Jun 06, 2014

NASA Satellite to Map Carbon Emissions Using Plant 'Glow'

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WASHINGTONNASA will try again this summer to launch an Earth observation satellite that will use a novel fluorescence sensor to help measure atmospheric CO2 and ultimately map carbon sources and carbon sinks on Earth.

A previous launch failure provided several unexpected bonuses in the run up to the scheduled July 1 launch of the $467.5 million Orbiting Carbon Observatory-2. The first attempt to launch the carbon-detecting sensor in 2009 did not reach orbit when a payload fairing, the clamshell-shaped cover protecting the satellite, failed to separate.

That failure was “heartbreaking,” Ralph Basilio, the Jet Propulsion Laboratory’s program manager for the Orbiting Carbon Observatory, said in an interview. “I’m an engineer. I was trained to solve problems, and we walked away with nothing to solve.”

NASA quickly decided to fund a second satellite, OCO-2 (a “carbon copy” of the first, Basilio joked), which is scheduled for launch from Vandenberg Air Force Base, Calif., on July 1 into a near-polar orbit. The JPL team working on the first satellite also remained in place to tweak the design for OCO-2.

The intervening five years gave Basilio’s team the gift of time to mitigate technical risks, including switching to a different booster, the Delta II, and upgrading the payload’s reaction wheels, mechanical steering devices used to control a satellite’s attitude in space. (Last year’s failure of reaction wheels on the Kepler Space Telescope prevented mission planners from pointing the planet-hunting satellite.)

The launch failure also gave OCO scientists a chance to work with managers of the Japanese Greenhouse Gases Observing Satellite (GOSAT) launched in 2009. GOSAT measures atmospheric CO2 and methane. As NASA mission managers readied the new payload, JPL research scientist Christian Frankenberg spotted something hidden in the Japanese data: solar-induced fluorescence, or glow, from chlorophyll in plants. Measuring fluorescence in the lab is relatively straightforward. Space-based observations were only possible over the ocean.

The glow is a signature of photosynthesis, the process by which plants convert sunlight into chemical energy. As vegetation absorbs sunlight, some of the light is dissipated as heat while some radiation is re-emitted at longer wavelengths as fluorescence.

The rub has always been that the brightness of our sun makes it impossible to observe this glow over land. On a hunch, Frankenberg applied a technique called Fraunhofer lines, the black lines of the electromagnetic spectrum where there is no light. The German optician Joseph Fraunhofer applied this insight to develop the diffraction grating used in most spectrometers.

The Japanese satellite uses a Fourier transform spectrometer. Frankenberg figured out a way to leverage Fraunhofer lines to design a grating spectrometer for OCO-2 that will allow the satellite for the first time to measure solar-induced fluorescence over land.

“Fluorescence data, when combined with the observatory’s atmospheric carbon dioxide measurements, will increase the value of the OCO-2 mission,” Basilio noted. The hoped-for result will be a treasure trove regional data on carbon sources and sinks collected along the approximately 10-kilometer swaths of the Earth covered by the NASA carbon observatory.

One reason is that simply measuring atmospheric CO2 doesn’t directly reflect what’s going in the ground. Hence, the mission’s new spectrometers will provide climate scientists with what Basilio called a “geophysical record” of what’s happening in the Earth’s carbon cycle.

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