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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
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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= (4f—7 - 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
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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
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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.
Representative terms from entire chapter:
distance measurements
