W. L. Sjogren

Galileo Gravity Results and the Internal Structure of Io

Doppler data generated with the Galileo spacecrafts radio carrier wave were used to measure Ios external gravitational field. The resulting triaxial field is consistent with the assumption that Io is in tidal and rotational equilibrium. The inescapable conclusion is that it has a large metallic core. If the core is a eutectic mixture of iron and iron sulfide, it comprises 20.2 ± 7.4 percent of the satellites total mass with a radius that is about 52 percent of Ios mean radius of 1821.3 kilometers; if the core is pure iron, it comprises 10.5 ± 3.7 percent of the total mass with a radius of about 36 percent of the mean radius.

Lunar Shape via the Apollo Laser Altimeter

Data from the Apollo 15 and Apollo 16 laser altimeters reveal the first accurate elevation differences between distant features on both sides of the moon. The large far-side depression observed in the Apollo 15 data is not present in the Apollo 16 data. When the laser results are compared with elevations on maps from the Aeronautical Chart and Information Center, differences of 2 kilometers over a few hundred kilometers are detected in the Mare Nubium and Mare Tranquillitatis regions. The Apollo 16 data alone would put a 2-kilometer bulge toward the earth; however, the combined data are best fit by a sphere of radius 1737.7 kilometers. The offset of the center of gravity from the optical center is about 2 kilometers toward the earth and 1 kilometer eastward. The polar direction parameters are not well determined.

Lunar Gravity: Preliminary Estimates from Lunar Orbiter

Tracking data fromt the lunar orbiters have been analyzed for information regarding Moons gravity field. These preliminary results include values of a set of harmonic coefficients throlugh degree 4 in the tesserals and degree 8 in the zonals. Implications regarding Moons mass distribution are discltssed: one implicationt is that Moon is nearly homogeneous.

Lunar gravity via Apollo 14 Doppler radio tracking.

Gravity measurements at high resolution were obtained over a 100-kilometer band from + 70� to -70� of longitude during the orbits of low periapsis altitude (approximately 16 kilometers). The line-of-sight accelerations are plotted on Aeronautical Chart and Information Center mercator charts (scale 1 : 1,000,000) as contours at 10-milligal intervals. Direct correlations between gravity variations and surface features are easily determined. Theophilus, Hipparchus, and Ptolemaeus are negative features, whereas Mare Nectaris is a large positive region. The acceleration profiles over Mare Nectaris are suggestive of a broad disk near the surface rather than a deeply buried spherical body. These data are in good agreement with the short arc of Apollo 12 lunar module descent data.

Lunar gravity via the Apollo 15 and 16 subsatellites

Dense Doppler tracking coverage of the Apollo 15 and 16 subsatellites over ten and eighteen day periods when periapsis altitudes were 15–50 km has provided detailed gravity mapping of the lunar frontside.Many new gravity features are revealed including one that does not correlate with any visible topographic structure. All unfilled craters sampled are negative anomalies. The mascons consistently produce gravity highs that load the surface with ≈800 kg cm−2 excess mass. The Orientale region is represented with a solution grid of 177 point masses that clearly show the ringed structure. The eastern limb is also displayed with a solution grid of point masses. The gravity variations over the central portion of the frontface are shown as line-of-sight acceleration contours in milligals.

Apollo 15 gravity analysis from theS-band transponder experiment

TheS-Band Transponder experiment used precision doppler tracking data of the command and service module, the lunar module and the subsatellite to provide detailed information about the near side gravity field. No special instruments are required other than the existingS-Band transponder used for real time navigation. The data consists of variations in the spacecraft speed as measured by the earth-based radio tracking system, which has a resolution of 0.65 mm/s.Initial data reduction has been concentrated on the low altitude CSM data (≈ 20 km) which provides new detailed gravity profiles of the Serenitatis and Crisium mascons. The results are in good agreement with Apollo 14 analysis and strongly suggest that the mascons are near surface features with a mass distribution per unit area of approximately 800 kg/cm2. The Apennines reveal themselves as a local gravity high of 85 mgal and Marius Hills likewise have a gravity high of 62 mgal.The subsatellite data is too sparse at present to definitely determine new gravity anomaly locations. The spacecraft is functioning well and a dense data block is being obtained, which will provide a new gravity map from ±95° longitude to ±30 latitude. Since periapsis altitudes are following relatively close to predicted altitudes, it seems fairly safe at this point to believe the subsatellite lifetime will be at least one year.

