Originating Technology/NASA Contribution

On December 7, 1972, roughly 5 hours and 6 minutes after launch, the crew of Apollo 17 took one of history’s most famous photographs. The brilliant image of the fully illuminated Earth, the African and Antarctic continents peering out from behind swirling clouds, came to be known as the “Blue Marble.” Today, Earth still sometimes goes by the Blue Marble nickname, but as the satellites comprising NASA’s Earth Observing System (EOS) scan the planet daily in ever greater resolutions, it is often the amount of green on the planet that is a focus of researchers’ attention.

NASA satellites offer insight on topics like climate change and disease outbreaks through data like this Normalized Difference Vegetation Index image showing vegetation in Africa in August 1994.
Earth’s over 400,000 known plant species play essential roles in the planet’s health: They absorb carbon dioxide and release the oxygen we breathe, help manage the Earth’s temperature by absorbing and reflecting sunlight, provide food and habitats for animals, and offer building materials, medication, and sustenance for humans. As part of NASA’s efforts to study our own planet along with the universe around it, the Agency’s EOS satellites have been accumulating years of valuable data about Earth’s vegetation (not to mention its land features, oceans, and atmosphere) since the first EOS satellite launched in 1997. Among the powerful sensors used is the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the NASA Terra and Aqua satellites. MODIS sweeps the entire Earth every few days, beaming back information gathered across 36 bands of visible and infrared light, yielding images that let scientists track how much of Earth is green over the course of seasons and years.

Monitoring the density and distribution of vegetation on Earth provides a means of determining everything from the impact of natural and human-induced climate change to the potential outbreak of disease. (Goddard Space Flight Center and U.S. Department of Defense researchers have determined, for example, that vegetation density can be used to pinpoint regions of heavy rainfall in Africa—regions ripe for outbreaks of rainfall-correlated diseases like mosquito-borne Rift Valley fever.)

While the Space Agency is continually seeking to upgrade the power and scope of its satellite sensors, it is also finding ways to bring that potent information-gathering capacity down to Earth. Scientists at Stennis Space Center developed one such tool that is placing some of those sensor capabilities in the hands of farmers and agricultural researchers on the ground.


In 1998, Mike Thurow, president of Plainfield, Illinois-based Spectrum Technologies Inc., met with Stennis scientists and learned about a new technology the Center had recently patented: a hand-held plant chlorophyll meter developed from Stennis work on satellite sensors. The meter measures two wavelengths of light—700 nanometers (nm) and 840 nm—to determine a plant’s chlorophyll content. Chlorophyll, the pigment found in green plants and algae that allows for the production of energy from light (photosynthesis), comes in two varieties: a and b. Chlorophyll a absorbs 700-nm light, while 840-nm light is unaffected by the chlorophyll but helps indicate the reflectiveness of a leaf’s physical surface. The NASA meter compares the ratio of 700-nm and 840-nm available light to the ratio of the same wavelengths of reflected light, arriving at a chlorophyll index value. Chlorophyll levels are strong indicators of health in green plants; the meter’s Stennis inventors demonstrated that, by detecting chlorophyll amounts, their technology could reveal plant stress caused by factors like heat, insects, disease, and lack of water or nutrients—up to 16 days before visible signs emerged.

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