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THE LAGEOS (Laser Geodynamic Satellite)
The LAGEOS spacecraft launched by NASA into Earth orbit on May 4,
1976, from Vandenburg Air Force Base, California, contains a message
plaque, addressed to human and other beings of the far distant
future. The spacecraft is a two-foot diameter aluminum sphere
weighing 903 lbs. (409 kg), covered with 426 reflecting surfaces,
giving it the appearance of a giant golf ball. Pulsed laser beams
transmitted from Earth ground stations are returned by the reflectors
on LAGEOS; the travel times are precisely measured, permitting ground
stations in different parts of the Earth to measure their separations
to better than one inch in thousands of miles.
One of LAGEOS' functions is to aid in the understanding of the Earth's
crustal motions. The outer crust of the Earth is composed of vast
plates which float on the semi-liquid mantle underneath. These plates
move with respect to each other about an inch per year. This is the
rate at which North and South America are moving away from Europe and
Africa, in what is called continental drift or plate tectonics.
For LAGEOS to be used as a standard for crustal motions of the Earth,
it must be placed in a highly accurate orbit. LAGEOS was launched
into a nearly circular orbit about 3,600 miles (5,800 km) above the
Earth. The spacecraft will orbit the Earth from pole to pole so that
it will be visible to laser transmitters in all parts of the world.
The stability of the orbit is so great it will not be dragged by the
atmosphere back to the Earth's surface for many millions of years.
LAGEOS PLAQUE
NASA and the spacecraft contractor, the Aerospace Systems Division of
the Bendix Corporation, asked Dr. Carl Sagan to design a LAGEOS
plaque.
The plaque is 4 inches by 7 inches (10 cm by 18 cm) stainless steel
plate. The spacecraft carries two identical copies included in its
interior. In its upper center it displays the simplest counting
scheme, binary arithmetic. The numbers one to ten in binary notation
are shown. At upper right is a schematic drawing of the Earth in
orbit around the Sun, and an arrow indicating direction of motion.
The arrowhead points to the right, the convention adopted for
indicating the future. All arrows accompanying numbers are "arrows of
time". Under the Earth's orbit is the binary number one, denoting the
period of time used on the plaque -- one revolution of the Earth, or
one year.
The remainder of the LAGEOS plaque consists of three maps of the
Earth's surface. The first map denotes the Earth 268 million years in
the past. All the continents are shown together in one mass. The
close fit of South America into West Africa was one of the first hints
that continental drift actually occurs.
The middle map represents the zero point in time for the other two
maps. It displays the present configuration of the planets.
The final map shows the Earth's surface 8.4 million years from now --
very roughly the estimated lifetime of the LAGEOS. Many important
changes in the Earth's surface are shown, including the drift of
California out into the Pacific Ocean.
Whoever comes upon the LAGEOS plaque needs only compare a current map
of the Earth's geography with that in the lower two maps to calculate
roughly the difference between his time and ours. Drift rates of
about an inch per year can, in fact be estimated by comparing the
bottom two maps. The same objective of LAGEOS and the method of
telling time on the plaque are identical.
Author: John R. Bannister
February 1977
NASA Aerospace Education Services Project
Oklahoma State University, Stillwater, Oklahoma
LASER GEODYNAMICS SATELLITE (LAGEOS) II
SPACELINK NOTE: LAGEOS II is targeted for launch aboard the Space Shuttle
(STS-52) in mid-October, 1992. Use the GO TO feature and enter STS-52 as the
keyword for more complete information on the mission.
The Laser Geodynamics Satellite (LAGEOS) II, like its predecessor launched
in 1976, is a passive satellite dedicated exclusively to laser ranging. Laser
ranging involves sending laser beams from Earthto the satellite and recording
the round-trip travel time. This measurement enables scientists to precisely
measure the distances between laser ranging stations on the Earth and the
satellite.
LAGEOS is designed to provide a reference point for laser ranging
experiments that will monitor the motion of the Earth's crust, measure and
understand the "wobble" in the Earth's axis of rotation, collect information on
the Earth's size and shape and more accurately determine the length of the day.
The information will be particularly useful for monitoring regional fault
movement in earthquake-prone areas such as California and the Mediterranean
Basin.
