Somewhere That’s GreenPrint
By Jennifer Henderson
Russia’s Sputnik, the first orbiting satellite, made Americans wonder what this small metallic eye could see from space. Not very much, as it turned out. Today, however, specialized instruments in space survey more than we might imagine, including subtle patterns of plant growth that no human eye could easily detect.
Since the 1980s, scientists at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, have monitored world food production with satellite remote sensors. Data from these satellites are fed into an algorithm called the Normalized Difference Vegetation Index (NDVI), which derives a number that correlates to a particular color on the spectrum of light. The contrast between areas where plants have absorbed visible or reflected near-infrared light produces images that reveal the degree of photosynthesis taking place within plants. In short, NDVI is a measurement of “greenness” or density of vegetative growth in a specific location. These near-real-time images are averaged across several days for more accuracy and then compared with those from previous months or years, allowing scientists to identify areas of drought or famine and even to predict outbreaks of infectious diseases that are contingent on changes in climate.
In 2006, researchers, including Assaf Anyamba, a geographer and remote sensing scientist with the University of Maryland-Baltimore County, used NDVI data and other variables to predict an outbreak of Rift Valley Fever in eastern Africa. This “risk map” gave health-care workers a six-week warning to prepare for the disease, which is carried by mosquitoes and transmitted to humans by mosquito bites or through contact with infected livestock. Where there’s an increase of green density, there’s more water, and more water means a greater risk for mosquito-borne disease. “Without such systematic, continuous Earth-system measurements from satellites, we would not be able to translate the information into outbreak predictions,” Anyamba says.
One surprising application is in archaeology. By comparing the light-reflecting differences between sand, soil, and rock formations, archaeologists believe they have identified ancient ruins and historical excavations in Troy, the ancient city in Turkey.
Jennifer Henderson is a Virginia Tech doctoral student in science and technology studies.