Revised August 25, 2022
The Vera C. Rubin Observatory science community is concerned about the increasing deployment of Low Earth Orbit communications satellite constellations which, if unchecked, could jeopardize the discoveries anticipated from Rubin Observatory when science operations begin in 2024. There was a session at the Rubin 2022 Project and Community Workshop discussing these challenges. Because Rubin Observatory is uniquely affected by these satellite constellations, the science team is pursuing mitigation strategies to reduce the impact of the satellites on Rubin Observatory science.
The Vera C. Rubin Observatory is nearing completion, and its Legacy Survey of Space and Time (LSST) will soon offer an unprecedented, detailed view of the changing sky. Starting in late 2024, Rubin Observatory will employ the 8.4-meter Simonyi Survey Telescope and the 3200-megapixel LSST Camera to capture about 1,000 images of the sky, every night, for ten years. Each image will cover a 9.6 square degree field of view, or about 40 times the area of the full Moon. Because of the telescope's large light-collecting area, each 30-second exposure will reveal distant objects that are about 20 million times fainter than those visible with the unaided eye. This large combination of light-collecting area and field of view on the sky is unprecedented in the history of optical astronomy, which is one of the reasons Rubin/LSST was the top-ranked ground-based project in the National Academy of Science 2010 Decadal Survey of Astronomy and Astrophysics.
LSST survey images will contain data for about 20 billion galaxies and a similar number of stars, and will be used for investigations ranging from cosmological studies of the Universe to searches for potentially dangerous Earth-impacting asteroids. However, the revolutionary discoveries anticipated from the Rubin Observatory LSST could be significantly degraded by the alarming pace of new deployments of Low Earth Orbit (LEO) communications satellite constellations.
In late May 2019, SpaceX launched the first 60 of its planned Starlink constellation of 42,000 communications satellites to LEO orbits at altitudes of about 550 km. Since then, SpaceX has launched every 3 weeks and soon will deploy its larger Gen2 Starlink satellites. Many other companies, including Amazon, OneWeb, E-Space, and AST SpaceMobile have also entered the race, and the number of satellites launched may exceed 400,000 over the next decade. The negative impact of these satellites on optical astronomy depends on the number and (critically) brightness of satellites. Rubin Observatory is an extreme case for the sensitivity of astronomical observations to satellite constellations because of its unprecedented ability to repeatedly monitor the sky widely and deeply. During the nominal 30-second visit to a sky patch, satellites in 400-600km LEO orbits typically move about 15 degrees across the sky (about four times the diameter of Rubin Observatory’s field of view), and are visible a few hours after sunset and before sunrise. With 400,000 satellites orbiting Earth, tens of thousands of satellites would be visible above the horizon and it would be difficult to find a circle of 9.6 square degrees anywhere on the sky that does not contain satellite streaks. Simulations of the LSST observing cadence and the full SpaceX satellite constellation show that as many as 30% of all LSST images would contain at least one Starlink satellite trail. With the planned constellations of 400,000 satellites at 400-600 km, all images in twilight will contain streaks. The OneWeb constellation at 1200 km will be visible all night long in Chilean summer. Measurements of the brightness of the current LEO satellites in their final orbits indicate that these trails would cause residual artifacts in the reduced data. If these LEO satellites can be darkened to 7th magnitude, then a new instrument signature removal algorithm can remove some of the residual artifacts. This is challenging due to apparent non-linear crosstalk between the 16 channels on each of the 189 CCDs, the cause of which is still under study. The bright main satellite trail would still be present, potentially creating bogus alerts and systematics at low surface brightness. Masking of these trails is not 100% perfect. This is a challenge for science data analysis, adding potentially significant effort.
If satellites were darkened to 7th magnitude, they would be 10x below saturation in LSST images. In this case, and with 1-arcminute wide software masks needed to fully mask the faint scattered light from the satellite streak, it is likely that only small fractions of pixels in the affected images—probably in the 1% to 10% range—would be rendered scientifically useless. If this estimate proves correct, the net fraction of lost LSST pixels would be in the 3% range, which corresponds to several months of observing time over the ten-year survey. In the case of the brighter streaks from the large reflective AST SpaceMobile satellites, entire CCDs will be saturated, losing 6-8% of the pixels in each exposure. There is no current understanding that most of the planned satellites will be fainter than 7th magnitude, and so the impact on LSST science may be significant.
However there is a larger challenge: because of the unprecedented large samples, LSST science will be limited by systematics rather than sample variance (area incompleteness). Of concern are various systematic effects that do not simply scale with the number of lost pixels—in other words, the residuals from these mitigation strategies on the science cases for which LSST was designed. For example, the LSST ability to detect asteroids approaching from directions interior to the Earth's orbit may be severely impacted because those directions are visible only during twilight when LEO satellites are brightest—nearly every LSST image taken at this time would be affected by at least one satellite trail. Precision cosmological studies are another example; they are very sensitive to small systematic effects, and might suffer from artifacts due to the removal or masking of the satellite tracks. At the low surface brightness of many LSST science programs, the trail is several hundred pixels wide. Perhaps the largest systematic will be the bogus alerts caused by glints from 400,000 LEOsats rolling to adjust orbits for collision avoidance.
The Rubin Observatory team is working closely with SpaceX engineers to jointly find ways to lessen the impact of the satellite streaks, though no silver bullet yet exists. It remains to be seen how faint Starlink Gen2 satellites will be as a function of sun-satellite-observer angle. In some cases, technical solutions developed by SpaceX may be adopted by other constellation operators. It is already known, however, that other operators of satellites in LEO will present a significant threat to the main mission of LSST: discovery of the unexpected.