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November 23, 2020

Our distant ancestors were closer to nature than most people living today. Because of outdoor lighting, urbanites are especially deprived of a clear view of the night sky. The Milky Way, a common sight just a few centuries ago, is not visible to much of the world's population. It's estimated that a third of humanity, including nearly 80% of North Americans and 60% of Europeans, can't view the Milky Way.[1]

Milky Way as seen over Bontecou Lake, New York

A panorama of the Milky Way as seen over Bontecou Lake, Dutchess County, New York. Bontecou Lake is in the Mid-Hudson Region of the Hudson Valley, about 80 miles north of New York City and its light pollution. (Wikimedia Commons image by Julian Colton.)

A major contributor to light pollution is sodium vapor lamp street lighting, in which an intense yellow-orange spectral lines of sodium vapor (actually a doublet at 588.9950 and 589.5924 nanometers) is excited in a plasma. These lines, called the sodium D-lines, are the basis of the sensitive flame test for sodium used by chemists.

Since the light emission from such street lights is concentrated at these optical wavelengths, their interference with astronomical imaging is mitigated by astronomers' use of ooptical filters. Sodium vapor street lighting has been superseded by much more energy efficient LED lighting. Although it's easier to craft LED fixtures that direct their light downward, where it's needed, rather than upwards towards the sky, the spectrum of most such lighting makes it more difficult to filter.[2]

Map of North American light pollution (NOAA)

Map of North American light pollution created by the US National Oceanic and Atmospheric Administration. Bright regions are correlated with high population density.

As I remarked in an earlier article (Light Pollution, February 4, 2013), there's also a correlation between a country's night illumination and its economic development.

While Japan and South Korea show strong illumination on such maps, North Korea is essentially dark.

(Map from the NOAA website.[3])

A sparsely populated island is great place to site an observatory away from light pollution. The Roque de los Muchachos Observatory (ORM) is located in the Canary Islands, a group of Spanish islands in the Atlantic Ocean about 60 miles (100 kilometers) west of Morocco. This observatory is second only to the Mauna Kea Observatory, Hawaii, for clear optical and infrared astronomy in the Northern Hemisphere. While suitably isolated from mainland light pollution, the islands are still inhabited by about two million people. The island of Tenerife, which also hosts the Teide Observatory, is home to nearly a million people, and it's about about 85 miles from ORM.

Spain has a Law for the Astronomical Quality of the Instituto de Astrofísica de Canarias (IAC) Observatories that requires municipalities in northern Tenerife, and on the island of La Palma where ORM is located, to control their light pollution.[1] In one case, the Tenerife city of Puerto de la Cruz is replacing most of its street lights with LED lights that have optics that filter out blue light and direct their light downwards.[1] Blue light creates a form of light pollution known as sky glow.[1]

Astronomy has benefited from the Space Age through its many space observatories, of which the Hubble Space Telescope is the best known. The same space technology that enables such observatories has also produced a few problems. While the occasional streaks of light from early satellites and spacecraft annoyed optical astronomers, there was a greater threat to radio astronomers. orbiting radio frequency sources, such as Direct broadcast satellites, contain powerful radio transmitters that beam signals down to Earth. As early as 1960 it was realized that there would be interference from out-of-band and spurious signals, as well as the direct signals, from these transmitters.[4]

An example of radio-frequency interference on a radio astronomical observation.

An example of radio frequency interference on a radio astronomical observation. These data are for the same OH/IR star observed at 1612 MHz by the Very Large Array. An Iridium satellite was about 22 degrees from the star in the right image. (Image from fig. 4.1 of Ref.4, used with permission of Prof. Gregory B. Taylor, University of New Mexico.)

Things have become significantly worse for astronomers with the recent launches of multiple low Earth orbit satellites (LEOS) into large Satellite constellations to provide low-latency satellite internet to remote regions not connected by high speed cable. While such connection has been of great benefit to many people,[5] this large population of satellites has caused alarm among both optical and radio astronomers.[6-10]

The International Astronomical Union (IAU) expressed concern about the threat to astronomy by constellations of communication satellites in June, 2019.[9] Such satellites reflect sunlight and they create bright streaks on astronomical images that interfere with astronomical observations.[6] These streaks are worst in the hours around sunset and sunrise when the satellites are not yet shadowed from the Sun by the Earth.[6] The number of illuminated satellites when the Sun is 18 degrees below the horizon would approach a thousand, with the numbers decreasing as night progresses.[9]

