In May 2019 SpaceX launched its first batch of 60 Starlink communication satellites, which surprised astronomers and laypeople with their appearance in the night sky. Astronomers have only now, a little over a year later, accumulated enough observations of constellation satellites like those being launched by SpaceX and OneWeb, and run computer simulations of their likely impact when fully deployed, to thoroughly understand the magnitude and complexity of the problem. This research informed the discussion at the Satellite Constellations 1 (SATCON1) workshop held virtually 29 June to 2 July 2020 and led to recommendations for observatories and constellation operators. The SATCON1 report concludes that the effects on astronomical research and on the human experience of the night sky range from “negligible” to “extreme.”
The text below reproduces the Executive Summary of the SATCON1 report. The version-of-record of the full report and the associated Working Group Technical Reports are available as the following PDF downloads:
Existing and planned large constellations of bright satellites in low-Earth orbit (LEOsats) will fundamentally change astronomical observing at optical and near-infrared (NIR) wavelengths. Nighttime images without the passage of a Sun-illuminated satellite will no longer be the norm. 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 astronomy facilities. 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. This report is the outcome of the Satellite Constellations 1 (SATCON1) workshop held virtually on 29 June–2 July 2020. SATCON1, organized jointly by NSF’s NOIRLab and AAS with funding from NSF, aimed to quantify better the impacts of LEOsat constellations at optical wavelengths and explore possible mitigations.
Recent technology developments for astronomical research — especially wide-field imaging on large optical telescopes — face significant challenges from the new ability in space and communication technologies to launch many thousands of LEOsats rapidly and economically. This troubling development went unnoticed by our community as recently as 2010, when New Worlds, New Horizons — the most recent National Academies’ decadal survey of astronomy and astrophysics — was issued. In the last year, the sky has changed, with growing numbers of satellite trails contaminating astronomical images.
Many astronomical investigations collect data with the requirement of observing any part of the sky needed to achieve the research objective with uniform quality over the field of view. These include studies that are among the highest priorities in the discipline: stellar populations in the Milky Way and neighboring galaxies; searches for potentially hazardous near-Earth objects; identification of gravitational wave sources such as neutron star mergers; and wide-area searches for transiting exoplanets. At a minimum, a fraction of the area being imaged is lost to the trails or significantly reduced in S/N (signal-to-noise ratio). However, many of these areas of research also include a time-critical aspect and/or a rare, scientifically critical target. Such a missed target, even with low probability, will significantly diminish the scientific impact of the project. For example, if a near-Earth object is not recovered, its orbital parameters are lost. If the transit of a promising super-Earth exoplanet candidate is missed, the orbital timing may not be recovered. If the optical counterpart of a gravitational wave source is lost in the few percent of pixels in satellite trails, its rapid fading may preclude subsequent identification. Detailed simulations beyond the scope of this workshop are required to better quantify the potential scientific cost of losing uniform full area coverage in these cases.
Even more challenging simulations are required to understand the impact on very large samples (e.g., from Vera C. Rubin Observatory) that are limited not by small number statistics but rather by systematic uncertainties. One measure of precision cosmology, for example, is the gravitational weak lensing shear that elongates faint galaxy images, and more complex modeling is needed to understand the major impact these satellites will have on this field.
Initial visibility simulations have shown the significant negative impacts expected from two communications-focused LEOsat constellations, Starlink (launched by Space Exploration holdings, LLC [SpaceX]), and OneWeb. For SATCON1, simulations were performed of the visibility of LEOsats with 30,000 second-generation Starlink satellites below 614 km and ~48,000 OneWeb satellites at 1200 km, in accord with the FCC filings for these projects. For all orbital heights, the visibility of sunlit satellites remains roughly constant between sunset and astronomical twilight (Sun 18 degrees below the horizon). The key difference between lower (~600 km) and higher (~1200 km) orbits is the visibility in the dark of night between astronomical twilights: higher altitude constellations can be visible all night long during summer, with only a small reduction in the number visible compared to those in the twilight.
Mitigation of the most damaging impacts on scientific programs is now being actively explored by the professional astronomy community worldwide. These investigations have benefited from collaboration with SpaceX, the first operator to launch a substantial constellation of LEOsats (538 satellites over 9 launches as of July 2020). 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.
