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4 Current Space Research in Gravitation At present, the U.S. space program is involved in gravitational research In several areas. The lunar ranging program and the con- t~nu~ng program to establish the motions of solar system bodies constitute an important ongoing research area. These are de- scribed in this chapter. The development of the magnetic gravity gyro experiment (the major gravitational program at NASA), the development of the cryogenic principle of equivalence experiment (supported by PACE and NSF), and the search for long-period gravitational radiation by tracking Galileo and Ulysses are de- scribed In subsequent chapters. LUNAR RANGING The optical retroreflector packages placed on the lunar surface by the Apollo Il. 14, and 15 missions and by Luna 21 make possible highly accurate laser distance measurements to the Moon. The large majority of the data through 1982 was acquired by the McDonald Observatory in Texas. The accuracy is typically 10 to 15 cm. Recently, three other lunar ranging stations have begun reg- ular observations. These stations are in France, Australia, and Hawaii. Three of the four stations have 1.5-m-diameter receiving 26
27 telescopes, and all four are expected to have su~nanosecond puise- length lasers soon. A precision of 1 to 2 cm for roughly 15 min of observing time has been demonstrated, and routine performance with similar accuracy ~ expected. Nordtvecit pointed out In 1968 that lunar range data would provide a sensitive test of whether the gravitational self-energy of the Earth obeys the principle of equivalence. From one point of view, this test gives a check on one of the fundamental assumptions of general relativity. From a different viewpoint, the lunar distance provides the best test at present of the superposition law for gravitation, and thus can be regarded as a fifth test of general relativity. Within the framework of conservative theories without a preferred frame, the size of the effect is given by Ad= (4f7 - 3)dN cmD where AN ~ 10 m and D is the difference in mean longitudes of the Moon and Sun. When combined with the value for 7 from the Viking time-delay measurements, the present lunar ranging results show ~ = I, as predicted by general relativity, with an uncertainty of 0.004. The lunar ranging measurements are likely to continue to play a substantial role in solar system tests of gravitational theory in the future. With the current improvements in measurement accuracy, an uncertainty of less than 0.001 for ~ is expected. A determination of geodetic precession for the lunar orbit should be possible soon, and wiP improve as a longer span of accurate data is obtained. In addition, lunar range data will give an independent check on the constancy of the Newtonian gravitational constant with an expected accuracy of better than one part in 10~t per year. Lunar range data also complement planetary distance measurements by helping to tie down some of the classical parameters needed in order to test relativistic predictions. ANALYSIS OF PLANETARY AND [UNA11 MOTION AI1 important phase of present research in gravitational phys- ics is the analysis of spacecraft tracking data, lunar laser ranging data, and planetary radar measurements in order to determine the dynamics of the solar system. The motions of the inner planets and
28 . the Moon provide our best tests of several gravitational phenom- ena; measurements of the relativistic time delay for electromag- netic signals passing near the Sun also are of major unportance. Striking progress has been made in the last few years by analyz- ing tracking data for the Viking Lander and Orbiter spacecraft, in combination with tracking data for other spacecraft, lunar ranging data, planetary radar data, and other solar system information. One advantage of combined solutions using all available data is that the time base for the observations extends over a longer period than for individual spacecraft missions. Even though the 1971-1972 tracking data for the Mariner 9 Mars Orbiter gave less accurate Earth-Mars distances than the Viking mission provided, the inclusion of these earlier Mariner 9 observations yields con- straints on the orbital motions over a substantially longer time. Since nongravitational forces on the planets and the Moon are neg- ligible, the integrated effects of non-Newtonian effects on orbital motions can be determined more accurately with the extended data sets. Although optical observations of the planets are less ac- curate than spacecraft tracking or radar measurements, they help to determine some orbit parameters to which distance measure- ments are less sensitive. Since the orbit of the Earth is common to all of the data types, improvements in it from one type of oh servation help to increase the strength of the other data types. Thus, all of the accurate data on the motions of the inner planets and the Moon need to be analyzed jointly as an integrated test of the extent to which solar system dynamics obey our current understanding of the laws of gravitation. It is not clear whether new tracking data for planetary or- biters or landers that is useful for gravitational physics will be obtained in the next decade. However, some new radar distance measurements to Mercury are being made, and they could provide substantial improvements in our knowledge of Mercury's perihe- lion precession. Lunar laser range data of improved accuracy and from a number of stations are now being obtained, as discussed previously. In addition, more refined analyses of the Viking track- ing data and of the effects of the asteroids on the motion of Mars are very much needed. In view of the complexity of the solar system en cl the great difficulty of modeling all of the interactions between the different bodies well enough to give their positions with accuracies on the order of one part in 10~2, it ~ essential that the work be carried out by at least two independent groups. Such
29 major analysis efforts currently are being carried out by the comas oration between MIT, Harvard University, and the Smithsonian Astrophysical Observatory and by the Jet Propulsion Laboratory of Cal Tech. The Task Group on Fundamental Physics and Chem- istry believes that strong continued support ~ needed for such independent but mutually supportive efforts in order to achieve continued progress on solar system dynarn~cs tests of gravitational physics during the next decade. More intensive programs of radar distance measurements to Mercury that make use of repeatedly observed "closure points" to reduce uncertainties from planetary topography would be valuable in providing improved tests of gravi- tation, as would increased accuracy for multiple-station lunar laser range measurements.
