Photo by Donald R Pettit, NASA

Tokyo at night as seen from the International Space Station. CLICK HERE for an slideshow of nighttime cities

11 September 2007—The size and shape of the “human footprint” on Earth might best be seen from space—and in the dark, say scientists. Christopher Elvidge, a researcher at the National Oceanic and Atmospheric Administration's National Geophysical Data Center in Boulder, Colo., and his collaborators have come up with a concept for a satellite mission, called NightSat, which would take pictures of Earth's surface at nighttime with a high-resolution, low-light camera. The purpose is to obtain a complete, cloud-free map of all nighttime lights on Earth on an annual basis.

Lights serve as a kind of geographic marker of human activity on the planet: they tell us information about the location of settlements, how land is used between urban areas, how dynamic population growth and migration is, and what impact urban settlements have on a region's natural resources, weather, and climate. Analyzing changes in nighttime lights over time can provide a model of how human civilization is progressing on the planet and how its relationship to the environment is affected. The concept is simple, so why haven't nighttime lights been mapped already?

The main issue is technology. Earth observation systems, most notably the Landsat program, have taken many years just to gather daytime images of all the geographic features on the planet, and many of them have been ill equipped to handle low-light photography. According to Elvidge, the only satellite sensor currently collecting data on global nighttime lights is the U.S. Air Force Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS). But with a resolution of just 2.7 kilometers from an orbit 830 km up, OLS does not provide adequate spatial resolution for the study of human activity. In addition, the camera has a limited amount of memory and is not self-calibrating, so it's impossible to make accurate brightness comparisons between cities.

To determine the necessary specs for NightSat, Elvidge and his collaborators from NASA and academic institutions tested data from two moderate- to high-resolution sources. The first was from a Kodak/Nikon DCS-760 camera aboard the International Space Station during a mission from November 2002 to May 2003. From an altitude of 390 km, the camera produced 60-meter resolution images, but some images were blurred by a mismatch between the camera exposure time and the velocity of the spacecraft. Nevertheless, the photographs demonstrated the ability to take moderate-resolution multicolored images from a satellite orbiter.

The second source was a Cirrus DCS camera capable of 1.5-meter resolution when it was flown on an aircraft at an altitude of about 14 km. The Cirrus camera flew over Las Vegas and Los Angeles in September 2004. The 1.5-meter-resolution data were then used to simulate images at 25-, 50-, 100-, 200-, and 742-meter resolutions—the last is the resolution of a camera on the next available U.S. Earth-sensing satellite, NPOESS (National Polar-orbiting Operational Environmental Satellite System), set for launch in 2009.

The conclusion: in order to retain relevant information about major streets and building clusters, NightSat would need a camera with a resolution between 50 meters to 100 meters. Additional requirements would be that the orbiter it was on should revisit locations about every 20 days and that it be sensitive to multiple types of light (sodium-based, mercury vapor, incandescent, halogen, metal halide, fluorescent, and so on) and infrared. (The NightSat mission concept will be detailed in a paper to be published later this year in the International Journal of Remote Sensing.) Given these requirements, just how much would a NightSat project potentially cost?

The most recent Landsat satellite, Landsat 7, launched in April 1999, came in at over US $800 million, though many feel the next mission, the Landsat Data Continuity Mission (LDCM), will cost substantially less. Elvidge says the cost of the NightSat mission needs to be roughly a tenth that of the Landsat mission to have any chance of being approved. So it must be built using more or less “off the shelf” components. Still, Elvidge feels that NightSat is well worth the effort and hopes it will become a reality. It might help that a soon-to-be-published report by the U.S. National Research Council's (NRC) Space Studies Board recommends that Earth-observation satellites being planned and deployed by NASA and NOAA include systems—like NightSat—that monitor the impact of human activity on Earth.

“I think [NightSat] would be the most detailed mapping of the spatial distribution of humans and human activities that we've ever had,” Elvidge says. “There would be nothing else like it.”