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The Goddard-managed Laser Geodynamics Satellite (LAGEOS) II, scheduled to launch aboard Space Shuttle Columbia in the fall of 1992, will help scientists monitor the motion of the Earth’s crust. LAGEOS II, like its predecessor LAGEOS I, launched in 1976, is a passive satellite dedicated exclusively to laser ranging.
LAGEOS II, built by the Italian Space Agency, Agenzia Spaziale Italiana (ASI), is a spherical satellite made of aluminum with a brass core. It is only 24 inches (60 cm) in diameter yet it weighs approximately 900 pounds (405 kg). This compact, dense design was selected to make the satellite’s orbit as stable as possible.
The solid outer 62 miles (100 kilometers) of the Earth, or lithosphere, is composed of rigid blocks, or plates. The plates move slowly, generally not faster than 6 inches (15 cm) a year. Although the motions are slow, the effects can be catastrophic. Plates may bump into one another, spread apart or move horizontally past one another. Most of the world’s mountain ranges, great earthquakes, and volcanic activity are caused by this movement. At mid-ocean ridges, the plates move away from one another and hot molten-mantle material, or magma, a more plastic layer of the Earth located between the crust and the core, rises and creates new oceanic lithosphere.
The theory of plate tectonics tries to explain how the continents got where they are today and predicts where they will be in the future. Scientists theorize that up until about 200 million years ago, there was one giant super-continent where the Atlantic Ocean is today. It was called Pangaea, which means "all lands." The word is a combination of the prefix pan, which means all and the Greek work gafa which means land or earth.
Then, about 180 million years ago, Pangaea started to break up into several continents. About 150 million years ago cracks began to appear in the northern continent. Rock, coming up from the magma level, poured up through the cracks, pushing the plates apart. Pangaea split into separate continents. The plates carrying the continents drifted away from each other with a low layer of rock forming between the plates.
The continents we now call North American and Europe kept moving apart. They move very slowly, about an inch (3 cm) a year. Today, they are about 3,000 miles (4828 km) apart. The low layer of rock gradually filled with water and became the Atlantic Ocean. The continents are still drifting apart at about the rate of one inch (3 cm) per year. Although the existence of Pangaea has never been proven, a look at the map of today’s world provides strong evidence in support of the theory. The continents look as if they are pieces of a giant jigsaw puzzle that could fit together to make one giant super-continent. The bulge of Africa fits the shape of the coast of North America while Brazil fits along the coast of Africa beneath the bulge. By studying the motions of the Earth’s crust, scientists are able to learn more about the precise locations on the Earth so their movements can be studied.
Scientists also use the data to look at the Earth’s axis of rotation which causes a movement or "wobble" at the north pole. The Earth is not perfectly round, and the material beneath the Earth’s surface and its atmosphere is not equally distributed around the imaginary line around which the Earth spins. This causes a small "wobble" in the Earth’s rotation, much like the motion of a child’s top. The spin rate varies by only a few thousandth’s of a second which changes the length of the Earth’s day.
Understanding the Earth’s wobble and rotation rate provides important insight into how mass shifts above and within the Earth. Through measurements using the LAGEOS satellites, scientists can detect changes in polar motion to an accuracy of 2 inches (5 cm) and changes in the length of the day to within one ten-thousandth of a second.
The Earth’s gravity plays a large part in controlling the orbit of a satellite. Because the Earth is neither perfectly round nor equally dense through its interior, gravity varies from place to place around the globe. In addition, tides , caused by the gravity of the Sun and the Moon pulling on the Earth, cause the Earth’s mass to shift in a space of hours. Using laser ranging with the LAGEOS satellites, scientists can measure large-scale changes in gravity. Studying these changes helps scientists understand better the properties, as well as the strength and behavior of materials deep within the Earth.
The satellites are designed to help scientists study the geodynamics of Earth. Geodynamics is the study of the motion of the earth and the forces and processes active in the interior of the Earth and how they affect the features of the Earth’s crust.
The LAGEOS II project is a joint program between NASA and the Italian Space Agency (ASI). The satellite, designed to be identical to LAGEOS I, was built by ASI using LAGEOS I drawings, handling fixtures, dummy spacecraft and other materials provided by NASA’s Goddard Space Flight Center (GSFC), Greenbelt, Md. GSFC technicians also made optical measurements on the satellite necessary for precision tracking. The two booster rockets are provided by ASI, and the launch, aboard Space Shuttle Columbia on the STS-52 mission, is provided by NASA.
In SLR, ground-based lasers, such as those located at GSFC, transmit intense, short pulses of light to retroreflector-equipped satellites such as LAGEOS II. The retroreflectors on the surface of LAGEOS II are three-dimensional prisms that reflect light, in this case a laser beam, back to its source. By recording the round-trip travel time for the pulse to be transmitted to the satellite and return to Earth, scientists can determine the location of the laser station on the Earth’s surface. Using this information scientists can determine accurately the distance between the stations. Thus, the relative distance between the locations, and hence their changes with time, can be determined. This enables scientists to study the motion of the Earth’s crust between various points on the Earth’s surface.
At least fourteen satellites equipped with retroreflectors have been launched by the U.S. and other countries for use in SLR. Reflectors have even been placed on the Moon by the Apollo astronauts and by unmanned Soviet probes. Laser ranging to the Moon has been done routinely since 1969, and has provided important information on relatively; the Moon’s shape, structure, motion and orbit.
Since the mid-1960s, when SLR was first demonstrated by NASA, many countries have developed and operated SLR systems. Today, the global community of more than 35 SLR stations cooperate in tracking these satellites and sharing their data, technological innovations and operational experiences. NASA has played a central role in establishing and coordinating programs and helping new international groups get started.
The global SLR network has provided a basic framework for the determination of plate motion. Despite the relatively slow rate of plate motion in most parts of the world, there have been some significant results.
Global plate motion studies by SLR systems have largely confirmed the expected motion for most plates, derived from geologic data averaged over several million years. They provide the first demonstrated proof that the plates move as the theory of plate tectonics suggests and that the motion over short-time scales is similar to that determined from long-term geologic averages.
While SLR measurements have largely confirmed the predictions of plate tectonics, there also are indications of some departures. For example. preliminary SLR results suggest that the northern portion of the Pacific Plate may be moving at a slightly different rate than the southern portion of the plate.
In addition, SLR studies of several plates near the U.S. west coast are having important implications for the earthquake hazard problem in California. They show scientists how stress is distributed and stored within the boundary zones between two large moving plates on the west coast.