Lunar gravity - Apollo 17

Reduction of doppler radio tracking of the orbiting spacecraft has shown consistency with Apollo 14 data results and has revealed new gravity anomalies. Large craters are negative anomalies while wrinkle ridge regions are positive. The Central highlands are mostly a positive anomaly except for the Apollo 16 landing site, which is in a negative area. A gravity high northwest of Theophilus is not easily explained.

Lunar global figure from mare surface elevations

Laser altimetry data from the Apollo 15, 16, and 17 missions show that the ringed maria surfaces lie on one particular reference surface and that the center of gravity is definitely displaced from the optical center. If these extensive surfaces are assumed to be near hydrostratic surfaces, then there must have existed a time in lunar history when lunar tides and/or internal processes were much different than they are today.

Lunar Orbiter Ranging Data: Initial Results

Data from two Lunar Orbiter spacecraft have been used to test the significance of corrections to the lunar ephemeris. Range residuals of up to 1700 meters were reduced by an order of magnitude by application of the corrections, with most of the residuals reduced to less than 100 meters. Removal of gross errors in the ephemeris reveals residual patterns that may indicate errors in location of observing stations, as well as the expected effects of Lunar nonsphericity.

RADIO-SCIENCE INVESTIGATION ON A MERCURY ORBITER MISSION

Abstract Results from Mariner 10 for Mercurys gravity field and results from radar ranging for elevations are reviewed. Implications of improving these results are discussed, as well as the opportunity to perform relativistic gravity tests with a future Mercury Orbiter. With a spacecraft placed in orbit with periherm at 400 km altitude, apherm at 16,800 km, period 13.45 h and latitude of periherm at +30 deg, a significant improvement in measurements of Mercurys gravity field and geophysical properties will result. The 2000 Plus mission that evolved during the European Space Agency (ESA) Mercury Orbiter assessment study (Hechler, 1994) can provide a global gravity field complete through the 25th degree and order in spherical harmonics. If after completion of the main mission, the periherm could be lowered to 200 km altitude, the gravity field could be extended to 50th degree and order. Also, a search for a Hermean ionosphere could be performed during the mission phases featuring Earth occultations. Because of its relatively large eccentricity and proximity to the Sun, Mercurys orbital motion provides one of the best solar system tests of general relativity. Consequently, a number of feasible relativistic gravity tests are described within the context of the parameterized post-Newtonian formalism. Current results on the relativistic precession of Mercurys perihelion are uncertain by 0.5%, and improvements are feasible with a Mercury Orbiter mission. Also, improved limits on a possible time variation in the gravitational constant G as measured in atomic units are feasible. Moreover, by including a space-borne ultrastable crystal oscillator (USO) or an atomic clock in the Mercury Orbiter payload, a new test of the solar gravitational redshift would be possible to an accuracy of one part in 10 4 with a USO, and to an accuracy of one part in 10 7 with an atomic standard. With an atomic clock and additional hardware for a multi-link Doppler system, including Doppler extraction on the spacecraft, the effect of Mercurys gravity field on the USOs frequency could be measured with an accuracy of one part in 10 6 . Other relativistic effects are discussed including the geodetic precession of the orbiters orbital plane about Mercury, a planetary test of the Equivalence Principle (Nordtvedt effect), and a solar conjunction experiment to measure the relativistic time delay (Shapiro effect).