The LAGEOS II project is a joint program between NASA and the Italian
space agency, Agenzia Spaziale Italiana (ASI), which built the satellite using
LAGEOS I drawings and specifications, handling fixtures, dummy spacecraft and
other materials provided by the Goddard Space Flight Center (GSFC), Greenbelt,
Md. GSFC also tested the corner-cube retroreflectors on the surface of LAGEOS
II. ASI provided the Italian Research Interim Stage (IRIS) and the LAGEOS
Apogee Stage (LAS), the two upper stages that will transport LAGEOS II to its
proper altitude and circularize its orbit. NASA is providing the launch aboard
Space Shuttle Columbia.
The Spacecraft
The LAGEOS II satellite 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 makes the
satellite's orbit as stable as possible.
The LAGEOS design evolved from several trade-offs that proved necessary to
achieve the program objectives. For example, the satellite had to be as heavy
as possible to minimize the effects of non-gravitational forces, yet light
enough to be placed in a high orbit. The satellite had to be big enough to
accommodate many retroreflectors, but small enough to minimize the force of
solar pressure.
Aluminum would have been too light for the entire body of the sphere.
Design engineers finally decided to combine two aluminum hemispheres bolted
together around a brass core. They selected the materials to reduce the
effects of the Earth's magnetic field. LAGEOS II should remain in orbit
indefinitely.
LAGEOS II has the dimpled appearance of a large golf ball. Imbedded into
the satellite are 426 nearly equally spaced, cube-corner retroreflectors, or
prisms. Most of the retroreflectors (422) are made of suprasil, a fused silica
glass. The remaining four, made of germanium, may be used by lasers of the
future. About 1.5 inches (3.8 cm) in diameter, each retroreflector has a flat,
circular front-face with a prism-shaped back.
The retroreflectors on the surface of LAGEOS II are three-dimensional
prisms that reflect light, in this case a laser beam, directly back to its
source. A timing signal starts when the laser beam leaves the ground station
and continues until the pulse, reflected from one of LAGEOS II's
retroreflectors, returns to the ground station.
Since the speed of light is constant, the distance between the station and
the satellite can be determined. This process is known as satellite laser
ranging (SLR). Scientists use this technique to measure movements of the
Earth's surface up to several inches per year. By tracking the LAGEOS
satellites for several years, scientists can characterize these motions and
perhaps correlate them with Earth dynamics observed on the ground.
Launch, Orbit Insertion And Data Collection
After the Shuttle releases LAGEOS II, two solid-fuel stages, the Italian
Research Interim Stage (IRIS) and the LAGEOS Apogee Stage (LAS), will engage.
The IRIS will boost LAGEOS II from the Shuttle's 184-mile (296 km) parking
orbit to the satellite injection altitude of 3,666 miles (5,900 km). The LAS
will circularize the orbit. This will be the first IRIS mission and will
qualify the IRIS, a spinning solid fuel rocket upper stage, for use in
deploying satellites from the Space Shuttle cargo bay.
LAGEOS II's circular orbit is the same as that of LAGEOS I, but at a
different angle to the Earth's equator: 52 degrees for LAGEOS II and 110
degrees for LAGEOS I. The complementary orbit will provide more coverage of the
seismically active areas such as the Mediterranean Basin and California,
improving the accuracy of crustal-motion measurements. It also may help
scientists understand irregularities noted in the position of LAGEOS I, which
appear to be linked to erratic spinning of the satellite itself.
LAGEOS II will undergo a very intensive tracking program in its first 30
days of flight. This will allow laser ranging stations to precisely calculate
and predict the satellite's orbit. By the end of the 30 days, full science
operations will have begun.
NASA operates 10 SLR stations. Four are Transportable Laser Ranging
Systems (TLRS), built to be moved easily from location to location. Four
Mobile Laser Ranging Systems (MOBLAS) are in semi-permanent locations in
Australia and North America, including GSFC. The University of Hawaii and the
University of Texas at Austin operate the other two NASA systems.
NASA and ASI have selected 27 LAGEOS II science investigators from the
United States, Italy, Germany, France, the Netherlands and Hungary. The
investigators will obtain and interpret the scientific results that come from
measurements to the satellite. By tracking both LAGEOS I and LAGEOS II,
scientists will collect more data in a shorter time span than with LAGEOS I
alone.
Data from LAGEOS II investigations will be archived in the Crustal
Dynamics Data and Information System (CDDIS) at GSFC. It will be available
worldwide to investigators studying crustal dynamics.