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New hints on the Pioneer anomaly

par Serge Reynaud - 23 mai 2009

The trajectory of Pioneer 10 and Pioneer 11 probes in the outer solar system was found not to fit the awaited one. This unexpected discovery has been published a decade ago by a team working at JPL, a NASA lab. A collaboration of french physicists working at ONERA, Géosciences Azur and Laboratoire Kastler Brossel have added new hints to this file by producing a new detailed analysis of the JPL data.

Launched in 1972 and 1973, the Pioneer 10 and 11 probes have been the first visitors of the giant planets Jupiter and Saturne. After having met successfully their primary mission, they have continued their travel to the frontiers of the solar system and sent back to Earth numerous observations of interest. In particular, they performed the first large scale test of the law of gravity and thus discovered a trajectory anomaly which is still not understood today.

The trajectories are monitored by Doppler tracking from the antennas of NASA Deep Space Network. The Doppler signals show a deviation from the prediction which has been characterized by the JPL team as as anomalous acceleration directed towards the center of the solar system with an amplitude of 0,9 nanometers per second squared. Although weak, this deviation is much larger than the instrumental resolution. Despite a lot of work, no convincing explanation has been proposed to date. Should the deviation be related to some anomalous behaviour of gravitation, it would have a large impact on fondamental physics, solar system physics and astrophysics.

The data used by the JPL team have been made available to an international collaboration with the aim of characterizing the anomaly in the best possible manner. In this context, a french collaboration has re-analyzed the Doppler data covering the period from November 1986 to July 1998 for Pioneer 10. To this aim, they have developed a specific orbitography software. They have thus confirmed the presence of a secular anomaly, with an amplitude roughly equal to that found in the JPL analysis.

They have also revealed modulated anomalies in the Doppler signal with periods corresponding to the diurnal and annual motions of Earth. These effects with annual, semi-annual and diurnal periods appear to be accounted for by a single angle associated to the geometry of signal propagation between Earth and the probe. The new hints, which have to be put in the already thick file of the Pioneer anomaly, seem to discard the interpretations involving only the probe itself or its environment.

More information

- Pioneer 10 Doppler data analysis : disentangling periodic and secular anomalies, Agnès Levy, Bruno Christophe, Philippe Berio, Gilles Metris, Jean-Michel Courty, Serge Reynaud, Advances in Space Research 43, 10 (2009) 1538-1544 [ hal-00322209]

- a news by Maurice Mashaal on the site (in french) of Pour la Science

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