OCO 2

OCO 2 — formally the Orbiting Carbon Observatory 2 — is an American Earth-observing satellite operated by NASA and dedicated to the precise measurement of atmospheric carbon dioxide and the monitoring of terrestrial plant productivity. Assigned NORAD catalog ID 40059 and international designator 2014-035A, it was launched on 1 July 2014 (UTC; 2 July local Australian time at the launch site) and remains operational in orbit as of this writing. The mission represents NASA's second attempt to place a dedicated high-precision carbon dioxide observatory in space, following the loss of the original Orbiting Carbon Observatory during a launch failure in 2009. OCO 2 holds the distinction of being only the second satellite globally capable of high-precision CO₂ column measurements, after Japan's GOSAT spacecraft.

Mission and Purpose

The central scientific objective of OCO 2 is to build a detailed, near-global picture of how carbon dioxide is distributed through Earth's lower atmosphere. Carbon dioxide is the principal long-lived greenhouse gas driving contemporary climate change, yet the surface-level sources and sinks that control its atmospheric concentration are not evenly or well understood at fine spatial scales. Ground-based monitoring networks, while precise, are sparse and geographically biased. OCO 2 was designed to help close this observational gap by collecting millions of individual column measurements per day, enabling scientists to identify regional patterns of CO₂ emission and uptake that no surface network could resolve on its own.

The spacecraft carries a high-resolution spectrometer system that analyzes sunlight reflected from Earth's surface and lower atmosphere in three narrow spectral bands. By measuring the degree to which carbon dioxide and molecular oxygen absorb specific wavelengths of near-infrared light, the instrument can infer the total column abundance of CO₂ along the line of sight between the satellite and the ground. This technique allows measurement of spatial gradients subtle enough to detect the seasonal "breathing" of the terrestrial biosphere — the drawdown of CO₂ during the Northern Hemisphere growing season and its release as vegetation senesces in autumn.

Alongside its primary CO₂ measurements, OCO 2 also collects data related to solar-induced fluorescence (SIF), a faint glow emitted by chlorophyll in actively photosynthesizing plants. Although SIF detection was not the spacecraft's primary design goal, OCO 2's spectrometers proved sensitive enough to detect this signal, yielding an important secondary data product that scientists use to track vegetation health, gross primary production, and the geographic extent of active plant growth across continents and seasons.

The mission came about directly because of the 2009 failure of its predecessor. The original Orbiting Carbon Observatory was destroyed when its payload fairing failed to separate during launch, causing the spacecraft to fall back into the atmosphere before reaching orbit. NASA and its partners determined that the science objectives were important enough to justify building and flying a near-identical replacement, which became OCO 2. The replacement mission inherited not only the scientific goals of its predecessor but also much of the instrument and spacecraft heritage, which helped contain development costs and schedule risks.

Orbit and Tracking

OCO 2 orbits Earth in a sun-synchronous orbit, the class designated SSO in tracking databases. Sun-synchronous orbits are polar or near-polar orbits in which the orbital plane precesses at a rate that keeps it aligned with the Sun at a roughly constant local solar time throughout the year. For an Earth-observing instrument that relies on sunlight for its measurements, this arrangement is particularly valuable: the spacecraft passes over any given point on Earth at approximately the same local time on each orbit, ensuring that illumination conditions remain consistent from one overpass to the next. This consistency is essential for making the long-term, climatologically meaningful comparisons that OCO 2's science requires.

The spacecraft maintains a nearly circular orbit at an altitude of approximately 706–708 km above Earth's surface, with an apogee of 708 km and a perigee of 706 km. The orbital inclination is 98.2°, consistent with a sun-synchronous configuration at this altitude. One complete orbit takes approximately 98.8 minutes, meaning OCO 2 completes just under fifteen revolutions of Earth per day. Over successive days, the ground track shifts westward with each orbit, allowing the satellite to build up near-global coverage within a repeating cycle.

