Terra Satellite and Earth Observing System (EOS)

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Terra Satellite and Earth Observing System (EOS)


The Terra satellite, formerly called EOS AM-1, is a multinational scientific research satellite that is circling Earth in a sun-synchronous orbit. The satellite was launched from Vandenberg Air Force Base, California, on December 18, 1999, aboard an Atlas II-AS launch vehicle. Terra is considered the flagship of the Earth Observing System (EOS), headed by the U.S. National Aeronautics and Space Administration (NASA). Terra began collecting data about the condition of Earth's environment and the changing climatic systems above Earth on February 24, 2000.

EOS, a part of NASA's Earth Science Enterprise (ESE), consists of a series of satellite missions in polar and low-inclination orbits about Earth designed to improve scientific understanding of Earth's complex global environmental system. The series of satellites are composed of various scientific instruments designed for long-term global observations of the atmosphere, biosphere, land surfaces, and oceans and other water bodies of Earth.

Historical Background and Scientific Foundations

Five instruments are carried aboard Terra designed to gather information on various aspects of climate and weather: ASTER, CERES, MISR, MODIS, and MOPITT. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is used to create maps of surface temperature, emissivity, reflectance, and land elevation. It provides high-resolution images in fourteen different parts of the electromagnetic spectrum. The Clouds and the Earth's Radiant Energy System (CERES) measures the radiation flux of Earth and provides estimates of cloud properties from all elevations of Earth's atmosphere.

The Multi-angle Imaging Spectroradiometer (MISR), which consists of nine different digital cameras, measures: 1) solar radiation reflected by Earth's surface and atmosphere in various directions and spectral bands; 2) concentrations of atmospheric aerosol particles, both created naturally and artificially by human activities, and by upper air winds and cloud cover; and 3) characteristics of land-surface properties such as locations of vegetation and distribution of land masses.

The Moderate-resolution Imaging Spectroradiometer (MODIS) gathers data in 36 spectral bands in order to image Earth with respect to cloud cover, radiation budget, and processes occurring in the oceans, on land, and in the lower atmosphere. The Measurements of Pollution in the Troposphere (MOPITT) monitors changes in pollution patterns and its effect in the lower atmosphere, measures upwelling infrared radiation, and calculates carbon monoxide (CO) concentrations in the lower atmosphere.

Terra is only one of numerous satellites launched as part of EOS. Other satellites launched in the last years of the twentieth century include: Orbview-2/SeaWiFS (Sea-viewing Wide Field-of-view Sensor), launched August 1, 1997; TRMM (Tropical Rainfall Measuring Mission), November 27, 1997; Landsat 7, April 15, 1999; QuikSCAT (Quick Scatterometer), June 19, 1999; ACRIMSAT (Active Cavity Radiometer Irradiance Monitor Satellite), December 20, 1999; and NMP/EO-1 (Earth Observing-1), November 21, 2000.

Satellites launched in the first few years of the twenty-first century include: Jason-1, December 7, 2001; Meteor-3M/Sage III (Stratospheric Aerosol and Gas Experiment III), December 10, 2001; GRACE (Gravity Recovery And Climate Experiment), March 17, 2002; Aqua, May 4, 2002; ADEOS II (SeaWinds)/Midori II, December 13, 2002; ICESat (Ice, Clouds, and land Elevation Satellite), January 12, 2003; SORCE (Solar Radiation and Climate Experiment), January 25, 2003; Aura, July 15, 2004; CloudSat, April 27, 2006; and CALIPSO, April 27, 2006.

As of November 2007, future EOS satellites to be launched include: OSTM, May 2008; OCO, September 2008; Glory, December 2008; Aquarius, March 2009; NPP, September 2009; LDCM, July 2011; and GPM (launch yet to be decided).


BIOSPHERE: The sum total of all life-forms on Earth and the interaction among those life-forms.

EMISSIVITY: The ratio of the radiation emitted by a surface to that emitted by a black body at the same temperature. The ability of a body to emit radiation.

FLUX: The measure of the flow of some quantity per unit area per unit time.

