and rapidly varying reflectivity and the effectiveness of experimental mitigations."
There's optimism about the ongoing collaboration. "I hope that the collegiality and spirit of partnership between astronomers and commercial satellite operators will expand to include more members of both communities and that it will continue to prove useful and productive," NOIRLab director Patrick McCarthy said in a press release. "I also hope that the findings and recommendations in the SATCON1 report will serve as guidelines for observatories and satellite operators alike as we work towards a more detailed understanding of the impacts and mitigations and we learn to share the sky, one of nature's priceless treasures."
