As NASA's new Orbiting Carbon Observatory moves closer to its planned launch next week, the team responsible for the spacecraft faces enormous challenges to fly the first-ever probe to map carbon dioxide levels across.
"OCO will be making one of the most challenging measurements of any atmospheric trace gas that has ever been made," said Charles Miller, OCO deputy principle investigator at the Jet Propulsion Laboratory, in Pasadena Calif.
OCO is poised to launch from California's Vandenberg Air Force Base atop a Taurus XL rocket on Feb. 24 to begin its carbon dioxide-hunting mission. The spacecraft will use three high-resolution spectrometers built by Hamilton Sundstrand Sensor Systems of Pomona, Calif., to measure carbon dioxide and oxygen molecules in the Earth's atmosphere based on the way those molecules absorb sunlight.
That data will then be used to show the specific regions where natural and man-made sources are producing carbon dioxide as well as highlighting areas, called sinks, where oceans and plants are removing carbon dioxide from the atmosphere. OCO will circle the Earth every 99 minutes, mapping the globe every 16 days from its near-polar, sun-synchronous orbit.
Hunting Earth's carbon
Through various activities including forest fires and the burning of fossil fuels, sources on Earth emit approximately 8 billion tons of carbon every year. But only half of that carbon remains in the atmosphere. The other half is hidden — absorbed by Earth's oceans, plants and soils, said Anna Michalak, OCO science team member from the University of Michigan in Ann Arbor.
"The relative fraction of carbon that is staying in the atmosphere versus going into plants and oceans varies dramatically from year to year," Michalak said. "We want to understand why plants and oceans are taking up as much carbon as they are ... so we can predict how they will behave in the future."
Once the spacecraft is in orbit, the OCO management team will spend up to 13 weeks confirming that the spacecraft and its subsystems are functioning properly, said Ralph Oscillo, OCO deputy project manager at JPL. Once OCO is found to be in good working order, the spacecraft will be maneuvered into position as the lead spacecraft in the A-Train, a constellation of five Earth observing satellitesflying in formation around the globe. After that, OCO scientists will spend months evaluating the initial data being returned by the onboard instrument to ensure the spectrometers are fully calibrated. Oscillo said science operations would begin in October or November when the OCO team plans to begin providing data showing the regional distribution of carbon dioxide.
Big challenges in tiny changes
The enormous challenges inherent in the mission are due to the small variations in the amounts of atmospheric carbon dioxide. Those levels range from a maximum of 362 carbon dioxide molecules in 1 million molecules of air, to a minimum of 351 carbon dioxide molecules in 1 million air molecules - a 0.3 percent difference, said David Crisp, principle investigator for the OCO at JPL.
The variation, while small, has big implications for scientists studying climate change. For OCO to do its job, it must accurately measure the minuscule changes in atmospheric carbon dioxide.
Slideshow: Month in Space: April 2013 To verify that OCO's measurements of atmospheric carbon dioxide concentrations are accurate, data gathered from the spacecraft will be compared with measurements obtained by ground stations, tall towers and airborne instruments as part of the National Oceanic and Atmospheric Administration's carbon dioxide research program, Michalak said.
OCO instrument data also will be compared with observations made by Japan Aerospace Exploration Agency's Greenhouse Gases Observing Satellite — or GOSAT — which was renamed Ibuki following its Jan. 23 launch. Ibuki is designed to measure carbon dioxide levels around the world. However, the two spacecraft carry very different instruments. Ibuki carries an interferometer designed to detect atmospheric methane and carbon dioxide. OCO employs a gradient spectrometer designed specifically to measure carbon dioxide, Crisp said.
"The objectives of the two missions are different," Crisp said. "GOSAT [Ibuki] is looking for carbon dioxide sources for treaty monitoring purposes. Those sources are a little easier to see than sinks, because sources are fairly intense and localized. We are looking for sinks which are much weaker, more distributed and harder to find."
The two satellites will cross orbit paths several times a day. "That gives us an opportunity to take nearly simultaneous measurements at a few points on the globe every day so the teams can compare the results," Crisp said.
The NASA budget includes $278 million for the entire OCO mission, which is scheduled to last two years. If the primary mission is successful, NASA officials could extend OCO's science operations well beyond 2011. The spacecraft carries enough fuel to remain in orbit for five to 10 years, Crisp said.
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