Eunice L. Lau

Study of the anomalous acceleration of Pioneer 10 and 11

Our previous analyses of radio Doppler and ranging data from distant spacecraft in the solar system indicated that an apparent anomalous acceleration is acting on Pioneer 10 and 11, with a magnitude a_P ~ 8 x 10^{-8} cm/s^2, directed towards the Sun (anderson,moriond). Much effort has been expended looking for possible systematic origins of the residuals, but none has been found. A detailed investigation of effects both external to and internal to the spacecraft, as well as those due to modeling and computational techniques, is provided. We also discuss the methods, theoretical models, and experimental techniques used to detect and study small forces acting on interplanetary spacecraft. These include the methods of radio Doppler data collection, data editing, and data reduction. There is now further data for the Pioneer 10 orbit determination. The extended Pioneer 10 data set spans 3 January 1987 to 22 July 1998. [For Pioneer 11 the shorter span goes from 5 January 1987 to the time of loss of coherent data on 1 October 1990.] With these data sets and more detailed studies of all the systematics, we now give a result, of a_P = (8.74 +/- 1.33) x 10^{-8} cm/s^2. (Annual/diurnal variations on top of a_P, that leave a_P unchanged, are also reported and discussed.)

Indication, from Pioneer 10 / 11, Galileo, and Ulysses data, of an apparent anomalous, weak, long range acceleration

Radio metric data from the Pioneer 10/11, Galileo, and Ulysses spacecraft indicate an apparent anomalous, constant, acceleration acting on the spacecraft with a magnitude

The mass, gravity field, and ephemeris of Mercury

\ensuremath{\sim}8.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\mathrm{cm}/{\mathrm{s}}^{2}

Io's gravity field and interior structure

, directed towards the Sun. Two independent codes and physical strategies have been used to analyze the data. A number of potential causes have been ruled out. We discuss future kinematic tests and possible origins of the signal.

A STUDY OF THE PIONEER ANOMALY: NEW DATA AND OBJECTIVES FOR NEW INVESTIGATION

This paper represents a final report on the gravity analysis of radio Doppler and range data generated by the Deep Space Network (DSN) with Mariner 10 during two of its encounters with Mercury in March 1974 and March 1975. A combined least-squares fit to Doppler data from both encounters has resulted in a determination of two second degree gravity harmonics, J2 = (6.0 ± 2.0) × 10−5 and C22 = (1.0 ± 0.5) × 10−5, referred to an equatorial radius of 2439 km, plus an indication of a gravity anomaly in the region of closest approach of Mariner 10 to Mercury in March 1975 amounting to a mass deficiency of about GM = −0.1 km3sec−2. An analysis is included that defends the integrity of previously published values for the mass of Mercury (H. T. Howard et al. 1974, Science 185, 179–180; P. B. Esposito, J. D. Anderson, and A. T. Y. Ng 1978, COSPAR: Space Res. 17, 639–644). This is in response to a published suggestion by R. A. Lyttleton (1980, Q. J. R. Astron. Soc. 21, 400–413; 1981, Q. J. R. Astron. Soc. 22, 322–323) that the accepted values may be in error by more than 30%. We conclude that there is no basis for being suspicious of the earlier determinations and obtain a mass GM = 22,032.09 ± 0.91 km3sec−2 or a Sun to Mercury mass ratio of 6,023,600 ± 250. The corresponding mean density of Mercury is 5.43 ± 0.01 g cm−3. The one-sigma error limits on the gravity results include an assessment of systematic error, including the possibility that harmonics other than J2and C22 are significantly different from zero. A discussion of the utility of the DSN radio range data obtained with Mariner 10 is included. These data are most applicable to the improvement of the ephemeris of Mercury, in particular the determination of the precession of the perihelion.

