SpaceX satellites now appearing in 1 in 5 of telescope’s twilight images
SpaceX's aspirations to blanket the Earth in high-speed internet through a constellation of orbiting satellites continues apace, and a new study demonstrates the significant mark they are already making on the world of astronomical imaging. As the number of satellites in low-Earth orbit has grown rapidly over the past two years, researchers have found they are now affecting almost a fifth of important twilight observations, though they describe the overall scientific impacts as small, for now.
SpaceX began launching its Starlink satellites into orbit in 2019, and in that same year astronomers were already noticing bright streaks of light appear in their images. Since then, concerns have grown over how these satellites might compromise the dark and quiet sky that scientists rely on to study the universe, or even inhibit the detection of hazardous near-Earth asteroids.
Today, there are almost 1,800 of them orbiting the Earth at an altitude of around 550 km (341 miles), and an analysis of archival images gathered by the Zwicky Transient Facility (ZTF) at Caltech's Palomar Observatory near San Diego between November 2019 and September 2021 shows how they are making their presence felt. This instrument scans the night sky every two day days, collecting observations of cosmic objects such as supernovae and near-Earth asteroids.
This analysis revealed a total of 5,301 streaks created by Starlink satellites, affecting almost a fifth of twilight observations. These observations are made at dawn or dusk and are vital for detecting asteroids that travel close to the Sun and through Earth's neighborhood.
"In 2019, 0.5 percent of twilight images were affected, and now almost 20 percent are affected," said Przemek Mróz, study lead author.
Despite this rapid uptick in streaky images, the researchers say that science operations are not being strongly impacted, with each streak affecting less than one-tenth of a percent of the pixels in each image, and weather conditions posing a bigger threat as it stands.
"There is a small chance that we would miss an asteroid or another event hidden behind a satellite streak, but compared to the impact of weather, such as a cloudy sky, these are rather small effects for ZTF," said study co-author Tom Prince.
With SpaceX planning to have as many as 10,000 satellites in orbit by 2027, the scientists anticipate nearly all twilight images taken at ZTF will contain at least one streak in the future. Moving forward, the scientists expect software to help avoid potential pollution of their images, which could predict the path of the satellites and enable them to schedule observations that avoid their path, or potentially mask their effects.
SpaceX is taking measures of its own to reduce the impacts of these satellites on astronomy. This includes adding sun visors to the satellites to reduce their reflectivity, with the first prototype launched in June of 2020. As part of their analysis, the authors of the study also examined the effectiveness of these visors, finding that they do in fact reduce the brightness of the satellites by a factor of about five, though this is still a little brighter than the levels demanded by the Satellite Constellations 1 (SATCON1) workshop, made up of astronomers, policymakers and experts in 2020.
"We don't expect Starlink satellites to affect non-twilight images, but if the satellite constellation of other companies goes into higher orbits, this could cause problems for non-twilight observations," Mróz says.
The research was published in the journal The Astrophysical Journal Letters.