About Terra



Earth's 4.5 billion year history is a study in change. Natural geological forces have been rearranging the surface features and climatic conditions of our planet since its beginning. Today, there is compelling scientific evidence that human activities have attained the magnitude of a geological force and are speeding up the rates of global changes. For example, carbon dioxide levels have risen 30 percent since the industrial revolution and about 40 percent of the world's land surface has been transformed by humans.

Scientists don't understand the cause-and-effect relationships among Earth's lands, oceans, and atmosphere well enough to predict what, if any, impacts these rapid changes will have on future climate conditions. Scientists need to make many measurements all over the world, over a long period of time, in order to assemble the information needed to construct accurate computer models that will enable them to forecast the causes and effects of climate change. The only feasible way to collect this information is through the use of space-based Earth "remote sensors" (instruments that can measure things like temperature from a distance). Consequently, NASA's Earth Science Enterprise has begun an international study of planet Earth that is comprised of three main components: 1) an Earth Observing System (EOS), consisting of a fleet of satellites specially designed to study the complexities of global change; 2) an advanced computer network for processing, storing, and distributing data (called EOSDIS); and 3) teams of scientists all over the world who will study the data.

On December 18, 1999, NASA launched the EOS "flagship"—EOS Terra—to begin collecting a new 18-year global data set on which to base future scientific investigations about our complex home planet.

The EOS Terra Spacecraft

Physically, the EOS Terra spacecraft is roughly the size of a small school bus. It carries a payload of five state-of-the-art sensors that will study the interactions among the Earth's atmosphere, lands, oceans, and radiant energy (heat and light). Each sensor has unique design features that will enable EOS scientists to meet a wide range of science objectives (see topics at right).

EOS Terra will orbit the Earth from pole to pole (8.4MB Quicktime Movie), descending across the equator in the morning when cloud cover is minimal and its view of the surface is least obstructed. The satellite's orbit will be perpendicular to the direction of Earth's spin, so that the viewing swaths from each overpass can be compiled into whole global images (4.4MB). Over time, these global images will enable scientists to show and tell the stories of the causes and effects of global climate change.

The sensors on EOS Terra will not actively scan the surface (such as with laser beams or microwave pulses). Rather, the sensors work much like a camera. Sunlight that is reflected by Earth, and heat that is emitted from Earth, will pass through the apertures of Terra sensor's (3.4MB). This radiant energy will then be focused onto specially designed detectors that are sensitive to selected regions of the electromagnetic spectrum, ranging from visible light to heat. The information produced by these detectors will then be transmitted back to Earth and processed by computers into images that we can interpret.

The five Terra onboard sensors are (see animations at right):

  • ASTER, or Advanced Spaceborne Thermal Emission and Reflection Radiometer;

  • CERES, or Clouds and Earth's Radiant Energy System;

  • MISR, or Multi-angle Imaging Spectroradiometer;

  • MODIS, or Moderate-resolution Imaging Spectroradiometer; and

  • MOPITT, or Measurements of Pollution in the Troposphere.

The life expectancy of the EOS Terra mission is 6 years. It will be followed in later years by other EOS spacecraft that take advantage of new developments in remote sensing technologies.

As we learn more about our home planet, new questions arise, drawing us deeper into the complexities of Earth's climate system. We don't know the answers to many other important questions, like: Is the current warming trend temporary, or just the beginning of an accelerating increase in global temperatures? As temperatures rise, how will this affect weather patterns, food production systems, and sea level? Are the number and size of clouds increasing and, if so, how will this affect the amount of incoming and reflected sunlight, as well as the heat emitted from Earth's surface? What are the causes and affects of ozone fluctuations? How will climate change affect human health, natural resources, and human economies in the future? NASA's Earth Science Enterprise in general, and EOS Terra in particular, will help scientists answer these questions, as well as some we don't even know to ask yet.

Learn more about Earth

While the major goal of NASA's Earth Observing System is to increase understanding of our changing planet, EOS Terra data are not limited to serving the needs of just the scientific community. Rather, the ultimate product of this mission is education in its broadest forms. There are many information resources and educational materials that NASA makes available to inform the general public on the goals, objectives, and results of its missions. For more details on how to obtain some of these information resources, visit the EOS Project Science Office Web site or stay tuned to this site, as the latest new data products from Terra will be featured here.

next: Air


by David Herring
March 1, 1999

EOS Terra

Terra Status Reports
More Animations

Terra Orbit
Animation of EOS Terra orbit

Instrument Pass Overlapping
Animation depicting instrument pass overlapping

Terra Received Reflected Light
Animation showing how Terra receives reflected light






