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 willorbit 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 becompiled 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
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