The U.S. company SpaceX plans to build a constellation of 42,000 low-Earth-orbit communication satellites called Starlink. A new study, led by Dr Przemysław Mróz from the Astronomical Observatory of the University of Warsaw, looks at the impact of satellites on ground-based astronomical research.

For a few years an American company SpaceX has been launching an increasing number of internet satellites into orbit around the Earth. The satellite network, also called a constellation, now includes nearly 1,800 members orbiting at altitudes of about 550 kilometres. Astronomers have expressed concerns that these objects, which can appear as streaks in telescope images, could hamper their scientific observations.

To quantify these effects, a team of researchers, led by Dr Przemysław Mróz from the Astronomical Observatory at the University of Warsaw, studied archival images captured by the U.S. National Science Foundation (NSF)-funded Zwicky Transient Facility (ZTF), an instrument that operates from Caltech’s Palomar Observatory near San Diego. ZTF scans the entire night sky every two days, cataloguing cosmic objects that explode, blink, or otherwise change over time. This includes everything from supernovae to near-Earth asteroids. The research team decided to focus on the study of the effects of Starlink satellites because these satellites currently represent the largest low-Earth orbit constellation and they have well-characterised orbits. Their work results have been published in The Astrophysical Journal Letters, a peer-reviewed scientific journal.

Streaks at dawn or dusk

The ZTF findings show that 5,301 satellite streaks appear in archival images taken between November 2019 and September 2021. According to the study lead author, the streaks are most apparent in so-called twilight observations at dawn or dusk, which are important for finding near-Earth asteroids that appear close to the sun in the sky. ZTF has discovered several asteroids of this nature, including 2020 AV2, the first asteroid spotted with an orbit that fits entirely within the orbit of Venus. “In 2019, 0.5 percent of twilight images were affected, and now almost 20 percent are affected,” as pointed out by Dr Przemysław Mróz.

“In the future, the scientists expect that nearly all of the ZTF images taken during twilight will contain at least one streak, especially after the Starlink constellation reaches 10,000 satellites, a goal SpaceX hopes to reach by 2027. We do not expect Starlink satellites to affect non-twilight images, but if the satellite constellations of other companies go into higher orbits, this could cause problems for non-twilight observations,” Dr Mróz said.

An asteroid behind a satellite streak

Yet despite the increase in image streaks, the new report notes that ZTF science operations have not been strongly affected yet. Study co-author Tom Prince, the Ira S. Bowen Professor of Physics at the California Institute of Technology, indicates that a single streak affects less than one-tenth of a percent of the pixels in a ZTF image. “There is a small chance that we would miss an asteroid or another event hidden behind a satellite streak, but compared to weather, such as a cloudy sky, these are rather small effects for ZTF,“ Prof. Prince revealed, “Software can be developed to mitigate potential problems, including software to predict the locations of the Starlink satellites, and thus avoid scheduling an observation when a satellite might be in the field of view. Software can also assess whether a passing satellite may have affected an astronomical observation, which would allow astronomers to mask, or otherwise reduce, the negative effects of the streaks,” he added.

The new study also looked at the effectiveness of visors on the Starlink satellites, which SpaceX added to the spacecraft beginning in 2020 to block sunlight from reaching the satellites. According to the ZTF observations, the visors reduce the satellite brightness by a factor of about five. That dims the satellites down to an apparent brightness level of 6.8 magnitude (the brightest stars are 1st magnitude and the faintest stars that we can see with our eyes are about 6th magnitude).

This is still not dim enough to meet standards outlined by the Satellite Constellations 1 (SATCON1) workshop in 2020, a gathering sponsored by the NOIRLab (National Optical-Infrared Astronomy Research Laboratory) and the AAS (American Astronomical Society) to bring together astronomers, policy makers, and other experts to discuss the impact of large satellite constellations on astronomy and society. The study group called for all LEO satellites to be at 7th magnitude or fainter, which would largely decrease their negative impact on the astronomical observations.

Further, the authors note that their study is specific to ZTF. Like ZTF, the Vera C. Rubin Observatory will also survey the sky nightly, but due to its more sensitive imager (an 8-metre class wide-field ground-based telescope), astronomers predict that it may be more negatively affected by satellite streaks than ZTF.