If you shrank Earth down to the size of a basketball, then by comparison its atmosphere would have roughly the thickness of a sheet of plastic wrap. All that is essential to us in our everyday lives is contained in this relatively thin sheet. Without its "wrapper," our world would be as cold and barren as the moon. Yet with it, the Earth is blanketed in all the essential ingredients-air, water, nutrients, and heat-needed to sustain the vast web of life. Over billions of years, the atmosphere has evolved into a complex system in which life and climate are intricately interwoven. For instance, we know that biological systems, as well as many of the physical processes such as temperature changes, are driven by the amount of sunlight that reaches the Earth's surface. Yet, when we measure the amount of incoming sunlight at the top of Earth's atmosphere, and compare this number with the amount of sunlight received at the surface, we find that 8 percent of the sunlight is missing! This number varies depending upon whether it is a clear, hazy, or cloudy day. Scientists aren't sure what percentage of the missing sunlight is due to the absorption and reflection of clouds; what percentage is being scattered by aerosols-trillions of tiny solid and liquid molecules-suspended in the atmosphere; or some combination of both clouds and aerosols. EOS Terra will enable scientists to find the missing sunlight and, subsequently, greatly improve the accuracy of their global climate models.

Now picture the atmosphere as a solid block of ice. With no impurities contained within it, the ice would appear translucent like glass, allowing light to shine through it. But if there were many tiny bubbles trapped within the ice, it would appear less clear and whitish, depending upon the number and density of the bubbles. This is because the bubbles would absorb and scatter light, rather than allowing it to pass through. Likewise, aerosol particles absorb and scatter sunlight in the atmosphere, thereby having a net cooling effect at the Earth's surface. Moreover, when they mingle with clouds, aerosols modify clouds' droplets, enhancing their ability to scatter and absorb more sunlight. Scientists have observed that clouds polluted with aerosols generally appear whiter and brighter than those unpolluted. We know that due to their interactions with Earth's radiant energy, clouds and aerosols play important roles in ongoing climate changes, yet the full extent of their contributions is not fully understood. The mystery lies partly in the fact that the physical and chemical structures of both clouds and aerosols change rapidly through time and space. So, one of the major challenges of the EOS Terra mission is to not only observe how and why clouds and aerosols change, but to also simultaneously measure their effects on the Earth's total radiant energy.

next: Land
back: About Terra

About Terra

On land-with the right mix of heat, light, nutrients, and water-millions of plant species thrive to feed and shelter millions of animal species. Not only do plants form the very foundations of their ecosystems, they also interact dynamically with the atmosphere to moderate climate through the exchanges of moisture, heat, and greenhouse gases. Plants' most significant influence on climate is their regulation of the global carbon cycle. Via "photosynthesis," plants take in carbon dioxide from the atmosphere and then use sunlight as energy to "fuse" carbon dioxide and water into complex molecules called "carbohydrates." The basic raw materials that feed Earth's biosphere, plants use carbohydrates as food and to make plant structures. Then, animals and humans consume plants to get these same carbohydrates that also serve as building blocks in our bodies. Both plants and animals breathe (or "respire"), thereby "burning" carbohydrates as fuel for metabolism and converting them back into separate water and carbon dioxide molecules that are eventually released back into the atmosphere. Hence, through biological respiration and decomposition of dead tissue, carbon is returned back to the atmosphere and the carbon cycle is complete.

As previously stated, scientists observe that the amount of carbon dioxide in the atmosphere has risen by 30 percent over the last 100 years, contributing to a 0.5°C increase in average global temperatures. Scientists recently discovered that these two trends have prolonged each growing season in the northern hemisphere over the last 20 years, resulting in more annual plant productivity. Some scientists theorize that increasing plant productivity will effectively act as a "storehouse" for the excess carbon dioxide in the atmosphere, and that the two trends will balance. However, new scientific data suggest that as the rate of increase in carbon dioxide accelerates, and temperatures rise, land plants could begin regulating their "evapotranspiration" (water loss due to evaporation) rates in order to conserve water. If this plant response occurs on a large scale, then computer models indicate that the greenhouse warming will be amplified over tropical land areas by as much as 50 percent over and above the current greenhouse warming trend.

The EOS Terra spacecraft will enable scientists to compare plant productivity with carbon dioxide and other important greenhouse gas levels, as well as temperature trends. These data will better enable scientists to predict how changes in the climate will impact Earth's ecosystems. The new data will also allow scientists to measure how certain human activities, such as biomass burning and deforestation (see before and after pics of deforestation), may be contributing to climate change.

next: Ocean
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About Terra

The ocean also harbors plant life-including microscopic plants called "phytoplankton," which serve as the foundation for the marine food chain. Like their land-based relatives, phytoplankton photosynthesize to produce carbohydrates. Then, small fish and whales eat the phytoplankton, bigger fish eat the smaller fish, and humans eat many of the bigger fish.

