A satellite's-eye view of continental drift
It's only an inert little satellite, about the size of a bushel basket. It has no radios, instruments, or moving parts. Yet it enables scientists to "see" inside Earth. It helps them measure the slow draftihng of continents, detect the strain of earthquake faults, and trace internal adjustments of our planet as it recovers from the glacial burden of the last ice age.
Such are the fruits of a little over half a decade's study by Lageos, the Laser Geodynamic Satellite.
The 410-kb (903 lb.) satellite, launched into a circular orbit 6,000 km (3, 730 miles) high in May of 1976, is a geodetic benchmark is the sky -- and is expected to last for millions of years. Slight changes in its orbital motion reflect variations in Earth's gravitational field. Tracked closely by laser pulses, these orbital changes can be made to reveal Earth's shape. Satellite tracking also shows up changes in Earth's rotation, shifts of mass within the planet, tidal effects, and even large-scale movements of ground water.
Recent research, for example, has detected what is probably a "rebound" of material within the planet in regions once depressed by the weight of ice age glaciers. J. O. Dickey, J. G. Williams, and C. F. Yoder of the NASA Jet Propulsion Laboratory and R. J. Eanes, B. E. Schultz, and B. D. Tapley of the University of Texas at Austin reported these findings last June in Nature.
Commenting on the research recently, Schutz explained that such a slow, longterm internal rebound from the glaciation would have been undetectable without the satellite. Now, however, after tracking stations around the world have accumulated over half a decade of data, such changes have begun to show up.
Tapley added that exchanges of momentum between the atmosphere and Earth can be followed as an aid to studies of weather and climatic variations. Precision measurement of distances between various locations on the planet's surface allow movements of only a few centimeters a year to be charted. Such movements occur along earthquake faults as well as between continents. Tapley added a local angle by noting that the data also help in tracing the drop in the seabed around the Texas Gulf Coast and the effects on ground-water displacement of geothermal energy projects.
In short, a great deal of diverse information can be extracted from tracking satellites whose motions are fine-tuned to the shape and mass distribution of Earth. While it receives relatively little publicity, satellite geodesy makes a unique and practical contribution to human knowledge and welfare.
But it also reflects the ongoing tension between using space technology openly for human benefit and keeping it secret for military purposes. To trace the details of Earth's continental drift, scientists need highly accurate position data. Such data are alos needed to target long-range missiles. So from the very beginning of satelite geodesy -- when tracking of the first US satellite revealed that Earth is slightly pear-shape -- there has been pressure for secrecy.
The first geodetic satellite -- that is, a satellite specially designed for geodesy -- was launched in October of 1962. But it was only shortly before the launching that the Pentagon yielded to strong pressure from US scientists and agreed to allow this satellite, called Anna, and its data to be freely used.
There have been a number of US geodetic satellites since then, with Lageos being the latest. Some have been expressly military. In fact, as veteran geodecist W. M. Kaula of the University of California at Los Angeles has noted, even today the geodetic results of satellite tracking are often secreted away by the military.
Kaula has called attention to the fact that there have been no new civilian gedetic missions since the Lageos project. While that satellite will be available indefinitely, geodecists would like types of data it can't provide, such as satellite-to-satellite measurements of relative motion. It should also bo possible to use satellites and their response to Earth's structure to study hidden details of mountain belts such as the roots of the Andes or the Alps. Kaula has noted, too, that satellite geodesy is unglamorous. It tends to be fairly low on the list of civilian space priorities. Yet slowly, often after years of tedious detailed tracking and analysis, it yields valuable insights into the hidden workings of our planet. Bomb test hazardm
Environmentalists concerned about the hazards of nuclear radiation should focus more attention on French weapons tests in the South Pacific. According to an official French investigation, these tests could be seriously contaminating that area.
The investigation was led by Haroun Tazieff, a geologist in charge of studies of environmental risks for the French prime minister's office. His team found that testing centered on the island of Mururoa was poorly monitored for radiation hazards. Also, it found that there may have been major releases of highly radioactive material buried at the site.
For example, one team member estimates that a storm in March of 1981 may have dispersed some 10 to 20 kg (22 to 44 lbs.) of plutonium waste from aerial tests that were stopped in 1974. The report notes that the monitoring "ignored the quantities [of such waste] introduced into the enviornment, their solubility and possible dispersal."
The team concluded that, while there may be relatively little direct short-term danger from the testing, it is possible that long-term hazards could develop if buried wastes leaked into the sea -- although there is insufficient information to assess this potential danger.
The report, as summarized in Nature, notes pointedly that "the absence of such information disarms defenders of the French nuclear test programme."