John D. Anderson

The Pioneer 10/11 spacecraft yielded the most precise navigation in deep space to date. However, their radio-metric 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 × 10-9 Hz/s and can be interpreted as a constant sunward acceleration of each particular spacecraft of aP = (8.74 ± 1.33) × 10-10 m/s2. The nature of this anomaly remains unexplained. Here we summarize our current knowledge of the discovered effect and review some of the mechanisms proposed for its explanation. Currently we are preparing for the analysis of the entire set of the available Pioneer 10/11 Doppler data which may shed a new light on the origin of the anomaly. We present a preliminary assessment of such an intriguing possibility.

Analysis of the radio-metric tracking data from the Pioneer 10 and 11 spacecraft at distances between 20--70 astronomical units from the Sun has consistently indicated the presence of an anomalous, small, and constant Doppler frequency drift. The drift is a blue-shift, uniformly changing at the rate of (5.99 +/- 0.01) x 10^{-9} Hz/s. The signal also can be interpreted as a constant acceleration of each particular spacecraft of (8.74 +/- 1.33) x 10^{-8} cm/s^2 directed toward the Sun. This interpretation has become known as the Pioneer anomaly. We provide a problem set based on the detailed investigation of this anomaly, the nature of which remains unexplained.

Radio Doppler data from the Galileo spacecraft's encounter with Amalthea, one of Jupiter's small inner moons, on 5 November 2002 yield a mass of (2.08 ± 0.15) × 10 ¹⁸ kilograms. Images of Amalthea from two Voyager spacecraft in 1979 and Galileo imaging between November 1996 and June 1997 yield a volume of (2.43 ± 0.22) × 10 ⁶ cubic kilometers. The satellite thus has a density of 857 ± 99 kilograms per cubic meter. We suggest that Amalthea is porous and composed of water ice, as well as rocky material, and thus formed in a cold region of the solar system, possibly not at its present location near Jupiter.

The Pioneer 10 and 11 spacecraft had exceptional deep-space navigational capabilities. The accuracies of their orbit reconstruction were limited, however, by a small, anomalous, Doppler frequency drift that can be interpreted as an acceleration of (8.74 +/- 1.33) x 10^{-8} cm/s^2 directed toward the Sun. We investigate the possibility that this anomaly could be due to a drag on the spacecraft from their passing through the interplanetary medium. Although this mechanism is an appealing one, the existing Pioneer radiometric data would require an unexpectedly high mass density of interplanetary dust for this mechanism to work. Further, the magnitude of the density would have to be nearly constant at distances ~ 20-70 AU. Therefore, it appears that such an explanation is very unlikely, if not ruled out. Despite this, the measured frequency drift by itself places a directly-measured, model-independent limit of \lessim 3 x 10^{-19} g/cm^3 on the mass density of interplanetary dust in the outer(~20-70 AU) solar system. Lower experimental limits can be placed if one presumes a model that varies with distance. An example is the limit \lessim 6 x 10^{-20} g/cm^3 obtained for the model with an axially-symmetric density distribution that falls off as the inverse of the distance. We emphasize that the limits obtained are experimentally-measured, in situ limits. A mission to investigate the anomaly would be able to place a better limit on the density, or perhaps even to measure it.

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.)

Radio Doppler data at X band (∼8.4 GHz) and spacecraft attitude control data were obtained during the encounter with comet 81P/Wild 2. The Doppler data set an upper bound of 5 × 10¹⁵ kg on the mass of the nucleus, about 100 times larger than theoretical estimates. The attitude control data indicate that a large dust particle with a mass in the range of 20 to 40 mg impacted the spacecraft about 15.5 s before closest approach.

The radio-metric Doppler tracking data from the Pioneer 10/11 spacecraft, from between 20-70 AU, yields an unambiguous and independently confirmed anomalous blue shift drift of a_t = (2.92 \pm 0.44)\times 10^{-18} s/s^2. It can be interpreted as being due to a constant acceleration of a_P = (8.74 \pm 1.33) \times 10^{-8} cm/s^2 directed towards the Sun. No systematic effect has been able to explain the anomaly, even though such an origin is an obvious candidate. We discuss what has been learned (and what might still be learned) from the data about the anomaly, its origin, and the mission design characteristics that would be needed to test it. Future mission options are proposed.

We present the discovery of mass anomalies on Ganymede, Jupiter's third and largest Galilean satellite. This discovery is surprising for such a large icy satellite. We used the radio Doppler data generated with the Galileo spacecraft during its second encounter with Ganymede on 6 September 1996 to model the mass anomalies. Two surface mass anomalies, one a positive mass at high latitude and the other a negative mass at low latitude, can explain the data. There are no obvious geological features that can be identified with the anomalies.

Radio Doppler and ranging data at X band (∼8.4 GHz), optical navigation data, and spacecraft attitude control data can be used to characterize the dust environment surrounding comet 81P/Wild 2 and possibly the mass and bulk density of its nucleus. As a secondary objective, a favorable solar conjunction prior to the cometary encounter on 2 January 2004 can be used to sound the Sun's corona at X band.