Starlink's Impact on Astronomy: A Deep Dive into Satellite Streaks with the Zwicky Transient Facility

The deployment of large satellite constellations like SpaceX's Starlink is revolutionizing internet access, but it also poses new challenges for ground-based astronomy. A recent study utilizing archival images from the Zwicky Transient Facility (ZTF) at Caltech's Palomar Observatory sheds light on the increasing presence of satellite streaks and their potential impact on astronomical observations.

The Zwicky Transient Facility: A Sentinel of the Night Sky

The Zwicky Transient Facility (ZTF) is a vital instrument for scanning the entire night sky every two days. Funded by the National Science Foundation (NSF) and an array of international partners it catalogs transient objects—those that change in brightness over time—such as supernovae, near-Earth asteroids, and other celestial phenomena. This makes it an ideal tool for assessing the impact of the growing number of artificial satellites. ZTF data are processed and archived by Caltech's IPAC astronomy center. NASA supports ZTF's search for near-Earth objects through the Near-Earth Object Observations program.

The Starlink Constellation and Its Effects

Since 2019, SpaceX has launched nearly 1,800 Starlink satellites into low-Earth orbit (LEO) at altitudes of approximately 550 kilometers. These satellites, designed to provide global internet coverage, reflect sunlight and appear as satellite streaks in telescope images. Astronomers have voiced concerns that these streaks could interfere with scientific observations, particularly those related to identifying potentially hazardous asteroids.

Key Findings of the ZTF Study:

• Increased Streak Frequency: The study, published in The Astrophysical Journal Letters, reveals a significant increase in satellite streaks. Over 5,301 streaks appeared in ZTF images taken between November 2019 and September 2021.
• Twilight Observations Most Affected: Streaks are most prominent in twilight observations, crucial for discovering near-Earth asteroids that appear close to the sun. 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.
• Growing Impact: The study lead author Przemek Mróz, a former Caltech postdoctoral scholar, notes a sharp increase in affected images. "In 2019, 0.5 percent of twilight images were affected, and now almost 20 percent are affected."
• Future Projections: Scientists anticipate that nearly all ZTF images taken during twilight will contain at least one streak, especially as the Starlink constellation expands to its projected 10,000 satellites by 2027.
• Impact on Non-Twilight Images: While Starlink satellites are not expected to affect non-twilight images, Mróz says that "if the satellite constellation of other companies goes into higher orbits, this could cause problems for non-twilight observations".

Mitigating the Impact: Solutions and Challenges

Despite the rise in satellite streaks, the ZTF team emphasizes that science operations have not been severely hampered. According to Tom Prince, the Ira S. Bowen Professor of Physics, Emeritus, at Caltech, a single streak affects less than one-tenth of a percent of pixels in a ZTF image.

Potential Mitigation Strategies:

• Software Solutions: Software can predict satellite locations, allowing astronomers to avoid scheduling observations when satellites are in the field of view. Software can also assess the potential impact of passing satellites and mask or reduce the negative effects of streaks in post-processing.
• Satellite Visors: SpaceX has implemented visors on its Starlink satellites to reduce their brightness. The ZTF study found that these visors reduce satellite brightness by a factor of about five, bringing them down to an apparent magnitude of 6.8.

Ongoing Challenges:

• Brightness Standards: Even with visors, Starlink satellites do not yet meet the brightness standards (seventh magnitude or fainter) recommended by the Satellite Constellations 1 (SATCON1) workshop, a gathering of astronomers, policymakers, and industry experts organized by the NOIRLab (National Optical-Infrared Astronomy Research Laboratory) and the AAS (American Astronomical Society).
• Next Generation Observatories: The study's authors highlight that the impact of satellite streaks may be more significant for future observatories like the Vera C. Rubin Observatory in Chile, which has a more sensitive imager than ZTF.

Collaboration and the Future of Astronomy

The increasing presence of satellite constellations in Earth orbit necessitates ongoing dialogue and collaboration between astronomers, satellite operators, and policymakers. The ZTF study provides valuable data for understanding the current impact of Starlink satellites and informing strategies to mitigate potential disruptions to astronomical research. Further research, technological advancements, and responsible space practices are crucial to preserving our ability to explore the cosmos.