OCO 2 flies as part of the A-Train (Afternoon Train) satellite constellation, a coordinated formation of Earth-observing spacecraft that share similar sun-synchronous orbits and cross the equator in close succession during the early afternoon local time. Flying in formation with other instruments allows scientists to correlate OCO 2's CO₂ measurements with complementary observations of clouds, aerosols, and other atmospheric properties made by neighboring satellites in the same overpass window. This coordination enhances the scientific value of each individual dataset.

The satellite's NORAD catalog ID, 40059, and its international designator, 2014-035A, allow it to be tracked continuously by ground-based radar networks. Its orbital parameters are updated regularly in publicly accessible two-line element sets, and its position at any moment can be computed from current ephemeris data.

Design and Operator

OCO 2 was built by Orbital Sciences Corporation under contract to NASA. The spacecraft has a launch mass of 454 kg, placing it in the category of a small to medium Earth-observing satellite. Orbital Sciences — which has since become part of Northrop Grumman — brought experience in small and medium satellite design to the program, and the OCO 2 bus drew on heritage from other missions developed by the same manufacturer.

NASA is the sole operator of OCO 2. The mission is managed under the agency's Science Mission Directorate and falls within the broader portfolio of Earth science satellites that NASA operates to study the planet's climate, land surface, oceans, and atmosphere. Science data from the mission are processed and archived by NASA's Jet Propulsion Laboratory and made publicly available to the research community.

The spacecraft's spectrometer package is the scientific heart of the mission. It uses three co-boresighted, high-resolution grating spectrometers to simultaneously observe in distinct near-infrared absorption bands associated with oxygen and carbon dioxide. The instrument's design must balance spectral resolution high enough to resolve individual absorption lines against the need to collect sufficient light reflected from Earth's surface — a challenge compounded by the fact that the satellite is moving rapidly relative to the ground. OCO 2 can be operated in multiple pointing modes, including nadir (pointing straight down), glint (pointing at the sunlit ocean surface, where specular reflection provides a brighter signal), and target mode (dwelling on a fixed surface location to enable validation against ground-based instruments).

Significance and Legacy

OCO 2 occupies a significant place in the history of climate-observing satellite missions. When it reached orbit in 2014, it became only the second satellite in the world capable of high-precision column measurements of atmospheric CO₂, after JAXA's GOSAT, which had been operating since 2009. The two missions are complementary in their sampling strategies: GOSAT observes from a similar altitude but uses a different pointing pattern that yields spatially sparser but longer-dwell measurements, while OCO 2 acquires dense along-track swaths optimized for spatial coverage.

Data from OCO 2 have contributed to a substantial body of peer-reviewed research. Studies using OCO 2 measurements have mapped regional CO₂ fluxes over the tropics, examined the atmospheric signature of major wildfire events, detected the drawdown of CO₂ over agricultural and forested regions, and provided new estimates of the relative contributions of different land and ocean regions to the global carbon budget. The satellite's solar-induced fluorescence data have proven especially valuable for understanding the response of global vegetation to drought, heat, and changing growing seasons.

The success of OCO 2 has directly informed the design of follow-on missions. NASA's OCO-3 instrument was developed using OCO 2 heritage and is now operating aboard the International Space Station, where its non-sun-synchronous orbit provides sampling at a wider range of local solar times and allows targeted observations of urban and industrial emission sources. Internationally, other agencies have developed or are developing high-precision CO₂ observing satellites that build on the scientific and technical precedents established by OCO 2 and GOSAT.

OCO 2 continues to orbit Earth as of this catalog entry, with no reentry date recorded. Its longevity has meant that the mission has accumulated an observational record spanning more than a decade, long enough to begin capturing interannual variability in the carbon cycle and to track long-term trends in regional CO₂ concentrations. A dataset of this length and consistency, from a single well-characterized instrument, has become an increasingly valuable scientific resource as researchers work to understand how Earth's carbon cycle is responding to both human emissions and the changing climate those emissions are driving.