Now, researchers are on the cusp of producing these maps not every decade, but every night. This will be a sea change in the way scientists study light at night, and how our dependence on it is changing our planet.
“To be honest, I don’t know what we’ll see. … Will we see just a steady glow all the time from cities? Or will we literally see cities blinking?” asks John Barentine, the program manager for the International Dark Sky Association. “The maps tell us so much about skyglow, but we have also learned an awful lot about humanity just by studying the patterns of where that light appears at night.”
Since November 2011, that data has come primarily from the new satellite, called Suomi NPP, which is an amalgam of a meteorologist’s name and an acronym soup that comprises NASA, NOAA, and the U.S. Department of Defense. It’s a weather satellite first and foremost, but it also has an instrument called the Visible Infrared Imager Radiometer Suite, which looks at Earth in visible light.
Researchers at NASA use VIIRS data to make maps of the Earth at night, but it’s not really designed for this purpose, explains Miguel Román of NASA’s Goddard Space Flight Center in Greenbelt, Md., who leads the team. “When you have an instrument like this, you end up with like a Swiss Army knife. You can do a lot of things, but you’re not particularly good at doing any of them,” he says.
The satellite’s first “black marble” image of the world, published in 2012, required 5,247 orbits and 20 terabytes—eight times the volume of the Library of Congress. It was a heavy lift, because the satellite isn’t really taking pictures. Rather, it is sensing different wavelengths of light to study multiple components of the Earth and its systems. It collects light in 22 different wavelengths, from green to near-infrared, much in the way a digital camera does—only its pixels capture stretches up to a half-mile wide. The pixels stream down from space into a ground station in Svalbard, Norway, and computer codes turn them into geophysical variables. Scientists write the codes based on the laws of nature and light, to the extent that we understand them.
“You have to have a science team that is constantly developing the theory, and understanding the behavior of that instrument. There’s a lot of work, and jobs, to ensure that we are not just taking pretty pictures that might end up as your iPhone background,” Román says.
His team manually generated the first black-marble maps, choosing pictures without clouds and moonshine. It took two months of work, and many of the satellite’s images weren’t usable. “With the moon, you’d just say, ‘Sorry, the moon is too hard to take out, and it’s too bright, so I can’t show whether that place is a city or it’s just moonshine.’ That’s when you have decided 50 percent of your data is trash,” Román says. “We said, ‘No. We don’t have 300 engineers building this satellite and launching it into space to trash it.’”