Global environmental change is one of the most pressing international concerns of the 21st Century. These changes may include the effects of global warming, stratospheric ozone depletion, and large-scale changes in land cover due to human activities such as biomass burning.
In the 1960s, the desire to monitor the Earth's environment was linked with new technology that enabled us to observe our planet from space - giving humans our first global view of Earth.
Before 1957, the word satellite meant one thing -- a small body that revolved around a larger astronomical body. Today we call these "natural satellites." In 1957, the Soviets launched the first "artificial satellite," Sputnik 1. Today there are hundreds of artificial satellites in orbit around the Earth. These satellites are used for many purposes, such as communications, weather forecasting, and navigation, as well as observing the Earth.
Satellites used to observe the Earth carry a variety of instruments to study the land, ocean, air and life, as well as their interactions. Scientists at NASA's Langley Research Center use several satellites now in orbit to measure some very specific atmospheric phenomena, such as the amount of ozone in the atmosphere, and to help them distinguish between environmental changes caused by humans and those that occur naturally.
The technology used on a satellite varies, depending on its mission. Computers aboard a satellite can receive, store and transmit information in the form of radio signals sent to and from stations on Earth. For Earth scientists, the receipt of the data begins a long process of determining what the data means. By incorporating the data into computer models (which use mathematical formulas called algorithms) researchers can simulate, or model, Earth's processes -- how the atmosphere, oceans and land surfaces interact as a system. Scientists hope that incorporating global satellite data into their computer models will help them better understand the interactive roles of Earth's systems, and help them predict how the Earth's environment will change over time.
The Measurement of Air Pollution from Satellites (MAPS) instrument produced the first global measurements of atmospheric carbon monoxide (CO) in 1981 when it flew aboard the Space Shuttle Columbia (STS-2). MAPS' most important finding was that air pollution is a worldwide phenomenon, not just a problem in industrialized countries. In 1981, and in subsequent shuttle flights in October 1984 and 1994, MAPS measured high values of CO pollution in the tropics caused by seasonal biomass burning. In 1997, MAPS will be mounted to the Russian space station Mir to monitor global CO levels during a year of seasonal changes.
The Earth Radiation Budget Experiment (ERBE) is made up of three satellites launched in the mid-1980s. Since then, ERBE has been the primary source of global data for studying the heating and cooling of the atmosphere. This data may tell us the extent to which global warming is occurring. ERBE technology also measures the effects of clouds on the exchange of energy between the sun, Earth and space. The ERBE sensors measure energy from the sun in various wavelengths: reflected shortwave solar radiation (light that does not reach the Earth but is reflected off clouds) and longwave emitted energy (the heat that is emitted into space by the Earth). By analyzing long-term measurements of these energy components, scientists can study the Earth's climate. ERBE has provided the most accurate data ever obtained on short- and longwave radiant energy, helping us better understand how clouds reflect and absorb sunlight, and the heat emitted by the Earth into space. NASA scientists have used this data to make important contributions to climate prediction by improving how clouds are represented in atmospheric models.
The Clouds and the Earth's Radiant Energy System (CERES) instrument is a follow-on to ERBE. CERES will be able to better identify cloud properties as well as help scientists better understand the Earth's energy budget. CERES will be launched in late 1997 aboard the Tropical Rainfall Measuring Mission (TRMM) spacecraft, as part of NASA's Mission To Planet Earth Program.
The Stratospheric Aerosol and Gas Experiment I (SAGE I) measured ozone, particles in the upper atmosphere (aerosols) and nitrogen dioxide from 1979 to 1981. Using a process called solar occultation, sensors on SAGE I measured sunlight coming through the atmosphere to determine how much sunlight was absorbed. The amount of absorption indicates the amount of various sunlight absorbing gases, like ozone, or aerosols, that are present. Solar occultation occurs as the satellite experiences sunrises and sunsets, when the light is not too bright to obscure readings. SAGE I produced the first global atmospheric data of this type.
SAGE II began operation in 1984 with the launch of the Earth Radiation Budget Satellite. SAGE II, which is still operating, provides global measurements of the vertical structure of ozone, nitrogen dioxide, water vapor and stratospheric aerosols. The SAGE II data helped scientists understand the causes and effects of the Antarctic ozone hole, and has made invaluable contributions to understanding the decline of stratospheric ozone over the Earth's mid-latitudes.
The Halogen Occultation Experiment (HALOE), launched in 1991 aboard the Upper Atmosphere Research Satellite (UARS), measures ozone and other atmospheric gases. Like SAGE I and II, HALOE uses the solar occultation technique; however, it measures visible infrared light and uses a filter which separates the gases according to their individual light "signatures." Analysis of the HALOE data proved conclusively that the Antarctic ozone hole was caused by human-produced chlorofluorocarbons (CFCs).
The adage, "a picture is worth a thousand words," holds especially true for satellite data - a global satellite "picture" can help scientists "see" the whole Earth and better understand its many interdependent systems. NASA will continue to study the Earth from space, and improve our satellite remote sensing abilities, through its ongoing Mission to Planet Earth Program.
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