The LAGEOS- III Mission

The Laser Geodynamic Satellite Experiment (LAGEOS-III) is a joint multinational program with collaboration from France, Germany, Great Britain, Italy, Spain and the Usa, to measure, for the first time, a quasi-stationary property of the Earth - its gravitational magnetic dipole moment (gravitomagnetism) as predicted by Einstein's theory of general relativity.

Today, almost eighty years after Einstein introduced his geometric theory of gravity, we have just begun to verify it. Of no less stature than the "tide producing" -M/r2 "electric component" of gravity is the inertial-frame defining "magnetic component" of gravitation -J/r3. To see this force in action: first, inject a satellite into a polar orbit about an earth-like mass idealized as not spinning with respect to the distant quasars. The satellite will remain in orbit in a continuous acceleration towards the center-of-mass of the attracting body under the influence of the Newtonian 1/r2 force, and its orbital plane will remain fixed in orientation with respect to distant quasars. Second, spin the central body, giving it angular momentum, and follow the trajectory of the satellite. Its orbital plane will experience a torque along the central body's rotation axis. The orbital plane will undergo a precessional motion in the direction of the central body's rotation. The mass in motion of the central body, or "mass current", produces a dipole gravitational field - the gravitomagnetic field. In the case of a satellite orbiting at two earth radii, the orbital plane will precess about the body axis of the earth at aproximately 32 mas/yr. This is the Lense-Thirring effect.

The idea behind the LAGEOS gravity measurement is simple, it proposes the use of the orbital planes themselves as a gyroscope.

Furthermore, through measurements of the LAGEOS satellites, one can detect changes in polar motion to within 2 inches (5 centimeters) and changes in the length of the day to within one ten-thousandth of a second. Improved measurements of these minute variations will lead to an increased understanding of their realationship to earthquakes, winds, tides and other mass changes within the Earth.

The LAGEOS satellite is composed of two aluminum hemispheres bolted together, with a brass cylindrical core along its body axis (the original axis of spin). It is covered by an array of 426 specially slaped prisms laser light is shone on them from Earth stations, and the travel time is used to determine the position of the satellite.

SATELLITE LASER RANGING

The laser ranging is done by scatering a Laser Beam through a telescope aimed at the satellite. There are Satellite Laser Ranging (SLR) stations scattered through the world. These stations use a suite of satellites to conduct daily measurements to a precision of less than 0.4 inches ( 1 centimeter).

International cooperation has long been a part of the SLR program, first demonstrated the in the 1960s. Since then, many countries have developed and operated fixed and transportable SLR systems. Today more than 30 countries cooperate in SLR measurements, sharing data, technology and operational experience. NASA has played a central role in the international SLR program by coordinating existing programs and helping new programs get started, and pioneering improvements in laser-ranging technology.

In 1989, NASA and some of the world's leading Earth scientist formed the Fiducial Laboratories for an Internationaal Science Network (FLINN). This permanent network of ground stations, spaced approximately 620 miles (1,000 kilometers) apart, includes many current NASA and international SLR stations. These stations also support other types of geophysical experiments including LAGEOS I, II and, in the future, III.

THE CURRENT LAGEOS III. PROJECT.

(A) The present LAGEOS III project will be devoted to improved analyses of the effect of the Earth albedo, this effect should be negligible in the measurement of the Lense-Thirring effect by the LAGEOS III experiment; measurement of spin axis and spin rate of LAGEOS and LAGEOS III and prediction of the dynamics of their spin orientation and of their spin rate, this will reduce the total error due to thermal thrust (both due to re-emission of Earth infrared radiation by the LAGEOS retroreflectors and to re-emission sunlight modulated by LAGEOS eclipses by the Earth); further study of geopotential and tidal perturbations in the LAGEOS III experiment. The Center for Space Research of the University of Texas at Austin has estimated that the static and the dynamical parts of the geopotential will presently introduce an error of about 2% of the Lense-Thirring effect; updated study of the injection requirements, by considering the improvements in the Earth gravity field solution and the use of the data from the "new" satellite LAGEOS II (launched in October 1992 by NASA and ASI, Italian Space Agency). Considering these two factors the corresponding total injection error should be negligible. Thus, the total error in the measurement of the Lense- Thirring effect is largely reduced with respect to the previuos studies. To see a picture about the burster click here.

(B) Analyses of the various and important applications of the LAGEOS III experiment in geodynamics and geodesy, and study of possible improvements in the important technique of laser ranging.

(C) OTHER RELATIVISTIC MEASUREMENTS

In addition to the measurement of the Lense-Thirring effect and to improved measurements in geodynamics and geodesy the LAGEOS III mission will provide other general relativistic and gravitational measurements. The purpose of this part of the proposal is to further analyze and identify the other possible gravitational tests achievable with LAGEOS III.

In 1977 Rubicam calculated the LAGEOS general relativistic perigee precession of about 3.3 arcsec/ year or about 195m/year. The problem of measuring the general relativistic perigee shift was then treated by Ashby and Bertotti in 1984. In 1989, Ciufolini and Matzner showed that from the analysis of the LAGEOS data, it is possible to have a measurement of the LAGEOS relativistic perigee precession which can be used to put limits on some alternative theories of gravity, such as some nonsymmetric gravitational theories. In 1993, Ciufolini and Nordtvedt, by analyzing LAGEOS ans satellite laser ranging data, have set a new limit of about less than 2x10-12 to a conceivable spatial anisotropy of the gravitational interaction.

The LAGEOS data have also been used to improve the limits to hypothetical deviations from the weak field inverse square law for gravity, in the range of few thousand km.

Then, we propose to study the improvements in testing general relativity, that will derive from the use of LAGEOS III data, in particular: the improvements in the limits on a conceivable anisotropy of the gravitational interaction, and on some related PPN parameters, achievable by using the data of LAGEOS, LAGEOS II, LAGEOS III and other laser ranged satellites (in collaboration with Michael Soffel and Kenneth Nordtvedt); the improved measurements of the general relativistic precession of the LAGEOS III perigee; the possibility of improving the accuracy ( or at least to have and additional alternative measurment in the field of Earth) of the PPN parameter y, which measures curvature generated by mass, for example this migth be achieved by choosing a special orbit for LAGEOS III with high eccentricity (in collaboration with Richard Matzner and Bruno Bertotti); improved limits on the strength of a hypothetical 5th force, in the range of few thousand km, migth be placed using LAGEOS III data.

The LAEFF group, led by Juan Pérez Mercader, will investigate on the general relativistic many body system consisting of LAGEOS, LAGEOS II and LAGEOS III in the gravitational field of Earth. By accurately measuring the parameters of this system over a time span, one can compare the observations with the theoretical predictions derived from computer simulations and get information non only about general relativistic phenomena, but also about interesting physical parameters, such as Newton's constant and the Earth moment of inertia (Juan Pérez-Mercader), while also checking the existence of chaotic fluctuations in gravitational systems, first predicted by Poincaré in the XIX, and recently numerically checked by Laskar et al.

Finally, we shall investigate on other possibly observable, general relativistic, orbital effects and on other potential improvements in testing general relativity and gravitation which might be performed by using data from laser ranged satellites and from LAGEOS III (in collaboration with John Anderson, Bruno Bertotti, Fernando de Felice, Ronald Hellings, Richard Matzner, Kenneth Nordtvedt and Juan Pérez Mercader).