In their studies of global climate change, NASA's science teams are using Earth-observing satellites to map everything from floods and wildfires to El Ninos and phytoplankton. They track sea ice breakups and make daily scans of temperatures from the bottom of the troposphere to the top of the stratosphere. Wonder where smog-producing ozone is concentrated? Data from NASA's spacecraft reveal the ugly picture.

But the space agency cannot draw a global profile of carbon dioxide (CO2). This abundant greenhouse gas is increasing and, as it does, average global temperatures rise with it.

In a 2007 report, the U.S. Climate Change Science Program pointed to a rough balance between the amount of naturally formed carbon that's absorbed from the atmosphere and what's released into it. Nature's carbon is far more abundant than what humans produce, but man-made sources are growing. Between 1751 and 2003, 306-626 billion tons of carbon were added to the atmosphere as carbon dioxide, mostly through fossil fuel combustion, cement manufacturing and forest clearing. The range in those numbers underscores the uncertainties of current carbon sampling methods.

Scientists posit that about 60% of human emissions are absorbed. The rest stays in the atmosphere. While they say absorption is split 50:50 between oceans and soil, they are uncertain where the carbon is going when on the land. Clearly, the political drive to battle global warming will be on firmer scientific ground if concrete facts can be given.

To this end, NASA is set to add another satellite to its inventory. About the size of a phone booth, the Orbiting Carbon Observatory isn't even as big as some of the instruments on the space agency's larger Earth-observing fleet.

"We could put the spacecraft inside the TES instrument on Aura," quips OCO Principal Investigator David Crisp of NASA's Jet Propulsion Laboratory. His reference is to the Tropospheric Emission Spectrometer on a school-bus sized atmospheric sleuth that the space agency launched in 2004.

Built by Orbital Sciences Corp. in Dulles, Va., the 972-lb. OCO will become the newest member of the Afternoon Constellation, or A-Train, of Earth-observing spacecraft funded by NASA, France and Canada.

It is set for a 1:53 a.m. PST launch on Feb. 23 by an Orbital Sciences XL 3110 booster from Vandenberg AFB, Calif. The solid-fueled Taurus will raise OCO into an initial 398-mi.-high orbit for a 10-day systems and instrument checkout. Over the next 20 days, the spacecraft's own propulsion system will raise it to an altitude of 438 mi. where it will be inserted at the front of the A-Train.

The observatory will be the train's sixth member, followed, in order, by NASA's Aqua and CloudSat, France's Calipso and Parasol, with Aura as the caboose.

Their Sun-synchronous orbit has a near polar inclination. They circle the Earth every 99 min. and pass the same ground track every 16 days, crossing the equator at 1:30 p.m. local time. The A-Train stretches 3,800 mi. in space, but orbital speeds of 4.7 mps. mean they pass the same ground spot so rapidly their observations can be coordinated. OCO will be only 3.3 min. ahead of Aqua.