The fight for dark skies

Recently, a large radio telescope detected low frequency radio waves from dozens of Starlink satellites in low Earth orbit. These unintended signals emanating from the onboard electronics could interfere with astronomical research.

These same satellites are also increasing the brightness of the night sky, which is significantly affecting optical astronomy research.

The culprit in both cases is satellite constellations, which are groups of artificial satellites working together as a system. The number of small satellites in low Earth orbit started growing rapidly in 2019, when companies such as SpaceX and OneWeb began to build the capacity for global internet coverage. Low orbits mean the signal travel times are shorter, but at such a close range it takes a lot of satellites to cover the planet. Because of shrinking launch costs, the satellite business is booming.

The numbers are eye-popping. There are around 10,000 satellites orbiting over our heads, a number that has doubled in the last four years. By 2030, the number is expected to grow to 75,000. SpaceX alone will account for 40,000 of these satellites as part of its Starlink constellation.

Radio astronomers are worried. Quiet radio skies have allowed some spectacular recent discoveries, such as the first image of a massive black hole. The radio interference from Starlink satellites falls near a frequency of 150 MHz, which is protected for astronomy by the International Telecommunication Union. There are no international regulations governing such emissions from spacecraft.

In contrast, terrestrial radio astronomy is well protected. In 1958, the National Radio Astronomy Observatory in West Virginia benefited from federal legislation that declared 13,000 square miles as the National Radio Quiet Zone. In this vast area, there are no cell towers, and the use of mobile phones and WiFi networks is highly restricted. This radio-quiet “oasis” is unique. Other radio observatories have avoided radio interference by choosing sites in remote parts of Australia, Chile and South Africa.

The good news is that the astronomers and the tech companies are talking. In January, the National Science Foundation and SpaceX signed an agreement to limit interference from the burgeoning number of their satellites.

For most of human history, the stars blazed in a dark night sky. But starting with the Industrial Revolution, and accelerating in the past 50 years, the stars have been disappearing as the population has grown and people have moved into cities flooded with artificial light. In the last decade, the night sky in North America and Europe has been getting brighter by 10 percent per year.

Optical astronomers are sounding the alarm about the impact of satellites on their research. Satellites catch the sun’s light when they are near the horizon and leave ugly streaks across deep images made with large telescopes. For the last 20 years, only 3 percent of Hubble Space Telescopes images were marred by satellite trails, but that could grow to 50 percent by the 2030s. Astronomy’s flagship survey tool, the Vera Rubin Observatory, is about to take data. One-third of its images may be affected.

There is some good news on this front too. In the agreement with the National Science Foundation, SpaceX has committed to changing the design and coatings on its satellites to make them invisible to the naked eye and reduce their impact on sensitive astronomical cameras. Meanwhile, astronomers are eying ever-more remote locations, like Antarctica and the far side of the moon.

Radio interference is an esoteric problem, but the loss of dark skies is something everyone can understand. More than 80 percent of the world and 99 percent of European and North American populations live under light-polluted skies. Most people in the industrialized world have never seen the beautiful arc of the Milky Way. A child born today in an urban or suburban setting will only be able to see 100 stars by his or her 18th birthday.

So enjoy, if you can, the roughly 2,500 stars you can see with the naked eye at a truly dark location. Preserving dark and quiet skies is a way to protect an essential part of our cultural heritage.

Chris Impey is a professor of astronomy at the University of Arizona. He is the author of hundreds of research papers on observational cosmology and education and popular books on black holes, the future of space travel, teaching cosmology to Buddhist monks, how the universe began, how the universe will end, and exoplanets. His massive open online courses have enrolled over 400,000 people.