SAN
FRANCISCO – As NASA's new Orbiting Carbon
Observatory (OCO) 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 (JPL), 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 satellites flying 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.
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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