SATCON1 was attended by over 250 astronomers and engineers from commercial operators (mainly from SpaceX since they are furthest along in their work on this issue), as well as other stakeholders, and reached a number of conclusions and recommendations for future work. The organizers hope that the collegiality and spirit of partnership between these two communities will expand to include other operators and observatories and continue to prove useful and productive. Our findings and recommendations should serve as guidelines for observatories and satellite operators alike to use going forward, even as we work toward a more detailed understanding of the impacts and mitigations.
Our findings and recommendations are listed below.
The projected surface density of bright satellites in constellations is greatest near the horizon and during twilight. For this reason, LEO constellations disproportionately impact science programs that require twilight observations, such as searches for near-Earth objects (NEOs), distant Solar System objects and optical counterparts of fleeting gravitational wave sources. 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.
We find two step-functions in impact based on the brightness of the satellites: naked-eye visibility and instrument sensor calibration range. If satellites are visible to the naked eye, the scope of impact expands to include non-professional, unaided-eye observers including amateur astronomers and astrophotographers, and possibly indigenous peoples and members of religions that observe the sky for calendar-keeping. Satellites whose apparent brightnesses are below unaided-eye visibility can have a much more severe impact on astronomical science if they are bright enough to cause non-correctable artifacts in the camera sensors. For fainter satellites, of course, the trail itself remains and must be dealt with. In the cases where it might be impossible to fully mask or remove trails, a bright enough satellite could induce systematic errors impacting some science investigations.
Satellites below 600 km
LEOsat constellations below 600 km 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 (see the examples mentioned above). Since these orbits are closer to Earth, satellites at these altitudes will be brighter than the same satellites would be at higher orbital altitudes. The reduced range makes them more likely to exceed the unaided-eye brightness threshold if operators fail to design with this criterion in mind.
Satellites above 600 km
Satellites above 600 km 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.
Approaches to mitigate LEOsat impacts on optical-NIR astronomy fall into six main categories.
Launch fewer or no LEOsat constellations. This is the only option identified that can achieve zero impact.
Deploy satellites at orbital altitudes no higher than ~600 km.
Darken satellites by lowering their albedo, shading reflected sunlight, or some combination thereof.
Control each satellite’s attitude in orbit so that it reflects less sunlight to Earth.
Remove or mask satellite trails and their effects in images.
Avoid satellite trails with the use of accurate ephemerides.
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.
Support development of a software application for observation planning available to 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. Current simulation work provides a strong basis for the development of such an application.
Support selected detailed simulations of the effects on data analysis systematics and data reduction signal-to-noise impacts of masked trails on scientific programs affected by satellite constellations. Aggregation of results should identify any lower thresholds for the brightness or rate of occurrence of satellite trails that would significantly reduce their negative impact on the observations.
LEOsat operators should perform adequate laboratory Bi-directional Reflectance Distribution Function (BRDF) measurements as part of their satellite design and development phase. This would be particularly effective when paired with a reflectance simulation analysis.
Reflected sunlight ideally should be slowly varying with orbital phase as recorded by high etendue (effective area × field of view), large-aperture ground-based telescopes to be fainter than 7.0 Vmag + 2.5 × log(rorbit / 550 km), equivalent to 44 × (550 km / rorbit) watts/steradian.
Operators must make their best effor to avoid specular reflection (flares) in the direction of observatories. If such flares do occur, accurate timing information from ground-based observing will be required for avoidance.
Pointing avoidance by observatories is achieved most readily if the immediate post-launch satellite configuration is clumped as tightly as possible consistent with safety, affording rapid passage of the train through a given pointing area. Also, satellite attitudes should be adjusted to minimize reflected light on the ground track.
Support 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. Such observations require facilities spread over latitude and longitude to capture Sun-angle-dependent effects. In the longer term, support a comprehensive satellite constellation observing network with uniform observing and data reduction protocols for feedback to operators and astronomical programs. Mature constellations will have the added complexity of deorbiting of the units and on-orbit aging, requiring ongoing monitoring.
Determine the cadence and quality of updated positional information or processed telemetry, distribution, and predictive modeling required to achieve substantial improvement (by a factor of about 10) in publicly available cross-track positional determination.
Adopt a new standard format for publicly available ephemerides beyond two-line-elements (TLEs) in order to include covariances and other useful information. The application noted in Recommendation 2 should be compatible with this format and include the appropriate errors.