Researchers on the ground have combed through a trove of images taken by orbiting astronauts to reveal unprecedented details about light pollution streaming from Earth's major cities.
Since the Mercury missions in 1960s, NASA astronauts have taken more than a million hand-held pictures of Earth. With the introduction of digital time-lapse imaging from the International Space Station (ISS), in the last decade the collection grew rapidly. Many of these NASA and ESA images are cataloged and publicly released in high-resolution on the Gateway to Astronaut Photography of Earth. Visitors can search photos of specific locations on the world map or search by coordinates.
More than a tool for public outreach, the astronauts' images have scientific value in mapping the distribution and intensity of lights visible at night. Scientists from the Universidad Complutense de Madrid (Spain) and the Cégep de Sherbrooke (Canada), together with members of the public, have worked on a project called Cities at Night.
In 2012, during a conference in Beijing, I met Alejandro Sanchez de Miguel, the young leading project scientist from Madrid, who shared this idea and the first results. The immense potential of the program was very evident, and we talked about a possible citizen-science project based on it.
Now, after three years of work, the team officially announced the program and its early results at the recent General Assembly of the International Astronomical Union. The aim is to produce a global color map of the entire Earth at night. The first released version of the map presents selected densely populated areas.
Revolution in Resolution
You've probably seen maps of Earth's night lights derived from Defense Meteorological Satellite Program images. These show resolutions of about 1 km per pixel, good enough to resolve towns and villages. NASA's Night Lights, known as the Black Marble and introduced in 2012, provides the improved resolution of 750 meters (a half mile). These maps have been great tools for educators and science communicators, but they not convey color or spectral information and do not resolve fine details necessary for advanced studies on light pollution.
But Earth-watchers were quick to realize the potential of images taken by astronauts aboard the International Space Station. While DMSP satellites orbit at altitudes near 830 km, the ISS orbits at roughly half that. Also, astronauts have access to an arsenal of lenses that range from an 8-mm Nikon fisheye to 1,200-mm super telephoto.
Like astrophotographers on Earth, challenged by constantly moving celestial objects (due to Earth's rotation) and long exposures, ISS photographers are also limited by the station's rapid velocity over the ground below (8 km per second). Nighttime photography demands longer exposures, higher sensitivities, and faster lenses. In order to freeze the motion and capture a sharper image, astronaut Don Pettit fashioned a barn-door tracker from available parts onboard, which enabled the first motion-compensated night time imagery from the ISS. (Pettit is also an avid stargazer and astrophotographer — both from his homes in Texas and in orbit.) ESA's NightPod, a motorized tripod installed in 2012, compensates for the station's speed and the motion of Earth below. It was built to Pettit's specifications at the request of ESA astronaut Andre Kuipers. Learn more about Don Pettit and astronaut photography on the short video "The ISS Image Frontier".
Thanks to these improvements, some images taken by astronaut aboard the Space Station can resolve details down to only a few meters per pixel resolution. According to Sanchez, on rare occasions and with advanced processing methods, the resolution can even reach only a meter per pixel — good enough to resolve individual streetlights. A great example is the Valencia (Spain) image with 6-m resolution.
However, the typical resolution range of astronaut images used in the project are 20 to 200 m per pixel (for example London at night), still far better than previous maps. The Cities at Night team has worked with more than 130,000 images since last year. Most of these were single photos made with lenses of 40-mm focal length and higher.
While astronauts' photography does not cover the entire Earth, the ISS's orbital inclination of 52° gives them an excellent stage for observing most populated areas of the world. (You can explore hundreds of located ISS images on this world map.) Thanks to this resolution power the team were able to estimate the total cost of street light energy consumption in the European Union which is more than 6 billion euros per year.
Monitoring Light Pollution
NASA's Johnson Space Center provided images in their original (Raw) format, which enabled the team to analyze data in separate color channels (RGB) and access valuable spectral information. Sanchez and his colleagues could also calibrate the cameras' spectral response, make flat fields for each lens, and stitch neighboring images together to create mosaics, starting with some of the world's capitals.
For example, a comparison of images of Milan taken in 2012 and 2015 are very revealing. The newer image shows how the city center is now dominated by brighter, bluer LED lights, having replaced older (but "warmer") high-pressure sodium streetlighting. This sudden change shows greater light pollution and a shift to wavelengths with greater impact on human health and the environment (See why blue light at night is harmful.) Such environmental monitoring is an important potential of the project.
According to Christopher Kyba (German Research Center for Geoscience), who studies the ecological impact of artificial lights, "The ISS images are currently the only way to effectively study the global transition to solid state lighting (LEDs)." He notes that that these images have many uses beyond monitoring the spread of light pollution (and the consequent loss of starry skies). For example, they can play a role in many aspects of city planning: demographics, economics, ecological effects, conservation, and circadian disruption. They have even been used in studies that trace the causes and incidence of breast cancer and other diseases.
Skyglow and City Lights
The ISS images record light directly straight up into the sky. However, according to studies by Chris Luginbuhl and others, the main cause of the skyglow we see at ground level is light beamed sideways, just above horizontal. On their long glancing path through the atmosphere, photons scatter much more strongly than those directed toward the zenith.
An important part of the new study led by Sanchez is to correlate what's seen in the astronauts' images with the light pollution seen from the ground. The photos do contain evidence of diffuse skyglow, but up to now this had not been measured quantitatively. Prior surveys such as the World Atlas of Artificial Night Sky Brightness indirectly estimated skyglow brightness based on satellite images. However, amateur astronomers now routinely measure skyglow using portable devices such as the Sky Quality Meter or by even smartphone apps. (Learn how to rate your skyglow.) The Cities at Night team implemented some of these ground-based measurements in the project. This space-ground correlation, done for the first time, is an important step in creating more reliable estimates of actual light pollution.
"This project is incredibly important for science and really doesn't cost a lot of money," Kyba says. "It would be a real shame if it's not successful." If you're interested in supporting it, the project recently started a citizen-science effort and crowdfunding on Kickstarter to continue and extend the map.
Here is a (machine-generated) English translation of "Spatial, temporal and spectral variation of light pollution and its sources: methodology and results" by Sanchez and his team.