LAGEOS, Laser Geodynamics Satellite or Laser Geometric Environmental Observation Survey, are a series of two scientific research

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...

s designed to provide an orbiting

laser ranging A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...

benchmark for geodynamical studies of the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

. Each satellite is a high-density passive laser reflector in a very stable

medium Earth orbit A medium Earth orbit (MEO) is an Earth-centered orbit with an altitude above a low Earth orbit (LEO) and below a high Earth orbit (HEO) – between above sea level.

(MEO).

Function and operation

The spacecraft are

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

-covered

brass Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other wi ...

spheres with diameters of and masses of , covered with 426 cube-corner

retroreflector A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects radiation (usually light) back to its source with minimum scattering. This works at a wide range of angle of incidence, unlike a planar mirro ...

s, giving them the appearance of giant

golf ball A golf ball is a special ball designed to be used in the game of golf. Under the rules of golf, a golf ball has a mass no more than , has a diameter not less than , and performs within specified velocity, distance, and symmetry limits. Like g ...

s. Of these retroreflectors, 422 are made from fused silica glass while the remaining 4 are made from germanium to obtain measurements in the infrared for experimental studies of reflectivity and satellite orientation. They have no on-board sensors or electronics, and are not attitude-controlled. They orbit at an altitude of , well above

low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...

and well below

geostationary orbit A geostationary orbit, also referred to as a geosynchronous equatorial orbit''Geostationary orbit'' and ''Geosynchronous (equatorial) orbit'' are used somewhat interchangeably in sources. (GEO), is a circular geosynchronous orbit in altitu ...

, at orbital inclinations of 109.8 and 52.6 degrees. Measurements are made by transmitting pulsed laser beams from Earth

ground station A ground station, Earth station, or Earth terminal is a terrestrial radio station designed for extraplanetary telecommunication with spacecraft (constituting part of the ground segment of the spacecraft system), or reception of radio waves f ...

s to the satellites. The laser beams then return to Earth after hitting the reflecting surfaces; 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. The LAGEOS satellites make it possible to determine positions of points on the Earth with extremely high

accuracy Accuracy and precision are two measures of '' observational error''. ''Accuracy'' is how close a given set of measurements ( observations or readings) are to their '' true value'', while ''precision'' is how close the measurements are to each o ...

due to the stability of their orbits. The high mass-to-area ratio and the precise, stable (

attitude Attitude may refer to: Philosophy and psychology * Attitude (psychology), an individual's predisposed state of mind regarding a value * Metaphysics of presence * Propositional attitude, a relational mental state connecting a person to a propo ...

-independent) geometry of the LAGEOS spacecraft, together with their extremely regular orbits, make these satellites the most precise position references available.

Mission goals

The LAGEOS mission consists of the following key goals: * Provide an accurate measurement of the satellite's position with respect to Earth. * Determine the planet's shape (

geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...

). * Determine

tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...

movements associated with

continental drift Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pl ...

. Ground tracking stations located in many countries (including the US, Mexico, France, Germany, Poland, Australia, Egypt, China, Peru, Italy, and Japan) have ranged to the satellites and data from these stations are available worldwide to investigators studying crustal dynamics. There are two LAGEOS spacecraft, LAGEOS-1 launched in 1976, and LAGEOS-2 launched in 1992. , both LAGEOS spacecraft are routinely tracked by the ILRS network.

Time capsule

LAGEOS-1 (which is predicted to re-enter the atmosphere in 8.4 million years) also contains a 4 in × 7 in plaque designed by

Carl Sagan Carl Edward Sagan (; ; November 9, 1934December 20, 1996) was an American astronomer, planetary scientist, cosmologist, astrophysicist, astrobiologist, author, and science communicator. His best known scientific contribution is research on ex ...

to indicate to future humanity when LAGEOS-1 was launched. The plaque includes the numbers 1 to 10 in binary. In the upper right is a diagram of the Earth orbiting the Sun, with a binary number 1 indicating one revolution, equaling one year. It then shows 268,435,456 years in the past (binary: 2²⁸), indicated by a left arrow and the arrangement of the Earth's continents at that time (during the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

period). The present arrangement of the Earth's continents is indicated with a 0 and both forward and backward arrows. Then the estimated arrangement of the continents in 8.4 million years with a right facing arrow and 8,388,608 in binary (2²³). LAGEOS itself is shown at launch on the 0 year, and falling to the Earth in the 8.4 million year diagram.

Launch data

* LAGEOS 1, launched 4 May 1976, NSSDC ID 1976-039A, NORAD number 8820 * LAGEOS 2, deployed 23 October 1992 from

STS-52 STS-52 was a '' Space Transportation System'' ( NASA Space Shuttle) mission using Space Shuttle ''Columbia'', and was launched on October 22, 1992. Crew Backup crew Mission highlights Primary mission objectives were deployment of ...

, NSSDC ID 1992-070B, NORAD number 22195

References

External links

* {{official website, http://lageos.gsfc.nasa.gov/
LAGEOS-1, -2 (International Laser Ranging Service)

LAGEOS-1 page at US National Space Science Data Center

LAGEOS-2 page at US National Space Science Data Center

LAGEOS-1, -2 (eoPortal.org)

LAGEOS
video (1976) at AParchive.com
LAGEOS Collection, The University of Alabama in Huntsville Archives and Special Collections

Bob Spencer Collection, The University of Alabama in Huntsville Archives and Special Collections
Files of Robert Spencer
planner
on the LAGEOS project. NASA satellites orbiting Earth Time capsules Laser ranging satellites Italian Space Agency Satellite series Gravimetry satellites