SPACE shuttle Discovery sits on launch pad 39-A at the Kennedy Space Center with a payload designed to open a new era in planetary exploration. It is the Upper Atmosphere Research Satellite (UARS) - the leadoff spacecraft in Mission to Planet Earth.That's the US National Aeronautics and Space Administration's name for its share of a global research effort to study our planetary environment and the changes humans are making to it. It signifies that skills and knowledge learned in surveying other solar-system bodies from a spacecraft's perspective now will be used to study our home planet. With its launch targeted for Sept. 12, the new satellite is expected to begin that exploration this fall with the first ever coordinated study of the structure, chemistry, energy balance, and physical action of Earth's middle atmosphere. This big slice of air extends roughly between the heights of 15 to 100 kilometers (10 to 60 miles). It includes the stratosphere with its protective ozone layer, which will be a prime target for UARS's instruments. Carl Reber, UARS project scientist at the NASA Goddard Space Flight Center in Greenbelt, Md., notes that NASA began planning this study in 1977, long before it conceived of Mission to Planet Earth. "But," he adds, "we fit right into that concept." Knowing what goes on in the middle atmosphere is one of the keys to understanding what is happening to the atmosphere generally. Below that region - between the stratosphere and the ground - is the region meteorologists call the troposphere. That's where we live and where most of the weather occurs. No one knows how these two regions interact. However, reviewing the UARS mission in the American Geophysical Union weekly EOS, Dr. Reber points out that an estimated "70 percent of the total stratosphere air mass is exchanged with the troposphere each year." He further notes that "the fact that current [computer] forecast models that include the stratosphere provide better forecasts than the models that omit the stratosphere suggests that significant coupling exists." EBER also notes concern about possible human-caused atmospheric changes, of which the Antarctic ozone hole "is probably the most visible manifestation." He explaines: "Such changes, occurring in both the troposphere and the upper atmosphere, have far-reaching consequences and may eventually set constraints governing life on Earth. These potential threats highlight the need for a program of scientific research directed toward improving our knowledge of the physical and chemical processes occurring in Earth's upper atmosphere." Commenting further in a telephone interview, Reber explains that an understanding of what is happening to the ozone layer can only be gained through a comprehensive understanding of all the middle-atmospheric processes. That is why the UARS mission stresses coordination of many types of observations and related theoretical studies. This emphasis is reflected in the spacecraft itself. Measuring nearly 11 meters (35 feet) long, UARS has a mass of 6,600 kilograms (14,500 pounds), including 2,400 kilograms of instruments. These 10 instruments measure a variety of chemical and physical quantities, including winds. Their observations are coordinated so tightly that Reber says the UARS team thinks of the entire spacecraft as a single instrument rather than as a platform for 10 sensors. In operation, that "instrument" is to be controlled with precision. It must be pointed with an accuracy of 0.1 of a degree. Its orientation in space must also be determined with such precision that computer processing can refine the pointing accuracy to within 0.03 of a degree.