Periodic water temperature shifts, as in an El Nino or La Nina, can have dramatic effects on the health and distribution of phytoplankton, which in turn can have negative effects on those animals that depend upon phytoplankton for nutrition. Therefore, scientists plan to use EOS Terra data to closely monitor the abundance and distribution of phytoplankton all over the world. By precisely measuring even slight variations in ocean color, scientists can accurately estimate the abundance of the microscopic marine plant. Because phytoplankton selectively absorb red and blue light, and reflect green, scientists can measure the difference between incoming sunlight and that which is reflected back up into the atmosphere. This difference in sunlight represents the amount being used for photosynthesis, which allows scientists to derive the abundance of phytoplankton.

Covering more than 70 percent of the Earth's surface and containing 97 percent of its surface water, the ocean has been labeled "the heat engine of global climate" due to its influence on the timing and patterns of climate change. The ocean possesses a vast capacity to convert and store sunlight as heat. Because heat rises, it eventually escapes the ocean and rises up into the atmosphere where it directly influences temperature and precipitation patterns, and indirectly influences land vegetation through either precipitation or drought events. As previously mentioned, ocean temperature trends not only affect weather patterns, but they also affect the marine biosphere (see SST<>NDVI series). When the sea surface is cold-as during a La Nina-this allows the cold, deeper waters to flow upward, bringing critical life-sustaining nutrients with them. Yet, when sea surface temperatures increase-as during an El Nino-there is less vertical cycling of water, which means nutrients at the surface become more scarce and phytoplankton productivity drops. Larger fish and mammals that require phytoplankton to survive must either starve or move to where the food source is more abundant (see Picture of starving seals). South American fishermen who fish the Pacific named this phenomenon and know well that El Nino is a time when their livelihood is interrupted.

How will the frequency and severity of El Nino affect ocean productivity? How will concentrations of phytoplankton shift in response to changes in ocean circulation? How will changes in ocean productivity affect ocean transparency, and therefore impact the exchange of heat between the ocean and atmosphere? The EOS Terra satellite will measure these parameters with unprecedented accuracy to help scientists answer these questions.

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return to: Terra Spacecraft Fact Sheet

About Terra

New! Terra Spacecraft Quicktime VR Model. A VR model of the Terra spacecraft which you can manipulate to view all sides and angles (via Terra site).

  Terra QTVR Model

Terra was successfully launched on December 18, 1999, from Vandenberg Air Force Base. (4.6MB)

Terra Launch

Solar Panel Deployment
Terra's solar panel supplies the 3 kilowatts of power needed by the spacecraft. Its deployment is the first major event after Terra separates from its fairing. (3.3MB)

Terra Solar Panel Deployment

High Gain Antenna Deployment
Communication to and from Terra will be conducted through a high gain antenna and NASA's Tracking and Data Relay Satellite System (TDRSS). This allows data to be collected independent of the satellite passing over ground stations. (1.6MB)

Terra High Gain Antenna Deployment

Terra's Orbit
Terra will operate in a near-circular, sun-synchronous orbit with an inclination of 98.2 degrees. The spacecraft will cross the equator twice each orbit—once at 10:30AM (local time) moving from north to south, and once at 10:30PM from south to north. (4.2MB)

Terra Orbit

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)
ASTER will provide high-resolution images of land surface, water, ice, and clouds in visible, short-wave IR, and thermal IR wavelengths. (3MB)
· ASTER web site

ASTER Scanning Swath

Clouds and the Earth's Radiant Energy System (CERES)
CERES will measure the Earth's energy balance—comparing the amount of energy from the sun that reaches the Earth's surface vs. the amount of heat energy emitted by the Earth and its atmosphere. (3.2MB)
· CERES web site

CERES Scanning Swath

Multi-angle Imaging Spectro-Radiometer (MISR)
MISR will observe the Earth's surface from nine different angles simultaneously, providing 3D images and information about the angular properties of clouds and particulates. (3.9MB)
· MISR web site

MISR Scanning Swath

Moderate-resolution Imaging Spectroradiometer (MODIS)
MODIS will provide comprehensive measurements of ocean life (phytoplankton), land vegetation, cloud cover, and fires. (3.2MB)
· MODIS web site

MODIS Scanning Swath

Measurements of Pollution in the Troposphere (MOPITT)
MOPITT will measure two important pollutants—carbon monoxide and methane—in the lower atmosphere (troposphere). (3.3MB)
· MOPITT web site (Canadian Space Agency)
· MOPITT web site (University of Toronto)

MOPITT Scanning Swath

Combined Swaths of Terra's Instruments
Terra will conduct many of its observations simultaneuosly, allowing for new ways of integrating different types of data. (3.3MB)

Terra Instrument Scanning Swaths
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