Launch in the fog. Click to enlarge.

At early morning press time, NASA reported that the Observatory had passed its initial health check, established communication with both ground-based and spaceborne links, deployed its solar panels, and was “power positive”—i.e. generating its own power. OCO-2 has a planned operational life of two years.

Science Instrument. OCO-2’s science instrument, comprising three parallel high-resolution, near-infrared spectrometers in a common structure, will collect 24 measurements per second across a field of view of approximately three square kilometers, or one square mile. The narrow field of view allows OCO-2 to take more measurements that are not obscured by clouds, which can compromise accuracy, and to better identify carbon sources and sinks on a regional scale.

The spectrometers will view Earth through an f/1.8 Cassegrain telescope, and are designed to measure the absorption of sunlight reflected off of Earth’s surface by carbon dioxide and molecular oxygen, as viewed in the near-infrared part of the electromagnetic spectrum.

The sunlight rays entering the spectrometers will pass through the atmosphere twice—once as they travel from the Sun to the Earth, and then again as they bounce off from the Earth’s surface to the OCO-2 instrument. Carbon dioxide and molecular oxygen molecules in the atmosphere absorb light energy at very specific colors or wavelengths.

The OCO-2 instrument uses diffraction grating to separate the inbound light energy into a spectrum of multiple component colors. Each spectrometer will focus on a different range of colors, and will detect the specific colors that are absorbed by carbon dioxide and molecular oxygen. Measurements from all three spectrometers will be analyzed and integrated to precisely estimate the atmospheric concentration of carbon dioxide across the globe.

The instrument, which was designed by Hamilton Sunstrand and built by the California Institute of Technology’s Jet Propulsion Laboratory (JPL), is almost identical to the original Orbiting Carbon Observatory (OCO) Earth Science mission instrument, (earlier post), which launched on 24 February 2009 aboard a Taurus launch vehicle. OCO failed to reach orbit after a fairing did not separate, and was presumed to have plunged into the Pacific ocean somewhere near Antarctica. (Earlier post.)

Spacecraft. The OCO-2 spacecraft was built by Orbital Sciences Corporation on an elongated variant of Orbital’s LEOStar-2 science satellite platform. The 454-kilogram (999 lb) spacecraft is about the size of a telephone booth, with two solar panels each extending roughly two meters, or seven feet, from the spacecraft body. A 35 Ah nickel-hydrogen battery will provide power when the sun is eclipsed from the spacecraft. Launch services for the Delta II rocket were provided by United Launch Services, a joint venture of Boeing and Lockheed Martin.

Mission. The launch was conducted within a thirty-second launch window, which was necessary to enable OCO-2 to join the Afternoon Constellation, nicknamed the “A-Train”. The Afternoon Constellation, so named because it crosses the equator at approximately 1:30 PM local time every day, consists of five existing satellites in tight formation, collecting simultaneous data on aerosols, clouds, cloud ice, carbon sinks, carbon sources, ozone, particulates, and atmospheric water vapor.

As depicted above, the international Afternoon Constellation includes OCO-2, GCOM-W1, Aqua, CALIPSO, CloudSat, PARASOL, and Aura. GCOM-W1, Aqua, CALIPSO, CloudSat, and Aura are currently on orbit. On 18 December 18 2013, PARASOL ceased operation, fully exiting the A-Train. The instruments on these precisely engineered satellites make almost simultaneous measurements of clouds, aerosols, atmospheric chemistry, and other elements critical to understanding Earth’s changing climate. The footprint of each of the A-Train’s instruments is shown: active instruments aboard CALIPSO/CALIOP and CloudSat/CPR are indicated with dashed lines. This illustration color-codes instrument swaths—the area of Earth’s surface, or the surface of its atmosphere, over which data is collected—based on observed wavelength ranges. Microwaves (observed by both AMSRs, AMSU-A, CPR, MLS) are represented as red-purple to deep purple colors; yellow represents solar wavelengths (POLDER, OMI, OCO-2); gray represents solar and infrared wavelengths (MODIS, CERES); and red represents other infrared wavelengths (IIR, AIRS, TES, HIRDLS). Source: NASA. Click to enlarge.

OCO-2 will refine its polar orbit in following weeks as it moves into formation with the rest of the Afternoon Constellation, and will begin collecting data after instrument calibration and checkout. Scientists expect to be able to make preliminary data available beginning about 45 days after launch. Data will be acquired in three measurement modes:

• Nadir Mode: the instrument tracks and collects data directly beneath its orbit.

• Glint Mode: the instrument tracks and collects data where sunlight reflects off of the Earth, particularly the oceans.

• Target Mode: the instrument tracks and collects data on a specific surface target.

OCO-2’s primary data product will be estimates of atmospheric columns of carbon dioxide in the Earth’s atmosphere. In all, four data products will be generated:

• Level 1B - a unique recording of all soundings (approximately 74,000) made during a single spacecraft orbit.

• Level 2 - as previously mentioned, estimates of atmospheric columns of carbon dioxide, with geographic locations, as well as profiles of surface pressure, surface albedo, aerosol content, water vapor, temperature, and solar-induced chlorophyll fluorescence, all in cloud-free conditions.

• Level 3 - Monthly gridded global maps of atmospheric carbon dioxide concentrations.

• Level 4 - Monthly maps of global carbon dioxide sources and sinks.

The first spacecraft to measure atmospheric carbon dioxide is the Greenhouse Gases Observing Satellite (GOSAT), also referred to as Ibuki. GOSAT was successfully launched by the Japan Aerospace Exploration Agency (JAXA) on 23 January 2009, and has been operating since then. NASA researchers were able to use GOSAT data to refine their own algorithms for OCO-2, which collects data at almost 100 times GOSAT’s rate of data collection.