by Douglas Messier
A new report recommends “immediate, well-funded, comprehensive, and collaborative work” to implement a series of measures to mitigate the negative impacts that large satellite constellations on ground-based astronomy.
The report, whose executive summary was published last week, includes 10 recommendations for observatories and constellation operators that include the development of software to identify and mask satellite trails and designs changes to lessen the reflectivity of satellites. (The full list of recommendations are below.)
The report is a result of the Satellite Constellations 2 (SATCON2) workshop held in July. SATCON2 brought together more than 350 astronomers, satellite operators, dark-sky advocates, policy experts and other stakeholders from 40 countries. The gathering was conducted to determine how to implement recommendations developed during the SATCON1 workshop in 2020.
Large satellite constellations being launched by SpaceX, OneWeb and other companies are causing satellite trails in images taken by ground-based astronomers.
“The steady and now increasingly rapid growth of the number of satellites in low Earth orbit has the potential to transform the appearance of the night sky. While individual satellites can likely be designed to be invisible to the unaided eye, their trails will be easily visible to even entry-level amateur astronomy equipment and will be billions of times brighter than the sensitivity of major research facilities. In addition to the trails, diffuse brightening of the entire natural dark sky by scattering of reflected sunlight from satellites, up to a factor of 2–3 according to some calculations, is an increasingly real possibility,” the workshop organizers said in a press release.
“Beyond astronomy, there may be a variety of environmental impacts, including acceleration of climate change through upper atmosphere deposits, degradation of ocean and land health at sites of launches and re-entry, and increasing density of orbital debris,” the press release added.
The group noted that the International Astronomical Union (IAU) has called for the establishment of a new Centre for the Protection of the Dark and Quiet Sky from Satellite Constellation Interference. IAU recently teamed with the United Nations Office for Outer Space Affairs, the Instituto de Astrofísica de Canarias and the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory to hold the Dark and Quiet Skies for Science and Society II conference to address issues with satellite constellations.
“We are on the threshold of fundamentally changing a natural resource that since our earliest ancestors has been a source of wonder, storytelling, discovery, and understanding of ourselves and our origins. We transform that at our peril,” the SATCON2 organizers said.
- SATCON2 Executive Summary
- SATCON2 Press Conference
- SATCON1 Report
- SATCON1 Press Conference
- Dark & Quiet Skies 2020 Report
- IAU Statement on Satellite Constellations
- AAS Statement on Satellite Constellations
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.
For Constellation Operators
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 effort 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.
For Observatories and Operators in Collaboration
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.