SUN-SYNCHRONOUS ORBIT: Satellite orbit structured to keep the satellite directly above a fixed solar time as that time (area on the ground) moves around the rotating Earth. A sun-synchronous satellite will always look down on places that see the sun at a given height in the sky: shadows cast on the landscape just below the satellite will always be the same length for objects of identical height. Also called heliosynchronous orbit.

TROPOSPHERE: The lowest layer of Earth's atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth's surface.

UPWELLING: The vertical motion of water in the ocean by which subsurface water of lower temperature and greater density moves toward the surface of the ocean. Upwelling occurs most commonly among the western coastlines of continents, but may occur anywhere in the ocean. Upwelling results when winds blowing nearly parallel to a continental coastline transport the light surface water away from the coast. Subsurface water of greater density and lower temperature replaces the surface water and exerts a considerable influence on the weather of coastal regions. Carbon dioxide is transferred to the atmosphere in regions of upwelling.

Impacts and Issues

NASA is involved with studying Earth and, specifically, its atmosphere, oceans, land surface, water and oceans, ice and snow cover, and glaciers and polar ice sheets, along with their effect on weather and climate. In order to acquire the most comprehensive data into the global environment, NASA has created the Earth Science Enterprise, which brings together many scientific fields such as atmospheric science, biology, geology, meteorology, oceanography, and physics to explore how Earth's land, air, water, and life forms all work together to form the Earth's biosphere. By performing these activities, NASA scientists hope to be able to collect additional information to better predict the changing climate and how it will affect Earth and human beings in the future.

The series of Earth-observing satellites is the main physical component of the EOS project. It also includes a technologically advanced data system for retrieving and storing this information and a team of scientists who research the data.

For instance, CloudSat observed more than 150 tropical cyclones during its first six months in operation after its launch in April 2006. Data gathered from these storms were compared with weather data taken from ground-based stations. For the first time, a storm's intensity was measured between the top of the storm down to the land or water surface. Although the technology is not yet matured, satellites will likely be used more frequently in the near future to predict storm intensity in order to warn people of impending danger.

Managed by NASA's Goddard Space Flight Center, with inputs made by the Jet Propulsion Laboratory and the Langley Research Center, the NASA Earth Observing System is an important coordinated project to help humans better understand and protect Earth. As the flagship of EOS, the Terra satellite is expected to gather data at least into 2015 on which scientists can base investigations about Earth.

Because of increasing concerns over the future of Earth's global climate, the work performed by EOS scientists will greatly help to make educated and informed decisions about such important subjects as land erosion, melting of icecaps, tropical storms, rising flood waters, global warming, ozone depletion, and myriad other weather-related problems facing the world.

See Also Climate Change Science Program; Meteorology; Satellite Measurements.



1999 EOS Reference Handbook: A Guide to NASA's Earth Science Enterprise and the Earth Observing System, edited by Michael D. King and Rovnold Greenstone. Greenbelt, MD: NASA/Goddard Space Flight Center, 1999.

Committee on Earth Science and Applications from Space, National Research Council. Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond. Washington, DC: National Academies Press, 2007.

Observation of the Earth System from Space, edited by Jakob Flury. New York: Springer, 2006.

U.S. Geological Survey. Earth Observing-1 Extended Mission. Reston, VA: U.S. Department of the Interior, U.S. Geological Survey, 2003.

Web Sites

“About NASA's Earth Observing System.” National Aeronautics and Space Administration, November 7, 2007. <http://eospso.gsfc.nasa.gov> (accessed November 11, 2007).

“About the Terra Spacecraft.” National Aeronautics and Space Administration. <http://terra.nasa.gov/About> (accessed November 11, 2007).

“NASA Data May Help Improve Estimates of a Hurricane's Punch.” NASA Earth Observatory, National Aeronautics and Space Administration, November 1, 2007. <http://earthobservatorynasa.gov/Newsroom/NasaNews/2007/2007110 125761.html.> (accessed November 11, 2007).

William Arthur Atkins

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Terra Satellite and Earth Observing System (EOS)

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