Search for a standard explanation of the Pioneer anomaly

Radio Doppler data generated by the Deep Space Network (DSN) from four encounters of the Galileo spacecraft with Io, Jupiters innermost Galilean satellite, are used to infer Ios gravitational quadrupole moments. By combining the four flybys into a single solution for the gravity field, the response of Io to the second degree tidal and rotational potentials is accurately determined. This is characterized by the value of the second degree potential Love number k 2 = 1.2924 ± 0.0027. We construct interior models that satisfy constraints imposed by the mean radius R = 1821.6 ± 0.5 km, the mean density p = 3527.8 ± 2.9 kg/m 3 , and the normalized axial moment of inertia C/M R 2 = 0.37685 ± 0.00035. The gravitationally derived figure of Io has principal axes (c < b < a) a = 1830.0 ± 0.5 km, b = 1819.2 ± 0.5 km, and c = 1815.6 ± 0.5 km, consistent with the shape determined by imaging. Gravitational and other data strongly suggest that Io is in hydrostatic equilibrium. In this case, models of Ios interior density show that Io almost certainly has a metallic core with a radius between 550 and 900 km for an Fe-FeS core or between 350 and 650 km for an Fe core. Io is also likely to have a crust and a partially molten asthenosphere, but their thicknesses cannot be separately or uniquely determined from the gravitational data.

Tests of general relativity using astrometric and radio metric observations of the planets

The Pioneer 10/11 spacecraft yielded the most precise navigation in deep space to date. However, their radiometric tracking data has consistently indicated the presence of a small, anomalous, Doppler frequency drift. The drift is a blue shift, uniformly changing with a rate of ~6 × 10-9Hz/s and can be interpreted as a constant sunward acceleration of each particular spacecraft of aP = (8.74±1.33) × 10-10m/s2 (or, alternatively, a time acceleration of at = (2.92±0.44) × 10-18s/s2). This signal has become known as the Pioneer anomaly; the nature of this anomaly remains unexplained. We discuss the current state of the efforts to retrieve the entire data sets of the Pioneer 10 and 11 radiometric Doppler data. We also report on the availability of recently recovered telemetry files that may be used to reconstruct the engineering history of both spacecraft using original project documentation and newly developed software tools. We discuss possible ways to further investigate the discovered effect using these te...

Radar and spacecraft ranging to Mercury between 1966 and 1988

The data from Pioneer 10 and 11 shows an anomalous, constant, Doppler frequency drift that can be interpreted as an acceleration directed towards the Sun of aP = (8.74 ± 1.33) × 10-8cm/s2. Although one can consider a new physical origin for the anomaly, one must first investigate the contributions of the prime candidates, which are systematics generated on board. Here we expand upon previous analyses of thermal systematics. We demonstrate that thermal models put forth so far are not supported by the analyzed data. Possible ways to further investigate the nature of the anomaly are proposed.

RADIO-SCIENCE INVESTIGATION ON A MERCURY ORBITER MISSION

Abstract Current least squares fits to solar system data, including transit circle observations of the terrestrial and giant planets, radar observations of the terrestrial planets, Mariner 9 range fixes to Mars, and Pioneer 10 11 range fixes to Jupiter, have yielded some new results of interest to experimental relativity. Solutions have been obtained for the PPN parameters β and γ, the solar gravitational quadrupole moment J 2 , a time variation in the gravitational constant G , and four Nordtvedt parameters. The existing planetary data provide no significant result on the Nordtvedt effect. Under the assumption that the precession of the perihelion of Mercury is caused exclusively by planetary perturbations, general relativity, and the gravitational quadrupole moment, a value of (2.5 ± 1.6) × 10 −6 is obtained for J 2 . There are various combinations of the PPN parameters β, γ, α 1 , and α 3 that will also yield the observed precession of the perihelion, but there is no real empirical evidence for PPN values which disagree with general relativity. A positive upper bound of 1.4 × 10 −10 yr −1 is indicated for G G from existing data. A few months of Viking data are needed to refine this result, and at least 2.5 yr of Viking data are needed to determine the Nordtvedt effect to the same accuracy as the current lunar laser result.

Measurements of Space Curvature by Solar Mass

Improved solutions have been obtained for the orbit and equatorial cross-section of Mercury using radar ranging data spanning 22 years. These data have yielded new results on the precession of Mercurys perihelion and better limits on a possible time variation in the gravitational constant G.