TWO satellites circling our planet are opening a new adventure in self-knowledge for humanity.
They are beginning the global-scale study of Earth as a complete environmental system. It's an exploration that will continue into the next century.
The European Space Agency's Earth Resources Satellite (ERS-1), launched in July, and the United States Upper Atmosphere Research Satellite (UARS), launched in September, also act as heralds of International Space Year (ISY) in 1992. As part of the 500th-anniversary commemoration of Columbus's transatlantic exploration, ISY's main theme is space-based exploration of the entire planet.
Columbus's achievements enlarged Europeans' world view. The new space-based exploration has the more ambitious goal of helping all humanity to gain a global perspective. As Hubert Curien, founder president of the European Association for the International Space Year, explained in the association's newsletter: "What is needed is careful consideration as to how the Earth might be managed more intelligently."
It will take several decades of worldwide research on land and sea, as well as from space, to assemble the scientific knowledge on which to base planetary management. That is the task of the international global-change research program, which the US National Academy of Sciences calls "the most ambitious scientific enterprise ever undertaken."
The ERS-1 and UARS satellites are forerunners of the satellite systems that will carry out this research. They illustrate the sophistication with which those satellites will operate.
For example, John Frederick, professor of atmospheric sciences at the University of Chicago in Illinois, notes that "the difference between current satellites, which were designed in the 1970s, and the UARS is like the difference between a brand-new Rolls-Royce and a beat-up old Volkswagen." Nine instruments on the 6,500-kilogram (7,200-ton) spacecraft are used to detect 16 different chemical compounds between altitudes of 10 to 80 kilometers (6.2 to 49.6 miles). They can measure air temperatures, wind s peeds, solar radiation, and incoming cosmic-ray particles.
UARS data should give scientists a three-dimensional moving picture of the complex, physical and chemical processes of the upper atmosphere. Dr. Frederick compares these to "a big chemical soup with dozens of ingredients.A change anywhere will be felt everywhere else."
Among the most important changes the UARS team wants to monitor are those affecting the ozone layer, which absorbs ultraviolet radiation. Moving along its 584-kilometer-high orbit that is inclined 57 degrees to the equator, UARS can "see" most of that layer. Its 20-month nominal mission extends over one annual Antarctic ozone-hole cycle and two of the Arctic's milder wintertime ozone-depletion cycles.
ERS-1 has taken equally advanced capabilities into its 800-kilometer (496-mile) near-polar orbit. Its imaging radar can map surface details as small as 25 meters across day and night through clear or cloudy skies. Other instruments can measure ocean winds by detecting water-surface roughness, sense infrared (heat) radiation, and measure surface heights to a few centimeters' accuracy.
That kind of observational sophistication "marks the beginning of a new era for the European Earth-observing and environmental activities," says Jean-Marie Luton, director-general of the European Space Agency. He sees this as an era in which planetary changes will be continuously monitored. To provide such continuity, ESA plans to orbit a successor satellite - ERS-2 - in 1994. Then large polar-orbiting platforms with many instruments would take over the observing job. ESA hopes to launch the first two in
1997 and 1998.
ESA considers these platforms to be a key element in its long-term Earth-observing plan. But Mr. Luton points out that they are only part of a larger international effort. As he explains in the European space year association's newsletter: "The European platform should be seen in this context alongside the Earth Observing System of the United States and the Japanese platforms. The objective is that each of these missions will provide complementary data sets...."
But the platforms' future is uncertain. ESA members have yet to fully approve and fund the scheme, while the US has scrapped the concept altogether.
The US National Aeronautics and Space Administration (NASA) platform plan drew sharp criticism for several years. Critics charged that relying on a few massive platforms is risky and costly. The NASA-appointed Engineering Review Advisory Committee reexamined the plan last summer and recommended scrapping the platforms in favor of smaller, simpler satellites.
NASA spokesman Brian Dunbar says that scientific teams are now considering how to do this. Among other things, they have to decide which instruments to fly, as they may not be able to orbit all the instruments planned.
These will be tough decisions. The global-change program aims to understand how a multitude of components interact to form and run the planetary environment system. NASA's original plan featuring the platforms was to orbit the first-ever integrated system for observing these complex interactions. As Mr. Luton noted, its observations were to have been coordinated with those of the proposed European and Japanese platforms. Simultaneous operation of the instruments on these different platforms was considere d crucial to gaining a comprehensive overview of the planet's environmental machinery.
It remains to be seen how the shift in US plans will affect the European and Japanese programs. Whatever happens, no one has questioned that the need for international cooperation is vital for the success of the Mission to Planet Earth, NASA's term for its part of the effort.
Whatever satellites finally reach orbit, Earth scientists face a massive data-handling challenge. Comparing the rush of data from a full-scale planetary observing system to what previous Earth-scanning satellites have sent is like comparing Niagara Falls to a rivulet. There have been continuing doubts about data-management plans.
Meanwhile, the ERS-1 and UARS teams are learning the first lessons in planetary monitoring. These include learning how to deal with the unexpected.
"The most important result ... may well turn out to be something we haven't thought of - a total surprise," says Alden Roche of Lockheed Corporation in Palo Alto, Calif., one of the UARS's principal investigators.