Department of Physics University of Maryland, College Park, MD 20742
LAGEOS is a high-density geodetic satellite launched by NASA on 4 May 1976 [Johnson, et al., 1976]. Using a network of laser ranging stations, GSFC/NASA has obtained extremely accurate information on the orbital motion of LAGEOS and on the time-dependent evolution of its orbital parameters. The development of short-pulse laser ranging systems and better models for atmospheric refractive effects have improved the ability to locate the spacecraft or measure geodetic position and terrestrial crustal motion, but these systems do not measure the satellite rotational motion or gyroscopic effects. It is because of the potential for a second scientific use of the LAGEOS spacecraft, a use in modern physics quite distinct from their use in modern geodesy, that we seek to determine several subtle effects caused entirely by the spin rotation of these nearly spherically symmetric spacecraft. Because of the very precise orbital information, the LAGEOS satellites can be very effective tools in performing a precise measurement of one of the most interesting consequences of General Relativity, in the concept advanced for the "LAGEOS III" experiment to measure the Lense-Thirring effect [Ciufolini, 1986]. Thus an important requirement for the success of this experiment proves to be a precise knowledge of the orientation of the spin axes of the satellites [Ries, et al., 1988]. Recently such a body of directly observed data on the orientation of the spin axis of LAGEOS I has been obtained [Currie, et al., 1992], [Currie, et al., 1995].
From the point of view of the basic physics governing a spinning artificial satellite, the LAGEOS satellite is a nearly ideal candidate for developing a verifiable theoretical model of the rotational dynamics as influenced by the Lorenz and tidal forces. This theory can then be the basis for the study of "photon rocket thrust", that is, the response of the spacecraft to the combined radiation fields of the sun (visible wavelength) and of the earth (IR wavelengths) on the orbital elements of the satellite. However, in order to address the question of the adequacy of a given theoretical model, precise direct observations of the orientation of the spin axis must be available. A body of such data has been obtained by our observations and from various other sources and analyzed for the first time within a University of Maryland program conducted over the past five years. The techniques used in these observations, the analysis methods and the results of this program will be presented. This approach has determined the orientation of the spin axis generally to an accuracy much better than one degree and the rotation rate to much better than 1 percent.
In addition, a comparison between the observational data and predictions of various theoretical models and the results of various indirect techniques will be given. This will include the predictions of the models of Bertotti and Iess [Bertotti and Iess, 1991], Miller and Habib [Habib, et al., 1994], and Vokrouhlicky and Farinella [Farinella, et al., 1995]. These comparisons will be made with both computer integrations starting from the initial conditions of the satellite launch, and, where available, integrations starting from a more-recent epoch, the initial conditions defined by the measurements from the University of Maryland program. Comparisons will also be presented between our direct observations and, first, the indirect information obtained by the theoretical analysis of observations of the orbital acceleration [Rubincam, 1990], and, second, the change in the orbital acceleration caused by the passage of the satellite through the shadow of the earth [Ries, et al., 1993], [Robbins, et al., 1994].
Finally, in order to increase the time interval for the comparison to theory, data from other sources have been analyzed. The results of the analysis of observations performed at the AMOS facility on Maui, and the observations using various techniques performed in the late 1970s and early 1980s by the Lincoln Laboratories of MIT [Rork, 1996] and other groups [Kissell, 1992] will be presented. Candidate systems and new procedures proposed to obtain observations in the future will also be addressed.
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