The problem will only become worse, since there will be tens of thousands of satellites launched in the coming years to provide broadband Internet globally.[6] SpaceX (Hawthorne, California) has already launched more than 650 of its planned 12,000 Starlink satellites.[6] London-based OneWeb plans a constellation of 48,000 satellites.[6] The Amazon Kuiper Systems has US-government approval to launch 3,236 satellites.[6] Satellites orbiting at elevations more than 600 kilometers will cause a major problem, since they are sunlit longer, and OneWeb's planned constellation orbiting at 1,200 kilometers is in this category.[6]

Estimates are that the trails of constellation satellites will be bright enough to saturate detectors on large telescopes.[9] The International Astronomical Union notes that there are no internationally agreed rules or guidelines on the brightness of orbiting man made objects.[6,9] However, satellite operators are aware of the problem, and SpaceX, creator of the Starlink constellation and the first company to start launching such satellites, has been at the forefront of mitigation strategies.[6] When the Starlink constellation is complete, it will have about 12,000 satellites, hundreds of which will be visible at a time.[6]

Figure caption

Starlink satellites were imaged shortly after launch in November, 2019, with a 4-meter telescope at the Cerro Tololo Inter-American Observatory by astronomers Clara Martínez-Vázquez and Cliff Johnson.

The tiling effect is caused by gaps between CCD imaging chips.

(NSF National Optical-Infrared Astronomy Research Laboratory/CTIO/AURA/DELVE image, released under a Creative Commons License. Click for larger image.)

SpaceX has tried several things to reduce Starlink satellite visibility, such as changing the orientation the satellites so they direct less reflected sunlight to the ground. They even painted one satellite black, but that disturbed the thermal management of its electronics.[6-7] Presently, the satellites are fitted with a sunshade that blocks sunlight from reflecting from their antennas, which are the main sources of light.[6-7]

Satellite constellations can interfere with radio telescope signals. While such telescopes as the Square Kilometre Array in South Africa are in a radio-quiet zone that forbids radio emitters such as cellphones, it isn't protected from the thousands of radio-emitting satellites.[10] Of main concern are the range of frequencies used to detect organic molecules and water in space.[10] Starlink satellites emit in a frequency range of 10.7-12.7 gigahertz, and its predicted that molecules such as the amino acid, glycine, will soon be beyond detection.[10] Although the International Telecommunication Union has reserved some frequencies for radio astronomy, modern radio telescopes now operate over most of the frequency spectrum.[10]

Tony Beasley, Director of the National Radio Astronomy Observatory, is quoted in Science as saying,
"SpaceX is legally transmitting inside one of their bands and there are going to be impacts for anyone trying to do radio astronomy... These spectrum allocations represent the goals and intent of society. We make [them] to enable commerce and to enable defense and all kinds of activities. We have to come to a solution that satisfies all these to some extent."[10]


  1. Mark Halper, "Canary Islands installs street lights that protect night skies," LEDs Magazine, July 31, 2018.
  2. Lamp Spectrum and Light Pollution, Flagstaff Dark Skies Coalition.
  3. Light Pollution - Artificial Sky Brightness, National Oceanic and Atmospheric Administration Web Site.
  4. Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses (2007), Chapter: 4 Technical Aspects of Protection for the Scientific Use of the Radio Spectrum, The National Academies Press, Washington, DC., https://doi.org/10.17226/11719.
  5. Eric Ralph, "SpaceX Starlink aids Native American tribe: 'It catapulted us into the 21st century'," Teslarati, October 8, 2020.
  6. Alexandra Witze, "How satellite ‘megaconstellations’ will photobomb astronomy images," Nature News,August 26, 2020, doi: 10.1038/d41586-020-02480-5.
  7. Anthony Tyson and Joel Parriott, "Dark skies and bright satellites," Science, vol. 369, no. 6511 *September 25, 2020),, p. 1543, DOI: 10.1126/science.abe8973.
  8. C. Walker, et al., "Impact of Satellite Constellations on Optical Astronomy and Recommendations Toward Mitigations," Bulletin of the American Astronomical Society, vol. 52 (2020), https://doi.org/10.3847/25c2cfeb.346793b8.
  9. Understanding the Impact of Satellite Constellations on Astronomy, International Astronomical Union Press Release iau2001, February 12, 2020.
  10. Daniel Clery, "Starlink already threatens optical astronomy. Now, radio astronomers are worried," Science, October 9, 2020.

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