3.3 Available Doppler data

The Pioneer 10 and 11 spacecraft were in space for more than three decades. The original 2002 study [27Jump To The Next Citation Point] of the Pioneer anomaly (see Section 5) used approximately 11.5 years of Pioneer 10 and 3.5 years of Pioneer 11 Doppler data. These data points were obtained from the late phases of the respective missions, when the spacecraft were far from the Earth and the Sun.

An effort was launched at JPL in June 2005 to recover more Pioneer Doppler data, preferably the entire Doppler mission records, if possible: almost 30 years of Pioneer 10 and 20 years of Pioneer 11 Doppler data, most of which was never used previously in the investigation of the anomalous acceleration. This task proved much harder than anticipated, due to antiquated file formats, missing data, and corrupted files (see discussion below and [397Jump To The Next Citation Point]).

Below we discuss these two sets of Pioneer 10 and 11 Doppler data.

3.3.1 Pioneer Doppler data available in 2002

The anomalous acceleration of the Pioneer spacecraft was first reported in 1998 [24Jump To The Next Citation Point]. This effort utilized Pioneer 10 data from 1987 to 1995, and a shorter span of Pioneer 11 data to obtain accelerations of aP 10 = (8.09 ± 0.20) × 10−10 m ∕s2, and aP 11 = (8.56 ± 0.11) × 10−10 m ∕s2, respectively, using JPL’s Orbit Determination Program (ODP). The Pioneer 10 result was also verified by the Aerospace Corporation’s Compact High Accuracy Satellite Motion Program (CHASMP), which yielded −10 2 aP 10 = (8.65 ± 0.03) × 10 m ∕s. The errors quoted are the statistical formal errors produced by the fitting procedure.

In 2002, JPL published the results of a study that, to this date, remains the definitive result on the Pioneer anomaly [27Jump To The Next Citation Point] (see Section 5). In this study, a significantly expanded set of Doppler data was utilized. For Pioneer 10, the data set covered the period between January 3, 1987 and July 22, 1998. This corresponds to heliocentric distances between 40 and 70.5 AU. The data set contained 19,403 Doppler data points. For Pioneer 11, the data set was smaller: the trajectory of the spacecraft between January 5, 1987 and October 1, 1990 was covered, corresponding to heliocentric distances between 22.42 and 31.7 AU, with 10,252 Doppler data points.

The 2002 JPL study was also the first to combine Pioneer 10 and Pioneer 11 results. The study attempted to estimate realistic errors, e.g., errors due to physical or computational systematic effects. This approach resulted in the value of aP = (8.74 ± 1.33) × 10−10 m ∕s2, which is now the widely quoted “canonical” value of the anomalous acceleration of Pioneer 10 and 11.

3.3.2 The extended Pioneer Doppler data set

The recovery of radiometric Doppler data for a mission with such a long duration was never attempted before. It presents unique challenges, as a result of changing data formats, changes in navigational software and supporting hardware, changes in the configuration of the DSN (new stations built, old ones demolished, relocated, or upgraded), and the loss of people [397Jump To The Next Citation Point]. Even physically locating the data proved to be a difficult task, as incomplete holdings were scattered among various archives. Nonetheless, as of November 2009, the transfer of the available Pioneer Doppler data to modern media formats has been completed.

Initially, the following sources for Doppler data were considered:

During these data collection efforts, multiple serious problems were encountered (see details in [397Jump To The Next Citation Point]), including

Despite the unanticipated complexities, as of late 2009 the transfer of the available Pioneer Doppler data to modern media formats has been completed. Initial analysis of these data is under way, and it appears that while Pioneer 10 data prior to February 1980 is not usable, coverage is nearly continuous from that data until the end of mission. Similarly, for Pioneer 11, good data is available from mid-1978 until the loss of coherent mode in late 1990. Further details will be reported as appropriate upon the conclusion of this initial data analysis.

To summarize, there exists more than 20 years of Pioneer 10 and more than 10 years of Pioneer 11 data, a significant fraction of which had never been well studied for the purposes of anomaly investigation. This new, expanded data set may make it possible to answer questions concerning the constancy and direction of the anomalous acceleration.

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